Evaluation of two different direct-sampling ion-trap mass-spectrometry methods for monitoring halocarbon compounds in air

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FIELD ANALYTICAL CHEMISTRY AND TECHNOLOGY 4(1):14–30, 2000

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Abstract: Two different direct-sampling ion-trap massspectrometry (DSITMS) methods are evaluated for mon-itoring trace levels of volatile organic compounds (VOCs)in air. The first is based on the use of a sample introduc-tion system that mixes the air sample into a heliumstream prior to introduction into the ion trap through anopen-split interface. The second utilizes a valve and useszero air to flush the contents of the sample loop into theion trap. Unique features of this system are its use of airin place of helium as a buffer gas for the ion trap, andthe optimization of experimental parameters to maintainsensitivity and unit mass resolution. Dichlorodifluoro-methane (CFC12) and carbon tetrachloride (CCl 4) wereemployed as test compounds for this study. Figures ofmerit for both sample introduction methods were com-parable. Detection limits were approximately 50 parts perbillion by volume in MS, selected ion monitoring (SIM),and MS/MS modes. Analysis speeds were on the order of20 s or less per sample. The sensitivity of the ion trap,inherent selectivity of MS/MS, and fast response times ofthese sample introduction systems make these DSITMStechniques suitable for many applications that requireon-line, real-time monitoring of VOCs in air. � 2000 JohnWiley & Sons, Inc. Field Analyt Chem Technol 4: 14–30,2000Keywords: halocarbons; chlorofluorocarbons; volatileorganic compounds; ion trap mass spectrometry; tan-dem mass spectrometry; direct sampling mass spec-trometry; air monitoring

Introduction

There is an ever-increasing interest in air quality and inassessing the impact of human activities on the Earth’s at-

Correspondence to:Peter T. Palmer*Current address:Advanced Medicine Inc., 280 Utah Ave., South San

Francisco, CA 94080.Contract grant sponsor: NASAContract grant number: NAS 9-19410

� 2000 John Wiley & Sons, Inc.

mosphere. Since Rowland and Molina initially postulatedthe link between chlorofluorocarbons (CFCs) and ozone de-pletion in the early 1970s,1 a number of research groups havedevoted significant efforts into tracking the changes in at-mospheric concentrations of these compounds.2 The intro-duction of CFC replacement compounds into commercialuse necessitates the development of newmethods to monitorambient levels of these compounds in the atmosphere.3,4Re-cent passage of the Clean Air Act requires the monitoringof hundreds of VOCs to ensure compliance with Environ-mental Protection Agency (EPA) emission limits. A host ofother applications, including process control, fence-postmonitoring, stack monitoring, engine exhaust analysis, andhuman breath analysis require advanced technology for sen-sitive, selective monitoring of specific contaminants in air.

Most commercially available, laboratory-based instru-mentation is unsuitable for meeting these applications. Theever-increasing need for timely, high-quality atmosphericcomposition data continues to drive the development of newtechnology for on-line, in-situ monitoring of VOCs in air.5,6

Depending on the environment and application, the requireddetection limits may range from percent levels down toparts-per-trillion by volume (ppt). In some cases, a moregeneral detector is required to analyze for a variety of com-pounds (i.e., CFCs, VOCs), whereas in others specificitytoward a specific target compound or limited set of com-pounds is more important. An overriding constraint is theability to perform the analysis on site versus collecting thesample for subsequent laboratory analysis. Indeed, many ap-plications require fast cycle times to monitor rapid changesin concentration and composition.

The utility of mass spectrometry for detecting and iden-tifying components in complex mixtures is well recognized.Several mass spectrometers have been developed specifi-cally for field applications. Bruker’s Mobile EnvironmentalMass Spectrometer (MEM) can be equipped with a numberof sampling modules for air, water, and soil analysis.7–11

Although this system has a proven ability for on-site anal-

Evaluation of Two Different Direct-Sampling Ion-TrapMass-Spectrometry Methods for Monitoring HalocarbonCompounds in Air

Peter T. Palmer, 1 Carla Remigi, 1 and Dane Karr* 2

1Department of Chemistry and Biochemistry, San Francisco State University, San Francisco, California 941322Teledyne Electronic Technologies, Mountain View, California 94043

Received March 17 1999; revised September 15, 1999; accepted October 1, 1999

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FIELD ANALYTICAL CHEMISTRY AND TECHNOLOGY—2000 15

ysis, it is mobile rather than truly portable. More recently,efforts have focused on developing smaller instrumentationthat can be carried into the field on a truck or ported to asite by one or more individuals. Arnold, Meuzelaar, and co-workers developed a man-portable quadrupole-based GC/MS system.12–14Other groups have developed miniaturizedsector,15 time-of-flight,16 and ion cyclotron resonance-typemass spectrometers.17,18 Several field-portable mass spec-trometers are now available from a number of commercialvendors.

