Sector field mass spectrometers in ICP-MS

Review

Sector field mass spectrometers in ICP-MS

Norbert Jakubowskia,* , Luc Moensb, Frank Vanhaeckeb

aInstitut fur Spektrochemie und angewandte Spektroskopie, Postfach 10 13 52, D-44013 Dortmund, GermanybGhent University, Laboratory of Analytical Chemistry, Proeftuinstraat 86, B-9000 Ghent, Belgium

Received 23 April 1998; accepted 17 August 1998

Abstract

A new generation of sector field mass spectrometers, with improved analytical figures of merit at even lower prices, iscommercially available, giving a strong impetus to the development of inductively coupled plasma mass spectrometry (ICP-MS) sector field instrument applications in the analytical community. It is the aim of this paper to give an overview of theseinstruments, to introduce some basic concepts, to discuss their peculiarities and performance, and to present some selectedexamples of analytical applications to demonstrate the ‘state of the art’.q 1998 Elsevier Science B.V. All rights reserved.

Keywords:Inductively coupled plasma mass spectrometry; Sector field; High mass resolution

1. Introduction

Inductively coupled plasma mass spectrometry(ICP-MS) has matured into one of the most successfulmethods in atomic spectrometry, owing to its highdetection power and true multi-element capabilities[1, 2]. Nevertheless, the analytical figures of meritare limited by spectroscopic and non-spectroscopicinterferences. Spectroscopic interferences are causedby atomic or molecular ions having the same nominalmass as the analyte isotope of interest. They maydisturb or even obscure the true analytical signal, sothat the accuracy of the determination as well as thedetection limits for the elements investigated may beconsiderably deteriorated if low resolution instru-ments are applied. The sources from which the differ-ent interfering species may arise are manifold. Coldboundary layers at the skimmer and sampler orificehave been mentioned in the early days of ICP-MS [3].In the actual development, this influence is reduced

more and more by increasing diameters of the samplerand skimmer orifice. Nevertheless, these cones stillplay an important role in the appearance of molecularspecies. Secondary discharges taking place in theinterface region can be an additional source forsome specific interferences [4], and shielding of theplasma torch from the induction coil, often applied inup-to-date instrumentation, becomes an importanttool for their reduction. Without exaggeration, it canbe stated that the history of ICP-MS is, to a majorextent, also the history of a permanent fight againstspectroscopic interferences as a basically aggravating,but inevitable, phenomenon. To date, no well-accepted general model to explain all the differentcontributions exists and, at best, people have learnedto live with these entities and many different, mainlyhyphenated, techniques have been considered for thereduction of interferences, a detailed discussion ofwhich is given elsewhere [5].

Hyphenated techniques at the front end of the ICP-MS include the application of alternative sampleintroduction and aerosol processing techniques, such

Spectrochimica Acta Part B 53 (1998) 1739–1763

0584-8547/98/$ - see front matterq 1998 Elsevier Science B.V. All rights reserved.PII: S0584-8547(98)00222-5

* Corresponding author. e-mail: [email protected]

as electrothermal vaporization or laser ablation, toavoid sample dissolution and thus interferences other-wise caused by solvents. Some of the most powerfulhyphenated techniques aim at preconcentration oftraces, matrix-trace separation or solvent removal,which can enhance the sensitivity of interferedisotopes and/or reduce interferences caused by thematrix or by the solvents used. Another front-endtechnique, the ‘Cold Plasma’ approach, is well-estab-lished now for most quadrupole-based instrumentsand is helpful to overcome the interferences by mole-cular ions of the argon discharge gas (for a moredetailed discussion see reference [6]). Other techni-ques are provided as interface techniques in the regionbetween the ICP and the mass spectrometer. A veryrecent example of this is the insertion of a ‘CollisionCell’, formerly already well-established in organicmass spectrometry [7]. In this cell, a gas mixtureflows into a hexapole or octopole rod assembly, oper-ated as a bandpass filter with rf high voltage only.Depending on the gas mixture, molecular ions canexchange their charge with gas constituents and thusbecome neutralized, which efficiently reduces argon-based interferences.

Although some approaches have been appliedsuccessfully, none of these can cope with the problemin general. They all are limited to some specific inter-ferences or are applicable for some selected matricesor elements only. It has been recognized since theoutset of ICP-MS that the only general method toovercome limitations from spectroscopic interfer-ences is the application of high mass resolution.Although mass spectrometers of different types canbe operated at higher mass resolution, until nowonly ICP-MS instruments based on double focusingsector field mass spectrometers (ICP-SFMS) havebeen commercially available. Although availablesince 1988, they have not found widespread accep-tance until recently, when the high costs of the initialgeneration of instruments were considerably reducedwith the introduction of a second generation of sectorfield instruments. This gave strong impetus to thedevelopment of ICP-SFMS applications in the analy-tical community, which is reflected in an increasingnumber of publications and a growing interest in theanalytical performance of this technique. It is the aimof this paper to give an overview of the commerciallyavailable ICP-SFMS instruments, to introduce some

basic concepts, to discuss the peculiarities and perfor-mance of these instruments, and to present someselected examples of analytical applications todemonstrate the ‘state of the art’.

To avoid confusion, the following acronyms will beused throughout this paper, irrespective of whether ornot these are in agreement with other literature: ICP-MS is used whenever general aspects of the methodare concerned; ICP-QMS is used whenever the massanalyser in question is a low resolution quadrupolefilter; ICP-SFMS is used whenever an instrumentwith sector fields is applied. The latter instrumentscan be operated at low mass resolution (LMR) or athigh mass resolution (HMR).

2. Fundamentals

2.1. Spectroscopic interferences

Spectroscopic interferences may be subdivided intoseveral groups, as they can be attributed to thepresence of isobaric atomic ions, multiply chargedions and molecular ions of various origin. Isobaricoverlap exists where isotopes of different elementscoincide at the same nominal mass, and the maximummass resolution setting of commercial ICP-SFMSinstruments is by far not sufficient to overcome inter-ferences of this kind. Therefore, an alternativeapproach to cope with this problem is used. Foreach element—with the exception of only indium—at least one isotope can be found free from isobaricoverlap, but in many cases this will not be the mostabundant isotope. Multiply charged ions will be foundin the mass spectrum at a positionm/z, wherez is theion charge. In particular, doubly charged ions of themain matrix constituents are frequent contributors.Molecular ions may consist of atoms of the dischargegas and its contaminants, and/or components of thesolvent and the matrix.

Among all these spectroscopic interferences, multi-ply charged ions and molecular ions cause the mostsevere problems and thus their origin shall briefly bediscussed. Ions with double or multiple charge can begenerated by any of the ionization processes in theplasma and interface, depending on the ionizationenergies of the elements; in particular, this happensas a result of secondary discharges in the interface

N. Jakubowski et al. / Spectrochimica Acta Part B 53 (1998) 1739–17631740

region. Molecular interferences may be introducedby the analytical sample itself, such as oxideswhich, owing to their high bond strength, have areal chance of ‘surviving’ the passage through thehot zone of the plasma. They may also arise from

the reagents used for sample pretreatment. Anotherimportant source of molecular ions is formation ofcluster ions from the dominant species in theplasma, which preferentially occurs in cool plasmaboundaries (possibly at the walls of the skimmer)or in the sampling and expansion areas of theinterface.

2.2. Mass resolution

Whether or not an interfering ion signal will beseparated from an analytical signal depends on themass difference and the resolution of the instrument.Mass resolutionR is generally defined as

R� m=Dm �1�whereDm is the mass difference necessary to achievea valley of 10% between two neighbouring peaks ofidentical intensity at massm andm^ Dm. However,since the intensities of neighbouring peaks are rarelyidentical, an alternative definition is much moreuseful in praxi. In this definitionDm is derived fromthe peak width at the points in the profile which corre-spond to 5% of the peak height of a single interfer-ence-free peak. This will lead to the same value as inthe 10% definition, if the neighbouring peaks aresymmetric and equally high.

How useful high mass resolution can be will bediscussed using as an example the determination ofTi in a solution containing HNO3 and H2SO4 as maincomponents and Ca as a trace element. Fig. 1(a)shows the analytical situation for the main isotopeof Ti at mass 48 for a sector field instrument operatedat a resolution of 300. At this resolution setting, thedifferent contributions coming from the solvents andother constituents of the sample cannot be identifiedor separated, and therefore they all contribute to therecorded profile (black profile in Fig. 1(a)). In Fig.1(b) the situation changes, when the mass resolutionis increased toR� 3000. The signal from Ti can nowbe clearly separated from interfering components,except from the signal of Ca. This is an isobaric inter-ference and needs an even higher resolution, as isshown in more detail in Fig. 1(c). Here, a resolutionof R � 8000 is sufficient to identify the Ca interfer-ence, but for separation an even higher resolution isrequired (R � 10 500). From this point of view, thehighest resolution setting would be necessary to

N. Jakubowski et al. / Spectrochimica Acta Part B 53 (1998) 1739–1763 1741

Fig. 1. Signal from Ti at mass 48 simulated by a computer program(Finnigan MAT, Bremen, Germany) for a solution containing HNO3

and H2SO4 as the main and Ca as the minor component. The masswindow is shown at mass 48: (a) in low resolution mode (R� 300);(b) in medium resolution mode (R� 3000); (c) in high resolutionmode (R � 8000). Black signal: signal resulting from isotope ofinterest and from interfering ions. Gray signal: isotope of interest.

separate all contributing interfering species, but froman analytical point of view a resolution ofR� 3000would already be sufficient, because Ca can be deter-mined in this sample by using its isotope at mass 44,which—although spectrally interfered by CO2

1—canbe quantified in the same run atR� 3000 and used forcorrection at mass 48.

Typical examples of spectroscopic interferencesare compiled in Table 1, juxtaposing the affectedatomic species with the interfering ion and the massresolution required for peak separation. It can be seenfrom this table that the lowest resolution is required toseparate the main isotope of Si,28Si, from 14N2, aninterference mainly caused by the use of HNO3. Addi-tionally it can be seen that, in many cases, a resolutionof R � 3000 will be sufficient to eliminate interfer-ences due to polyatomic ions, but some examples areleft, i.e. 80Se1 and 40Ar2

1, and 75As1 and 40Ar35Cl1,for which resolution settings of more than 7500 arerequired. Commercial high resolution instruments, asconsidered in this paper, provide a resolution some-what above 10 000, so that most of the interferencesshown in Table 1 can clearly be overcome. Neverthe-less, high mass resolution is not a panacea to all types

of spectroscopic interferences. Most of the isobaricinterferences cannot be resolved, as has beenmentioned, and even some polyatomic argides,hydrides and oxides of matrix components require aresolution higher than or close to the upper end of theachievable range.

