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Practicum Spectroscopy LIM
Fall 2010
Laser Ablation-Inductively CoupledPlasma-Mass Spectrometry: acquiring
basic skills and solid samplemeausurements
Jorge Ferreiro, study degree Chemistry, 5th semester, [email protected] Lauber, study degree Chemistry, 5th semester, [email protected]
Assistant: Mohamed Tarik
Abstract: LA-ICP-MS is a widely used method for trace and ultratrace elementdetection in solid samples. To get intense signals and low oxidation rates, dailyoptimization for ICP was necessary. Adjusting parameters were nebulizer gas flowrate, auxiliary gas flow rate, plasma gas flow, RF-power and ion lens voltage; firstone being mainly important for optimal signals and low oxide formation. For MS,different dwell times can be set up, showing higher sensitivity for lower dwell time.After daily optimization the LOD and sensitivity for 30 elements of the referencematerial NIST 610 were calculated, showing higher LODs for unsensitive elements.A qualitative analysis of a 5 centime Swiss coin showed presence of Al, Cu, Na,Ni and Zn. Using a reference material and software Lamtrace a quantitativeanalysis of a steel sample was performed. In total 17 elements were identified andthe results were accurate except for Cu and Cr.
Zurich, the 21.10.2010
J. Ferreiro A. Lauber
1 Introduction
Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) isa widely used method for elemental analysis in solids. The sample is ablated witha pulsed laser beam and introduced as aerosol into the ICP-MS, where it’s ionized(ICP) and detected (MS).
Figure 1: Standard set-up for LA-ICP-MS equipement. [1]
The sample is placed into an airtight, closed ablation-cell which is flushed witha constant He-flow (1 Lmin−1). Laser pulses with certain amount of energy arefocussed on a solid sample surface. The pulse ablates material from the surfacecreating a microplasma of ions and neutral particles, which are carried by He/Arto the plasma of the ICP. Two working modes for ablation are used: scanningmode and drilling mode.
The inductively coupled plasma (ICP) atomizes and ionizes the aerosol producedby the laser. Plasma is a state of matter made of ions, electrons and atoms reachinga temperature of about 8000 K. A radio frequency (RF) coil surrounds a part ofthe torch. The plasma is ignited with a high voltage spark, which ionizes a first Ar-atom. The generated electron is accelerated and ionizes further Ar-atoms leadingto a cascade reaction. The RF-coil supplies the plasma torch with energy to keepit stable. The plasma torch consists of three concentric tubes, which are usuallymade from quartz. These are shown as the outer tube, middle tube, and sampleinjector. The gas (usually Ar) used to form the plasma is passed between the outerand middle tubes at a flow rate of 12 - 18 Lmin−1. A second gas flow, the auxiliarygas, passes between the middle tube and the sample injector normally at 1 Lmin−1
and is used to change the position of the base of the plasma relative to the tube
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and the injector. A third gas flow, the nebulizer gas, also flowing normally at 1Lmin−1 carries the sample, in the form of a fine-droplet aerosol, from the sampleintroduction system.
In ICP-MS the ion beam comes through the interface into the mass analyzer. Therole of the interface is to transport the ions efficiently from the plasma, which is atatmospheric pressure (760 Torr) to the mass spectrometer analyzer region, whichis at approximately 10−6 Torr. The interface consists of two metallic cones (firstknown as sampler cone and second one known as skimmer cone) with very smallorifices, which intermediary space is maintained at a pressure of 2 Torr with amechanical roughing pump and are normally water-cooled. To focus the ions forthe detector an ion lens system (mostly hexapole rods) are used.
For the separation by means of a detection of the different elements of the sample,one uses a mass spectrometer (MS). There are many different mass spectrometerswith different characteristics and functionalities. For ICP-MS Quadrupole-MS (Q-MS) is widely used because cheap and the sensitivity is sufficient for most analysis.The collimated ion beam enters the mass separator which usually consists of fourhyperbolic rods. Each opposite pair of rods are connected with a DC and an ACvoltage. According to Mathieu’s equations for a given DC and AC amplitude,only ions of a given m
zratio will have stable oscillating trajectories and reach the
detector. Widely used detectors are electron- and photomultiplier.