Ion-trap mass spectrometry (ITMS) has generated intenseinterest in recent years and promises to be the high-perform-ance mass analyzer of the future.19,20 It is small, relativelysimple, and inexpensive. It is recognized as one of the mostsensitive mass spectrometers currently available. It has ex-cellent experimental versatility and is capable of collectingelectron ionization (EI), chemical ionization (CI), and se-quential stages of MS (i.e., MS/MS,MS/MS/MS,MSn) data.This tandem mass spectrometry capability is particularlyvaluable for real-time monitoring applications, in which ad-ditional stages of MS can be used to tailor the selectivity ofthe analysis to the compound and matrix of interest. Collec-tively, these features make the ion trap well suited for avariety of air monitoring applications.

A number of research groups are actively pursuing theuse of ion traps for air monitoring applications. Wise andGuerin at Oak Ridge National Laboratories have made greatprogress developing, reporting, and applying a set of tech-niques collectively referred to as DSITMS.21 Bruker devel-oped the Chemical and Biological Mass Spectrometer(CBMS) for detecting chemical warfare agents.22Meuzelaarand co-workers developed the first field-portable ion-trap in-strument with MS/MS capabilities.23,24Wise et al. also re-ported on the development of a field-portable ion-trap sys-tem.25 Researchers at Los Alamos National Laboratoryreported on the development of a miniaturized ion trap forVOC analysis.26,27 Without a doubt, further efforts towardminiaturizing ion-trap mass spectrometers will render themmore attractive for field applications.

The application requirements often dictate the choice ofa sample introduction system. Various investigators haveevaluated leak-type interfaces, transfer line GC, membrane,glow discharge (GD) ionization sources, and atmosphericpressure ionization (API) sources for these applications. Therelative merits and deficiencies of these sample introductionsystems can be evaluated by comparing analytical criteria,including detection limits, specificity, and response times,and practical considerations, such as space, weight, power,and hardware requirements.

Transfer-line GC/MS is particularly valuable insofar thatit provides a second dimension of information via chro-matographic separation in time in addition to the informationthat can be derived frommass spectrometric data.28Low ppbdetection limits and 6-s cycle times have been reported forbenzene, toluene, and xylene mixtures.29 Cryotrapping andthermal desorption steps can be used with transfer-lineGC/MS to reduce detection limits to low ppt levels.

Membrane inlets provide a means of sample enrichment.

A number of investigators have reported the use of mem-brane inlets in conjunction with ITMS for monitoring VOCsin air.30–32Although membrane inlets have the drawback ofpoor sensitivity toward more polar VOCs, slow responsetimes, and memory effects, there have been a number ofrecent advances in this area that have addressed these defi-ciencies. Cisper, Gill, Townsend, and Hemberger describeda system that uses a two-stage hollow membrane inlet, jetseparator, Finnigan ITD, and Teledyne HST-1000 FNF ac-cessory to detect 20 ppt of toluene in EI/MSmode.33Amem-brane inlet was used along with charge-exchange ionizationto achieve low-ppb detection limits for a series of polar com-pounds, representing a 4–20-fold improvement over EI.34

Finally, a thin silicone membrane deposited on a micropo-rous polypropylene support fiber was used to reduce re-sponse times to less than 1 min.35

The first use of a GD source in conjunction with ITMSwas reported by McLuckey, van Berkel, Goeringer, andGlish, who demonstrated detection limits of 2 ppt for 2,4,6-dinitrotoluene.36,37 More recently, Gordon and co-workersused a GD source, a Finnigan ITMS ion trap, and a TeledyneHST-1000 FNF accessory to establish detection limits onthe order of 1–10 ppb for nonpolar and polar VOCs in bothEI/MS and EI/MS/MS modes.30,31Although the GD sourcepresents additional problems with respect to complexity,added weight, and power requirements, its strong pointis its ruggedness and applicability to nonpolar and polarVOCs.