2.3. Double focusing

A mass resolution of up to 10 000 and more, whichshall be defined here as ‘high’ resolution, is usuallyachieved with double focusing instruments on thebasis of a magnetic and an electric sector field.These instruments have an even longer tradition inMS than quadrupoles, although technically moresophisticated. When supposing that a non-divergingbeam of mono-energetic ions of massm, which wereaccelerated in an electric field with a potential differ-enceUa, is injected perpendicularly into a magneticsector field B, then all ions will move on a circle withradius rm due to the Lorentz force of the magneticfield. Under these conditions the ratiom/z (z� ioniccharge) can be calculated from:

m=z� B2r2m=2Ua �2�

with Ua, B andrm as defined above.In the usual configuration, the magnetic field has

the properties of an optical lens. Thus the ion beam,with a cross-section defined by an entrance slit, will befocused by the sector field to an image of this slitbehind the field where an exit slit can be used forfurther refinement. From this point of view, themagnetic field has directional focusing properties.The image of the entrance slit, however, can beconsiderably broadened if the ions have a certainenergy spread. The problem is that the magneticfield has energy dispersion properties, too, as can beseen from Eq. (2), where different energies will resultin different radii in the magnetic field. Such differ-ences in ion energy can be caused by the ICP ionsource itself as well as by the ion extraction process.Therefore, a second analyser is required which has anenergy dispersion too, but opposite to that of themagnetic sector. Such an analyser can be realized byan electric sector field which is simply a cylindrical,or better a toroidal sector condensor. The energydispersion of the electric sector is used to compensatefor that of the magnetic device, so that finally only


Table 1Mass resolution (R) necessary to resolve typical interferences

Molecular ion Nuclide Resolution

R , 300012C2

1 24Mg1 160514N2

1 28Si1 95812C16O1 28Si1 155714N16O1H1 31P1 96815N16O1 31P1 145816O2

1 32S1 180112C16O2

1 44Ca1 128135Cl16O1 51V1 257235Cl16O1H1 52Cr1 167140Ar12C1 52Cr1 237540Ar14N1 54Fe1 208840Ar16O1 56Fe1 250240Ar23Na1 63Cu1 279032S16O2

1 64Zn1 1952

R: 3000–750032S16O1 48Ti 1 251932S2

1 64Zn1 4261R: 7500–10 00040Ar35Cl1 75As1 777540Ar2

1 80Se1 9688

mass dispersion is left. For a better understanding, thefunctioning of a combined device which consists of anelectric and a magnetic sector can best be madeobvious from the ion trajectories calculated by useof the ‘SIMION’ program. In Fig. 2(a), ion trajectoriesare shown for a device which consists of a 908 magnetoperated at 4770 Gauss and a 608 electric sector with avoltage of 1 and 2 410 volts. The mono-energeticions emerging from the entrance slit S1 with an angleof 78 have been accelerated to 8000 volts. The trajec-tories are shown for mass 90, 100 and 110. The opera-tional conditions have been chosen in this example sothat only ions with a mass of 100 can reach the exit slitS2, after which the detector is located. Although theions have a certain angular spread, they are focused bythe magnetic as well as by the electric field. From thispoint of view, both sector fields have directional(angular) focusing properties. In Fig. 2(b), the situa-tion is shown where only ions of mass 100 are trans-mitted through the entrance slit, but with an energy

spread of 500 eV. These ions are now not well-focused by the magnet, and for a single magneticdevice the resolving power would be worse. Onlyby use of an electric sector field are all ions well-focused into the exit slit owing to the energy disper-sion of the electric sector. The combined systemfocuses both in angle and energy, and this is thereason why this arrangement often is called doublefocusing. It should be pointed out that these doublefocusing conditions can only be realized with specificcombinations of the electric and magnetic sector fieldangles and appropriate geometries.

In the resulting instrument, the mass resolution isdetermined by the slit widths. With the slits fullyopen, the instrument can be operated in a low resolu-tion mode which is characterized by peaks with atrapezoidal shape. This peak shape is advantageousif an instrument is operated in a peak jumping modebecause small changes in the mass positioning willstill lead to the same intensity value. By decreasing


Fig. 2. SIMION calculations of ion trajectories in a double focusing instrument with a 908 magnet operated at 4770 Gauss and a 608 electricsector with a voltage of1 and 2 410 V; Ua � 8000 V (see Ref. [77]): (a) ion trajectories for mass 90, 100 and 110 are shown for mono-energetic ions emerging from the entrance slit S1 with an angle of 78; (b) ion trajectories for mass 100 with an energy spread of 500 eV.

the slit widths, the resolution will be increased. Animportant difference to quadrupole operation shouldbe pointed out here. With a double focusing instru-ment, a variation of the peak width with the mass canbe observed, while the resolution is constant over thewhole mass range, whereas a quadrupole mass analy-ser in normal operation displays constant peak widthwith mass and thus varying mass resolution.

3. Instruments

An ICP-SFMS instrument as discussed above isschematically shown in Fig. 3. Ion source, samplinginterface and subsequent lens system are necessary asin quadrupole-based instruments. However, sometechnical peculiarities of double focusing instrumentsshould be mentioned here. A major difference is therequirement of a high voltage for acceleration. Aswith MS instruments for organic analysis, highvoltage is usually applied to the ion source and inter-face, keeping the analyser at ground potential. Aspecial lens system, consisting of a quadrupole lenswith an electrode configuration similar to a quadru-pole mass analyser but operated with dc voltages only,is normally used both to focus the ions into the massanalyser and to shape the circular peak profile of theion beam behind the skimmer to the rectangularprofile of the entrance slit. The heart of a sectorfield instrument consists of the entrance slit, themagnetic and electrostatic sector field and the exitslit. The position sequence of the two analyser compo-nents is optional: traditionally the ESA is placedbefore the magnetic sector field, but nowadays the

so-called ‘reverse geometry’ [11] with the ESAbehind the magnetic sector is usually considered tobe advantageous because the high ion currents fromthe source are first reduced by mass analysis, and onlyions of the selected mass are subjected to the subse-quent energy analysis. This helps to improve the‘abundance sensitivity’ as well as to reduce thenoise level.

The first ICP-SFMS instrument in the market wasthe ‘PLASMATRACE I’ introduced in 1988 by VG(Winsford, UK) [8]. The MS part was a modifiedversion of an instrument which had been designed toserve the needs of organic MS. It was arranged with a‘Nier–Johnson geometry’ with a 708 electrostaticsector and a 358 laminated magnetic sector. The reso-lution could be continuously varied from 400 to 8000by motor control of the slit widths. Design of the ionsource and the interface was based on a ICP-QMSsystem of the same manufacturer. An acceleratingvoltage of about 4 kV was applied to the interface.Two independent detectors were used for ion detec-tion: (1) a Faraday cup for the high ion currents ofmajor sample components, and (2) a secondary elec-tron multiplier (SEM) operated in counting mode forminor components and traces. In 1994, an improvedversion, the ‘PLASMATRACE II’, with a reversegeometry was released (Fisons/VG, now availablefrom Micromass, Manchester, UK). Besides a changeof the geometry, the main innovation was an increaseof the accelerating voltage to 6 kV, so that the maxi-mum mass resolution was raised to 10 000. An alter-native double focusing ICP-MS instrument, the‘PLASMA 54’, was introduced by VG (VG Elemen-tal, Winsford, UK) in 1994. This instrument wasdeveloped for dedicated, simultaneous high precisionisotope ratio measurements with seven moveableFaraday collectors. Additionally, a fixed Daly detectorcan be used for trace analysis. Although a doublefocusing geometry was chosen for this instrument, itcan only be operated in a low resolution mode. Similarto this concept is a new multicollector instrumentfrom ‘nu instruments’ (Wrexham, UK), which usesa newly developed zoom lens for variable dispersion.This allows the instrument to be operated with fixedinstead of moveable collector arrangements, whichhelps to keep instrumental costs low. A new compactinstrument, the ‘AXIOM’, has been announced by‘VG Elemental’ recently. For this instrument, a single


Fig. 3. Sketch of an ICP-SFMS system.

detector can be used for scanning purposes or, alter-natively, a multicollector arrangement can be installedfor high precision isotope ratio measurements.

Another manufacturer, Jeol (Tokyo, Japan) intro-duced an ICP-SFMS instrument, the ‘JMS PLAS-MAX 1’, in 1991 and a successor, the ‘JMSPLASMAX 2’ was launched to the market someyears ago. This instrument is also based on a reverseNier–Johnson geometry with an accelerating voltageof 6000 V applied to the interface. The mass resolu-tion may be varied continuously up to 12 000.

Following a different design principle, the‘ELEMENT’ instrument from Finnigan MAT(Bremen, Germany) was introduced in 1993 [9]. It isbased on a purpose-built analyser system with anextremely compact design [10]. Concerning the ionacceleration, the high voltage (8 kV) is applied tothe whole analyser behind the interface system, sothat operation of the ICP and the interface is possibleat ground potential without any high voltage hazard.Three fixed resolution settings can be selected bycomputer control: 300, 4000 and 10 000. For iondetection, a secondary electron multiplier is usedwhich can be operated in analogue or pulse countingmode, simultaneously.

A new sector field instrument, the ‘ISOPROBE’,

has recently been introduced to the market by Micro-mass (Manchester, UK). In this instrument, a collisioncell is used instead of an electric sector for energyfocusing. This arrangement makes use of the factthat collisions of ions in a hexapole cell loaded witha gas is an effective means for reduction of the ionenergy spread. As a result, the instrument can be oper-ated at medium resolution settings ofR� 2500, evenwith a single magnetic device.

4. Operational characteristics

4.1. Spectrum scan

With sector field devices, several scan modes arepossible. Eq. (2) indicates that ions of different masscan be found at different radii, so that mass separationis achieved. Simultaneous registration now is possibleif multiple detectors are located at different radii. Thiscan be done continuously using a photoplate ormodern photoplate-like electronic devices [11] ordiscontinuously by a multidetector arrangement[12], and offers the advantage of a true simultaneousmulti-element analysis, which, of course, results inconsiderably better precision in comparison to any


Fig. 4. Mass window showing the interfence of40Ar16O1 on the main isotope56Fe1 at (a) mass resolution ofR� 300; (b) mass resolution ofR�3000. Instrument ‘ELEMENT’; concentration of Fe� 10 ng/ml of Merck II standard solution; scan mode: electric scan.

scanning procedure. A more practical way to acquire amass spectrum is to set one detector at a fixed positionand to vary the magnetic field strengthB in a so-calledB-scan, which is the most important mode of opera-tion. Due to self-induction in the magnet coils, thescan speed is limited, but nevertheless a whole massspectrum can be aquired in less than 300 ms. An alter-native and even faster scan mode, electric scanning(E-scan), can be realized by variation of the acceler-ating voltage, withB and rm at fixed values. At firstglance, the E-scan may appear advantageous, but dueto a loss of sensitivity it can not cover the whole massrange and is therefore restricted to a partial mass rangeonly.

According to the feature of constant resolution, thepeak width of sector field instruments increases withmass, so that at aboutm/z� 238 the peaks are slightlybroader than observed with the usual quadrupoleinstruments. In the low mass range, the signals areneedle-like, and the distances between two massesare very large, so that in a B-scan, a major part ofthe scan time is wasted in background measurement.Peak hopping techniques following a list of prese-lected masses are therefore much more effective inmulti-element analysis. Peak hopping is considerablyfavoured by the trapezoidal shape of the peak profiles(at low mass resolution), as shown before in Fig. 1.

4.2. Mass resolution modes

The question arises as to which mass resolutionshould be selected for measuring an interferedisotope. As mentioned earlier, on the one hand thisis determined by the resolution theoretically requiredto resolve the spectral interference (defined in Eq. (1))while, on the other hand, the sensitivity is an impor-tant parameter which influences the selection becausenarrowing the beam defining slits and thus increasingthe resolution has to be paid for by a loss in ion trans-mission. This is illustrated by a typical example,which is presented in Fig. 4. In this figure, the massspectrum atm/z� 56 is shown at mass resolutions of300 and 2500. With LMR a broad, unresolved butintense peak is obtained, whereas at a mass resolutionof 2500, 56Fe1 can clearly be separated from40Ar16O1. However, the signal intensity is decreasedby about one order of magnitude and this is the reasonwhy HMR is only applied if required to separate

analyte isotopes from interfering ions. In this exam-ple, the intensity of the interfering polyatomic ionspecies by far exceeds that of the analyte isotopeand, due to the tailing of the highly abundant signal,full baseline separation requires a resolution signifi-cantly above the theoretical value given in Table 1. Ingeneral, the theoretical value is only a lower estimateof the resolution required in praxi.