For Q-MS a number of instrumental parameter can also be adjusted. The peak-hopping mode of operation is generally selected for quantitative analysis. In sucha case the dwell time is the period of time that is spend collecting data at a partic-ular mass. Starting with the lowest mass to be analyzed, the instrument collectsdata (in form of counts-per-second (cps)) for a duration of the selected dwell timeand then jumps to the next mass chosen and so on. Normally higher dwell timesprovoke a smaller error but also fewer mass sweeps in total acquistion period. Toget a quantitative analysis of an unknown sample natural internal standards arewidely used.
(CA)S =(CA)R · (Iis)R(Cis)R · (IA)R
· (Cis)S · (IA)S(Iis)S
(1)
C indicates concentrations (in ppm), I inentistiy (in cps), A analyte, S sample, isinternal standard and R reference material. For all kind of analytical measurementsthere are always two very important parameters which are given: limit of detectionLOD and sensitivity S. The LOD describes the lowest amount of substance whichcan be distinguished from the background:
LOD = 3 · σnetS
(2)
3
where σnet is the background standard deviation and S is the sensitivity given by
S =I
c(3)
where I is the intensity an c the concetration.
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2 Experimental
2.1 General
A quadrupole ICP-MS instrument Perkin Elmer SCIEX ELAN 6100 DRCII, controlled by the software called ELAN was used. For laser ablation a solidstate Nd:YAG laser LSX-213, CETAC Technologies (λ = 213 nm) and a solidstate Nd: YAG laser LSX-500, CETAC Technologies (λ = 266 nm) were used.A reference material silicate glass NIST 610 was used for the daily optimization.For the data evaluation Lamtrace and the program R were used.
2.2 Daily optimization
In order to produce low oxidation rates in the ICP ThO+
Th+should be lower than
0.5%. To monitor complete atomization and ionization U+
Th+≈ 1. U+ and Th+ are
used because both ions have similar ionization energies, masses and concentrations(in NIST 610) and their major isotope has an abundance of > 99%. Adjustingparameters were nebulizer gas flow rate, auxiliary gas flow rate, plasma gas flowrate and ion lens voltage. The nebulizer gas flow rate (0.6 - 0.9 Lmin−1) andauxiliary gas flow rate (0.6 - 0.9 Lmin−1) were changed in a bigger range than theplasma gas flow rate (16.5 - 17.5 Lmin−1). The RF-power was kept at 1350 W.
2.3 Acquisition of transient signals and mass spectra
The dwell time influences the homogeneity of transient signals. The effect of threedifferent dwell times (4ms, 10ms, 50 ms) was investigated on 30 elements of NIST610. For six major elements (Ag, Cu, Li, Ni, Pb, Si) the transient signal wasmeasured.
2.4 Mass spectra acquistion
To identify the elements within NIST 610 two mass spectra were acquiered (oneblank and one NIST 610). To acquire good mass spectra the multi channel analyzer(MCA) was set to measure 20 points per mass. For the elements Ag, Li, Ni, Pb
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and Si the resolution R (in NIST 610 and blank) was calculated.
R =m
∆m(4)
2.5 Analysis of unknown sample
The mass spectrum of a 5 centime Swiss coin was acquiered in hole drilling abla-tion mode to identify all present elements in the sample. The mass spectrum ofthe background was measured. For the detected elements a transient signal wasacquired.
2.6 Analysis of steel sample
After daily optimization of the ICP-MS system a steel sample was analyzed in holedrilling mode. In total 10 transient signals were measured: 2 times standard, 6times sample and finally 2 times standard to assure that the measurement sequencedidn’t alter the conditions in the plasma. Ni was used as internal standard toperform the quantification of the sample [2].