API sources are another promising alternative for achiev-ing lower detection limits. Ketkar and co-workers demon-strated low-ppt level detection limits for chemical warfareagents GB and VX in air using an Extrel API tandem quad-rupole instrument.38,39 Kenny and Crenshaw demonstrateddetection limits of 100 ppb for thioglycol in air using aTAGA APCI tandem quadrupole mass spectrometer.40Morerecently, Ma, Gilbert, and Shadman employed APIMS toobtain detection limits of 1 ppt for methane, water, oxygen,and carbon dioxide in air.41

The focus of this research was the implementation andevaluation of two different leak-type sample introductionsystems for monitoring trace levels of VOCs in air viaDSITMS. The first is a prototype direct air sampling systemdeveloped by Scientific Instrument Services based on a de-sign originated at Oak Ridge National Laboratories.42 Thisdevice is henceforth referred to as the real-time air monitor(RTAM). It mixes the air sample with a helium stream andthen passes the resulting mixture through an open-split in-terface prior to introduction into the ion trap. The secondsystem employs a valve in the sample loop configuration tointroduce discrete volumes of an air sample into the ion trap.This device is henceforth referred to as the valve inlet. Keyfeatures of this system are its use of air in place of heliumas the buffer gas and the subsequent optimization of ion-trapparameters to maintain sensitivity and unit mass resolution.Figures of merit (i.e., detection limits, sample sizes, andanalysis times) and performance characteristics are de-scribed for each inlet system and results are compared toprevious work reported in the literature.

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FIG. 1. Schematic of RTAM.

FIG. 2. Schematic of valve-based air-sampling interface.

Experiment

The test compounds chosen for this work were CFC12and CCl4. This choice was guided by the availability of gasstandards on hand and the fact that these compounds evi-dence few problems with carryover and adsorption onto sur-faces. Concentration units are reported on a per-volumebasis(e.g., ppb is equivalent to parts per billion by volume),which, unlike the units of mg/m3, are independent of tem-perature and pressure. For reference, note that 50 ppb cor-responds to 0.25, 0.28, and 0.31 mg/m3 for CFC12, CFC11,

and CCl4, respectively. Gas standards were prepared from a1-ppm mixture of trichlorofluoromethane (CFC11), CFC12,and CCl4 in air (Matheson Specialty Gases, Allentown, PA).Lower concentrations were prepared with the use of a com-puterized gas dilution system (Environics, Manchester, CT)and a balance gas of zero air. Concentrations of CFCs in airfrom 10 ppb to 1 ppm were prepared daily and stored in 10l Tedlar bags (Alltech, Waukegan, IL). The zero air used asa balance gas for the dilutions and the bath gas for the valve-based sample introduction system was 99.999% purity, low-VOC-grade air (Matheson Specialty Gases, Allentown, PA).


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The helium was 99.995% purity and subsequently passedthrough a multibed filter (Alltech, Waukegan, IL). Althoughtrichlorofluoromethane was present in all the standards usedin this study, it was not quantitated because of a relativelylarge hydrocarbon background ion at m/z 85.

The RTAM depicted in Figure 1 is a prototype inlet sys-tem (Scientific Instrument Services, Ringoes, NJ) based ona design originated by researchers at Oak Ridge NationalLaboratories.42 This inlet system mixes an air sample into ahelium stream. The combined stream is then passed throughan open-split interface prior to introduction into the ion trap.On/off and metering valves regulate air, helium, and open-split flow rates. Small diaphragm pumps actuate air andopen-split flow. A 12-in. length of 75-�m ID deactivatedfused silica was used as a transfer line between the open-split interface and the ion trap. Both the transfer line andopen-split interface were heated to 120�C. Tuning of theRTAM involves setting helium, air sample, and split flowsfor optimum sensitivity and resolution. This is basically anempirical procedure as true flows through the inlet are un-known when this device is connected to the ion trap. Sampleflow is controlled by a 7-turn metering valve and was set toprovide a sufficient flow of the sample through the systemand minimize hysteresis. In practice, the optimal setting ofthe air metering valve was about 2 turns. Lower and highersample flows (corresponding to more or less than 2 turns onthe metering valve, respectively) did not improve sensitivityand only functioned to increase or decrease the amount oftime required for the air sample to enter and clear the inlet.Actual sample flows are in the 1-l/min range. Helium flowis controlled by a 15-turn metering valve and was set toprovide optimal mass resolution. A helium metering valvesetting of six to seven turns provided better than unit massresolution. Lower helium flows (corresponding to less thansix turns on the metering valve) degraded mass resolution,whereas higher helium flows (corresponding to more thanseven turns) increased resolution but reduced sensitivity.Ac-tual helium flows are also in the 1-l/min range. Split flow iscontrolled by a seven-turn metering valve and was set to pullthe desired flow of the combined mixture of air and heliumpast the open split interface. This valve was set to two turns

