Rapid monitoring of volatile organic compounds: a comparison between gas chromatography/mass spectrometry and selected ion flow tube mass spectrometry

Research Article

Received: 22 August 2013 Revised: 23 September 2013 Accepted: 25 September 2013 Published online in Wiley Online Library

Rapid Commun. Mass Spectrom. 2014, 28, 10–18

10

Rapid monitoring of volatile organic compounds: a comparisonbetween gas chromatography/mass spectrometry and selected ionflow tube mass spectrometry

Vaughan S. Langford1, Ian Graves2 and Murray J. McEwan1,3*1Syft Technologies Ltd, 3 Craft Pl, Christchurch, New Zealand2Hill Laboratories, Private Bag 3205, Hamilton 3240, New Zealand3University of Canterbury, Christchurch, New Zealand

RATIONALE: The gold standard for monitoring volatile organic compounds (VOCs) is gas chromatography/massspectrometry (GC/MS). However, in many situations, when VOC concentrations are at the ppmv level, there arecomplicating factors for GC/MS. Selected ion flow tube mass spectrometry (SIFT-MS) is an emerging technique formonitoring VOCs in air. It is simpler to use and provides results in real time.METHODS: Three different experiments were used for the comparison. First SIFT-MS was applied to monitor theconcentrations of 25 VOCs in a mixture at concentrations up to 1 ppmv using only a generic database for known kineticdata of three reagent ions (H3O+, NO+ andO2

+) with each VOC. In experiment 2, a side-by-side comparison wasmade of 17VOCs at concentrations between 1 ppmv and 5 ppbv after small corrections had beenmade to the SIFT-MS kinetic data. Ina third experiment, a side-by-side comparison examined two groups of samples received for commercial analysis.RESULTS: In experiment 1, 85% of the VOC concentrations were within 35% of their stated values without anycalibration of the SIFT-MS instrument. In experiment 2, the two techniques yielded good correspondence between themeasured VOC concentrations. In experiment 3, good correlation was found for VOCs from three of the samples.However, interferences from some product ions gave over-reported values in one sample from the SIFT-MS instrument.CONCLUSIONS: These three experiments showed that GC/MS was better suited to monitoring samples containinglarge numbers of VOCs at high concentrations. In all other applications, SIFT-MS proved simpler to use, was linearwith concentration over a much wider concentration range than GC/MS and provided faster results. Copyright ©2013 John Wiley & Sons, Ltd.

(wileyonlinelibrary.com) DOI: 10.1002/rcm.6747

Gas chromatography/mass spectrometry (GC/MS) is themost widely used technique for analysing mixtures of volatileorganic compounds (VOCs).[1,2] The capillary column in GCprovides physical separation between compounds and themass spectrometer provides the identification. Databases areavailable with retention indices covering polar and non-polarcolumns for more than 70 000 compounds. An even moreextensive database containing the electron ionization massspectra of more than 210 000 compounds is available to assistin the identification of analytes.[3] However, although theGC/MS technique has found such widespread use, there aresome difficulties associated with sampling VOCs fromgaseous and liquid headspace samples. In many cases a pre-concentration step is required. This may entail a purge-and-trap technique for the analysis of VOCs in solution. Herean inert gas is bubbled through the sample to displace theVOCs, followed by trapping and pre-concentration of theanalytes on an adsorption cartridge. Alternatively, for gas

* Correspondence to: M. J. McEwan, Syft Technologies Ltd,3 Craft Pl, Christchurch, New Zealand.E-mail: [email protected]

Rapid Commun. Mass Spectrom. 2014, 28, 10–18

analysis, the sample is passed through an adsorption trap thathas greater affinity for VOCs than for the bulk gas, providingpre-concentration. In both cases, trapped VOCs are releasedlater from the substrate by using thermal desorption andpassed to the GC/MS sample injector. They are then separatedin the GC column for subsequent MS detection, identificationand quantitation. Reliable quantitation using GC/MS requiresfrequent calibration across the measuring range.

