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 (wileyonlinelibrary.com) DOI: 10.1002/rcm.6747
Rapid monitoring of volatile organic compounds: a comparison between gas chromatography/mass spectrometry and selected ion flow tube mass spectrometry Vaughan S. Langford1, Ian Graves2 and Murray J. McEwan1,3* 1
Syft Technologies Ltd, 3 Craft Pl, Christchurch, New Zealand Hill Laboratories, Private Bag 3205, Hamilton 3240, New Zealand 3 University of Canterbury, Christchurch, New Zealand 2
RATIONALE: The gold standard for monitoring volatile organic compounds (VOCs) is gas chromatography/mass spectrometry (GC/MS). However, in many situations, when VOC concentrations are at the ppmv level, there are complicating factors for GC/MS. Selected ion flow tube mass spectrometry (SIFT-MS) is an emerging technique for monitoring 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 the concentrations of 25 VOCs in a mixture at concentrations up to 1 ppmv using only a generic database for known kinetic data of three reagent ions (H3O+, NO+ and O+2 ) with each VOC. In experiment 2, a side-by-side comparison was made of 17 VOCs at concentrations between 1 ppmv and 5 ppbv after small corrections had been made to the SIFT-MS kinetic data. In a 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 any calibration of the SIFT-MS instrument. In experiment 2, the two techniques yielded good correspondence between the measured 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 containing large numbers of VOCs at high concentrations. In all other applications, SIFT-MS proved simpler to use, was linear with concentration over a much wider concentration range than GC/MS and provided faster results. Copyright © 2013 John Wiley & Sons, Ltd.
Gas chromatography/mass spectrometry (GC/MS) is the most widely used technique for analysing mixtures of volatile organic compounds (VOCs).[1,2] The capillary column in GC provides physical separation between compounds and the mass spectrometer provides the identification. Databases are available with retention indices covering polar and non-polar columns for more than 70 000 compounds. An even more extensive database containing the electron ionization mass spectra of more than 210 000 compounds is available to assist in the identification of analytes.[3] However, although the GC/MS technique has found such widespread use, there are some difficulties associated with sampling VOCs from gaseous and liquid headspace samples. In many cases a preconcentration step is required. This may entail a purgeand-trap technique for the analysis of VOCs in solution. Here an inert gas is bubbled through the sample to displace the VOCs, followed by trapping and pre-concentration of the analytes on an adsorption cartridge. Alternatively, for gas
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* 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 that has greater affinity for VOCs than for the bulk gas, providing pre-concentration. In both cases, trapped VOCs are released later from the substrate by using thermal desorption and passed to the GC/MS sample injector. They are then separated in the GC column for subsequent MS detection, identification and quantitation. Reliable quantitation using GC/MS requires frequent calibration across the measuring range. The purpose of the present study is to provide a comparison between accepted GC/MS methodology and selected ion flow tube mass spectrometry (SIFT-MS). SIFT-MS is an emerging technique that offers significant advantages over GC/MS in a range of scenarios through its simplicity of operation and real-time analysis of VOCs. SIFT-MS is one of several quantitative techniques that might be described as real-time, such as proton transfer mass spectrometry (PTR-MS)[4,5] and fast GC.[6] However, SIFT-MS does not suffer the variability of product ion ratios that are a function of the drift fields in PTR-MS and has the advantage of no chromatography columns with corresponding elution time and phase overload problems of fast GC. The principles of SIFT-MS technology have been well summarised elsewhere.[7,8] SIFT-MS is distinct from most MS analytical techniques in that it can be used as a real-time
Copyright © 2013 John Wiley & Sons, Ltd.
Rapid monitoring of VOCs absolute VOC analyser without the requirement for calibration curves to calculate or determine analyte concentrations. It quantifies VOCs on the basis of the ratio of product ion count to reagent ion count.[7] However, to do this, knowledge of the kinetic parameters of ion-molecule reaction rates and product ion distributions for the reagent ion reactions with each analyte is required together with the sample flow rate and instrument mass 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 with the carrier gas and the rate coefficients and product ion distributions 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 no direct comparisons with other analytical techniques on multiple analytes have been made. The purpose of this study is to provide a much more extensive comparison of SIFT-MS with GC/MS for the quantification of multiple VOCs in a sample and to explore the circumstances under which SIFT-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 utilize a regulatory method for analysing whole air samples. The canister sampling methods of the United States Environmental Protection Agency (US EPA) were deemed most appropriate as they are whole air methods. The best known of these are the toxic organic compendium methods, 14A and 15 (TO-14A[14] and TO-15[15]). We report here the results of a comparison between GC/MS and SIFT-MS on selected compounds from the US EPA TO-14 and TO-15 compendium methods.
EXPERIMENTAL Chemicals and standards
Rapid Commun. Mass Spectrom. 2014, 28, 10–18
acetone acrylonitrile acetonitrile benzene bromomethane 1,3-butadiene carbon disulfide carbon tetrachloride chlorobenzene 1,2-dibromoethane 1,2-dichlorobenzene ethyl acetate ethyl benzene
isooctane methyl ethyl ketone (butanone) naphthalene propylene styrene tetrachloroethylene tetrahydrofuran toluene 1,1,1-trichloroethane trichloroethylene 1,3,5-trimethylbenzene vinyl chloride
The mixtures containing the standards were placed in 1-L Tedlar sample bags (SKC Inc., Eighty-Four, PA, USA) and their concentrations were measured without any calibration using the existing SIFT-MS data base that provides an ’absolute’ measurement as long as the relevant ion-molecule kinetic data are known. To examine the response of the SIFT-MS instrument with concentration, serial dilutions were prepared as follows. Zero air (1 L) was placed in the sample bag using a precision 1-L gas syringe (Vitalograph, Ennis, Ireland). Calibration gas was spiked into zero air by using gas-tight syringes (SGE, Melbourne, Australia) of appropriate volume. Because Tedlar film is relatively permeable to moisture, the calibration has effectively been made in moderately humidified zero air. The series of dilutions provided a range of concentrations between 1 ppmv and 5 ppbv. The GC/MS instrument was calibrated from the known concentrations of the 25 standards of Table 1. Known volumes of the standards were removed from the supplied cylinder via a 1-L gas-tight syringe (SGE Analytical Science Pty Ltd, Ringwood, Australia) and combined with humidified air in a 15-L Silonite® canister (Entech Instruments Inc., Simi Valley, CA, USA) to give a nominal concentration of 10 parts per billion by concentration (ppbv). An internal standard was similarly made to a concentration of 50 ppbv from a 1 ppmv stock containing bromochloromethane, 4-bromofluorobenzene (BFB), chlorobenzene-d5 and 1,4-difluorobenzene (Custom Gas Solutions Inc.). Calibration of the GC/MS system was performed and the instrument performance assessed in accordance with US EPA TO-15.[15] Initial calibration of the GC/MS instrument was performed by injecting a constant volume of the 50 ppbv internal standard and varying volumes of the calibrating standard gases such that the calibration range went from 0.5 to 50 ppbv at six concentration levels. Daily compliance was determined by the assessment of the BFB mass spectrum, instrument blank and 10 ppbv standard response, again according to US EPA TO-15.[15] Experiment 2 used a sub-section of 17 VOCs from the list of standard compounds shown in Table 1 for direct comparison between SIFT-MS and GC/MS and these are
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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 air using a Peak ZA010 zero air generator (Peak Scientific, Inchinnan, UK) and an activated charcoal scrubber (Alltech Associates Inc., Deerfield, IL, USA). The humidified zero air was confirmed to be contaminant free before use.[15] Pressure measurement during the certification of canisters and preparation of standards and samples were carried out with 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 purchased from three laboratories supplying certified mixtures of analytes for the TO-14 and TO-15 standards at a nominal concentration of 1 ppmv and stated accuracy of 5% and with a balance of nitrogen. The 25 VOCs chosen are shown in Table 1. The laboratories supplying the standards were Linde Spectra Environmental Gases, Alpha, NJ, USA, TO-14A Standard; Global Calibration Gases LLC, Palmetto, FL, USA, TO-15 plus; Custom Gas Solutions, Durham, NC, USA, TO-15 plus.