to give optimal sensitivity. Higher and lower settings (cor-responding to more or fewer turns on the metering valve,respectively) did not seem to have as large of an effect onsensitivity as anticipated, with less than an order of magni-tude difference in signal between fully open and fully closed.This is mostly likely due to the fact that it is the length andinner diameter of the transfer line that controls flow into theion trap. Actual flow into the ion trap is difficult to ascertainbut is estimated to be slightly larger than the 1-ml/min nom-inal helium flow based on the pressures indicated on the iongauge. Once optimized, these valve settings needed no fur-ther adjustments on a day-to-day basis and were usedthroughout this study.

The valve inlet, depicted in Figure 2, uses a 10-port valve(VICI, Houston, TX) equipped with a 15-�l loop and con-figured in the sample loop mode. A 10-ml/min full-scalemass flow controller (Edwards, Wilmington, MA) provideda constant 0.3-ml/min air flow through the valve and transferline and into the ion trap. The valve, sample loop, and at-tached lines were enclosed in a valve oven heated to 100�C.A 1-m section of 0.25 mm deactivated fused silica transferline (Alltech, Waukegan, IL) connected the valve outlet tostandard transfer line inlet into the ion trap. Samples weretransferred into the sample loop using a gas-tight syringe.Approximately 10 times the volume of the sample loop (1ml) was used to flush the lines and fill the sample loop.Samples were loaded into the loop and the valve switchedto the inject position to flush the sample into the ion trap.The valve was returned to the load position 50 s later. Thiscycle was repeated every minute to perform replicate anal-yses during a single data-acquisition event.

All mass spectra were collected on a Teledyne 3DQDis-covery� ion-trap mass spectrometer (Mountain View, CA).Pressure measurements were made with a Granville–Phil-lips model 342 mini ion gauge (Boulder, CO) calibrated forN2. For all analyses, an ionization time of 50 ms, emissioncurrent of 20 microamperes, scan range of 70–130 m/z, and10 microscans were employed. SIM experiments were per-formed with the use of filtered noise fields (FNF), whichallowed only the ions of interest (m/z 101–103, 117–121)to remain in the trap. MS/MS experiments also used FNF to

TABLE 1. Detection limits of various sample introduction systems in conjunction with mass spectrometers for real-time air monitoring.

Inlet system Mass spectrometer Target compound(s) Detection limits Reference

Mass flow controller Finnigan ion trap Trichloroethylene �10 ppm 43Metering valve Finnigan ITMS ion trap CCl4, chlorobenzene 0.25–125 ppb 44Open-split interface Finnigan ITMS ion trap Ethylbenzene, styrene low ppb 45Open-split interface Extrel Questor quadrupole Toluene, methylene chloride 12–36 ppb 46Open-split interface Finnigan ITMS ion trap Various VOCs low ppb 47RTAM Finnigan ITS41 ion trap Various VOCs low ppb 42RTAM Finnigan Magnum ion trap Various VOCs low ppb 48RTAM Teledyne 3DQ ion trap Various VOCs low ppb 49RTAM Finnigan ITS40 ion trap Various VOCs low ppb 50This work Teledyne 3DQ ion trap CFC11, CCl4 50 ppb

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FIG. 4. Calibration curves for CFCs analyzed in MS mode with the use of the RTAM. The triangles represent a plot of average intensity of m/z 101 fromCFC11, and the squares represent a plot of average intensity of m/z 119 from CCl4.

FIG. 3. Total and selected ion chromatograms and background-subtracted mass spectrum from replicate analyses of 50 ppb CFCs in air in MS mode withthe use of RTAM.


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FIG. 6. Calibration curves for CFCs analyzed in MS/MS mode with the use of the RTAM. The triangles represent a plot of average intensity of m/z 66(from a precursor ion of m/z 101) from CFC11, and the squares represent a plot of average intensity of m/z 82 (from a precursor ion of m/z 117) fromCCl4.

FIG. 5. Calibration curves for CFCs analyzed in SIM mode with the use of the RTAM. The triangles represent a plot of average intensity of m/z 101from CFC11, and the squares represent a plot of average intensity of m/z 119 from CCl4.