The purpose of the present study is to provide a comparisonbetween accepted GC/MSmethodology and selected ion flowtube mass spectrometry (SIFT-MS). SIFT-MS is an emergingtechnique that offers significant advantages over GC/MS ina range of scenarios through its simplicity of operation andreal-time analysis of VOCs. SIFT-MS is one of severalquantitative techniques that might be described as real-time,such as proton transfer mass spectrometry (PTR-MS)[4,5] andfast GC.[6] However, SIFT-MS does not suffer the variabilityof product ion ratios that are a function of the drift fields inPTR-MS and has the advantage of no chromatographycolumns with corresponding elution time and phase overloadproblems of fast GC.

The principles of SIFT-MS technology have been wellsummarised elsewhere.[7,8] SIFT-MS is distinct from mostMS analytical techniques in that it can be used as a real-time

Copyright © 2013 John Wiley & Sons, Ltd.

Table 1. Selected compounds from the US EPA TO-14Aand TO-15 methods used for calibration of the SIFT-MSinstrument from certified gas standards. See text fordetails. The nominal concentration is 1.0 ppmv with aquoted accuracy 5%

acetone isooctaneacrylonitrile methyl ethyl ketone (butanone)acetonitrile naphthalenebenzene propylenebromomethane styrene1,3-butadiene tetrachloroethylenecarbon disulfide tetrahydrofurancarbon tetrachloride toluenechlorobenzene 1,1,1-trichloroethane1,2-dibromoethane trichloroethylene1,2-dichlorobenzene 1,3,5-trimethylbenzeneethyl acetate vinyl chlorideethyl benzene

Rapid monitoring of VOCs

absolute VOC analyser without the requirement for calibrationcurves to calculate or determine analyte concentrations. Itquantifies VOCs on the basis of the ratio of product ion countto reagent ion count.[7] However, to do this, knowledge of thekinetic parameters of ion-molecule reaction rates and production distributions for the reagent ion reactions with each analyteis required together with the sample flow rate and instrumentmass discrimination factors. The analysis is based on equations(11) and (12) in Smith and Španěl.[7] In SIFT-MS technology,the ions are subjected to near thermal collision energies withthe carrier gas and the rate coefficients and product iondistributions are well defined.It has been established in a number of earlier studies that

SIFT-MS measures VOCs accurately in real time[9–13] but nodirect comparisons with other analytical techniques onmultiple analytes have been made. The purpose of this studyis to provide a much more extensive comparison of SIFT-MSwith GC/MS for the quantification of multiple VOCs ina sample and to explore the circumstances under whichSIFT-MS might replace GC/MS.SIFT-MS best lends itself to the analysis of air and a

convenient way of making the comparison was to utilizea regulatory method for analysing whole air samples. Thecanister sampling methods of the United StatesEnvironmental Protection Agency (US EPA) were deemedmost appropriate as they are whole air methods. The bestknown of these are the toxic organic compendiummethods, 14A and 15 (TO-14A[14] and TO-15[15]). Wereport here the results of a comparison between GC/MSand SIFT-MS on selected compounds from the US EPATO-14 and TO-15 compendium methods.

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EXPERIMENTAL

Chemicals and standards

Helium (>99.99%, BOC, Auckland, New Zealand) and nitrogen(99.995% Southern Gas, Rolleston, New Zealand) were used.Humidified zero air was produced on site from compressed airusing a Peak ZA010 zero air generator (Peak Scientific,Inchinnan, UK) and an activated charcoal scrubber (AlltechAssociates Inc., Deerfield, IL, USA). The humidified zero airwas confirmed to be contaminant free before use.[15]

Pressure measurement during the certification of canistersand preparation of standards and samples were carried outwith an PG7-50.00-PSIA digital pressure gauge (APG, Logan,UT, USA).Three quite different experiments were undertaken in

this study.Experiment 1 was the quantification by the SIFT-MS

instrument of mixtures of 25 VOCs that had been purchasedfrom three laboratories supplying certified mixtures ofanalytes for the TO-14 and TO-15 standards at a nominalconcentration of 1 ppmv and stated accuracy of 5% and witha balance of nitrogen. The 25 VOCs chosen are shown inTable 1. The laboratories supplying the standards were LindeSpectra Environmental Gases, Alpha, NJ, USA, TO-14AStandard; Global Calibration Gases LLC, Palmetto, FL, USA,TO-15 plus; Custom Gas Solutions, Durham, NC, USA,TO-15 plus.