Table 1. Selected compounds from the US EPA TO-14A and TO-15 methods used for calibration of the SIFT-MS instrument from certified gas standards. See text for details. The nominal concentration is 1.0 ppmv with a quoted accuracy 5%
V. S. Langford, I. Graves and M. J. McEwan Table 2. Compounds used to spike cans in comparative SIFT-MS and GC/MS experiments
Analyte acetone acetonitrile acrylonitrile benzene butanone carbon disulfide chlorobenzene ethylbenzene isooctane (2,2,4trimethylpentane) naphthalene styrene tetrachloroethene tetrahydrofuran toluene 1,1,1-trichloroethane trichloroethene 1,3,5-trimethylbenzene
Pre-spike mix
Supplier Mallinckrodt Baker Inc. Merck Chromspec Distributors Ltd Pure Science Ltd BDH Laboratory Supplies Fisher Scientific Sigma Aldrich Acros Merck
Mix Mix Mix Mix
2 3 2 1
Sigma Aldrich Sigma Aldrich Sigma Aldrich Merck Merck Sigma Aldrich Merck Supelco
Mix Mix Mix Mix Mix Mix Mix Mix
3 2 2 1 2 2 1 3
Mix 1 Mix 1 Mix 2 Mix 1 Mix 1
shown in Table 2 together with the suppliers of the VOC. Three liquid mixtures of these 17 VOCs were prepared from samples of the pure parent compounds based around the volatility of the compounds. Four canisters were spiked with varying amounts of the headspace above the liquids. The spiking process was performed in several steps using a 10-μL gas-tight syringe (SGE Analytical Science Pty Ltd). The cans were pressurised to 5 psig with humidified zero air. The SIFT-MS instrument was then used to provide rapid quantification of the contents to confirm that the levels were within the linear range required by the GC/MS system (target levels were 0.5 to 50 ppbv). A small correction was made to several of the SIFT-MS-derived concentrations from the kinetic data so that the analyte concentrations were within 10% of the concentrations of the calibrated mixtures. A small correction also had to be made when sampling from canisters at 5 psig for the SIFT-MS instrument because normally it analyses samples at atmospheric pressure. Although not in the mixture, seven other compounds for which both instruments were set up to measure were also monitored for their background levels. Experiment 3 in which samples submitted to the analytical laboratory for analysis were subjected to a side-by-side GC/MS and SIFT-MS comparison. Two different groups of samples were selected for analysis: one group was of gas samples from soil that had been contaminated by an oil spill and one sample from a clandestine methamphetamine
Table 3. Results of SIFT-MS calibration versus the certified gas standards – the quantification of SIFT-MS is based on generic kinetic data Compound and reagent ion benzene NO+ O+2 toluene NO+ O+2 ethylbenzene H3O+ NO+ 1,3,5-trimethylbenzene H3O+ NO+ O+2 styrene NO+ O+2 naphthalenea H3O+ NO+ O+2 propene H3O+ O+2 1,3-butadiene NO+ isooctane NO+
Product m/z
Kinetics references
Ratio of measured to certified concentration
LOD/ppbv
LOQ/ppbv
C6H+6 C6H+6
78 78
[17] [17]
0.66 0.89
0.23 0.16
0.52 0.32
C7H+8 C7H+8
92 92
[17] [17]
0.91 0.90
0.10 0.09
0.25 0.20
C8H10.H+ C8H+10
107 106
[17] [17]
0.83 0.88
0.18 0.16
0.37 0.35
C9H12.H+ C9H+12 C9H+12
121 120 120
[17] [17] [17]
1.06 1.06 0.98
0.14 0.26 0.10
0.31 0.52 0.24
C8H+8 C8H+8
104 104
[18] [18]
1.07 0.94
0.21 0.09
0.42 0.21
C10H+9 C10H+8 C10H+8
129 128 128
[19] [20] [20]
0.64 0.58 0.48
0.65 0.67 0.37
1.24 1.30 0.77
C3H+7 C3H+6
43 42
[21] [21]
0.86 1.34
0.27 0.20
0.50 0.38
C4H+6
54
[18]
0.94
0.05
0.16
C8H+17
113
[22]
0.80
0.29
0.58
Product ion
(Continues)
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Rapid monitoring of VOCs Table 3. (Continued) Compound and reagent ion carbon tetrachloride O+2 O+2 1,1,1-trichloroethane O+2 O+2 chloroethene O+2 O+2 trichloroethene O+2 O+2 O+2 tetrachloroethene O+2 O+2 O+2 chlorobenzene NO+ NO+ O+2 O+2 dichlorobenzenesa NO+ NO+ O+2 O+2 methyl bromidea O+2 1,2-dibromoethanea O+2 acetonitrile H3O+ acrylonitrile H3O+ carbon disulfide O+2 acetone H3O+ NO+ butanone NO+ O+2 ethyl acetate H3O+ tetrahydrofuran NO+ O+2 a
Product m/z
Kinetics references
Ratio of measured to certified concentration
LOD/ppbv
LOQ/ppbv
119 117
[23] [23]
0.76 0.76
0.40 0.30
0.92 0.77
CH3C35Cl 37Cl+ CH3C35 Cl+2
99 97
[23] [23]
0.65 0.70
0.62 0.34
1.17 0.65
+ C2H35 3 Cl + C2H37 Cl 3
62 64
[23] [23]
0.65 0.70
0.11 0.37
0.25 0.82
C2H35Cl37Cl+2 + C2H35Cl37 2 Cl C2H35Cl+3
134 132 130
[23] [23] [23]
1.01 1.01 1.08
1.83 0.41 0.33
3.61 0.87 0.73
37 + C35 2 Cl2 Cl2 35 37 + C2 Cl3 Cl + C35 2 Cl4
168 166 164
[23] [23] [23]
1.17 1.17 1.21
1.33 0.56 0.54
2.80 1.22 1.30
+ C6H35 5 Cl 37 + C6H5 Cl + C6H35 5 Cl 37 + C6H5 Cl
112 114 112 114
[24] [24] [24] [24]
0.77 0.75 0.91 0.83
0.23 0.99 0.43 0.45
0.48 2.08 0.84 0.84
37 + C6H35 4 Cl Cl + C6H35 Cl 4 2 35 37 + C6H4 Cl Cl 35 + C6H4 Cl2
148 146 148 146
[18] [18] [18] [18]
0.75 0.69 0.94 0.83
0.73 0.62 0.33 0.22
1.59 1.24 0.83 0.52
94
[24]
0.39
0.20
0.41
107
[18]
0.79
0.05
0.11
CH3CN.H+
42,60,78
[25]
0.46
0.11
0.21
CH2CHCNH+
54,72,90
[19]
0.69
0.10
0.20
76
[26]
1.15
0.16
0.36
C3H7O+ NO+.C3H6O
59,77 88
[27] [18]
0.59 0.86
0.34 0.52
0.60 0.95
NO+.C4H8O C4H8O+
102 72
[27] [27]
0.47 0.48
0.26 0.61
0.52 1.13
89,107
[28]
1.22
0.19
0.36
71 71
[29] [29]
0.65 0.82
0.25 0.28
0.48 0.52
Product ion + C35Cl37 2 Cl C35Cl+3
+ CH79 3 Br + C2H79 4 Br
CS+2
CH3COOC2H5.H+ C4H7O+ C4H7O+
Some contamination was found from the bag substrate and the bag background has been subtracted.