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3 Results
3.1 Daily optimization
Table 2 in the appendix summarizes the optimal parameter values for each labday.
t s
Icps
10
102
103
104
105
106
107
108
0 25 50 75 100 125 150 175 200 225
Nebulizer Gas Flow (L/min)
0.8 0.80.7 0.70.6
t s
Icps
10
102
103
104
105
106
107
108
0 25 50 75 100 125 150 175 200 225
Auxiliary Gas Flow (L/min)
0.9 0.90.75 0.750.65
Figure 2: (Top) Nebulizer gas flow rate for daily optimization: The best working con-ditions (low oxide formation and intense signal) are achieved with a flow rate of0.8 Lmin−1. (Bottom) Auxiliary gas flow rate for daily optimization: With a flowrate of 0.8 Lmin−1 best signals are achieved. Both paramters were changed in awide range. Colorcodes: Al(red), Na (black), Si (green), Th (blue), ThO (brown),U (yellow).
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t s
Icps
10
102
103
104
105
106
107
108
0 25 50 75 100 125 150 175 200 225
Plasma Gas Flow (L/min)
17.5 17.517.0 17.016.5
Figure 3: (Top) Plasma gas flow rate for daily optimization: Good working conditions areachieved with a flow rate of 17.5 Lmin−1. Although the effects of larger increaseor decrease of the plasma flow rate wasn’t sudied. (Bottom) Ion lens voltagecalibration: The calibration should yield a good linear fit for the meausred points.The unit for y-axis is voltage-level. Colorcodes: Al(red), Na (black), Si (green),Th (blue), ThO (brown), U (yellow).
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3.2 Acquistion of transient signals and mass spectra
t s
Icps
102
103
104
105
106
0 25 50 75 100 125 150
tdwell =4ms
t s
Icps
102
103
104
105
106
0 25 50 75 100 125 150
tdwell =10ms
Figure 4: (Top) Transient signal acquistion for dwell time = 4ms in hole drilling modefor six elements: The transient signals are very homogeneous. (Bottom) Transientsignal acquistion for dwell time = 10 ms in hole drilling mode for six elements:The signals begin to tickle and lose homogeneity. All signals descrease constanteleydue to the hole drilling mode. Colorcodes: Ag (blue), Cu(dark green), Li (red),Ni (light green), Pb (dark blue), Si (dark red).
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t s
Icps
102
103
104
105
106
0 25 50 75 100 125 150
tdwell =50ms
Figure 5: Transient signal acquistion for dwell time = 50 ms in hole drilling mode for sixelements: Ag (blue), Cu(dark green), Li (red), Ni (light green), Pb (dark blue),Si (dark red). The strong tickeling of transient signals increases more comparingto 4ms and 10 ms. So it’s clearly better to settle a lower dwell time. All signalsdescrease constanteley due to the hole drilling mode.
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3.3 Mass spectra acquistion
m z
Icps
1
10
102
103
104
105
106
0 15 30 45 60 75 90 110 130 150 170 190 210 230 250
23 Na
40 Ar
ArNArCl
Ar2
m z
Icps
110
102
103
104
105
106
0 15 30 45 60 75 90 110 130 150 170 190 210 230 250
55 Mn59 Co
95 Mo107 Ag
120 Sn121 Sb
140 Ce 159 Tb 175 Lu
197 Au
208 Pb209 Bi
232 Th238 U
40 Ar
Figure 6: (Top) Mass spectra of gas blank: Very high intensities are shown for masses40 and 80, which correspond to the plasma gas 40Ar+ respectively the Ar-dimer80Ar+2 . The signals in the low mass range show up due to non-metal elements ofthe matrix. (Bottom) Mass spectra of NIST 610: All desired NIST 610 elementsare detected.