Copyright © 2013 JohnRapid Commun. Mass Spectrom. 2014, 28, 10–18

The mixtures containing the standards were placed in 1-LTedlar sample bags (SKC Inc., Eighty-Four, PA, USA) andtheir concentrations were measured without any calibrationusing the existing SIFT-MS data base that provides an’absolute’ measurement as long as the relevant ion-moleculekinetic data are known. To examine the response of theSIFT-MS instrument with concentration, serial dilutionswere prepared as follows. Zero air (1 L) was placed in thesample bag using a precision 1-L gas syringe (Vitalograph,Ennis, Ireland). Calibration gas was spiked into zero airby using gas-tight syringes (SGE, Melbourne, Australia) ofappropriate volume. Because Tedlar film is relativelypermeable to moisture, the calibration has effectively beenmade in moderately humidified zero air. The series ofdilutions provided a range of concentrations between 1ppmv and 5 ppbv.

The GC/MS instrument was calibrated from the knownconcentrations of the 25 standards of Table 1. Known volumesof the standards were removed from the supplied cylinder viaa 1-L gas-tight syringe (SGE Analytical Science Pty Ltd,Ringwood, Australia) and combined with humidified air in a15-L Silonite® canister (Entech Instruments Inc., Simi Valley,CA, USA) to give a nominal concentration of 10 parts per billionby concentration (ppbv). An internal standard was similarlymade to a concentration of 50 ppbv from a 1 ppmv stockcontaining bromochloromethane, 4-bromofluorobenzene(BFB), chlorobenzene-d5 and 1,4-difluorobenzene (CustomGas Solutions Inc.). Calibration of the GC/MS system wasperformed and the instrument performance assessed inaccordance with US EPA TO-15.[15]

Initial calibration of the GC/MS instrument was performedby injecting a constant volume of the 50 ppbv internalstandard and varying volumes of the calibrating standardgases such that the calibration range went from 0.5 to 50 ppbvat six concentration levels. Daily compliance was determinedby the assessment of the BFB mass spectrum, instrumentblank and 10 ppbv standard response, again according toUS EPA TO-15.[15]

Experiment 2 used a sub-section of 17 VOCs from thelist of standard compounds shown in Table 1 for directcomparison between SIFT-MS and GC/MS and these are

wileyonlinelibrary.com/journal/rcmWiley & Sons, Ltd.

Table 2. Compounds used to spike cans in comparativeSIFT-MS and GC/MS experiments

Analyte SupplierPre-spike

mix

acetone MallinckrodtBaker Inc.

Mix 1

acetonitrile Merck Mix 1acrylonitrile Chromspec

Distributors LtdMix 2

benzene Pure Science Ltd Mix 1butanone BDH Laboratory

SuppliesMix 1

carbon disulfide Fisher Scientific Mix 2chlorobenzene Sigma Aldrich Mix 3ethylbenzene Acros Mix 2isooctane (2,2,4-trimethylpentane)

Merck Mix 1

naphthalene Sigma Aldrich Mix 3styrene Sigma Aldrich Mix 2tetrachloroethene Sigma Aldrich Mix 2tetrahydrofuran Merck Mix 1toluene Merck Mix 21,1,1-trichloroethane Sigma Aldrich Mix 2trichloroethene Merck Mix 11,3,5-trimethylbenzene Supelco Mix 3