Rapid Commun. Mass Spectrom. 2014, 28, 10–18
of the canister was measured on receipt at the laboratory and the samples were then diluted with zero air to give a positive pressure. The dilution factor was approximately two. SIFT-MS measurements In the current study a transportable Voice200® SIFT-MS instrument (Syft Technologies Ltd, Christchurch, New Zealand) was used.[13,16] In the Voice200® instrument, three
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laboratory that had undergone some remedial clean up procedures. For these analyses, ambient air or soil vapour was sampled using critical orifice restricted sampling trains (that had been internally coated with Silonite® to minimise surface interactions) 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 nominally 180 mL min–1 with sampling completed when 127 Torr of residual vacuum remained within the canister. The pressure
V. S. Langford, I. Graves and M. J. McEwan reagent ions (H3O+, NO+ and O+2 ) are generated in a microwave discharge of moist air at a total pressure about 0.35 Torr. The resulting ions are mass-selected by a quadrupole mass filter upstream of the flow tube and injected into a helium carrier gas at a pressure around 0.6 Torr. These ions are convected along the curved flow tube and are sampled through an aperture at the end of the flow tube. The region immediately behind the aperture lens is pumped by a split-flow turbo pump and the ions then enter the downstream quadrupole mass spectrometer where they are mass-selected and counted. Switching between the reagent ions by the upstream quadrupole occurs typically in 10 ms allowing a seamless analysis of analyte concentrations using all three reagent ions. To avoid analyte loss during the transfer process from the sample container to the instrument, the instrument was equipped with a heated passivated inlet providing direct access to the reaction tube. This inlet was connected to canisters using a Silonite®-coated Micro-QT™ valve (Entech Instruments Inc.) and Tedlar bags via a short length of Teflon tubing. The sample time for each product ion resulting from the analyte reaction with the reagent ions was typically 3.7 s. For the current comparison, the Voice200® SIFT-MS instrument was shipped to the R.J. Hills Laboratory in Hamilton, New Zealand.
’absolute’ in the sense that no concentration calibrations were undertaken prior to this analysis. The analytical results are simply the end product of the insertion of the relevant kinetic parameters for each ion-molecule reaction and the instrument software then takes the ratio of the product ion counts to the reagent ion counts to derive the concentrations.[7] The following comments are relevant to the measurements reported in Table 3. For each analyte, there is the possibility of examining three different reagent ion reactions. However in a matrix of 25 or so analytes, a number of the product ions may have the same mass (isobaric) and hence the isobaric ion products from a reagent ion cannot be included in the analyses. In practice when different reagent ions’ reactions with the matrix yield results for an analyte that differ significantly, only the reagent ion yielding the lower concentration is used because of the potential for mass overlaps. In the current matrix, the reagent ions used to quantify the designated analyte are shown in Table 3. In addition, the reagent ion reactions of some structural isomers may be similar for each isomer and in that case only a total of all the possible isomers that are present
(a) GC/MS measurements GC/MS analysis was carried out by R.J. Hill Laboratories Limited. This laboratory is accredited to the ISO 17025 standard for US EPA Compendium Method TO-15 and analyses were carried out according to this standard. The analysis of the canisters was made as follows. Air from the canister (typically 125 mL) was transferred by an autosampler (model 7410D, Entech Instruments Inc.) and pre-concentrated (model 7150D, Entech Instruments Inc.) prior to GC/MS analysis. The pre-concentrated sample was then thermally desorbed 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 had a 60 m × 0.32 mm i.d. BP-1 analytical column with a film thickness of 1 μm (SGE Analytical Science Pty Ltd). Helium carrier gas was used, with the head pressure set at 11.7 psig yielding an average linear velocity of 36 cm s–1. The gas chromatograph oven was held at 35°C for 4 min, increased to 110°C at 4°C min–1 held for 0.1 min then increased to 220°C at 15°C min–1 and held for 5 min, giving a total run time of 36.2 min. Mass spectra were collected after a delay of 4 min from m/z 29 to 160 until 10 min; the mass range was then changed to m/z 34 to 270 for the remainder of the run.
(b)
RESULTS AND DISCUSSION Experiment 1
14
The concentrations of the 25 analytes listed in Table 1 as measured by the Voice200® instrument are shown in Table 3 expressed as a ratio of the certified concentration. These measured values are based solely on the rate coefficients and product branching ratios of the relevant reagent ion reactions reported in the literature (column 4 of Table 3) and are
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Figure 1. Example calibration data for SIFT-MS of measured concentration against certified concentration for aromatics (a) and halogenated hydrocarbons (b).
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Rapid monitoring of VOCs can be found. In a mixture where there is a large disparity in concentrations between analytes, the limiting factor is whether the reagent ion number density is reduced by reactions with an analyte. In practice linearity is experienced between parts per trillion by volume and mid parts per million by volume. At higher analyte concentrations, a dilution of the sample mixture is required. The results presented in Table 3 demonstrate that of the 50 reagent ion determinations of the analyte concentrations based on the existing database kinetic data, 86% of the results are within 35% of the expected concentrations. Some of the outliers may simply be a result of the sampling method. An example is the results for naphthalene where likely desorption loss onto the Tedlar bag would have resulted in a low result for all three reagent ions. The results for the ketones, acetone and butanone were also surprising as a calibration afterwards of acetone using a calibrated permeation tube (Vici Metronics, Poulsbo, WA, USA) gave excellent correlation within 10% of the stated concentration of 3.85 ppmv from the permeation tube. We also show in the two right-hand columns of Table 3 the limits of detection (LODs) and limits of quantitation (LOQs) for the listed analytes in the configuration of the Voice 200 used for the comparison and these were obtained as outlined
previously.[11] These values were found by monitoring the product ion mass for each analyte in a 6-L Entech canister that had only been used for zero quality air. A demonstration of the response of the SIFT-MS instrument with analyte concentration is shown in Fig. 1. A series of selected volatiles at ppmv concentrations was prepared in Tedlar bags and the samples were then sequentially diluted with zero air to generate a series of measurements at different concentrations between 5 ppbv and 1 ppmv. The linearity of the concentrations in all cases as expressed by R2, the coefficient of determination, over the three orders of magnitude examined was R 2 ≥0.997. We have shown elsewhere that a linear concentration response over five orders of magnitude is typical.[13] Figure 1(a) shows the linearity of the response for the named hydrocarbons and Fig. 1(b) the response for the named chlorinated hydrocarbons. Experiment 2: GC/MS and SIFT-MS side-by-side comparison of spiked canisters The target levels of volatiles were in the low- to mid-ppbv range. The canisters were tested using both SIFT-MS and GC/MS. The results of the comparison study are shown
Table 4. Side-by-side comparison of GC/MS and SIFT-MS of four different mixtures containing 17 VOCs with listed concentrations shown as ppbv
a
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These analytes were not included in the mix but were simply in the methods for analysis of both instruments. They represent signal backgrounds. b C2-alkylbenzenes is the total of ethylbenzene and the three xylene isomers for the SIFT-MS study. In this experiment, only ethylbenzene was added to the VOCs in the mixtures. c C3-alkylbenzenes is the total of all isomers for SIFT-MS. In this test, only 1,3,5-trimethylbenzene was added to the VOCs in the mixtures. d Acetonitrile was not measured by the GC/MS instrument.