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3.4 Analysis of unknown sample
m z
Icps
110
102
103
104
105
106
0 15 30 45 60 75 90 110 130 150 170 190 210 230 250
23 Na
60 Ni
27 Al
65 Cu
68 Zn
t s
Icps
102
103
104
105
106
107
108
0 25 50 75 100 125
Figure 7: (Top) Mass spectra of coin: In order to indentify the elements possibly presentin the sample the whole mass spectrum was acquiered skipping m
z = 40 (40Ar+)and m
z = 80 (80Ar+2 ) to protect the detector from high concentration of Ar-species.(Bottom) Transient signals for identified elements: After identfication transientsignals were meausured showing very homogenous distributioon despite of Na.
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3.5 Analysis of steel sample
Element Corrected bg./cps bg./cps Calc. Concentration/ppm LOD/ppmB 35(13) 108(10) 41(12) 30(2)Ti 6800(1500) 333(15) 1500(300) 13.9(7)V 31000(1300) 664(33) 363(2) 1.04(7)Cr 24700(850) 410(13) 309(3) 0.88(7)Co 260000(10000) 131(10) 3240(15) 0.51(5)Cu 98000(3000) 153(9) 4900(85) 2.0(2)Zr 230000(82000) 120(10) 1200(210) 0.19(2)Nb 600000(150000) 130(7) 4300(960) 0.30(3)Mo 18200(800) 152(13) 641(8) 1.5(1)Ag 725(48) 121(9) 12.5(5) 0.69(6)Sn 11200(331) 113(9) 184(6) 0.60(6)Sb 8100(260) 114(4) 108(3) 0.48(6)Ce 14000(17000) 112(12) 58(59) 0.17(1)Ta 350000(96000) 116(7) 2800(700) 0.30(3)W 76000(4000) 106(6) 2012(35) 0.9(1)
Table 1: Calculated concentrations, background(bg.), background correction and LOD for steelsample. Ni was used as internal standard. Ti, Co, Cu, Nb, Ta and W have highconcentrations in steel. One can’t figure out a trend for LOD because normally thevalues increase with heavier elements, but in general most elements have a very lowLOD so they are easily detected.
4 Discussion
4.1 Daily optimization
The nebulizer gas flow rate showed the largest influence on the signal and formationof oxides. Auxiliary gas flow rate and plasma gas flow didn’t have such a largeinfluence. Notice this results are for the intervalls in which the paramters werechanged. Obviously the size of the particles in the aerosol will determine thequalitity of the signal. Normally the plasma gas flow will also have a stronginfluence on the sample mainly on the oxide formation but in this case the ratewasn’t nearly changed.
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4.2 Acquisition of transient signals and mass spectra
A higher dwell time leads to a more homogenous background and slightly betterintensity. In general higher dwell times lead to a longer sweep which can resultin a loss of signal. So in this case the suggestion is to take a dwell time between0.1 ms up to 10 ms because the mass scan is performed hopping in steps of 1 m
z
although the scan range is big.
4.3 Mass spectra acquistion
With one point per mass (MCA = 1) one can’t calculate a resolution for twoadjacent signals since there is only a maximal value but it isn’t possible to drawa line shape. Although the scan speed is higher than for multiple points perpeak. One observes for heavier elements better mass resolution for two adjacentpeaks. Because of isobaric and polyatomic interferences normally one has a loss ofsensitivity.
4.4 Analysis of unknown samples
Al, Cu, Na, Ni and Zn were identified through the mass spectrum of the coin. Forall signals the transient signal was very homogenous despite for Na, which is oneof the major contamination sources for analysis. Na is present in salt which ispresent on human skin. So the coin was always in contact with Na. The signalappaered to be inhomogenous because the analysis was performed in hole drillingmode and the Na is probably only present on the surface.
4.5 Analysis of steel sample
Despite of Co and Cr for all detected elements the concentration was very accuratecompared to the certified sample.
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5 Literature
[1] Thomas R., A beginner’s guide to ICP-MS, Spectroscopy Tutorial
[2] Longerich H.P., Jackson S.E., Günther D., J. of Analytical Spectrometry,1996,11, 899-904.