Table 3. Results of SIFT-MS calibration versus the certified gas skinetic data

Compound andreagent ion Product ion Product m/z

Kirefe

benzeneNO+ C6H6

+ 78O2

+ C6H6+ 78

tolueneNO+ C7H8

+ 92O2

+ C7H8+ 92

ethylbenzeneH3O

+ C8H10.H+ 107

NO+ C8H10+ 106

1,3,5-trimethylbenzeneH3O

+ C9H12.H+ 121

NO+ C9H12+ 120

O2+ C9H12

+ 120styreneNO+ C8H8

+ 104O2

+ C8H8+ 104

naphthalenea

H3O+ C10H9

+ 129NO+ C10H8

+ 128O2

+ C10H8+ 128

propeneH3O

+ C3H7+ 43

O2+ C3H6

+ 421,3-butadieneNO+ C4H6

+ 54isooctaneNO+ C8H17

+ 113

V. S. Langford, I. Graves and M. J. McEwan

wileyonlinelibrary.com/journal/rcm Copyright © 2013 John

12

shown in Table 2 together with the suppliers of the VOC.Three liquid mixtures of these 17 VOCs were preparedfrom samples of the pure parent compounds based aroundthe volatility of the compounds. Four canisters were spikedwith varying amounts of the headspace above the liquids.The spiking process was performed in several steps using a10-μL gas-tight syringe (SGE Analytical Science Pty Ltd). Thecans were pressurised to 5 psig with humidified zero air. TheSIFT-MS instrument was then used to provide rapidquantification of the contents to confirm that the levels werewithin the linear range required by the GC/MS system (targetlevels were 0.5 to 50 ppbv). A small correction was made toseveral of the SIFT-MS-derived concentrations from the kineticdata so that the analyte concentrations were within 10% of theconcentrations of the calibrated mixtures. A small correctionalso had to be made when sampling from canisters at 5 psigfor the SIFT-MS instrument because normally it analysessamples at atmospheric pressure. Although not in the mixture,seven other compounds for which both instruments were setup tomeasurewere alsomonitored for their background levels.

Experiment 3 in which samples submitted to the analyticallaboratory for analysis were subjected to a side-by-sideGC/MS and SIFT-MS comparison. Two different groups ofsamples were selected for analysis: one group was of gassamples from soil that had been contaminated by an oilspill and one sample from a clandestine methamphetamine

tandards – the quantification of SIFT-MS is based on generic

neticsrences

Ratio of measured tocertified concentration LOD/ppbv LOQ/ppbv

[17] 0.66 0.23 0.52[17] 0.89 0.16 0.32

[17] 0.91 0.10 0.25[17] 0.90 0.09 0.20

[17] 0.83 0.18 0.37[17] 0.88 0.16 0.35

[17] 1.06 0.14 0.31[17] 1.06 0.26 0.52[17] 0.98 0.10 0.24

[18] 1.07 0.21 0.42[18] 0.94 0.09 0.21

[19] 0.64 0.65 1.24[20] 0.58 0.67 1.30[20] 0.48 0.37 0.77

[21] 0.86 0.27 0.50[21] 1.34 0.20 0.38

[18] 0.94 0.05 0.16

[22] 0.80 0.29 0.58

(Continues)

Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2014, 28, 10–18

Table 3. (Continued)