V. S. Langford, I. Graves and M. J. McEwan in Table 4. The VOCs in italics represent background levels for compounds that were in the analytical methods for each instrument but not in the mixtures. Overall the agreement between the two quite different techniques is very good from a mixture containing 17 VOCs. Differences in concentrations of greater than 30% were observed for only styrene (higher for SIFT-MS than for GC/MS), and acetone and carbon disulfide (both lower in all mixtures than the concentrations reported by GC/MS). We are not sure why lower concentrations of acetone were found in the SIFT-MS measurements as the association reaction of NO+ with acetone was used to determine the concentration 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 Syft Voice200® instrument.
Experiment 3: Side-by-side comparison of four commercial samples The comparative results for this study are shown in Table 5. The first sample set came from a site where soil had been contaminated by leakage from an underground fuel storage tank. Samples were taken from the crawl space surrounding the tank and from soil where fuel seepage had occurred. The comparative results are shown for the VOCs listed in column 1, column 2 represents background signals for each analyte, column 3 represents analyte concentrations from the air sampled from the crawl space, and column 4 those from soil samples. The second sample included in Table 5 came from air samples from a remediated clandestine methamphetamine (’P’) laboratory and these results are shown in column 5. The following observations can be made. Good agreement between the two techniques was found for non-contaminated samples such as the ambient
Table 5. Comparison of analyte concentrations in two commercial samples analysed at Hill Laboratories. See the text for more details on the samples
a
16
C2-alkylbenzenes gives the total of ethylbenzene and the three xylene isomers for SIFT-MS; speciation reported for GC/MS has been summed. b C3-alkylbenzenes gives the total for all isomers for SIFT-MS; speciation reported for GC/MS has been summed. c Not reported for GC/MS due to interference from 2-methylbutane. d Not reported for GC/MS.
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Rapid Commun. Mass Spectrom. 2014, 28, 10–18
Rapid monitoring of VOCs samples and the samples from the crawl space (columns 2 and 3). In the contaminated samples (column 4, soil gas), good correlation was obtained for the smaller aromatic hydrocarbons (benzene, toluene and C2-alkylbenzenes). However, there was poor agreement for a number of the other compounds in the soil sample. The reason for lower correlation in the soil sample is that the soil was saturated with hydrocarbons from the fuel and these hydrocarbons generate mass overlaps with the SIFT-MS product ions from the analytes included in the method. Under these conditions, SIFT-MS is susceptible to interference when large numbers of multiple components at higher concentrations are present. However, in the comparative analysis of the residuals from the ’P’ lab, good correlation was obtained between the two techniques with the additional benefit from SIFT-MS of all VOCs present being observed. The GC/MS method does not report methanol due to the background in the canisters from the manufacturing process.
CONCLUSIONS
Rapid Commun. Mass Spectrom. 2014, 28, 10–18
We thank John Gray for assistance with preparatory and follow-up work for the trial, Daniel Milligan and Barry Prince for helpful discussions (all Syft Technologies Ltd) and Alastair Boyd of Hill Laboratories for sample preparation and helpful discussions.
REFERENCES [1] Current Practice of Gas Chromatography-Mass Spectrometry, (Ed: W. M. A. Neilson). Marcel Dekker, New York, 2001. [2] M. C. McMaster. GC/MS: A Practical Users Guide, (2nd edn.). John Wiley, New Jersey, 2008. [3] NIST/EPA/NIH Mass Spectral Database (NIST11) and NIST Mass Spectral Search Program (version 2.0g). U.S. Dept. of Commerce, Standard Reference Data Program, Gaithersburg, MD, 2011. [4] W. Lindinger, A. Hansel, A. Jordan. Online monitoring of volatile organic compounds at pptv levels by means of proton transfer reaction-mass spectrometry (PTR-MS). Medical applications, food control and environmental research. Int. J. Mass Spectrom. 1998, 173, 191. [5] R. S. Blake, P. S. Monks, A. M. Ellis. Proton transfer reaction mass spectrometry. Chem. Rev. 2009, 109, 861. [6] M. S. Klee, L. M. Blumberg. Theoretical and practical aspects of fast gas chromatography and method translation. J. Chromatogr. Sci. 2002, 40, 234. [7] D. Smith, P. Španěl. Selected ion flow tube mass spectrometry for on-line trace gas analysis. Mass Spectrom. Rev. 2005, 24, 661. [8] C. G. Freeman, M. J. McEwan. Rapid analysis of trace gases in complex mixtures using selected ion flow tube-mass spectrometry. Aust. J. Chem. 2002, 55, 491. [9] P. Španěl, J. R. Cocker, B. Rajan, D. Smith. Validation of the SIFT technique for trace gas analysis of breath using the syringe injection method. Ann. Occupat. Hyg. 1997, 41, 373. [10] J. Kubista, P. Španěl, K. Dryahina, C. Workman, D. Smith. Combined use of gas chromatography and selected ion flow tube mass spectrometry for absolute trace gas quantification. Rapid Commun. Mass Spectrom. 2006, 20, 563. [11] D. B. Milligan, G. J. Francis, B. J. Prince, M. J. McEwan. Demonstration of selected ion flow tube MS in the parts per trillion range. Anal. Chem. 2007, 79, 2537. [12] G. J. Francis, D. B. Milligan, M. J. McEwan. Detection and quantification of chemical warfare agent precursors and surrogates by selected ion flow tube mass spectrometry. Anal. Chem. 2009, 81, 8892. [13] B. J. Prince, D. B. Milligan, M. J. McEwan. Application of selected ion flow tube mass spectrometry to real time atmospheric monitoring. Rapid Commun. Mass Spectrom. 2010, 24, 1763. [14] Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air: Method TO-14A, (2nd edn.), U.S. Environmental Protection Agency, Research Triangle Park, NC, EPA 600/625/R-96/010b, January 1997. [15] Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air: Method TO-15, (2nd edn.), U.S. Environmental Protection Agency, Research Triangle Park, NC, EPA 600/625/R-96-010b, January, 1997. [16] www.syft.com. [17] P. Španěl, D. Smith. Selected ion flow tube studies of the reactions of H3O+, NO+ and O+2 with several aromatic and aliphatic hydrocarbons. Int. J. Mass Spectrom. 1998, 181, 1. [18] Syft Technolgies Ltd Database, 2013.
Copyright © 2013 John Wiley & Sons, Ltd.