4.3. ICP parameters

Optimization of the ion yields in ICP-SFMS withrespect to generator power and nebulizer gas flow givethe same optimum values for all elements [13], butthis may strongly depend on the specific instrumentand its optimization. Neither atomic mass nor ioniza-tion energy show a significant influence. Any effectsof this kind are reduced significantly by the highkinetic energy of the ions gained by the high accel-eration voltage in combination with energy focusingby the ESA.

5. General analytical performance

Before going into the details of some selectedexamples from different fields of application, a shortintroductory survey of the analytical performance ofhigh resolution instruments shall be given as an illus-tration of the ‘state of the art’.

5.1. Sensitivity

ICP-SFMS generally offers higher sensitivity incomparison to QMS owing to two reasons. First, theion transmission of quadrupoles may be mass-depen-dent, leading to losses at the higher end of the massscale, whereas this is not the case for magnetic sectordevices. Second, the lens systems of double focusinginstruments do not need a central beam stop (photonstop) for noise reduction, where additional losses canoccur. Sensitivity values for ICP-SFMS instrumentsrange from 107 (for Li) to 109 cps (for U) permg/ml inlow resolution mode if a Meinhard nebulizer is used[14]. Values of more than 108 cps permg/ml for In arestate of the art, but have also been reported for the newgeneration of quadrupole instruments [15]. Of course,the sensitivity can significantly be improved if highefficiency nebulizers (hyphenation) are applied.


The abundance sensitivity of ICP-QMS instrumentsis slightly superior to that of ICP-SFMS instruments.The manufacturers of ICP-SFMS instruments guaran-tee an abundance sensitivity which, in the low resolu-tion mode, is somewhat worse than that forquadrupole instruments, but any increase of the reso-lution will improve the abundance sensitivity suchthat values of 1027 can be realized. These are compar-able to the best values of quadrupoles.

5.2. Matrix effects

Theoretically, matrix effects (or rather non-spectro-scopic interferences, for a long time discussed interms of space charge effects), should be reduced byapplication of ion acceleration by high voltages.However, up to now this has only been investigatedexperimentally for some of the instruments underdiscussion and it was shown that matrix effects arecomparable to those for quadrupole-based instruments[16].

5.3. Background noise

The double bending of the flight path in a sectorfield instrument in combination with the entrance andexit slit is an effective means for reduction of the non-spectral background down to 0.1 cps, which is up toone order of magnitude lower than the best valuesreported for quadrupole instruments, and can evenbe improved by reduction of the slit width. Valuesof less than 0.001 cps have been reported recentlyfor a resolution setting of 12.000 [17]. These lownoise levels yield a significant improvement in thedetection limits for element determinations, espe-cially at an increased data acquisition time, as shallbe demonstrated later.

5.4. Precision, dynamic range and accuracy

As to the sources of statistical errors influencing theprecision of isotope intensity ratio measurements,fluctuations of aerosol generation by pneumatic nebu-lizers and instabilities of the plasma itself are usuallysupposed to be the most important. These effects can,of course, be eliminated by simultaneous registrationwith a multicollector arrangement, and indeed a preci-sion (relative standard deviation: RSD) in the range of1023% has been achieved with such equipment in a

low resolution mode. But even with a single detectorarrangement, a precision of typically 0.04% RSD hasbeen obtained by ICP-SFMS with peak jumping inlow resolution mode [18]. For quadrupole-basedinstruments, reported values for commercial instru-ments typically amount to 0.1% RSD. However,under optimum, but non-standard conditions, a valueof 0.02% RSD has been reported [19]. The typicalisotope ratio precision for quadrupole-based instru-ments is comparable to that obtained for a doublefocusing instrument operated atR� 3000 [20], thusgiving rise to triangular peaks as well. The improve-ment to 0.04% RSD mentioned above, in low resolu-tion mode, must therefore be attributed to the meritsof flat top peak profiles.

In ICP-MS, the dynamic range which can becovered by a linear calibration function is usuallylimited by the detection system. For sector field aswell as for quadrupole-based instruments, the use ofa SEM is standard. Such a detector can be operated inboth analogue and pulse counting modes, so that alinearity of up to nine orders of magnitude is achiev-able. The accuracy depends, of course, strongly on thequality of the calibration procedure and the referencematerials or the methods used for comparison. If unin-terfered isotopes are selected for evaluation, no differ-ence is to be expected between quadrupole and sectorfield instruments.

5.5. Hyphenated and interface techniques

Even with peak hopping techniques, double focus-ing instruments are slightly slower than quadrupoleinstruments, because for each stepwise change ofthe magnetic field a certain settling time, which isapproximately a millisecond per mass unit passed, isrequired. Therefore, the number of isotopes which canbe measured are limited when transient signals arerecorded. This is the case with sample introductiontechniques such as electrothermal vaporization(ETV), laser ablation (LA), liquid chromatography(LC) and gas chromatography (GC). For a longtime, sector field instruments were assumed not tobe fast enough for the detection of transient signals,but this does not hold true for the new generation ofinstruments equipped with laminated magnets. Thecapabilities of ICP-SFMS in combination with GCor LA were demonstrated in recent work [21]. In


both cases, transient signals of less than 5 s durationwere measured for a selected number of isotopes (,10). In comparison to quadrupoles, characterized byan excellent time-resolving power, this may appearinferior from a general point of view, but it will besufficient for the majority of applications.

The combination of ICP-SFMS with high efficiencynebulizers or other sample introduction systems can,on the one hand, be used to improve the versatility asdiscussed for LA and GC coupling or, on the otherhand, to further improve the sensitivity and thus thelimits of detection. A desolvation system combinedwith a conventional pneumatic nebulizer has beenused by Tittes et al. [22] to improve the detectionlimits for analysis of V, Cr and Ga in HCl; improve-ments of up to one order of magnitude resulting indetection limits below 1 pg/ml could be achieved. Itshould be mentioned that desolvation systems areoften an essential component of high efficiency nebu-lization systems, so that the improvement may betwofold: the high efficiency of these nebulizers maydirectly improve the sensitivity and, additionally,solvents which otherwise may cause spectroscopicinterferences can be removed from the aerosols by

desolvation. Hence, techniques recently used forhigh efficiency nebulization have been coupled toICP-SFMS, such as ultrasonic nebulization (USN)[23], thermospray nebulization (TSN) [24], hydraulichigh pressure nebulization (HHPN) [25] and directinjection nebulization (DIN) [26]. In particular, ultra-sonic nebulization has been applied to different areasof application to achieve ultimate detection limits atfg/ml levels, as will be discussed in more detail later.A microconcentric nebulizer (Cetac MCN 6000) hasbeen coupled to an ICP-SFMS instrument and sensi-tivities of up to 5 × 109 cps per ppm have beenmeasured for the elements Os, Pb and U with a sampleuptake rate of 0.04 ml/min only [27]. From these data,an overall sample utilization efficiency of 0.3% can becalculated, which is comparable with that obtained forthermal ionization mass spectrometry.

The ‘Cold Plasma’ technique has also been success-fully applied in ICP-QMS. Of course, it can also beused to improve detection limits in ICP-SFMS andinitial results show that the improvements are evenmore pronounced [28]. First, operational conditionsof a cold plasma can be used to avoid ionization ofAr and therefore all Ar-based molecules are


Table 2Detection limits using hot and cold plasma conditions

Isotope Detection limit, ng/L (ppt) Resolution Isotope Detection limit, ng/L (ppt) Resolution

7Li 0.012 cb 300 74Ge 0.7 h 3009Be 0.75 ha 300 88Sr 0.09 c 30011B 5.4 h 300 90Zr 0.08 h 30023Na 0.3 c 300 93Nb 0.0008 h 30024Mg 0.2 c 300 98Mo 0.2 h 30027Al 0.4 c 300 106Pd 0.2 c 30028Si 11 h 3000 107Ag 0.4 c 30039K 0.4 c 8000 114Cd 0.5 c 30044Ca 1.5 c 300 115In 0.2 c 30048Ti 0.4 h 3000 120Sn 0.25 h 30051V 0.07 h 3000 121Sb 0.07 h 30052Cr 0.24 c 3000 138Ba 0.7 h 30055Mn 0.14 c 3000 181Ta 0.0009 h 30056Fe 0.9 c 3000 184W 0.15 h 30059Co 0.14 c 3000 194Pt 1.3 c 30060Ni 0.3 c 300 197Au 0.8 h 30063Cu 0.2 c 300 205Tl 0.06 c 30064Zn 0.2 c 300 208Pb 0.12 c 30069Ga 0.4 c 300 209Bi 0.07 h 300

a h: hot plasma conditions.b c: cold plasma conditions.

significantly reduced, so that high mass resolutiondoes not have to be applied, thus avoiding a loss ofsensitivity. Second, even under normal plasma condi-tions, an improvement in the sensitivity by one orderof magnitude has been reported for an ICP-SFMSinstrument [28] when applying a ‘Shielded Torch’.A ‘Shielded Torch’ influences both the potentials inthe plasma and in the interface, so that as a result, theion energy spread is reduced and therefore transmis-sion in the double focusing device is improved.

5.6. Detection limits

Two different types of detection limits shall beconsidered here. First, there is an instrumental detec-tion limit which can be realized under optimum condi-tions with pure standards. Second, in practical work,this important figure of merit depends on the analyti-cal procedure applied and the matrix investigated. Thedetection limit of the method can be quite differentfrom the instrumental detection limit, and this will bediscussed in more detail in the next section.

For demonstration of instrumental detection limits,Table 2 presents data for some selected elements [29].The performance demonstrated at hand by this tablehas been achieved in a customer laboratory. Sincethese detection limits depend very much on solventblanks and overall cleanliness, they are not identicalto the instrumental detection limits guaranteed by themanufacturer but come close to them and can be takenhere as an example of the ultimate ‘state of the art’.For all those elements for which no blanks and inter-ferences are contributing, detection limits are at lowppq levels. Even for interfered elements such as Si, K,Ca, Cr and Fe, for which resolution settings between3000 and 8000 are necessary, detection limits at lowppt levels have been realized.

From an analytical point of view, such improveddetection limits for an increased number of elementsis the main reason why the number of ICP-SFMSinstrument users is rapidly growing, and the methodis being introduced in many new fields of application.

6. Applications

By offering high resolution as a general solutionfor most spectroscopic interferences, ICP-SFMSobviously has opened new possibilities and has

facilitated analytical procedures in existing and newfields of application, as will be illustrated in this section.

6.1. High tech materials

6.1.1. Semi-conductor manufacturingDue to the high price of the first ICP-SFMS instru-

ments, the early customers mainly came from thesemi-conductor industry, where the quality controlof high purity reagents and solvents is a most impor-tant issue. For this purpose, both the high resolutioncapability and the low detection limits of the instru-ments proved to be important features. For instance,for the determination of 26 elements, present atconcentration levels below 10 ng/ml in concentratedH2SO4 used as a 3:1 mixture with H2O2 for etching Siwafers, ICP-SFMS analysis of H2SO4 diluted tenfoldallowed the sample throughput to be significantlyincreased [30]. Indeed, the method replaced singleelement graphite furnace atomic absorption spectro-metry (GFAAS) and quadrupole-based ICP-MS, forwhich an evaporation stage to remove componentsotherwise leading to polyatomic interferences priorto the determination of Li, Ti, V, Cr, Mn, Co, Niand Zn was necessary. In addition, the risk of contam-ination was greatly reduced and results were found tobe precise and accurate.

Requirements for the measurement of impurities inultra-pure water used in the semi-conductor industrieshave greatly increased with the development of verylarge scale integrated circuits. Ultra-pure water is ofcritical importance for the cleaning and etching of theSi substrate. In this context, colloidal silicon is one ofthe most important contaminants and the desiredimpurity level for Si is now below 1 ng/ml, whichrequires an analytical method capable of determining0.1 ng/ml. Takaku et al. demonstrated that ICP-SFMSallows the spectroscopic interference at mass 28,mainly caused by CO1 and N2

1 (and accounting forseveral 100 ng/ml Si equivalents) to be eliminated,but that an instrumental background signal, mainlyoriginating from the plasma torch, hampered thedetermination [31]. Use of appropriate materialsand/or of preconcentration by evaporation, however,made it possible to determine Si at the ng/ml level.