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isolate the precursor ions (m/z 101 for CFC11, m/z 117 forCCl4), effect collision-induced dissociation (CID), and gen-erate the product ions (m/z 66 for CFC11, m/z 82 for CCl4).Typically, a 0.14-V FNF amplitude was applied for a 10-msinterval to effect collision-induced-dissociation for theMS/MS data. Automatic sensitivity control (ASC), a featuresimilar to automatic gain control (AGC) on Finnigan ion-trap instruments, was not used in this work, as the experi-mental parameters were optimized for maximum sensitivityand the gas standards analyzed spanned less than two ordersof magnitude. Teledyne Discovery version 1.0 software wasused to acquire the data. Teledyne Sequel version 1.0 andDiscovery version 2.0 were used for computing intensitiesfor gas standards and generating printouts of ion chromato-gram plots and mass spectra. Excel (Microsoft, Seattle,WA)was used to generate calibration curves and compute stan-dard deviations and least-squares fits.

Results and Discussion

Real Time Air Monitor (RTAM)

This sample introduction system operates in a continuousmode of operation, where the inlet is challenged with an air

sample or background gas. Lab air was used as the back-ground gas, which is adequate for the purposes of this workgiven that ambient concentrations of CFCs are typically lessthan 1 ppb. Data were acquired over a 7-min time period,during which 10 cycles of lab air (background gas) followedby a gas standard were presented to the sample inlet for20-s intervals, respectively. This process is illustrated in Fig-ure 3, which shows total and selected ion chromatogramsfor a series of 10 replicate analyses of a 50-ppb mixture ofCFCs in air, with the ion trap operating in MS mode. In-strument response to the sample was nearly instantaneous,with the signal increasing to its maximum value within1–2 s after presenting the sample to the inlet, and the signalreturning to the original baseline 1–2 s after removing thesample from the inlet.

Figures 4–6 show calibration curves for CFC11 andCCl4

analyzed in MS, SIM, and MS/MS modes of operation, re-spectively. The graphs show excellent linearity over the con-centration range examined, with correlation coefficientshigher than 0.9994 with the exception of CCl4 in MS/MSmode (R2 � 0.998). Detection limits were conservativelyestimated to be on the order of 50 ppb in all three modes ofoperation. Although the use of SIM and MS/MS did notprovide better detection limits than MS mode, these modes

FIG. 7. Total and selected ion chromatograms and background-subtracted mass spectrum from replicate analyses of 50 ppb CFCs in air in MS mode withthe use of the valve inlet equipped with a 15-�l sample loop.


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of operation are more appropriate for monitoring target com-pounds in complex mixtures. Reproducibility was reason-able, with relative standard deviations less than 5% for allbut the lowest concentration standards.

Numerous investigators have used the RTAM or similarsystems for analysis of VOCs in air. These studies involvedintroducing the air sample either discretely or continuouslyinto an ion trap or a quadrupole mass spectrometer. Datafrom these studies are provided in Table 1 for comparison.The results of this work show detection limits comparableto those previously reported. It is interesting to note thatTeledyne originally reported that the use of FNF would pro-vide detection limits up to three orders lower in magnitudeby eliminating unwanted ions from the trap. Although Wiseand co-workers found a two-orders-of-magnitude reductionin detection limits for VOCs in water using the TeledyneHST-1000 FNF accessory on a Finnigan ITMS instrument,49

a similar reduction in detection limits was not found whenVOCs in air were monitored (M. B. Wise, personal com-munication). These results are further corroborated in thiswork in which detection limits in MS, SIM, and MS/MS

modes were identical. This may be due to the fact that theair samples analyzed in this study contained only three com-ponents above the approximately 50-ppb detection limit, andhence obviate the full capabilities of FNF for reducing chem-ical background and realizing the improvement in sensitivityreported by the manufacturer.

Several improvements to the RTAM design were identi-fied over the course of this work. Leaks in the inlet wereindicated by an increase in the ion-trap manifold pressurewhen the heliummetering valve was closed or the split valvewas opened. Better connections of the tubing to the pumpsare warranted. The Tygon tubing used in the system willresult in the loss of more polar compounds, contamination,and carryover. This tubing should be replaced with heatedsilco-steel grade tubing to provide more leak-tight connec-tions and a cleaner sample path. Although the use of theopen-split interface in this inlet is essential for minimizingresponse time and memory effects, it leads to the consump-tion of large amounts of gases. Helium was consumed at arate of approximately 100 psi/h from a 300-cubic-foot-capacity gas cylinder and air samples at rates of up to

FIG. 8. TIC profile from injection of 100 ppm CFCs in air with the use of a valve inlet equipped with a 0.25-ml sample loop.