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The following observations can be made from this comparative study between SIFT-MS and GC/MS undertaken at a laboratory approved to conduct USA EPA TO15[15] compendium methods. The SIFT-MS analysis of certified gas standards in experiment 1 resulted in satisfactory quantitative agreement for most analytes (Table 3) although the instrument had not been pre-calibrated for those compounds – generic gas-phase kinetic data applicable to all SIFT-MS instruments were used. The measured data were then used to make minor adjustment to the kinetic parameters to calibrate the specific SIFT-MS instrument used in the trial. We have found those reagent ionanalyte reactions that are most susceptible to small changes from the published data for ion-molecule reactions (for example, whether the flow tube heater is on or off) are those where association reactions occur (some NO+ reactions) and some 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 confirmed that SIFT-MS offers a viable alternative to GC/MS for monitoring VOCs down to trace levels (pptv) and has the marked benefits of not needing the pre-concentration/ desorption steps that are required by GC/MS for trace level analysis. Further, SIFT-MS is a real-time method and provides results in seconds, it does not require different columns for polar and non-polar analytes, it is much simpler to use and has a linear response over a much wider concentration range than GC/MS. The two techniques are complementary and there are situations where one might be preferred over the other. For example in an analysis of an unknown sample containing a large number of VOCs in the same matrix, GC/MS would be the method of choice as was evidenced in the contaminated soil sample. Nevertheless, in many applications SIFT-MS offers clear advantages over accepted traditional methods for VOC analysis. In particular, SIFT-MS provides unique mass spectrometry based opportunities in real-time ambient air monitoring, because it is robust, easily deployable, easy to use, very sensitive, and no preconcentration and sample preparation is required. We have shown elsewhere[11,12] that it can also analyse down to the pptv level after only 15 min warm up.
Acknowledgements
V. S. Langford, I. Graves and M. J. McEwan [19] D. B. Milligan, P. F. Wilson, C. G. Freeman, M. Meot-Ner, M. J. McEwan. Dissociative proton transfer reactions of H+3 , N2H+ and H3O+ with acyclic, cyclic and aromatic hydrocarbons and nitrogen compounds and astrochemical implications. J. Phys. Chem. A 2002, 106, 9745. [20] P. F. Wilson, C. G. Freeman, M. J. McEwan. Reactions of small hydrocarbons with H3O+, O+2 and NO+ ions. Int. J. Mass Spectrom. 2003, 229, 143. [21] A. J. Midey, S. Williams, S. T. Arnold, I. Dotan, R. A. Morris, A. A. Viggiano. Rate constants and branching ratios for the reactions of various positive ions with naphthalene from 300 to 1400 K. Int. J. Mass Spectrom. 2000, 195/196, 327. [22] S. T. Arnold, A. A. Viggiano, R. A. Morris. Rate constants and product branching fractions for the reactions of H3O+ and NO+ with C2–C12 alkanes. J. Phys. Chem. A 1998, 102, 8881. [23] P. Španěl, D. Smith. Selected ion flow tube studies of the reactions of H3O+, NO+ and O+2 with some chloroalkanes and chloroalkenes. Int. J. Mass Spectrom. 1999, 184, 175.
[24] P. Španěl, D. Smith. Selected ion flow tube studies of the reactions of H3O+, NO+ and O+2 with several aromatic and aliphatic monosubstituted halocarbons. Int. J. Mass Spectrom. 1999, 189, 213. [25] P. Španěl, D. Smith. Selected ion flow tube studies of the reactions of H3O+, NO+ and O+2 with several amines and some other nitrogen-containing molecules. Int. J. Mass Spectrom. 1998, 176, 203. [26] P. Španěl, D. Smith. Selected ion flow tube studies of the reactions of H3O+, NO+ and O+2 with some organosulphur molecules. Int. J. Mass Spectrom. 1998, 176, 167. [27] D. Smith, T. Wang, P. Španěl. Analysis of ketones by selected ion flow tube mass spectrometry. Rapid Commun. Mass Spectrom. 2003, 17, 2655. [28] P. Španěl, D. Smith. SIFT studies of the reactions of H3O+, NO+ and O+2 with a series of carboxylic acids and esters. Int. J. Mass Spectrom. 1998, 172, 137. [29] T. Wang, P. Španěl, D. Smith. A selected ion flow tube study, SIFT, of the reactions of H3O+, NO+ and O+2 with several Nand O-containing heterocyclic compounds in support of SIFT-MS. Int. J. Mass Spectrom. 2004, 237, 167.
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Rapid Commun. Mass Spectrom. 2014, 28, 10–18
Revised: 23 September 2013
Accepted: 25 September 2013
Published online in Wiley Online Library
Rapid Commun. Mass Spectrom. 2014, 28, 10–18 (wileyonlinelibrary.com) DOI: 10.1002/rcm.6747
Rapid monitoring of volatile organic compounds: a comparison between gas chromatography/mass spectrometry and selected ion flow tube mass spectrometry Vaughan S. Langford1, Ian Graves2 and Murray J. McEwan1,3* 1
Syft Technologies Ltd, 3 Craft Pl, Christchurch, New Zealand Hill Laboratories, Private Bag 3205, Hamilton 3240, New Zealand 3 University of Canterbury, Christchurch, New Zealand 2
RATIONALE: The gold standard for monitoring volatile organic compounds (VOCs) is gas chromatography/mass spectrometry (GC/MS). However, in many situations, when VOC concentrations are at the ppmv level, there are complicating factors for GC/MS. Selected ion flow tube mass spectrometry (SIFT-MS) is an emerging technique for monitoring 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 the concentrations of 25 VOCs in a mixture at concentrations up to 1 ppmv using only a generic database for known kinetic data of three reagent ions (H3O+, NO+ and O+2 ) with each VOC. In experiment 2, a side-by-side comparison was made of 17 VOCs at concentrations between 1 ppmv and 5 ppbv after small corrections had been made to the SIFT-MS kinetic data. In a 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 any calibration of the SIFT-MS instrument. In experiment 2, the two techniques yielded good correspondence between the measured 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 containing large numbers of VOCs at high concentrations. In all other applications, SIFT-MS proved simpler to use, was linear with concentration over a much wider concentration range than GC/MS and provided faster results. Copyright © 2013 John Wiley & Sons, Ltd.
Gas chromatography/mass spectrometry (GC/MS) is the most widely used technique for analysing mixtures of volatile organic compounds (VOCs).[1,2] The capillary column in GC provides physical separation between compounds and the mass spectrometer provides the identification. Databases are available with retention indices covering polar and non-polar columns for more than 70 000 compounds. An even more extensive database containing the electron ionization mass spectra of more than 210 000 compounds is available to assist in the identification of analytes.[3] However, although the GC/MS technique has found such widespread use, there are some difficulties associated with sampling VOCs from gaseous and liquid headspace samples. In many cases a preconcentration step is required. This may entail a purgeand-trap technique for the analysis of VOCs in solution. Here an inert gas is bubbled through the sample to displace the VOCs, followed by trapping and pre-concentration of the analytes on an adsorption cartridge. Alternatively, for gas
10
* 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 that has greater affinity for VOCs than for the bulk gas, providing pre-concentration. In both cases, trapped VOCs are released later from the substrate by using thermal desorption and passed to the GC/MS sample injector. They are then separated in the GC column for subsequent MS detection, identification and quantitation. Reliable quantitation using GC/MS requires frequent calibration across the measuring range. The purpose of the present study is to provide a comparison between accepted GC/MS methodology and selected ion flow tube mass spectrometry (SIFT-MS). SIFT-MS is an emerging technique that offers significant advantages over GC/MS in a range of scenarios through its simplicity of operation and real-time analysis of VOCs. SIFT-MS is one of several quantitative techniques that might be described as real-time, such as proton transfer mass spectrometry (PTR-MS)[4,5] and fast GC.[6] However, SIFT-MS does not suffer the variability of product ion ratios that are a function of the drift fields in PTR-MS and has the advantage of no chromatography columns with corresponding elution time and phase overload problems of fast GC. The principles of SIFT-MS technology have been well summarised elsewhere.[7,8] SIFT-MS is distinct from most MS analytical techniques in that it can be used as a real-time
Copyright © 2013 John Wiley & Sons, Ltd.