Compound andreagent ion Product ion Product m/z

Kineticsreferences

Ratio of measured tocertified concentration LOD/ppbv LOQ/ppbv

carbon tetrachlorideO2

+ C35Cl237Cl+ 119 [23] 0.76 0.40 0.92

O2+ C35Cl3

+ 117 [23] 0.76 0.30 0.771,1,1-trichloroethaneO2

+ CH3C35Cl 37Cl+ 99 [23] 0.65 0.62 1.17

O2+ CH3C

35 Cl2+ 97 [23] 0.70 0.34 0.65

chloroetheneO2

+ C2H335Cl+ 62 [23] 0.65 0.11 0.25

O2+ C2H3

37Cl+ 64 [23] 0.70 0.37 0.82trichloroetheneO2

+ C2H35Cl37Cl2

+ 134 [23] 1.01 1.83 3.61O2

+ C2H35Cl2

37Cl+ 132 [23] 1.01 0.41 0.87O2

+ C2H35Cl3

+ 130 [23] 1.08 0.33 0.73tetrachloroetheneO2

+ C235Cl2

37Cl2+ 168 [23] 1.17 1.33 2.80

O2+ C2

35Cl337Cl+ 166 [23] 1.17 0.56 1.22

O2+ C2

35Cl4+ 164 [23] 1.21 0.54 1.30

chlorobenzeneNO+ C6H5

35Cl+ 112 [24] 0.77 0.23 0.48NO+ C6H5

37Cl+ 114 [24] 0.75 0.99 2.08O2

+ C6H535Cl+ 112 [24] 0.91 0.43 0.84

O2+ C6H5

37Cl+ 114 [24] 0.83 0.45 0.84dichlorobenzenesa

NO+ C6H435Cl37Cl+ 148 [18] 0.75 0.73 1.59

NO+ C6H435Cl2

+ 146 [18] 0.69 0.62 1.24O2

+ C6H435Cl37Cl+ 148 [18] 0.94 0.33 0.83

O2+ C6H4

35Cl2+ 146 [18] 0.83 0.22 0.52

methyl bromidea

O2+ CH3

79Br+ 94 [24] 0.39 0.20 0.411,2-dibromoethanea

O2+ C2H4

79Br+ 107 [18] 0.79 0.05 0.11acetonitrileH3O

+ CH3CN.H+ 42,60,78 [25] 0.46 0.11 0.21acrylonitrileH3O

+ CH2CHCNH+ 54,72,90 [19] 0.69 0.10 0.20carbon disulfideO2

+ CS2+ 76 [26] 1.15 0.16 0.36

acetoneH3O

+ C3H7O+ 59,77 [27] 0.59 0.34 0.60

NO+ NO+.C3H6O 88 [18] 0.86 0.52 0.95butanoneNO+ NO+.C4H8O 102 [27] 0.47 0.26 0.52O2

+ C4H8O+ 72 [27] 0.48 0.61 1.13

ethyl acetateH3O

+ CH3COOC2H5.H+ 89,107 [28] 1.22 0.19 0.36

tetrahydrofuranNO+ C4H7O

+ 71 [29] 0.65 0.25 0.48O2

+ C4H7O+ 71 [29] 0.82 0.28 0.52

aSome contamination was found from the bag substrate and the bag background has been subtracted.


13

laboratory that had undergone some remedial clean upprocedures. For these analyses, ambient air or soil vapour wassampled using critical orifice restricted sampling trains (thathad been internally coated with Silonite® to minimise surfaceinteractions) into specially cleaned, certified and evacuated(to 50 mTorr) Silonite®-coated stainless steel canisters (1.4 or 6 L,Entech Instruments Inc.). Samples were collected at nominally180 mL min–1 with sampling completed when 127 Torr ofresidual vacuum remained within the canister. The pressure


of the canister was measured on receipt at the laboratory andthe samples were then diluted with zero air to give a positivepressure. The dilution factor was approximately two.

SIFT-MS measurements

In the current study a transportable Voice200® SIFT-MSinstrument (Syft Technologies Ltd, Christchurch, NewZealand) was used.[13,16] In the Voice200® instrument, three


(a)

(b)


14

reagent ions (H3O+, NO+ and O2

+) are generated in amicrowave discharge of moist air at a total pressure about0.35 Torr. The resulting ions are mass-selected by aquadrupole mass filter upstream of the flow tube and injectedinto a helium carrier gas at a pressure around 0.6 Torr. Theseions are convected along the curved flow tube and aresampled through an aperture at the end of the flow tube.The region immediately behind the aperture lens is pumpedby a split-flow turbo pump and the ions then enter thedownstream quadrupole mass spectrometer where they aremass-selected and counted. Switching between the reagentions by the upstream quadrupole occurs typically in 10 msallowing a seamless analysis of analyte concentrations usingall three reagent ions. To avoid analyte loss during thetransfer process from the sample container to the instrument,the instrument was equipped with a heated passivated inletproviding direct access to the reaction tube. This inlet wasconnected to canisters using a Silonite®-coated Micro-QT™

valve (Entech Instruments Inc.) and Tedlar bags via a shortlength of Teflon tubing. The sample time for each production resulting from the analyte reaction with the reagent ionswas typically 3.7 s. For the current comparison, the Voice200®

SIFT-MS instrument was shipped to the R.J. Hills Laboratoryin Hamilton, New Zealand.