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FIG. 9. Profile mass spectra at times (a) and (b) from Figure 8.


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1 l/min when this inlet was being operated. Although thisshould not be a major limitation for lab studies, it could bea problem in the field, where limited amounts of helium and/or gas standards are available. The 75-�m ID fused-silicatransfer line became clogged with particulate matter twiceover the course of these studies. This problem was readilyapparent from the sudden reduction in instrument responseand was rectified by replacing the transfer line. This inletsystem should include a particulate filter to prevent suchoccurrences.

The RTAM provides a means for real-time monitoring ofVOCs in air with detection limits in the low-ppb range. Re-sponse times are on the order of seconds, and the inlet maybe operated in either a continuous or discrete fashion. Theinlet is rugged, with the usual exceptions for capillary trans-fer lines. For field use, the helium requirement presents anadditional item for transport but provides a well-establishedbuffer gas for the ion trap. Once the probe is optimized, itcan operate for weeks without further adjustment, althoughthis observation may not hold true for an environment with

rapid changes in meteorological conditions (i.e., tempera-ture, humidity). A potential problem with this inlet is theintroduction of excessive amounts of water into the ion trapwhen very humid environments are being monitored. Thiscould possibly lead to a reduction in sensitivity due to un-wanted ion-molecule reactions yielding protonated ions (i.e.,water chemical ionization). Although this could be exploitedfor some compounds, it would be more desirable to controlthis process by regulating water levels in the ion trap. Notethat although lab air was used as the background for thiswork and the humidity levels were as high as 80% at thetime the data were acquired, no adverse effects were ob-served in the mass spectral data.

Valve Inlet

Since the use of helium as a buffer gas for the ion trapwas described by Stafford et al.,51 nearly all commercial iontraps have been operated with a nominal 1-ml/min heliumflow. It is not well known that air can be used in place of

FIG. 10. TIC profile from injection of 1 ppm CFCs in air with the use of a valve inlet equipped with a 15-�l sample loop.

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helium as a buffer gas, albeit with somewhat reduced sen-sitivity and mass resolution. Bruker’s CBMS uses air as abuffer gas. Cameron et al. reported that air acts as a buffergas in a manner similar to helium to enhance sensitivity bycollision damping.52More recently, Lammert andWells pro-vided an excellent description of the theoretical and exper-imental basis behind the use of air in place of helium as abuffer gas.53 This work provides the lowest reported detec-tion limits for monitoring VOCs in air using DSITMS andair as a buffer gas. The valve inlet operates in a discretemode of operation, as opposed to the continuous mode forthe RTAM. Typical data acquisitions in this work involvedsetting up the ion trap for a 10-min data acquisition, duringwhich a series of 10 replicate injections were performed.This is illustrated in Figure 7, which shows total and selectedion chromatograms for a series of 10 replicate analyses of a100-ppb mixture of CFCs in air, with the ion trap set toacquire a product-ion spectrum of m/z 117 from CCl4.

It was theorized that larger sample loop volumes wouldprovide lower detection limits. A 0.25-ml sample loop wasinitially employed. Figure 8 shows the total ion chromato-gram from an analysis of a 100 ppm CFC standard in air.54

Instrument response to the sample was fairly rapid, with adelay of only a few seconds between switching the valveand the signal reaching its peak value. Some hysteresis wasindicated by a more than 10 s delay for the signal to returnto its original baseline. This is most likely due to the timenecessary for residual sample to be pumped out of the iontrap. This hypothesis was further corroborated by a surge inpressure from a nominal level of 8� 10�5 torr to amaximumof 6 � 10�4 torr when the valve was switched from the loadto the inject position, with a slow return of the pressure toits nominal value afterwards. Although the zero air gas usedto flush the sample into the trap was regulated at a flow rateof 0.3 ml/min, it is most likely the vacuum in the ion trap,the length, and ID of the transfer line that control the flowof the sample. Figure 9 shows profile mass spectra fromtimes (a) and (b) shown in Figure 8. Spectrum (a) showspoor mass resolution. This can be attributed to the increasedpressure in the ion trap and space charge conditions. Spec-trum (b) shows mass resolution returning to nominal levelsas time passes and the pressure decreases. Obviously, thisconfiguration of the valve inlet provided unacceptable massresolution and sensitivity for air monitoring. One possiblesolution would be to improve the conductance in the trap.Alternately, the transfer line length between the valve andthe ion trap could be increased to minimize the pressuresurge, although this will increase response times as well. Inthis work, the effect of reducing sample loop size was ex-plored as a means to minimize the pressure surge and theresulting degradation of mass resolution.