Rapid monitoring of VOCs absolute VOC analyser without the requirement for calibration curves to calculate or determine analyte concentrations. It quantifies VOCs on the basis of the ratio of product ion count to reagent ion count.[7] However, to do this, knowledge of the kinetic parameters of ion-molecule reaction rates and product ion distributions for the reagent ion reactions with each analyte is required together with the sample flow rate and instrument mass 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 with the carrier gas and the rate coefficients and product ion distributions 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 no direct comparisons with other analytical techniques on multiple analytes have been made. The purpose of this study is to provide a much more extensive comparison of SIFT-MS with GC/MS for the quantification of multiple VOCs in a sample and to explore the circumstances under which SIFT-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 utilize a regulatory method for analysing whole air samples. The canister sampling methods of the United States Environmental Protection Agency (US EPA) were deemed most appropriate as they are whole air methods. The best known of these are the toxic organic compendium methods, 14A and 15 (TO-14A[14] and TO-15[15]). We report here the results of a comparison between GC/MS and SIFT-MS on selected compounds from the US EPA TO-14 and TO-15 compendium methods.
EXPERIMENTAL Chemicals and standards
Rapid Commun. Mass Spectrom. 2014, 28, 10–18
acetone acrylonitrile acetonitrile benzene bromomethane 1,3-butadiene carbon disulfide carbon tetrachloride chlorobenzene 1,2-dibromoethane 1,2-dichlorobenzene ethyl acetate ethyl benzene
isooctane methyl ethyl ketone (butanone) naphthalene propylene styrene tetrachloroethylene tetrahydrofuran toluene 1,1,1-trichloroethane trichloroethylene 1,3,5-trimethylbenzene vinyl chloride
The mixtures containing the standards were placed in 1-L Tedlar sample bags (SKC Inc., Eighty-Four, PA, USA) and their concentrations were measured without any calibration using the existing SIFT-MS data base that provides an ’absolute’ measurement as long as the relevant ion-molecule kinetic data are known. To examine the response of the SIFT-MS instrument with concentration, serial dilutions were prepared as follows. Zero air (1 L) was placed in the sample bag using a precision 1-L gas syringe (Vitalograph, Ennis, Ireland). Calibration gas was spiked into zero air by using gas-tight syringes (SGE, Melbourne, Australia) of appropriate volume. Because Tedlar film is relatively permeable to moisture, the calibration has effectively been made in moderately humidified zero air. The series of dilutions provided a range of concentrations between 1 ppmv and 5 ppbv. The GC/MS instrument was calibrated from the known concentrations of the 25 standards of Table 1. Known volumes of the standards were removed from the supplied cylinder via a 1-L gas-tight syringe (SGE Analytical Science Pty Ltd, Ringwood, Australia) and combined with humidified air in a 15-L Silonite® canister (Entech Instruments Inc., Simi Valley, CA, USA) to give a nominal concentration of 10 parts per billion by concentration (ppbv). An internal standard was similarly made to a concentration of 50 ppbv from a 1 ppmv stock containing bromochloromethane, 4-bromofluorobenzene (BFB), chlorobenzene-d5 and 1,4-difluorobenzene (Custom Gas Solutions Inc.). Calibration of the GC/MS system was performed and the instrument performance assessed in accordance with US EPA TO-15.[15] Initial calibration of the GC/MS instrument was performed by injecting a constant volume of the 50 ppbv internal standard and varying volumes of the calibrating standard gases such that the calibration range went from 0.5 to 50 ppbv at six concentration levels. Daily compliance was determined by the assessment of the BFB mass spectrum, instrument blank and 10 ppbv standard response, again according to US EPA TO-15.[15] Experiment 2 used a sub-section of 17 VOCs from the list of standard compounds shown in Table 1 for direct comparison between SIFT-MS and GC/MS and these are
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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 air using a Peak ZA010 zero air generator (Peak Scientific, Inchinnan, UK) and an activated charcoal scrubber (Alltech Associates Inc., Deerfield, IL, USA). The humidified zero air was confirmed to be contaminant free before use.[15] Pressure measurement during the certification of canisters and preparation of standards and samples were carried out with 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 purchased from three laboratories supplying certified mixtures of analytes for the TO-14 and TO-15 standards at a nominal concentration of 1 ppmv and stated accuracy of 5% and with a balance of nitrogen. The 25 VOCs chosen are shown in Table 1. The laboratories supplying the standards were Linde Spectra Environmental Gases, Alpha, NJ, USA, TO-14A Standard; Global Calibration Gases LLC, Palmetto, FL, USA, TO-15 plus; Custom Gas Solutions, Durham, NC, USA, TO-15 plus.
Table 1. Selected compounds from the US EPA TO-14A and TO-15 methods used for calibration of the SIFT-MS instrument from certified gas standards. See text for details. The nominal concentration is 1.0 ppmv with a quoted accuracy 5%
V. S. Langford, I. Graves and M. J. McEwan Table 2. Compounds used to spike cans in comparative SIFT-MS and GC/MS experiments
Analyte acetone acetonitrile acrylonitrile benzene butanone carbon disulfide chlorobenzene ethylbenzene isooctane (2,2,4trimethylpentane) naphthalene styrene tetrachloroethene tetrahydrofuran toluene 1,1,1-trichloroethane trichloroethene 1,3,5-trimethylbenzene
Pre-spike mix
Supplier Mallinckrodt Baker Inc. Merck Chromspec Distributors Ltd Pure Science Ltd BDH Laboratory Supplies Fisher Scientific Sigma Aldrich Acros Merck
Mix Mix Mix Mix
2 3 2 1
Sigma Aldrich Sigma Aldrich Sigma Aldrich Merck Merck Sigma Aldrich Merck Supelco
Mix Mix Mix Mix Mix Mix Mix Mix
3 2 2 1 2 2 1 3
Mix 1 Mix 1 Mix 2 Mix 1 Mix 1