Accurate ultra-trace analysis of GaAs is also ofgreat importance for the semi-conductor industry.For this purpose, Becker et al. developed an elegant


sample preparation procedure [32]. This procedureaims at a selective volatilization of the matrix, byconverting both matrix elements (Ga and As) intothe corresponding chlorides in a stream of Ar andgaseous Cl2. As a result of the low boiling points ofboth GaCl3 and AsCl3, the matrix elements can beselectively removed by volatilization while theelements of interest are retained in the sample residue.Subsequently, this residue was dissolved in hot HNO3

and the solution obtained was analysed using either anICP-QMS or an ICP-SFMS instrument, the lattershowing significantly better detection capabilities.

6.1.2. Superconductors and opto-electronicsHigh purity lanthanide compounds are used in the

production of semi-conductors, high temperaturesuperconductors and opto-electronics. To determinelanthanide impurities in high purity Y2O3 andGd2O3, Takaku et al. used a sector field instrument,operated at a low resolution setting of 400 [33]. Detec-tion limits on the order of 10 pg/g in the solid materialwere found for most lanthanides. However, in Gd2O3,interferences by GdO1, GdH1 and GdOH1 hamperedthe determination of Tb, Yb, Tm and Lu. To eliminatethese interferences, a resolution setting of up toR .20 000 would be required. Since the instrument usedwas limited to a resolutionR � 10 000, the authorschose to measure the signal intensity of the doublycharged ions of the analytes at the low resolutionsetting. Detection limits were thus deteriorated by

approximately a factor of 100, but were still in theng/g range. Actual concentrations measured in99.99% pure Y2O3 and Gd2O3 ranged between someng/g and several 1000 ng/g and turned out to stronglyvary between samples of the same purity grade.

6.1.3. CeramicsThe analysis of ceramic materials is difficult for

methods requiring sample dissolution since highlyconcentrated strong digestion acids and long reactiontimes are often necessary [34]. An example of such amatrix is SiC. For digestion, a mixture of HNO3,H2SO4 and HF was applied. When sulphuric acid isused, the determination of several transition elementsby ICP-QMS is seriously hampered by the formationof polyatomic sulphur-containing species, as has beendiscussed in connection with Fig. 1. A more detaileddiscussion of this problem for SiC analysis is givenelsewhere [35]. For this matrix, strong interferencesare also observed which are caused by the matrix itselfand which can hamper the determination of Ca, Sc, Ti,Ni and Ga. This is illustrated by the mass spectrum inFig. 5 for the determination of Sc. This figure shows amedium resolution (R� 3000) mass spectrum atm/z�45. For ICP-QMS, the determination of the mono-isotopic Sc would not be possible because the inter-fering component cannot be corrected for by a matrixmatched solution. By application of high mass resolu-tion, separation of the isotope of interest from theabundant polyatomic ions originating from the Simatrix is possible. Another approach described forthis matrix, which is useful for ICP-QMS too, is theremoval of the matrix by evaporation of Si as SiF4.The result of this procedure is also shown in Fig. 5 forthe matrix induced interferences on mass 45. Byincreasing the reaction time for the evaporation,complete removal of this kind of interference can beobserved. ICP-SFMS was therefore applied to controlthe completeness of a matrix removal technique sothat, as a result, ICP-QMS can be ultimately used.

Jakubowski et al. compared the results obtained byICP-QMS in combination with an HPLC on-line tech-nique for matrix-trace separation and preconcentra-tion of trace elements with application of ICP-SFMSfor analysis of very pure Al2O3 powders for interferedas well as for uninterfered elements [36]. With ICP-SFMS, limits of detection are one to two orders ofmagnitude lower than with ICP-QMS without


Fig. 5. Analytical signal for45Sc in the analysis of SiC after diges-tion and application of a matrix removal technique. Evaporationtime is increasing fromD (original solution after digestion) toA.Instrument: ‘ELEMENT’ with integration time 500 ms and 30points per peak (from Ref. [35]).

preconcentration for uninterfered elements—at leaston condition that blank contamination is not the limit-ing factor—and similar to values obtained by ICP-QMS with preconcentration. For interfered elements,both ICP-SFMS with high mass resolution and ICP-QMS with matrix separation lead to improved limitsof detection, but with the advantage that the ICP-SFMS approach is universally applicable.

6.1.4. Liquid scintillatorExtreme detection power was required in the

‘BOREX’ project by Mousty et al., aiming at theuse of a new type of liquid scintillation detector forthe direct measurement of solar neutrinos with ener-gies below 1 MeV and thus for the study of the energyproducing fusion reactions in the sun [37]. The detec-tor, situated in the Gran Sasso cavern in Italy, must becharacterized by an extremely high radiochemicalpurity and thus a low background, in order to makethe detection of rare events such as the impact of thesolar neutrinos possible. The content of U and Th inthe scintillator therefore had to be below 1–0.1 fg/g.Mousty et al. succeeded in the detection of around0.5 fg/g of U and reported a detection limit of0.3 fg/g for Th in the scintillator. It was necessary to

preconcentrate the impurities, and actual concentra-tions in the order of 10 fg/g were measured in theconcentrate.

6.1.5. SteelICP-SFMS instruments also prove to be most useful

for the analysis of pure and alloyed steel and of coat-ings on these materials. Trace impurities mayseriously deteriorate the physico-chemical propertiesof the material and make it unsuitable for its purpose.Quality control and monitoring during production aretherefore necessary. Important elements nowadaysinclude Sb, Bi, Pb Sn and P, which can influence thephysico-chemical properties even atmg/g levels.Phosphorus is difficult to determine at such low levelswith ICP-QMS instruments, but with ICP-SFMSinstruments its determination atmg/g is withoutmajor difficulties. Limits of detection in real steelsamples have been calculated for P and S to be 50and 900 pg/ml, respectively, in solution or 0.6 and1mg/g, respectively, in the solid. In both cases, thelimitations are set by blank values [38]. Additionally,in the same run, the concentrations of transitionelement impurities and other minor and trace consti-tuents can be determined at levels in the medium ng/g


Fig. 6. Detection limits and resolution setting for 50 elements; 3 s criterion applied to 10 replicate measurements of a blank solution. ‘VGPLASMATRACE I’ at 5 s dwell time per peak (from Ref. [40]).

range. In the work of Finkeldei et al., it was shownthat P had been overestimated previously in manysteel reference standard materials [39]. The accuracyof the analysis with ICP-SFMS was satisfactory, ascould be demonstrated using photometry, but thelatter analysis technique was more laborious andtime-consuming.

6.2. Environment

6.2.1. Rain and surface waterThe low detection limits of ICP-SFMS, down to the

fg/ml range when used at low resolution, prove to bean important feature in many applications, amongothers in the analysis of natural waters. Yamasaki etal. analysed river, lake and rain water and explored theultimate limits of detection [40]. For this purpose, theutmost attention was paid to avoiding contaminationfrom any source (acids, vials, laboratory air). Fig. 6shows the detection limits (3 s criterion applied toblank measurements) obtained for 50 elements aswell as the resolution used. With few exceptions,sub pg/ml detection limits were reported. Whenmeasured at an intermediate resolution of 3000, detec-tion limits around 100 fg/ml were found for the tran-sition elements. This is orders of magnitude belowvalues reported for quadrupole-based instruments,where polyatomic interferences seriously hamper themeasurement of the analyte signal. Only for As, a highresolution (R. 7500) was used, leading to a detectionlimit of 10 pg/ml. Non-interfered elements weremeasured at low resolution, and for some, the reporteddetection limits are as low as 1 fg/ml. Althoughextreme precautions were taken, the limits of detec-tion reported are determined by blanks, and theauthors explicitly mention high blanks for Si, Ir andPt. Yamasaki et al. can be considered the currentrecord holders of the lowest instrumental ICP-MSdetection limit [40]. By measuring a blank solutionfor 1 h at mass 235, an instrumental detection limitfor 235U of 0.07 fg/ml was determined. Obviously,these experimental conditions are far from beingrealistic, but the authors also reported real-life 40-element analysis of lake, river and rain water, provingthe applicability of the method for routine analysis ofactual samples.

Narasaki and Cao have used a hydride generationsystem coupled to an ICP-SFMS instrument to deter-

mine both As and Se in river water samples afterappropriate sample preparation [41]. A resolutionsetting of 10 000 was used to separate the analyteion signals (75As1 and 82Se1) from those of40Ar35Cl1 and40Ar2H2

1. Increased As or Se concentra-tions were established for some sample locationsalong the Arakawa river. This was attributed to formermining activities in the corresponding regions anddissolution of ores in the river water.

6.2.2. Estuarine and sea waterThe trace analysis of sea water using ICP-MS is a

challenging analytical task due to the low concentra-tions involved and the high level of dissolved solids(salt). For multi-element analysis of sea water samplesusing ICP-QMS, sample preparation cannot be limitedto dilution as a result of the occurrence of many spec-tral interferences and a pronounced signal suppres-sion. To overcome the aforementioned limitations, avariety of separation/preconcentration methods havealready been used in combination with ICP-QMS.Rodushkin and Ruth succeeded in the analysis ofsaline water samples using an ICP-SFMS instrumentafter simple four- to fivefold dilution of the samples[42]. Depending on the presence of polyatomic ions,which show the same nominal mass as the isotopes ofinterest, either the low (R� 300) or the medium (R�3000) resolution setting was used. In order to evaluatethe accuracy of this approach, three reference materi-als from the Canadian National Research Council(NRC) were analysed: open ocean sea water(NASS-4), coastal sea water (CASS-2) and estuarinewater (SLEW-2). For all but two of the elementsstudied, the agreement between the experimentalresults and the corresponding certified values wasexcellent.

6.2.3. Snow and iceThe Greenland and Antarctic snow and ice caps are

well-preserved and detailed archives, consisting ofsuccessive datable layers and hence, permit past andrecent variations in the composition of the earth’satmosphere to be studied. In order to obtain someinformation concerning the heavy metal content ofthe atmosphere throughout history, Barbante et al.studied the metal content of such snow samples as afunction of depth [43]. As a result of the extremelylow concentration levels involved (low and sub pg/g


levels), an extremely sensitive technique is requiredfor this purpose and extreme precautions have to betaken during sample collection, storage, treatment andanalysis to avoid contamination. It was demonstratedthat ICP-SFMS offers limits of detection sufficientlylow for the direct analysis of polar snow samples (noanalyte preconcentration required), while as a result ofthe multi-element capabilities, the sample consump-tion could be limited to only a few ml of sample. Formost of the elements investigated, the instrument usedwas operated at the low resolution setting (R� 300).For the determination of a number of transitionmetals, a resolution setting of 3000 was necessary toavoid spectral overlap with polyatomic ions. As theelements studied can be considered as tracers of thenatural (crustal, marine, biogenic, volcanic) or anthro-pogenic origin of airborne deposited material, theresults obtained permit conclusions to be drawnconcerning their origin and long range transport.

6.2.4. SoilThe applicability of ICP-SFMS for routine multi-

element analysis, using both the high and low resolu-tion mode of the instrument, is demonstrated by themulti-element analysis of soil extracts by Latkoczy etal. [44]. Aiming at the characterization of heavy metal

binding and mobility in soils of different compositionand from different climate zones, this work required areliable and fast multi-element method. It was shownpossible to determine 18 elements of interest in asingle sequence: Al, Mn, Co, Cu, Zn, Sr, Se, Mo,Cd, Sn, Hg and Pb were measured at the low resolu-tion setting, and subsequently Si, P, V, Cr, Fe and Niwere determined at a medium resolution setting of3000. The analysis time per sample was 2 min,permitting measurements of up to 20 samples perhour on a routine basis. Trace element concentrationsranging from themg/ml to sub ng/ml levels could bedetermined with a sample consumption of only 2 ml.