[3] Uriano G.A.;Certifacte of Analysis - Standard Reference Material 1261a -AISI 4340 Steel, National Bureau of Standards, 1981, 56.
[4] Uriano G.A.;Certifacte of Analysis - Standard Reference Material 1262a -AISI 4340 Steel, National Bureau of Standards, 1981, 57.
6 Appendix
6.1 Tables
Parameter 1st day 2nd day 3rd dayNebulizer gas flow rate [l/min] 0.8 0.8 0.85Auxiliary gas flow rate [l/min] 0.8 0.8 0.8
Plasma gas flow [l/min] 17.52 17.53 17.5
Table 2: Optimal parameter values summarized for each experimental day. The parameters re-mained quite constant for each working day. The nebulizer gas flow rate and auxiliarygas flow rate were changed in bigger ranges than the nebulizer gas flow. Although thelast parameter will have a strong influence over the measurements. For the first daythe oxide formation rate and complete ionization/atomization were calculated. ThO+
Th+ =0.2% and U+
Th+ = 91%
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Element bg./cps bg. corr./cps LOD/ppm S/cps ppm−1
Li 144 49 16(4) 28(7)Na 1676 12400 14(3) 100(21)Al 5305 12000000 100(27) 28(8)Si 32780 581000 9913(2665) 12(3)Ca 1854 385000 2162(571) 100(26)Cr 600 74300 49(14) 173(51)Mn 695 6670 4(1) 206(54)Fe 530 8910 177(70) 205(81)Co 176 1980 2.1(5) 182(46)Ni 178 7390 11(3) 150(39)Cu 210 17400 11(3) 138(35)Zn 241 18400 236(114) 51(25)Mo 225 95 8(2) 400(97)Pd 165 23800 - -Ag 178 79.1 3.5(8) 260(56)Cd 170 32200 21(7) 147(48)Sn 153 4860 2.3(5) 535(128)Sb 160 69100 2.3(5) 291(66)Te 154 61500 - -Ba 153 3690 6(2) 582(150)Ce 139 27700 0.7(2) 555(135)Tb 151 220000 0.7(2) 566(145)Lu 136 251000 0.8(2) 501(134)Pt 154 212000 - -Au 138 195 2.7(8) 128(39)Pb 151 2880 2.5(6) 289(63)Bi 179 6250 1.8(4) 241(54)Th 131 86200 1.1(3) 334(85)U 161 150000 1.2(3) 321(76)
Table 3: Background (bg.), corrected background, LOD and sensitivity for 30 analytes of NIST610: Elements with high LODs have a very low sensitivity.
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Isotope Resolution7Li 729Si 2960Ni 76206Pb 219184W 204
Table 4: Calculated Resolutions for 5 elements of NIST 610: For heavier elements the resolutionis higher. Resolution was calculated at FWMH.
Element Calc. concentration/ppm Sample 1262 a/ppm Ref. 1261 a/ppm Rel. Error /%11B 41(12) 25 5 8747Ti 1500(300) 850 200 87.851V 363(2) 400 110 86.753Cr 309(3) 3000 6900 69.759Co 32.40(15) 3000 320 213365Cu 4900(85) 5100 420 90.190Zr 1200(210) 2000 90 91.493Nb 4300(960) 3000 220 92.595Mo 641(8) 700 1900 94.9107Ag 12.5(5) 11 4 196.0118Sn 184(6) 160 100 220121Sb 108(3) 120 42 45.7140Ce 59(58) 15 14 61.1181Ta 2800(700) 2100 210 141.4282W 2012(35) 2000 170 92.5
Table 5: Calculated concentrations, Sample 1262 a and reference (ref.) material 1261a and rela-tive errors. The relative errors are high since the reference material was only used forcalibration. Although the calculated concentrations are very accurate for some elements.[2],[3]
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6.2 Program codes
6.3 Lamtrace files
6.4 Labjournal
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