A 15-�l sample loop was employed in an attempt to re-duce the pressure surge and improve mass resolution. Figure10 shows the total ion chromatogram from an analysis of a1 ppm CFC standard in air. Instrument response to the sam-ple was again fairly rapid, with a delay of only a few secondsbetween switching the valve and the signal reaching its peak

value. Hysteresis was greatly reduced compared to the useof the 0.25-ml loop, most likely due to the smaller samplesize involved. No increase in pressure was observed whenthe valve was switched from the load to the inject position.Figure 11 shows the profile mass spectra from times (a) and(b) shown in Figure 10. The spectra indicated near unit massresolution across the peak profile. This 15-�l sample loopwas employed for the remainder of these experiments.

The scan functions and instrumental parameters em-ployed for these experiments were critical to their successfulimplementation. Charge exchange as opposed to EI was thedominant ionization mechanism, with the majority of chargestored in the form of the ion. This was noted by Cameron�O2

et al.50 and by Lammert and Wells,53 and confirmed in thiswork by a reduction in signal intensity when ion was�O2

selectively ejected from the ion trap with the use of FNFduring ionization. To take advantage of this, an RF masscutoff of m/z 30 and FNF were employed in the ionizationscan table to selectively store m/z 32 during ionization. Apostionization time was not required in this scan table, asthe charge exchange process occurred very rapidly. The pri-mary challenge in using air as a buffer gas was retuning theion trap to provide the unit mass resolution required for thiswork. A lower FAST frequency and higher scan ratesseemed to give the best resolution. Lower FAST frequenciesresult in a lower qeject level, which agrees with the work ofLammert and Wells. Higher scan rates may help to “punch”the ion through the neutral air molecules in the trap andminimize degradation of resolution. A more detailed discus-sion of the theoretical basis for optimization of the ion trapwith air used as a buffer gas may be found in Lammert andWells.53

Figures 12–14 show calibration curves for CFC11 andCCl4 in MS, SIM, and MS/MS modes of operation, respec-tively. The graphs show good linearity over the concentra-tion range examined, with correlation coefficients higherthan 0.994 for all the regression lines. Detection limits wereconservatively estimated to be on the order of 50 ppb, withthe exception of CCl4 in MS/MS mode, in which the detec-tion limit was somewhat higher. Figure 14 shows a smallerslope for CCl4 in MS/MS mode compared to that shown inFigure 6 with the RTAM used. This observation can be ex-plained by a low CID efficiency. The reason for this may berelated to the presence of neutral air molecules in the trap.Although the use of SIM and MS/MS did not provide betterdetection limits than MSmode, these modes of operation aremore appropriate for monitoring target compounds in com-plex mixtures. Reproducibility was good, with relative stan-dard deviations less than 5% for all but the lowest concen-tration standards.

Little published work is available on detection limits forair analysis where air is used as a buffer gas in an ion trap.Data from studies that have been published are provided inTable 2 for comparison. Cameron et al. removed the Teflonspacers between the ion-trap ring electrode and end-cap elec-trodes to increase the conductance for operation of the sys-tem, accommodate higher air flows, and improve sensitivity.


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FIG. 11. Profile mass spectra at times (a) and (b) from Figure 10.

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FIG. 12. Calibration curves for CFCs analyzed in MS mode with the use of a valve inlet. The triangles represent a plot of peak intensity of m/z 101 fromCFC11, and the squares represent a plot of peak intensity of m/z 119 from CCl4.