6.2.5. Trace element speciationAs mentioned before, the scanning speed of modern

double focusing instruments is sufficiently high tomeasure fast, transient signals as produced, forinstance, by laser ablation and by chromatographiccolumns used in on-line speciation studies. Examplesof the latter are the coupling with high performanceliquid chromatography for on-line speciation of Fecomplexes with humic substances [45] and, morechallenging for the scanning speed, with capillarygas chromatography (CGC). Transient signals of afew seconds duration are produced when detecting


Fig. 7. Multi-element chromatogram after on-line CGC separation of organo-Sn (I: BuPr3Sn; II: Bu2Pr2Sn; III: Bu3PrSn); organo-Hg (A:MePrHg; B: EtPrHg; C: Pr2Hg); and organo-Pb (1: Me3PrPb; 2: Me2Pr2Pb; 3: Et3PrPb; 4: Et2Pr2Pb; 5: Pr4Pb) compounds in a synthetic standardsolution; Xe (0.1% in H2 mobile phase) as internal standard, obtained with an ‘ELEMENT’, Finnigan MAT, operated at the low resolution mode(from Ref. [46]).

species separated by on-line CGC. In Fig. 7 an exam-ple is shown of the simultaneous speciation of organo-metallic compounds present in a synthetic sample[46]. The transient signals of Sn, Hg and Pb weremonitored, together with the continuous Xe signal(Xe added to the CGC carrier gas), which acts as aninternal standard. Though this required variation ofthe magnetic field to bridge the mass gap betweenthe nuclides120Sn and 126Xe on the one hand and202Hg and208Pb on the other, accurate determinationof all species is possible. Obviously, the number ofnuclides that can be monitored simultaneously islimited and depends on the analyte concentration,the duration of the transient signals, the mass separa-tion between the nuclides of interest and the precisionrequired. From a practical point of view, doublefocusing instruments in which the high acceleratingvoltage is applied to the mass spectrometer whilst theinterface is held at ground potential, can be moreconveniently coupled to various chromatographic oralternative sample introduction systems.

6.3. Radionuclides

As a result of nuclear weapons testing, nuclear fuelreprocessing and the Chernobyl accident, radioactivepollutants have been released into the environment.Even at low concentrations, these radioactive nuclidesmay present a risk to the health of living organisms,including men. Careful monitoring of their presencein different environmental compartments is thereforenecessary. The improved detection limits of ICP-SFMS turned out to be very useful for the determina-tion of low concentrations of long-lived radionuclides(99Tc, 226Ra, 232Th, 237Np, 238U, 239Pu and240Pu) [47].When an ultrasonic nebulizer was used, detectionlimits on the order of 1–10 fg/ml were obtained,corresponding to activities ranging between0.2 mBq/ml (226Ra) and 0.08 nBq/ml (232Th). In theenvironment, the radioactivity levels of these isotopesare of the order of 1 mBq/g or less, and with radio-metric methods samples need to be counted for at leastone day to collect 100 counts per gram sample. Appli-cation of ICP-SFMS is therefore very advantageous,since measurements of only 2.5 min are required toreach the detection limits stated above.

Yamamoto et al. reported the determination of99Tcin environmental samples [48]. After leaching, the

leachate was purified via solvent extraction and ionexchange to remove the isobaric interference due toRu (99Ru, 12.7% natural abundance) and to reduce theconcentration of dissolved matrix elements in the finalsolution. The resulting method is characterized by anabsolute detection limit of 0.25 pg of99Tc, corre-sponding to 0.16 mBq. In sediments from the EskEstuary in the Irish Sea,99Tc activities between24 mBq/g and 109 mBq/g were measured. The depthprofile of the measured activities reflects the trend inthe variation of the discharge from the nearby nuclearfuel reprocessing plant at Sellafield (UK).

Another radionuclide released into the environ-ment, mainly by nuclear weapon testing and fuelreprocessing, is129I. In order to achieve detectionlimits below 100 pg/ml in solutions of biologicalmaterials, Kerl et al. designed equipment for the intro-duction of gaseous I2 into the plasma, using flowinjection with on-line oxidation with concentratedperchloric acid of the iodide present in the samplesolution and gas/liquid separation [49]. To furtherfacilitate the determination and assure its accuracy,on-line standard addition was performed. A detectionlimit of 50 pg/ml was reported and the accuracy of themethod was demonstrated by the analysis of variousbiological standard reference materials to which aknown amount of129I had been added. Accurate deter-mination of natural I (127I) in the same materials waspossible (detection limit 100 pg/ml) by applying on-line isotope dilution; for this purpose the sample solu-tion was flow injected in a continuous flow of a129I-containing solution.

79Se is also an important long-lived radionuclide, ofwhich the concentration has to be determined in radio-active waste samples prior to their permanentdisposal. Using radioanalytical techniques, this deter-mination is time-consuming and usually separation of79Se from the other radionuclides is required. As aresult, Hoppstock et al. evaluated the possibility of79Se determination using ICP-SFMS [50]. AccurateICP-QMS determination of79Se is hampered by thepresence of a number of polyatomic ions(40Ar38ArH1, 40Ar39K1 and 63Cu16O1), doublycharged ions (158Gd21 and 158Dy21) and 79Br1. Themaximum resolution setting of the ICP-SFMS instru-ment used was insufficient to avoid all correspondingspectral overlap. Therefore, hydride generation wasused for sample introduction, permitting separation


of the analyte element from the majority of matrixcomponents. An extra gas wash bottle was includedin the experimental set-up to reduce the level of79Br1.Only the presence of40Ar38Ar1H1 could not beavoided and hence the limit of detection was restrictedto 0.1 ng/ml (instrument operated at the low resolu-tion setting:R� 300). However, this is sufficient forroutine control of many radioactive waste samples.

Becker et al. used an ICP-SFMS instrument oper-ated in the low resolution mode (R � 300) for thedetermination of radionuclides in an irradiated tanta-lum target [32]. For this determination, the radionu-clides of interest were separated from the highlyradioactive matrix (182Ta) using liquid–liquid extrac-tion. It was demonstrated that, in comparison toneutron activation analysis (NAA), ICP-SFMSpermitted more nuclides to be determined at theconcentration range of interest (1 ng/g–50mg/g).The application range could be further extended bycoupling HPLC on-line with the ICP-MS because, inthis way, isobaric overlap of radioactive nuclides withstable isotopes could be avoided. By using laser abla-tion as a means of sample introduction, researchers ofthe same group were even able to determine some

radioactive nuclides (e.g.99Tc, 129I, 232Th, 233U,237Np and238U) directly in some solid non-conductiveradioactive waste samples, such as cement or concrete[32]. Detection limits (, 1 ng/g) obtained using anICP-SFMS instrument operated at the low resolutionsetting were more than one order of magnitude betterthan those obtained using an ICP-QMS instrument.

6.4. Biomedical research

6.4.1. Body fluids and tissuesThe determination of physiologically and toxicolo-

gically important trace and ultra-trace elements inmaterials such as body fluids (blood, serum, urine,etc.) and tissues is difficult for ICP-QMS because, inaddition to interfering species such as oxides andargides, many species occur that originate from thematrix elements (C, Na, P, S, Cl, K, Ca). In Table 3,a number of analyte elements are listed together withinterfering species occurring in the analysis of humanserum and the spectral resolution required to resolvethe interferences. The table also shows estimates ofapparent concentrations due to the occurrence ofpolyatomic ions when ICP-QMS is used without any


Table 3Interferences and apparent concentrations in the quadrupole ICPMS analysis of a human serum reference material. Resolution required for theelimination of the interferences, reference concentrations and concentrations determined with HR-ICPMS. Finnigan MATElement

Element material (isotope) Polyatomic ion R needed Apparentcolic.(ng/ml)quad.ICPMS

Conc. (nglml) in human serum referencematerial

Certified HR-ICPMS

Al ( 27A1) 12C15N, 13C14N1 1454 4 (digest) 1.6 2.1 1.70 (SD� 0.54)12C14N1H1 919

V (51V) 35C116O1 2572 120 0.061 0.005 0.075 (SD� 0.008)Cr(52Cr) 40Ar12C1 2375 135 0.07 0.01 0.135(SD� 0.025)

35C116O1H1 1671Mn (55Mn) 39K1601 2670 5 0.7 0.3 0.701 (SD� 0.022)

37C11801 2034Fe (56Fe) 40Ar16O1 2502 300 2350 150 2153 (SD� 74)

40Ca16O1 2479Ni (60Ni) 44Ca16O1 3057 26 0.23

23Na37C11 2409Cu(63Cu) 40Ar23Na1 2790 300 1010 40 970(SD� 21)

31P16O21 1851

Zn (64Zn) 32S16O21 1952 270 870 20 750 (SD� 74)

As (75As) 40Ar35C11 7775 80 1.8 0.4Se (77Se) 40Ar37C11 9181 335 95 5

precaution or correction. Comparison with actualconcentrations, using a human serum reference mate-rial with normal trace element concentrations, allowsthe relative importance of the interferences to be esti-mated [51]. For elements such as Fe, Cu and Zn, theinterference is relatively low and procedures to reduceor correct for the interference can be shown to yieldaccurate results. When the apparent concentrationcaused by the interference is orders of magnitudehigher than the actual concentration (e.g. Ti, V, Cr,Ni), accurate determination can become impossible orat least very difficult. High resolution offers a conve-nient way to eliminate most interferences withoutfurther dedicated procedures. In Table 3, concentra-tions obtained with an ICP-SFMS instrument (R �3000) are listed; the agreement with certified or indi-cative values illustrates the accuracy of the method[52, 53]. As the maximum resolution of the instrumentused is limited to 7500, the As and Se signals couldnot be measured free from interference.

In the analysis of biological samples, sector fieldICP-MS used at the low resolution setting alsoenabled the concentrations of rare elements presentat concentrations below the limit of detection ofICP-QMS to be determined. Because they are chemi-cally inert, noble metals have long been considered asharmless. Today, however, it is known that Pd, Pt andtheir compounds, for instance, are among the mostpotent sensitizers with a high incidence of variousallergic reactions. The proliferation in the environ-ment of Pd, Pt and Rh from catalytic convertersused to reduce the emission of noxious exhaustgases by motorized vehicles, the release of Au, Pd

and other noble metals from dental alloys, the poten-tial leaching of noble metal catalyst residues frompolymers and the pollution by anti-cancer and anti-rheumatic Pt-containing and Au-containing drugs viathe sewage system, therefore call for watchfulness. AnICP-SFMS instrument was used by Dunemann et al.for the determination of noble metals in blood [54, 55]and urine [56]. The authors compare detection limitsof quadrupole-based [57] and sector field ICP-MS andreport values measured in blood and urine of healthypeople that were not exposed occupationally and werenot treated with noble metal-containing drugs. Table 4shows that detection limits, obtained with sector fieldICP-MS, are up to two orders of magnitude lower thanthose of ICP-QMS, though the latter method can belargely improved by methods such as chemicalpreconcentration and the application of electrothermalvaporization (ETV). Until recently, normal concentra-tions of, for example, Pt in urine, could only be deter-mined conveniently with adsorptive voltammetry;sector field ICP-MS offers comparable limits of detec-tion, the advantage of being less sensitive to interfer-ences, does not require total mineralization of thesample and is capable of multi-element analysis.