shown in Table 2 together with the suppliers of the VOC. Three liquid mixtures of these 17 VOCs were prepared from samples of the pure parent compounds based around the volatility of the compounds. Four canisters were spiked with varying amounts of the headspace above the liquids. The spiking process was performed in several steps using a 10-μL gas-tight syringe (SGE Analytical Science Pty Ltd). The cans were pressurised to 5 psig with humidified zero air. The SIFT-MS instrument was then used to provide rapid quantification of the contents to confirm that the levels were within the linear range required by the GC/MS system (target levels were 0.5 to 50 ppbv). A small correction was made to several of the SIFT-MS-derived concentrations from the kinetic data so that the analyte concentrations were within 10% of the concentrations of the calibrated mixtures. A small correction also had to be made when sampling from canisters at 5 psig for the SIFT-MS instrument because normally it analyses samples at atmospheric pressure. Although not in the mixture, seven other compounds for which both instruments were set up to measure were also monitored for their background levels. Experiment 3 in which samples submitted to the analytical laboratory for analysis were subjected to a side-by-side GC/MS and SIFT-MS comparison. Two different groups of samples were selected for analysis: one group was of gas samples from soil that had been contaminated by an oil spill and one sample from a clandestine methamphetamine
Table 3. Results of SIFT-MS calibration versus the certified gas standards – the quantification of SIFT-MS is based on generic kinetic data Compound and reagent ion benzene NO+ O+2 toluene NO+ O+2 ethylbenzene H3O+ NO+ 1,3,5-trimethylbenzene H3O+ NO+ O+2 styrene NO+ O+2 naphthalenea H3O+ NO+ O+2 propene H3O+ O+2 1,3-butadiene NO+ isooctane NO+
Product m/z
Kinetics references
Ratio of measured to certified concentration
LOD/ppbv
LOQ/ppbv
C6H+6 C6H+6
78 78
[17] [17]
0.66 0.89
0.23 0.16
0.52 0.32
C7H+8 C7H+8
92 92
[17] [17]
0.91 0.90
0.10 0.09
0.25 0.20
C8H10.H+ C8H+10
107 106
[17] [17]
0.83 0.88
0.18 0.16
0.37 0.35
C9H12.H+ C9H+12 C9H+12
121 120 120
[17] [17] [17]
1.06 1.06 0.98
0.14 0.26 0.10
0.31 0.52 0.24
C8H+8 C8H+8
104 104
[18] [18]
1.07 0.94
0.21 0.09
0.42 0.21
C10H+9 C10H+8 C10H+8
129 128 128
[19] [20] [20]
0.64 0.58 0.48
0.65 0.67 0.37
1.24 1.30 0.77
C3H+7 C3H+6
43 42
[21] [21]
0.86 1.34
0.27 0.20
0.50 0.38
C4H+6
54
[18]
0.94
0.05
0.16
C8H+17
113
[22]
0.80
0.29
0.58
Product ion
(Continues)
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Rapid monitoring of VOCs Table 3. (Continued) Compound and reagent ion carbon tetrachloride O+2 O+2 1,1,1-trichloroethane O+2 O+2 chloroethene O+2 O+2 trichloroethene O+2 O+2 O+2 tetrachloroethene O+2 O+2 O+2 chlorobenzene NO+ NO+ O+2 O+2 dichlorobenzenesa NO+ NO+ O+2 O+2 methyl bromidea O+2 1,2-dibromoethanea O+2 acetonitrile H3O+ acrylonitrile H3O+ carbon disulfide O+2 acetone H3O+ NO+ butanone NO+ O+2 ethyl acetate H3O+ tetrahydrofuran NO+ O+2 a
Product m/z
Kinetics references
Ratio of measured to certified concentration
LOD/ppbv
LOQ/ppbv
119 117
[23] [23]
0.76 0.76
0.40 0.30
0.92 0.77
CH3C35Cl 37Cl+ CH3C35 Cl+2
99 97
[23] [23]
0.65 0.70
0.62 0.34
1.17 0.65
+ C2H35 3 Cl + C2H37 Cl 3
62 64
[23] [23]
0.65 0.70
0.11 0.37
0.25 0.82
C2H35Cl37Cl+2 + C2H35Cl37 2 Cl C2H35Cl+3
134 132 130
[23] [23] [23]
1.01 1.01 1.08
1.83 0.41 0.33
3.61 0.87 0.73
37 + C35 2 Cl2 Cl2 35 37 + C2 Cl3 Cl + C35 2 Cl4
168 166 164
[23] [23] [23]
1.17 1.17 1.21
1.33 0.56 0.54
2.80 1.22 1.30
+ C6H35 5 Cl 37 + C6H5 Cl + C6H35 5 Cl 37 + C6H5 Cl
112 114 112 114
[24] [24] [24] [24]
0.77 0.75 0.91 0.83
0.23 0.99 0.43 0.45
0.48 2.08 0.84 0.84
37 + C6H35 4 Cl Cl + C6H35 Cl 4 2 35 37 + C6H4 Cl Cl 35 + C6H4 Cl2
148 146 148 146
[18] [18] [18] [18]
0.75 0.69 0.94 0.83
0.73 0.62 0.33 0.22
1.59 1.24 0.83 0.52
94
[24]
0.39
0.20
0.41
107
[18]
0.79
0.05
0.11
CH3CN.H+
42,60,78
[25]
0.46
0.11
0.21
CH2CHCNH+
54,72,90
[19]
0.69
0.10
0.20
76
[26]
1.15
0.16
0.36
C3H7O+ NO+.C3H6O
59,77 88
[27] [18]
0.59 0.86
0.34 0.52
0.60 0.95
NO+.C4H8O C4H8O+
102 72
[27] [27]
0.47 0.48
0.26 0.61
0.52 1.13
89,107
[28]
1.22
0.19
0.36
71 71
[29] [29]
0.65 0.82
0.25 0.28
0.48 0.52
Product ion + C35Cl37 2 Cl C35Cl+3
+ CH79 3 Br + C2H79 4 Br
CS+2
CH3COOC2H5.H+ C4H7O+ C4H7O+
Some contamination was found from the bag substrate and the bag background has been subtracted.
Rapid Commun. Mass Spectrom. 2014, 28, 10–18
of the canister was measured on receipt at the laboratory and the samples were then diluted with zero air to give a positive pressure. The dilution factor was approximately two. SIFT-MS measurements In the current study a transportable Voice200® SIFT-MS instrument (Syft Technologies Ltd, Christchurch, New Zealand) was used.[13,16] In the Voice200® instrument, three
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laboratory that had undergone some remedial clean up procedures. For these analyses, ambient air or soil vapour was sampled using critical orifice restricted sampling trains (that had been internally coated with Silonite® to minimise surface interactions) 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 nominally 180 mL min–1 with sampling completed when 127 Torr of residual vacuum remained within the canister. The pressure
V. S. Langford, I. Graves and M. J. McEwan reagent ions (H3O+, NO+ and O+2 ) are generated in a microwave discharge of moist air at a total pressure about 0.35 Torr. The resulting ions are mass-selected by a quadrupole mass filter upstream of the flow tube and injected into a helium carrier gas at a pressure around 0.6 Torr. These ions are convected along the curved flow tube and are sampled through an aperture at the end of the flow tube. The region immediately behind the aperture lens is pumped by a split-flow turbo pump and the ions then enter the downstream quadrupole mass spectrometer where they are mass-selected and counted. Switching between the reagent ions by the upstream quadrupole occurs typically in 10 ms allowing a seamless analysis of analyte concentrations using all three reagent ions. To avoid analyte loss during the transfer process from the sample container to the instrument, the instrument was equipped with a heated passivated inlet providing direct access to the reaction tube. This inlet was connected to canisters using a Silonite®-coated Micro-QT™ valve (Entech Instruments Inc.) and Tedlar bags via a short length of Teflon tubing. The sample time for each product ion resulting from the analyte reaction with the reagent ions was typically 3.7 s. For the current comparison, the Voice200® SIFT-MS instrument was shipped to the R.J. Hills Laboratory in Hamilton, New Zealand.