Similar improvements in conductance were not performedin this work. Mass resolution was empirically observed todecrease at higher air-flow rates, which agrees with the workof Cameron et al. and Lammert and Wells. Cameron notedthat the presence of additional helium had little effect oneither sensitivity or mass resolution. Cameron and co-work-ers also postulated that “air can serve as a buffer gas in theFinnigan ITD for ions of moderately low mass-to-chargeratios.”52 The work of Lammert and Wells corroborates this;provides more complete data showing the dependence be-tween mass resolution, mass, and air pressure; and showsunit mass resolution up to approximately 200 thompsons.53

It is also worthwhile to compare results from this inlet tothose from the RTAM. Detection limits were nearly identi-cal. Although the use of zero air and a discrete mode ofsample introduction with the valve system (versus lab airand a continuous mode with the RTAM unit) should theo-retically provide lower background levels and hence lowerdetection limits, this was not observed in this work. Re-sponse times for the valve inlet system were on the order of

5 s, which was not quite as fast as the near-instantaneousresponse times associated with the RTAM. This can be at-tributed to the low flow rate of zero air used to flush thesample into the ion trap. The data from this inlet systemresult in chromatograms in the form of peaks versus the box-car-type profiles observed with the RTAM, making the datamore amenable to integration via conventional mass-spec-trometry data systems. Although excessive levels of air nor-mally lead to shorter filament lifetimes, no filament burnoutwas observed over the course of this study. The most criticalconsiderations for using this inlet are tuning the ion trap toobtain unit mass resolution and the unknown effects of neu-tral air molecules on the CID process. Although unit massresolution is not maintained for m/z values greater than200 thompsons, this mass range is more than sufficientfor most VOCs. Note that while a helium gas cylinder isnot required with this inlet, a source of VOC-free air isneeded to flush the contents of the sample loop into themass spectrometer and provide a point of reference to indi-cate background levels of VOCs in the system for subse-


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quent quantitation. This VOC-free air could be supplied bya small gas cylinder or an air compressor coupled to a pu-rification filter.

Conclusions

This study has demonstrated the utility of both theRTAMand valve-based inlet systems coupled to an ion-trap massspectrometer for real-time monitoring of VOCs in air. Themerits of these analytical systems stem from their use of

relatively simple sample introduction systems for real-timemonitoring of VOCs, and an ion-trap mass analyzer, whichcouples excellent sensitivity, relatively low cost, and smallsize with the tandem mass spectrometry capability for selec-tive monitoring of VOCs. Detection limits for both systemsare on the order of 50 ppb in MS, SIM, and MS/MS modesfor both CFC11 and CCl4 in air. The RTAM results are com-parable to previous published work. The results with thevalve-based inlet with air used as a buffer gas have indicateddetection limits several orders of magnitude lower than those

FIG. 13. Calibration curves for CFCs analyzed in SIM mode with the use of a valve inlet. The triangles represent a plot of peak intensity of m/z 101from CFC11, and the squares represent a plot of peak intensity of m/z 119 from CCl4.

TABLE 2. Detection limits of various sample introduction systems with air used as a buffer gas in conjunction with ion-trap mass spectrometers forreal-time air monitoring.

Inlet system Mass spectrometer Target compound(s) Detection limits Reference

GC/membrane CBMS Pyridine, methyl salicylate, napthalene 30 pg–5 ng 32Leak valve Finnigan ITD Benzene, trichloroethylene, cyclohexane, acetone 1–40 ppm 52This work Teledyne 3DQ CFC11, CCl4 50 ppb

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previously published with unit mass resolution. MSmode ofoperation would most likely be used as a “survey” mode inair analysis applications, with SIM and MS/MS techniquesused for quantitation. SIM mode is more appropriate for afew target compounds where isobaric interferences are un-likely. MS/MS is the mode of choice for samples wheregreater specificity is required to quantitate a specific com-pound in a complex mixture. For many applications requir-ing real-time monitoring of CFCs and VOCs in air in terms,the sample introduction systems described here more thansuffice. Further refinement of these inlet systems and appli-cation and testing in the field are the subject of ongoingefforts by a variety of researchers. Continued improvementof these and other sample introduction systems, developmentof smaller, more portable ion traps, and feedback from laband field testing should render these DSITMS techniquesmore useful for a wider variety of air monitoring applica-tions.

Acknowledgments

Palmer acknowledges Carla Wong of NASA Ames Re-search Center for a generous loan of ion-trap instrumenta-tion, a gas-dilution system, and sundry supplies and stan-dards used in this work. Karr thanks Teledyne ElectronicTechnologies for support of his research and developmentefforts. The authors thank Rich Yelton, Marc Wise, SteveLammert, and Henk Meuzelaar for their input and com-ments.

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Documents

Evaluation of two different direct-sampling ion-trap mass-spectrometry methods for monitoring halocarbon compounds in air