An ICP-SFMS instrument coupled to size exclusionchromatography (SEC) has been applied by Wang etal. in LMR and HMR to investigate the identificationof inorganic elements in proteins of human serum andDNA fragments [58], demonstrating that ICP-SFMScan be a valuable tool to study the interaction ofessential and toxic metals with biomolecules whichotherwise would require organic mass spectrometricinstrumentation. As binding partners in biomolecules


Table 4Limits of detection (LOD) and values measured in normal humans (ng/l)

LOD LODElement Quadrupole ICPMS Double focusing ICPMS Concentration range in normal humans

Bloodafter microwave assisted high pressure ashing with HNO3; dilution factor 10 or 40 (quadrupole)Rh 620a 13a , 13a

Pd 390a 26a 32–78b

Pt 714a 32a 0.3–1.3b

Au 795a 48a 125–413b

Urine after UV photolysis; dilution factor 4Pt 0.24c 0.48–7.7c

a bibr[54]; bbibr[55]; cbibr[56].

Zn, Cu, Se, Cd, Pb, Th, U have been studied in LMRwhereas for Cr, Mn and Fe, a resolution ofR� 3000was required. In both cases, convincing detectionlimits were achieved. For example, Th and U weredetermined in a human serum reference material(National Institute of Standards and Technology,US) at ambient levels in the low pg/ml range inseveral fractions of molecular masses in the intervall10–630 kDa. In HMR mode the interaction of Cr(VI)added to DNA fragments at ppb levels was investi-gated. Cr(VI) is well known for its carcinogenic prop-erties and its strong oxidizing potential. Although thisCr species forms an oxoanion in solution, it could beshown that Cr binds as a cation in a specific DNAfragment during or after oxidation of the DNA, andjust this oxidation process explains its carcinogenicbehaviour.

6.4.2. FoodFor the analysis of carrots, Stu¨rup et al. fully

exploited the possibility of their ICP-SFMS instru-ment to switch between low and high resolutionsettings while measuring the same sample [59].Consecutively, different resolution settings betweenR � 400 andR � 10 000 were used to determinethe concentration of up to 60 trace elements, includingAs (ArCl1 interference at mass 75) and Eu (BaO1

interference at mass 153), for which a resolution ofR � 10 000 was used. The accuracy of the method

was demonstrated for three elements for whichcomparison was possible with certified concentrationsin a reference material from the Agricultural ResearchCentre of Finland (ARC). For Mn, Fe and Zn, con-centrations of respectively, 5.3 mg/kg, 12.6 mg/kgand 7.81 mg/kg measured with ICP-SFMS agreedwithin the experimental precision with the certifiedvalues of 4.90 mg/kg, 12.6 mg/kg and 6.9 mg/kg,respectively.

Augagneur et al. used ICP-MS for the determina-tion of the rare earth elements (REE) in wines ofdifferent origin [60]. The use of REE patterns as asoil characteristic is well-known in geochemistryand as the REE pattern of the vineyard soil is reflectedin the corresponding wine, determination of the REEin wines may permit identification and geographicallocation of their origin. The performance of differenttypes of ICP-MS instruments for the analysis of undi-luted wine samples was compared in this study. Themaximum resolution setting (R � 7500) of the ICP-SFMS instrument used is insufficient to avoid spectralinterference by oxide ions, but at the low resolutionsetting (R� 300) it offers improved detection limits(up to two orders of magnitude) when compared toquadrupole-based instrumentation. As the REEconcentration patterns were indeed observed toshow intercontinental and interregional variations,they show potential to be used for tracing imitationand falsification of wines.


Fig. 8. Isotopic signature (208Pb/206Pb vs 207Pb/206Pb) of snow samples (D) (1993–1996), geogenic material (V) (Sahara dust and variousmineral samples), leaded gasoline in Europe (X), leaded gasoline in Switzerland (B), and Broken Hill ores from Australia (BH) (from Ref.[61]).

6.5. Isotope ratios

6.5.1. Low resolutionICP-QMS instruments have been used not only for

trace element determination, but also for isotope ratiomeasurements. Obviously, the precision and accuracyof magnetic sector thermal ionization mass spectro-metry (TIMS) cannot be reached. With ICP-QMS,isotope ratios in aqueous standard solutions with asufficiently high concentration can routinely bemeasured with a precision of 0.1–0.2% RSD. In realsamples, reported values are typically of the order of0.5–1% RSD only, a precision however, which issufficient for many applications, such as tracer studiesin biological matrices, and a limited number of geolo-gical and identification purposes. ICP-SFMS instru-ments were shown to allow more precise isotoperatio measurements: under optimum conditions,RSD values on 10 measurements of the same samplewere found to be 0.04% RSD for both magnesium andlead isotope ratios [18]. This improvement, comparedto ICP-QMS, obviously widens the field of applica-tion of ICP-MS in isotope ratio studies.

6.5.2. Lead in snow and iceDoring et al. used a double focusing instrument (set

at low resolution) to determine the concentration andisotopic composition of Pb in snow and ice of highalpine sites [61]. In Fig. 8,208Pb/206Pb ratios areplotted versus207Pb/206Pb ratios measured in thesame samples. This figure allows the isotopic compo-sition of Pb in recent snow samples to be compared tothe isotopic composition of geogenic Pb and of Pb aspresent in leaded gasoline. Though the Pb emission byautomobiles has drastically decreased in the pastdecade, the authors were able to conclude fromthese data that cars still contribute as much to thePb pollution in high alpine areas as other anthropo-genic sources (waste incineration, industry) do. Thisresearch is remarkable because the Pb concentrationsin the snow samples were very low, ranging from 0.02to 9.7 ng/g, and determinations did not require morethan 0.8 ml of sample (microconcentric nebulization)while yielding an average standard error on the ratiosof 0.14% only. The advantages over TIMS in terms ofsample preparation, analysis time and samplethroughput are obvious and, in addition, the method

is not limited to elements with low first ionizationenergy.

6.5.3. Sr in prehistoric bone samplesUsing an ICP-SFMS instrument operated at the low

resolution setting (R � 300), Latkoczy et al. deter-mined Sr (86Sr/87Sr) isotope ratios in prehistorichuman bone samples [62]. In contrast to most otherelements, the isotopic composition of Sr varies innature as a result of natural radioactive decay (bdecay) of87Rb to 87Sr. Hence, the isotopic composi-tion of Sr in geological samples is strongly dependenton both the Rb/Sr concentration ratio and the age ofthe material. The Sr isotopic composition of vegeta-tion is in equilibrium with that of the local geologyand additionally, Sr passes through the food chainwithout significant fractionation of its isotopes.Hence, Sr isotope analysis of 7000 year old humanskeletons from a Neolithic settlement in Austriapermitted information on the provenance of singleindividuals to be obtained.

6.5.4. Radioisotope ratiosWhen compared to ICP-QMS, the accuracy and

precision of240Pu/239Pu isotope ratios measured witha double focusing instrument were shown to besubstantially better. Using an ultrasonic nebulizer,Kim et al. were able to measure the240Pu/239Pu ratioin a 21 pg/ml Pu solution with a precision (Poissonstatistics) of 2% and a deviation from the true value(certified by Oak Ridge National Laboratory) of 0.8%[47].

6.5.5. High resolutionProblems due to interfering polyatomic species,

hamper the determination of isotope ratios with ICP-QMS and often necessitate chemical separations.These become redundant when ICP-SFMS is used.Under ideal conditions, isotope ratios can bemeasured with a precision of better than 0.1% (RSDon 10 replicate measurements of the63Cu/65Cu ratio)[20]. The method has been used for isotopic analysisof various other elements such as Li, B, S, Ca and Fe[63].

6.5.6. Sediments, serum and urineThe results shown in Fig. 9 concern Antarctic sedi-

ment and human serum samples wherein Cu isotopes


were measured next to large peaks produced by matrixderived polyatomic species [20]. The Cu concentra-tion in the sediment digests was 10 ng/ml at most. Thehuman serum sample was diluted 10-fold prior to themeasurement, resulting in a Cu content in themeasured samples of approximately 100 ng/ml. Ascan be seen in Fig. 9, an RSD on five replicatemeasurements of the63Cu/65Cu ratio of 0.4–0.6%was found for the Antarctic sediment, whereas forthe serum samples the RSD (five measurements)was about 0.3%. Assuming that the isotopic composi-tion in both materials corresponds to the natural one,the mean values, corrected for mass bias, are accuratewithin the experimental uncertainty.

ICP-SFMS was also applied in a double labellingexperiment aimed at the study of the uptake andexcretion of Ca by the human body [64]. Ca isotopicratios were measured in urine after oral administrationof 44Ca and intravenous injection of42Ca. This methodis an alternative for ethically less acceptable radio-tracer experiments and for expensive and time-consuming studies using TIMS.

6.5.7. ICP-MS versus timsThe precision and accuracy of TIMS cannot be

reached with ICP-SFMS systems with a single detec-tor, let alone with ICP-QMS. However, a precisioncomparable to the precision of TIMS is achievablewith ICP-MS instruments equipped with a doublefocusing mass spectrometer and with multiple collec-tors. These instruments are becoming more and moreattractive for isotope ratio measurements for severalreasons. First of all, the high ionization efficiency of

the ICP facilitates high sensitivity measurements,even for those elements which are difficult to beionized by TIMS. In contrast to TIMS, ICP-SFMSdoes not show any fractionation related to the chemi-cal or physical properties of an element under inves-tigation. The latter effects lead to time-dependentvariations of isotope ratios and are difficult to correct.Although mass bias is more pronounced in ICP-SFMS, it is stable and can easily be calibrated [65].When measuring the87Sr/86Sr ratio, the multiplecollector system permits the86Sr/88Sr ratio to besimultaneously measured and to be used to normalizethe 87Sr/88Sr ratio and thus to eliminate the effect ofmass bias variation during the measurements. Thus,external precision values of 0.008% RSD wereobtained [66]. Elements such as Pb, the isotopiccomposition of which cannot be normalized withinternal standardization due to natural variations inthe isotopic composition, can be corrected for massdiscrimination by use of an external standard or byusing neighbouring isotopes of Tl,203Tl and 205Tl[67].

Various applications of isotope ratio measurementshave been described in the literature and a detailedoverview is given in a recent review by Becker andDietze [68]. Up to now, ICP-SFMS with multicollec-tor arrangements have been primarily applied togeochronology [69] and in nuclear research, but newapplications can be seen for trace element determina-tions in environmental analysis, material research andfor high purity chemicals by application of the isotopedilution technique [70].

7. Future instrumental developments

As a result of the increasing number of manufac-turers and instruments on the market and also offundamental research in ICP-MS, there is a permanentprocess of technical innovation in ICP-MS. Two differ-ent approaches are trend setting: on the one hand, newinstrumentation capable of high mass resolutionincreasingly appears in the field, and on the otherhand, new refined hyphenated and interface techniquesto avoid or to overcome spectroscopic interferences aresuccessfully applied. For the latter, two recent andpowerful approches for quadrupole-based instruments,the ‘Cold Plasma’ and the ‘Collision Cell’ approach,


Fig. 9. 63Cu/65Cu ratios in Antarctic sediment and human serumreference material. ‘ELEMENT’ (R� 3000) with 50 ms countingper peak in each of 1200 sweeps.

have been discussed previously. Of course, bothconcepts have also successfully been applied incombination with sector field instruments.

Concerning instrumental developments, a quadru-pole-based ICP-MS instrument with high mass reso-lution, which can resolve56Fe1 from 40Ar16O1, hasrecently been reported by Ying and Douglas [71].Although quadrupole mass analysers show strongsensitivity losses when operated at extreme resolutionsettings, it was shown in this investigation that suchlosses can be kept moderate if the quadrupole massanalyser is operated in a special ‘stability’ mode.