GC/MS measurements

GC/MS analysis was carried out by R.J. Hill LaboratoriesLimited. This laboratory is accredited to the ISO 17025standard for US EPA Compendium Method TO-15 andanalyses were carried out according to this standard. Theanalysis of the canisters was made as follows. Air from thecanister (typically 125 mL) was transferred by an autosampler(model 7410D, Entech Instruments Inc.) and pre-concentrated(model 7150D, Entech Instruments Inc.) prior to GC/MSanalysis. The pre-concentrated sample was then thermallydesorbed in a carrier gas stream and analyzed by GC/MS(7890A gas chromatograph and 5975C MSD mass spectrometer,Agilent, Santa Clara, CA, USA). The gas chromatograph hada 60 m× 0.32 mm i.d. BP-1 analytical column with a filmthickness of 1 μm (SGE Analytical Science Pty Ltd). Heliumcarrier gas was used, with the head pressure set at 11.7 psigyielding an average linear velocity of 36 cm s–1. The gaschromatograph oven was held at 35°C for 4 min, increasedto 110°C at 4°C min–1 held for 0.1 min then increased to220°C at 15°C min–1 and held for 5 min, giving a totalrun time of 36.2 min. Mass spectra were collected after adelay of 4 min from m/z 29 to 160 until 10 min; the massrange was then changed to m/z 34 to 270 for the remainderof the run.

Figure 1. Example calibration data for SIFT-MS of measuredconcentration against certified concentration for aromatics (a)and halogenated hydrocarbons (b).

RESULTS AND DISCUSSION

Experiment 1

The concentrations of the 25 analytes listed in Table 1 asmeasured by the Voice200® instrument are shown in Table 3expressed as a ratio of the certified concentration. Thesemeasured values are based solely on the rate coefficients andproduct branching ratios of the relevant reagent ion reactionsreported in the literature (column 4 of Table 3) and are


’absolute’ in the sense that no concentration calibrationswere undertaken prior to this analysis. The analyticalresults are simply the end product of the insertion of therelevant kinetic parameters for each ion-molecule reactionand the instrument software then takes the ratio of theproduct ion counts to the reagent ion counts to derivethe concentrations.[7]

The following comments are relevant to the measurementsreported in Table 3. For each analyte, there is the possibilityof examining three different reagent ion reactions. Howeverin a matrix of 25 or so analytes, a number of the productions may have the same mass (isobaric) and hence the isobaricion products from a reagent ion cannot be included in theanalyses. In practice when different reagent ions’ reactionswiththe matrix yield results for an analyte that differ significantly,only the reagent ion yielding the lower concentration is usedbecause of the potential for mass overlaps. In the currentmatrix, the reagent ions used to quantify the designated analyteare shown in Table 3. In addition, the reagent ion reactions ofsome structural isomers may be similar for each isomer and inthat case only a total of all the possible isomers that are present



can be found. In a mixture where there is a large disparity inconcentrations between analytes, the limiting factor is whetherthe reagent ion number density is reduced by reactions with ananalyte. In practice linearity is experienced between parts pertrillion by volume and mid parts per million by volume. Athigher analyte concentrations, a dilution of the sample mixtureis required.The results presented in Table 3 demonstrate that of the