From its operational principles, ion trap massanalysers come close to quadrupoles but can be usedfor storage of ions. High mass resolution has beenobtained in organic mass spectrometry with specialstorage conditions. As a special feature, ions caneven be stored in the trap under conditions of a colli-sion cell. This was shown to be advantageous for ICP-MS application by Koppenaal et al., who succeeded inachieving complete destruction of some selectedpolyatomic molecules, such as ArO1 and ArH1 [72].

The coupling of a time of flight (TOF) mass spec-trometer with an ICP has been investigated in moredetail by Hieftje’s group [73]. They recently demon-strated that, depending on the choice of the workingconditions, a mass resolution of more than 1000 maybe obtained in principle and the extremely high timeresolution of these instruments makes them ideallysuited to follow fast transient signals which has beendemonstrated using an ETV application [74].

Further improvements may also be expected formagnetic sector devices. For some applications eventhe mass resolution realized by modern instrumenta-tion is by far not sufficient to resolve all interferingspecies. Morita et al. described an ICP-SFMS instru-ment capable of a resolution ofR� 43000 [75]. Apartfrom the cost of such instruments, the low ion trans-mission efficiency, and thus sensitivity available atvery high resolution settings, is prohibitive for moregeneral use for the time being. For completeness, itshould be mentioned that, in principle, Fourier trans-form mass spectrometers (FTMS) are well suited torealize ultra-high mass resolution, but from a techni-cal point of view it is a challenge to realize the ultra-high vacuum conditions required for operation.Nevertheless, a first successful coupling of an ICPto a FTMS resulting in a resolution of more than

80 000 has been described recently [76]. Anotherapproach uses the fact that sector field devices ofspecific geometry can be operated in a simultaneousmeasuring mode. Cromwell and Arrowsmith have, forinstance, demonstrated that the Mattauch–Herzoggeometry, which is well-known for simultaneoussignal registration by photoplates, can also be oper-ated with an array detector for simultaneous detectionof ions over an extended mass range [11]. Such anarrangement can be expected to offer the advantagesof the highest sensitivity for multi-element conditionsand an improved precision for isotope ratio measure-ments.

This survey of some selected instrumentalapproaches demonstrates that interesting and promis-ing research is going on and that there is many achance for prolific innovation in the near futurebecause the inherent potential for instrumental devel-opment in ICP-MS still offers the opportunity to scorea hit in the competition for the highest and mostconvincing analytical performance.

8. Conclusion

ICP-SFMS is a high performance tool in the ever-lasting battle against spectroscopic interferences, inparticular when complex matrices from materialresearch, biological and medical samples, environ-mental samples, foods and soils are investigated.Such matrices not only induce interferences whichare well-known, but in complex samples, a numberof unexpected interferences may arise additionallyand confuse spectra and interpretation with the riskof erroneous quantification. ICP-SFMS is capable ofsolving most of the significant spectroscopic interfer-ence problems in real-life samples with the benefit ofconsiderably improved reliability for the analyticalresults, while simultaneously improving not onlysensitivity but also detection limits in most cases.Due to the high instrumental stabilities of magneticsector instruments, isotope ratios can be measuredwith a precision which comes more and more closeto that of thermal ionization mass spectrometry. Itopens analytical frontiers to new worlds of purityand ultra-trace analysis in elemental chemistry, andthe only serious limitation rising on the horizon is theubiquitous problem of blank values.


Acknowledgements

L.M. and F.V. thank the Fund for ScientificResearch (Flanders, Belgium) for financial support.N.J. thanks the Ministerium fuer Wissenschaft undForschung des Landes Nordrhein–Westfalen and theBundesministerium fuer Bildung, Wissenschaft,Forschung und Technologie. The advice and help ofProfessor Dr R. Dams, Dr D. Stuewer and Dr MichaelCampbell are highly appreciated.

References

[1] A. Montaser, D.W. Golightly (Eds.), Inductively CoupledPlasmas in Analytical Atomic Spectrometry, VCH, NewYork, 1992.

[2] K.E. Jarvis, A.L. Gray, R.S. Houk, Handbook of InductivelyCoupled Plasma Mass Spectrometry, Blackie, London, 1992.

[3] R.S. Houk, V.A. Fassel, G.D. Flesch, H.J. Svec, A.L. Gray,C.E. Taylor, Inductively coupled argon plasma as an ionsource for mass spectrometric determination of traceelements, Anal. Chem. 52 (1980) 2283–2289.

[4] K. Sakata, K. Kawabata, Reduction of fundamental polya-tomic ions in inductively coupled plasma mass spectrometry,Spectrochim. Acta Part B 49 (1994) 1027–1038.

[5] E.H. Evans, J.J. Giglio, Interferences in inductively coupledplasma mass spectrometry: a review, J. Anal. At. Spectrom. 8(1993) 1–18.

[6] S.D. Tanner, Characterization of ionization and matrixsuppression in inductively coupled ‘cold’ plasma mass spec-trometry, J. Anal. At. Spectrom. 10 (1995) 905–921.

[7] P. Turner, T. Merren, J. Speakman, C. Haines, Interfacestudies in the ICP-mass spectrometer, in: G. Holland, S.D.Tanner (Eds.), Plasmas Source Spectrometry—Developmentsand Applications, Special Publication (No. 202) of the RoyalChemical Society, Cambridge, 1997, pp. 28–34.

[8] N. Bradshaw, E.F.H. Hall, N.E. Sanderson, Inductivelycoupled plasma as an ion source for high-resolution massspectrometry, J. Anal. At. Spectrom. 4 (1989) 801–803.

[9] U. Giessmann, U. Greb, A new concept for elemental massspectrometry, Paper presented at the 2nd Regensburg Sympo-sium on Massenspektrometrische Verfahren der Elementspur-enanalyse, paper DV1, 1993.

[10] U. Giessmann, U. Greb, High resolution ICP-MS—a newconcept for elemental mass spectrometry, Fres. J. Anal.Chem. 350 (1994) 186–193.

[11] E.F. Cromwell, P. Arrowsmith, Novel multichannel plasma-source mass spectrometer, J. Am. Soc. Mass Spectrom. 7(1996) 458–466.

[12] A.J. Walder, D. Koller, N.M. Reed, R.C. Hutton, P.A. Freed-man, Isotope ratio measurement by inductively coupledplasma multiple collector mass spectrometry incorporating ahigh efficiency nebulization system, J. Anal. At. Spectrom. 8(1993) 1037–1041.

[13] I. Feldmann, W. Tittes, N. Jakubowski, D. Stuewer, U. Giess-mann, Performance characteristics of inductively coupledplasma mass spectrometry with high mass resolution, J.Anal. At. Spectrom. 9 (1994) 1007–1014.

[14] Specified sensitivity for the ‘ELEMENT’, ELEMENT DemoProgram, Vers. 1.95, Finnigan MAT, Bremen, 1998.

[15] E.R. Denoyer, D. Jacques, E. Debrah, S.D. Tanner, Determi-nation of trace elements in uranium: practical benefits of a newICP-MS lens system, At. Spectrosc. 16 (1995) 1–6.

[16] F. Vanhaecke, J. Riondato, L. Moens, R. Dams, Non-spectralinterferences encountered with a commercially available highresolution ICP-mass spectrometer, Fres. J. Anal. Chem. 355(1996) 397–400.

[17] H. Wildner, Application of inductively coupled plasma sectorfield mass spectrometry for the fast and sensitive determina-tion and isotope ratio measurement of non-metals in high-purity process chemicals, J. Anal. At. Spectrom. 13 (1998)573–578.

[18] F. Vanhaecke, L. Moens, R. Dams, P. Taylor, Precisemeasurement of isotope ratios with a double-focusingmagnetic sector ICP mass spectrometer, Anal. Chem. 68(1996) 567–569.

[19] A.L. Gray, J.G. Williams, A.T. Ince, M. Liezers, Noisesources in inductively coupled plasma mass spectrometry:an investigation of their importance to the precision of isotoperatio measurements, J. Anal. At. Spectrom. 9 (1994) 1179–1181.

[20] F. Vanhaecke, L. Moens, R. Dams, I. Papadakis, P. Taylor,Applicability of high-resolution ICP-mass spectrometry forisotope ratio measurements, Anal. Chem. 69 (1997) 268–273.

[21] M. Hamester, E. Schroeder, U. Greb, R. Pesch, Feststoffana-lyse mittels Laserablation und HR-ICP-MS, Poster presentedat the 3rd Symposium u¨ber massenspektrometrische Verfah-ren der Elementanalyse, Juelich, Germany, 1996.

[22] W. Tittes, N. Jakubowski, D. Stuewer, G. Toelg, J.A.C. Broe-kaert, Reduction of some selected spectral interferences ininductively coupled plasma mass spectrometry, J. Anal. At.Spectrom. 9 (1994) 1015–1020.

[23] R.C. Hutton, Sample introduction for ICP-MS: where the realproblems are solved, Paper presented at the 5th InternationalConference on Plasma Source Mass Spectrometry, Durham,UK, 1996.

[24] C. Thomas, N. Jakubowski, D. Stuewer, J.A.C. Broekaert,Thermospray device of improved design for application inICP-MS, J. Anal. At. Spectrom. 10 (1995) 583–590.

[25] N. Jakubowski, C. Thomas, D. Stuewer, I. Dettlaff, J. Schram,Speciation of inorganic selenium by inductively coupledplasma mass spectrometry with hydraulic high pressure nebu-lization, J. Anal. At. Spectrom. 11 (1996) 1023–1029.

[26] C. Thomas, N. Jakubowski, D. Stuewer, Application of a newthermospray device in ICP-MS with high mass resolution,Poster presented at the XXIX Colloquium SpectroscopicumInternationale Post Symposium ICP-MS, Wernigerode,Germany, 1995.

[27] Private communication (MM/PS/03), Iso-plasma trace—anew high-performance multicollector sector field ICP-HEX-MS system, Micromass, Manchester, UK, 1997.


[28] D. Wiederin, Order of magnitude improvement in sector-fieldICP-MS performance using a Pt guard electrode, Paperpresented at the 1998 Winter Conference on Plasma Spectro-chemistry, Scottsdale, AZ, USA, 1998.

[29] Detection limits for the ‘ELEMENT’, ELEMENT DemoProgram, Vers. 1.95, Finnigan MAT, Bremen, 1998.

[30] Fisons Instruments application note, High resolution ICP-MSanalysis of ultrapure H2SO4, Document No. VGE/SM/TM/175, 1993.

[31] Y. Takaku, K. Masuda, T. Takahashi, T. Shimamura, Deter-mination of trace silicon in ultra-high-purity water by induc-tively coupled plasma mass spectrometry, J. Anal. At.Spectrom. 9 (1994) 1385–1387.

[32] J.S. Becker, H.-J. Dietze, Double-focusing sector field induc-tively coupled mass spectrometry for highly sensitive multi-element and isotopic analysis, J. Anal. At. Spectrom. 12(1997) 881–889.

[33] Y. Takaku, K. Masuda, T. Takahashi, T. Shimamura, Deter-mination of trace impurity rare earth elements in high-purityrare earth element samples using high-resolution inductivelycoupled plasma mass spectrometry, J. Anal. At. Spectrom. 8(1993) 687–690.

[34] T. Graule, A. von Bohlen, J.A.C. Broekaert, E. Grallath, R.Klockenkaemper, P. Tschoepel, G. Toelg, Atomic emissionand atomic absorption spectrometric analysis of high-puritypowders for the production of ceramics, Fres. J. Anal. Chem.335 (1989) 637–642.

[35] F. Kohl, N. Jakubowski, R. Brandt, C. Pilger, J.A. Broekaert,C, New strategies for trace analyses of ZrO2, SiC, and Al2O3

ceramic powders, Fres. J. Anal. Chem. 359 (1997) 317–325.[36] N. Jakubowski, W. Tittes, D. Pollmann, D. Stuewer, J.A.C.