50 reagent ion determinations of the analyte concentrationsbased on the existing database kinetic data, 86% of theresults are within 35% of the expected concentrations. Someof the outliers may simply be a result of the samplingmethod. An example is the results for naphthalene wherelikely desorption loss onto the Tedlar bag would haveresulted in a low result for all three reagent ions. Theresults for the ketones, acetone and butanone were alsosurprising as a calibration afterwards of acetone using acalibrated permeation tube (Vici Metronics, Poulsbo, WA,USA) gave excellent correlation within 10% of the statedconcentration of 3.85 ppmv from the permeation tube. Wealso show in the two right-hand columns of Table 3 thelimits of detection (LODs) and limits of quantitation (LOQs)for the listed analytes in the configuration of the Voice 200used for the comparison and these were obtained as outlined

Table 4. Side-by-side comparison of GC/MS and SIFT-MS oconcentrations shown as ppbv

aThese analytes were not included in the mix but were simply insignal backgrounds.bC2-alkylbenzenes is the total of ethylbenzene and the three xyethylbenzene was added to the VOCs in the mixtures.cC3-alkylbenzenes is the total of all isomers for SIFT-MS. In this test, ondAcetonitrile was not measured by the GC/MS instrument.


previously.[11] These values were found by monitoring theproduct ion mass for each analyte in a 6-L Entech canister thathad only been used for zero quality air.

A demonstration of the response of the SIFT-MS instrumentwith analyte concentration is shown in Fig. 1. A series ofselected volatiles at ppmv concentrations was prepared inTedlar bags and the samples were then sequentiallydiluted with zero air to generate a series of mea-surements at different concentrations between 5 ppbvand 1 ppmv. The linearity of the concentrations in allcases as expressed by R2, the coefficient of determination,over the three orders of magnitude examined was R2

≥0.997. We have shown elsewhere that a linearconcentration response over five orders of magnitude istypical.[13] Figure 1(a) shows the linearity of the responsefor the named hydrocarbons and Fig. 1(b) the responsefor the named chlorinated hydrocarbons.

Experiment 2: GC/MS and SIFT-MS side-by-sidecomparison of spiked canisters

The target levels of volatiles were in the low- to mid-ppbvrange. The canisters were tested using both SIFT-MS andGC/MS. The results of the comparison study are shown

f four different mixtures containing 17 VOCs with listed

the methods for analysis of both instruments. They represent

lene isomers for the SIFT-MS study. In this experiment, only

ly 1,3,5-trimethylbenzenewas added to theVOCs in themixtures.


15


16

in Table 4. The VOCs in italics represent background levelsfor compounds that were in the analytical methods foreach instrument but not in the mixtures.Overall the agreement between the two quite different

techniques is very good from a mixture containing 17VOCs. Differences in concentrations of greater than 30%were observed for only styrene (higher for SIFT-MS thanfor GC/MS), and acetone and carbon disulfide (both lowerin all mixtures than the concentrations reported by GC/MS).We are not sure why lower concentrations of acetone werefound in the SIFT-MS measurements as the associationreaction of NO+ with acetone was used to determine theconcentration from the amplitude of the association ion peak,CH3COCH3.NO+ at m/z 88. As noted for experiment 1,subsequent experiments using two acetone permeation tubes(Vici Metronics) showed correct measurements for the SyftVoice200® instrument.

Table 5. Comparison of analyte concentrations in two commemore details on the samples

aC2-alkylbenzenes gives the total of ethylbenzene and the threehas been summed.

bC3-alkylbenzenes gives the total for all isomers for SIFT-MS; scNot reported for GC/MS due to interference from 2-methylbutdNot reported for GC/MS.


Experiment 3: Side-by-side comparison of fourcommercial samples

The comparative results for this study are shown in Table 5.The first sample set came from a site where soil had beencontaminated by leakage from an underground fuel storagetank. Samples were taken from the crawl space surroundingthe tank and from soil where fuel seepage had occurred. Thecomparative results are shown for the VOCs listed in column1, column 2 represents background signals for each analyte,column 3 represents analyte concentrations from the airsampled from the crawl space, and column 4 those fromsoil samples. The second sample included in Table 5came from air samples from a remediated clandestinemethamphetamine (’P’) laboratory and these results areshown in column 5. The following observations can bemade. Good agreement between the two techniques wasfound for non-contaminated samples such as the ambient

rcial samples analysed at Hill Laboratories. See the text for

xylene isomers for SIFT-MS; speciation reported for GC/MS

peciation reported for GC/MS has been summed.ane.



samples and the samples from the crawl space (columns2 and 3). In the contaminated samples (column 4, soil gas),good correlation was obtained for the smaller aromatichydrocarbons (benzene, toluene and C2-alkylbenzenes).However, there was poor agreement for a number of theother compounds in the soil sample. The reason for lowercorrelation in the soil sample is that the soil was saturatedwith hydrocarbons from the fuel and these hydrocarbonsgenerate mass overlaps with the SIFT-MS product ionsfrom the analytes included in the method. Under theseconditions, SIFT-MS is susceptible to interference whenlarge numbers ofmultiple components at higher concentrationsare present. However, in the comparative analysis of theresiduals from the ’P’ lab, good correlation was obtainedbetween the two techniques with the additional benefit fromSIFT-MS of all VOCs present being observed. The GC/MSmethod does not report methanol due to the background inthe canisters from the manufacturing process.

17

CONCLUSIONS

The following observations can be made from this comparativestudy between SIFT-MS andGC/MSundertaken at a laboratoryapproved to conduct USAEPATO15[15] compendiummethods.The SIFT-MS analysis of certified gas standards in

experiment 1 resulted in satisfactory quantitative agreementfor most analytes (Table 3) although the instrument had notbeen pre-calibrated for those compounds – generic gas-phasekinetic data applicable to all SIFT-MS instruments were used.The measured data were then used to make minor adjustmentto the kinetic parameters to calibrate the specific SIFT-MSinstrument used in the trial. We have found those reagent ion-analyte reactions that are most susceptible to small changesfrom the published data for ion-molecule reactions (forexample, whether the flow tube heater is on or off) are thosewhere association reactions occur (some NO+ reactions) andsome reactions that have multiple product ion pathways.The side-by-side comparisons of the two techniques for the

17 analytes examined in the mixtures (Table 4) also confirmedthat SIFT-MS offers a viable alternative to GC/MS formonitoring VOCs down to trace levels (pptv) and has themarked benefits of not needing the pre-concentration/desorption steps that are required by GC/MS for trace levelanalysis. Further, SIFT-MS is a real-time method and providesresults in seconds, it does not require different columns forpolar and non-polar analytes, it is much simpler to use andhas a linear response over a much wider concentration rangethan GC/MS. The two techniques are complementary andthere are situations where one might be preferred over theother. For example in an analysis of an unknown samplecontaining a large number of VOCs in the same matrix,GC/MS would be the method of choice as was evidenced inthe contaminated soil sample. Nevertheless, in manyapplications SIFT-MS offers clear advantages over acceptedtraditional methods for VOC analysis. In particular, SIFT-MSprovides unique mass spectrometry based opportunitiesin real-time ambient air monitoring, because it is robust,easily deployable, easy to use, very sensitive, and no pre-concentration and sample preparation is required. We haveshown elsewhere[11,12] that it can also analyse down to thepptv level after only 15 min warm up.


AcknowledgementsWe thank John Gray for assistance with preparatory andfollow-up work for the trial, Daniel Milligan and Barry Princefor helpful discussions (all Syft Technologies Ltd) andAlastair Boyd of Hill Laboratories for sample preparationand helpful discussions.

REFERENCES

[1] Current Practice of Gas Chromatography-Mass Spectrometry,(Ed: W. M. A. Neilson). Marcel Dekker, New York, 2001.

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[3] NIST/EPA/NIH Mass Spectral Database (NIST11) andNIST Mass Spectral Search Program (version 2.0g). U.S.Dept. of Commerce, Standard Reference Data Program,Gaithersburg, MD, 2011.

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Documents

Rapid monitoring of volatile organic compounds: a comparison between gas chromatography/mass spectrometry and selected ion flow tube mass spectrometry