Broekaert, Comparative analysis of aluminum oxide powdersby inductively coupled plasma mass spectrometry with lowand high mass resolution, J. Anal. At. Spectrom. 11 (1996)797–803.

[37] F. Mousty, R. Passarella, V. Pedroni, A. Polettini, P.R. Trinch-erini, Determination of U and Th at the sub-ppt levels in aliquid scintillator used for solar neutrino measurements, J.Trace Microprobe Tech. 10 (1992) 295–312.

[38] N. Jakubowski, D. Stu¨ewer, I. Feldmann, Analysis of steelsamples by use of high resolution ICP-and GD-mass spectro-metry, Paper presented at the 1995 Winter Conference onPlasma Spectrochemistry, Cambridge, UK, 1995.

[39] S. Finkeldei, G. Staats, ICP-MS. A powerful analytical tech-nique for the analysis of traces of Sb, Bi, Pb, Sn, and P in steel,Fres. J. Anal. Chem. 359 (1997) 357–360.

[40] S.I. Yamasaki, A. Tsumura, Y. Takaku, Ultratrace elements interrestrial water as determined by high-resolution ICP-MS,Microchem. J. 49 (1994) 305–318.

[41] H. Narasaki, J.Y. Cao, Determination of arsenic and seleniumin river water by hydride generation inductively coupledplasma–mass spectrometry with high resolution, Anal. Sci.12 (1996) 623–627.

[42] R. Rodushkin, Ruth, J, Determination of trace metals inestuarine and sea-water reference materials by high resolutioninductively coupled plasma mass spectrometry, J. Anal. At.Spectrom. 12 (1997) 1181–1185.

[43] C. Barbante, T. Bellomi, G. Mezzadri, P. Cescon, G. Scarponi,C. Morel, S. Jay, C. Van de Velde, C. Ferrari, C.F. Boutron,Direct determination of heavy metals at picogram per gramlevels in Greenland and Antarctic snow by double focusinginductively coupled plasma mass spectrometry, J. Anal. At.Spectrom. 12 (1997) 925–931.

[44] Ch. Latkoczy, T. Prohaska, G. Stingeder, W.W. Wenzel,Determination of trace elements in soil samples by means ofhigh resolution ICP-MS, in: Proceedings of the Fourth Inter-national Conference on the Biogeochemistry of TraceElements, Berkeley, CA, 1997, pp. 391–392.

[45] L. Rottmann, K.G. Heumann, Determination of heavy metalinteractions with dissolved organic materials in natural aqua-tic systems by coupling a high-performance liquid chromato-graphy system with an inductively coupled plasma massspectrometer, Anal. Chem. 66 (1994) 3709–3715.

[46] T. De Smaele, L. Moens, R. Dams, Coupling of GC withICPMS for trace metal speciation, in: G. Holland, S.D. Tanner(Eds.), Plasmas Source Spectrometry—Developments andApplications, Special Publication (No. 202) of the RoyalChemical Society, Cambridge, 1997, pp. 109–123.

[47] C.-K. Kim, R. Seki, S. Morita, S.I. Yamasaki, A. Tsumura, Y.Takaku, Y. Igarashi, M. Yamamoto, Application of a highresolution inductively coupled plasma mass spectrometer tothe measurement of long-lived radionuclides, J. Anal. At.Spectrom. 6 (1991) 205–209.

[48] M. Yamamoto, K.K. Syarbaini, A. Tsumura, K. Komura, K.Ueno, D.J. Assinder, Determination of low-level 99Tc inenvironmental samples by high resolution ICP-MS, J. Radio-anal. Nucl. Chem. 197 (1995) 185–194.

[49] W. Kerl, J.S. Becker, H.-J. Dietze, W. Dannecker, Determina-tion of iodine using a special sample introduction systemcoupled to a double-focusing sector field inductively coupledplasma mass spectrometer, J. Anal. At. Spectrom. 11 (1996)723–726.

[50] K. Hoppstock, J.S. Becker, H.-J. Dietze, Assessment of thedetermination of79selenium using double-focusing sector fieldICP-MS after hydride generation, At. Spectrosc. 18 (1997)180–185.

[51] J. Versieck, L. Vanballenberghe, A. De Kesel, J. Hoste, B.Wallaeys, J. Vandenhaute, N. Baeck, H. Steyaert, A.R. Byrne,F.W. Sunderman, Certification of a second-generation biolo-gical reference material (freeze-dried human serum) fortrace element determinations, Anal. Chim. Acta 204 (1988)63–75.

[52] L. Moens, P. Verrept, R. Dams, U. Greb, G. Jung, B. Laser,New high-resolution inductively coupled plasma mass spec-trometry technology applied for the determination of V, Fe,Cu, Zn and Ag in human serum, J. Anal. At. Spectrom. 9(1994) 1075–1078.

[53] J. Riondato, F. Vanhaecke, L. Moens, R. Dams, Determinationof trace and ultratrace elements in human serum with a doublefocusing magnetic sector inductively coupled plasma massspectrometer, J. Anal. At. Spectrom. 12 (1997) 933–937.

[54] J. Begerow, L. Dunemann, Mass spectral interferences in thedetermination of trace levels of precious metals in humanblood using magnetic sector field and quadrupole inductively


coupled plasma mass spectrometry, J. Anal. At. Spectrom. 11(1996) 303–306.

[55] J. Begerow, M. Turfeld, L. Dunemann, Determination ofphysiological palladium, platinum, iridium and gold levelsin human blood using double focusing magnetic sector fieldinductively coupled plasma mass spectrometry, J. Anal. At.Spectrom. 12 (1997) 1095–1098.

[56] J. Begerow, M. Turfeld, L. Dunemann, Determination ofphysiological platinum levels in human urine using magneticsector field inductively coupled plasma mass spectrometry incombination with ultraviolet photolysis, J. Anal. At. Spec-trom. 11 (1996) 913–916.

[57] E.R. Denoyer, P. Bru¨ckner, E. Debrah, Determination of traceimpurities in semiconductor-grade hydrofluoric acid andhydrogen peroxide by ICP-MS, At. Spectrosc. 16 (1995)12–15.

[58] J. Wang, R.S. Houk, D. Dreessen, D.R. Wiederin, Identifica-tion of inorganic elements in proteins in human serum and inDNA fragments by size exclusion chromatography and induc-tively coupled plasma mass spectrometry with a magneticsector mass spectrometer, J. Am. Chem. Soc. 120 (1998)5793–5799.

[59] S. Sturup, A. Bibak, A. Behrens, V. Gundersen, Multielemen-tal determinations using HR-ICPMS, Paper presented at the1996 Winter Conference on Plasma Spectrochemistry, FortLauderdale, FL, USA, 1996.

[60] S. Augagneur, B. Me´dina, I. Szpunar, R. Lobinski, Determi-nation of rare earth elements in wine by inductively coupledplasma mass spectrometry using a microconcentric nebulizer,J. Anal. At. Spectrom. 11 (1996) 713–721.

[61] T. Doring, M. Schwikowski, H.W. Ga¨ggeler, Determinationof lead concentrations and isotope ratios in recent snowsamples from high alpine sites with a double focusing ICP-MS, Fres. J. Anal. Chem. 359 (1997) 382–384.

[62] C. Latkoczy, T. Prohaska, G. Stingeder, M. Teschler-Nicola,Strontium isotope ratio measurements in prehistoric humanbone samples by means of high-resolution inductively coupledplasma mass spectrometry (HR-ICP-MS), J. Anal. At. Spec-trom. 13 (1998) 561–566.

[63] B. Klaue, J.F. Hogan, J.D. Blum, Measurement of lithium,boron, sulfur, calcium and iron isotope ratios by high resolu-tion ICP-MS, and their applications, Paper presented at the23rd Annual Conference of the Federation of AnalyticalChemistry and Spectroscopy Societies (FACSS XXIII),Kansas City, USA, 1996.

[64] S. Sturup, M. Hansen, C. Mølgaard, Measurements of44Ca:43Ca and42Ca:43Ca isotopic ratios in urine using highresolution inductively coupled plasma mass spectrometry, J.Anal. At. Spectrom. 12 (1997) 919–923.

[65] P.D.P. Taylor, P. De Bie`vre, A.J. Walder, A. Entwistle, Vali-dation of the analytical linearity and mass discriminationcorrection model exhibited by a multiple collector inductively

coupled plasma mass spectrometer by means of a set ofsynthetic uranium isotope mixtures, J. Anal. At. Spectrom.10 (1995) 395–398.

[66] A.J. Walder, P.A. Freedman, Isotopic ratio measurementusing a double focusing magnetic sector mass analyser withan inductively coupled plasma as an ion source, J. Anal. At.Spectrom. 7 (1992) 571–575.

[67] A.J. Walder, I. Platzner, P.A. Freedman, Isotope ratiomeasurement of lead, neodymium and neodymium–samariummixtures, hafnium and hafnium–lutetium mixtures with adouble focusing multiple collector inductively coupled plasmamass spectrometer, J. Anal. At. Spectrom. 8 (1993) 19–23.

[68] J.S. Becker, H.-J. Dietze, Double-focusing sector field induc-tively coupled mass spectrometry for highly sensitive multi-element and isotopic analysis, J. Anal. At. Spectrom. 12(1997) 881–889.

[69] A.N. Halliday, D.-C. Lee, J.N. Christensen, A.J. Walder, P.Freedman, C.E. Jones, C.M. Hall, W. Yi, D. Teagle, Recentdevelopments in inductively coupled plasma magnetic sectormultiple collector mass spectrometry, Int. J. Mass Spectrom.Ion Proces. 146–147 (1995) 21–33.

[70] J. Dahmen, M. Pfluger, M. Martin, L. Rottmann, G. Weich-brodt, Trace element determination of high-purity chemicalsfor the processing of semiconductors with high-resolutionICP-mass spectrometry using stable isotope dilution analysis(IDA), Fres. J. Anal. Chem. 359 (1997) 410–413.

[71] J.-F. Ying, D.J. Douglas, High resolution inductively coupledplasma mass spectra with a quadrupole mass filter, RapidComm. Mass Spectrom. 10 (1996) 649–652.

[72] D.W. Koppenaal, C.J. Barinaga, M.R. Smith, Performance ofan inductively coupled plasma source ion trap mass spectro-meter, J. Anal. At. Spectrom. 9 (1994) 1053–1058.

[73] D.P. Myers, G. Li, P.P. Mahoney, G.M. Hieftje, An induc-tively coupled plasma-time-of-flight mass spectrometer forelemental analysis. Part II: Direct current quadrupole lenssystem for improved performance, J. Am. Soc. Mass Spec-trom. 6 (1995) 400–410.

[74] P.P. Mahoney, S.J. Ray, G. Hieftje, Time-of-flight mass spec-trometry for elemental analysis, Appl. Spectrosc. 51 (1997)16A–28A.

[75] M. Morita, H. Ito, M. Linscheid, K. Otsuka, Resolution ofinterelement spectral overlaps by high-resolution inductivelycoupled plasma mass spectrometry, Anal. Chem. 66 (1994)1588–1590.

[76] K.E. Milgram, R.S. Houk, C.H. Watson, J.R. Eyler, Develop-ment and application of inductively coupled plasma (ICP)-Fourier transform ion cyclotron resonance (FTICR) massspectrometry, Paper presented at the 1998 Winter Conferenceon Plasma Spectrochemistry, Scottsdale, AZ, USA, 1998.

[77] M. Van Straaten, Analytical glow discharge mass spectrome-try: physical aspects and applications, Thesis work, Universityof Antwerp, Belgium, 1993.


Documents

Sector field mass spectrometers in ICP-MS