’absolute’ in the sense that no concentration calibrations were undertaken prior to this analysis. The analytical results are simply the end product of the insertion of the relevant kinetic parameters for each ion-molecule reaction and the instrument software then takes the ratio of the product ion counts to the reagent ion counts to derive the concentrations.[7] The following comments are relevant to the measurements reported in Table 3. For each analyte, there is the possibility of examining three different reagent ion reactions. However in a matrix of 25 or so analytes, a number of the product ions may have the same mass (isobaric) and hence the isobaric ion products from a reagent ion cannot be included in the analyses. In practice when different reagent ions’ reactions with the matrix yield results for an analyte that differ significantly, only the reagent ion yielding the lower concentration is used because of the potential for mass overlaps. In the current matrix, the reagent ions used to quantify the designated analyte are shown in Table 3. In addition, the reagent ion reactions of some structural isomers may be similar for each isomer and in that case only a total of all the possible isomers that are present
(a) GC/MS measurements GC/MS analysis was carried out by R.J. Hill Laboratories Limited. This laboratory is accredited to the ISO 17025 standard for US EPA Compendium Method TO-15 and analyses were carried out according to this standard. The analysis of the canisters was made as follows. Air from the canister (typically 125 mL) was transferred by an autosampler (model 7410D, Entech Instruments Inc.) and pre-concentrated (model 7150D, Entech Instruments Inc.) prior to GC/MS analysis. The pre-concentrated sample was then thermally desorbed 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 had a 60 m × 0.32 mm i.d. BP-1 analytical column with a film thickness of 1 μm (SGE Analytical Science Pty Ltd). Helium carrier gas was used, with the head pressure set at 11.7 psig yielding an average linear velocity of 36 cm s–1. The gas chromatograph oven was held at 35°C for 4 min, increased to 110°C at 4°C min–1 held for 0.1 min then increased to 220°C at 15°C min–1 and held for 5 min, giving a total run time of 36.2 min. Mass spectra were collected after a delay of 4 min from m/z 29 to 160 until 10 min; the mass range was then changed to m/z 34 to 270 for the remainder of the run.
(b)
RESULTS AND DISCUSSION Experiment 1
14
The concentrations of the 25 analytes listed in Table 1 as measured by the Voice200® instrument are shown in Table 3 expressed as a ratio of the certified concentration. These measured values are based solely on the rate coefficients and product branching ratios of the relevant reagent ion reactions reported in the literature (column 4 of Table 3) and are
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Figure 1. Example calibration data for SIFT-MS of measured concentration against certified concentration for aromatics (a) and halogenated hydrocarbons (b).
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Rapid monitoring of VOCs can be found. In a mixture where there is a large disparity in concentrations between analytes, the limiting factor is whether the reagent ion number density is reduced by reactions with an analyte. In practice linearity is experienced between parts per trillion by volume and mid parts per million by volume. At higher analyte concentrations, a dilution of the sample mixture is required. The results presented in Table 3 demonstrate that of the 50 reagent ion determinations of the analyte concentrations based on the existing database kinetic data, 86% of the results are within 35% of the expected concentrations. Some of the outliers may simply be a result of the sampling method. An example is the results for naphthalene where likely desorption loss onto the Tedlar bag would have resulted in a low result for all three reagent ions. The results for the ketones, acetone and butanone were also surprising as a calibration afterwards of acetone using a calibrated permeation tube (Vici Metronics, Poulsbo, WA, USA) gave excellent correlation within 10% of the stated concentration of 3.85 ppmv from the permeation tube. We also show in the two right-hand columns of Table 3 the limits of detection (LODs) and limits of quantitation (LOQs) for the listed analytes in the configuration of the Voice 200 used for the comparison and these were obtained as outlined
previously.[11] These values were found by monitoring the product ion mass for each analyte in a 6-L Entech canister that had only been used for zero quality air. A demonstration of the response of the SIFT-MS instrument with analyte concentration is shown in Fig. 1. A series of selected volatiles at ppmv concentrations was prepared in Tedlar bags and the samples were then sequentially diluted with zero air to generate a series of measurements at different concentrations between 5 ppbv and 1 ppmv. The linearity of the concentrations in all cases as expressed by R2, the coefficient of determination, over the three orders of magnitude examined was R 2 ≥0.997. We have shown elsewhere that a linear concentration response over five orders of magnitude is typical.[13] Figure 1(a) shows the linearity of the response for the named hydrocarbons and Fig. 1(b) the response for the named chlorinated hydrocarbons. Experiment 2: GC/MS and SIFT-MS side-by-side comparison of spiked canisters The target levels of volatiles were in the low- to mid-ppbv range. The canisters were tested using both SIFT-MS and GC/MS. The results of the comparison study are shown
Table 4. Side-by-side comparison of GC/MS and SIFT-MS of four different mixtures containing 17 VOCs with listed concentrations shown as ppbv
a
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These analytes were not included in the mix but were simply in the methods for analysis of both instruments. They represent signal backgrounds. b C2-alkylbenzenes is the total of ethylbenzene and the three xylene isomers for the SIFT-MS study. In this experiment, only ethylbenzene was added to the VOCs in the mixtures. c C3-alkylbenzenes is the total of all isomers for SIFT-MS. In this test, only 1,3,5-trimethylbenzene was added to the VOCs in the mixtures. d Acetonitrile was not measured by the GC/MS instrument.
V. S. Langford, I. Graves and M. J. McEwan in Table 4. The VOCs in italics represent background levels for compounds that were in the analytical methods for each instrument but not in the mixtures. Overall the agreement between the two quite different techniques is very good from a mixture containing 17 VOCs. Differences in concentrations of greater than 30% were observed for only styrene (higher for SIFT-MS than for GC/MS), and acetone and carbon disulfide (both lower in all mixtures than the concentrations reported by GC/MS). We are not sure why lower concentrations of acetone were found in the SIFT-MS measurements as the association reaction of NO+ with acetone was used to determine the concentration 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 Syft Voice200® instrument.
Experiment 3: Side-by-side comparison of four commercial samples The comparative results for this study are shown in Table 5. The first sample set came from a site where soil had been contaminated by leakage from an underground fuel storage tank. Samples were taken from the crawl space surrounding the tank and from soil where fuel seepage had occurred. The comparative results are shown for the VOCs listed in column 1, column 2 represents background signals for each analyte, column 3 represents analyte concentrations from the air sampled from the crawl space, and column 4 those from soil samples. The second sample included in Table 5 came from air samples from a remediated clandestine methamphetamine (’P’) laboratory and these results are shown in column 5. The following observations can be made. Good agreement between the two techniques was found for non-contaminated samples such as the ambient
Table 5. Comparison of analyte concentrations in two commercial samples analysed at Hill Laboratories. See the text for more details on the samples
a
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C2-alkylbenzenes gives the total of ethylbenzene and the three xylene isomers for SIFT-MS; speciation reported for GC/MS has been summed. b C3-alkylbenzenes gives the total for all isomers for SIFT-MS; speciation reported for GC/MS has been summed. c Not reported for GC/MS due to interference from 2-methylbutane. d Not reported for GC/MS.
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Rapid monitoring of VOCs samples and the samples from the crawl space (columns 2 and 3). In the contaminated samples (column 4, soil gas), good correlation was obtained for the smaller aromatic hydrocarbons (benzene, toluene and C2-alkylbenzenes). However, there was poor agreement for a number of the other compounds in the soil sample. The reason for lower correlation in the soil sample is that the soil was saturated with hydrocarbons from the fuel and these hydrocarbons generate mass overlaps with the SIFT-MS product ions from the analytes included in the method. Under these conditions, SIFT-MS is susceptible to interference when large numbers of multiple components at higher concentrations are present. However, in the comparative analysis of the residuals from the ’P’ lab, good correlation was obtained between the two techniques with the additional benefit from SIFT-MS of all VOCs present being observed. The GC/MS method does not report methanol due to the background in the canisters from the manufacturing process.
CONCLUSIONS
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We thank John Gray for assistance with preparatory and follow-up work for the trial, Daniel Milligan and Barry Prince for helpful discussions (all Syft Technologies Ltd) and Alastair Boyd of Hill Laboratories for sample preparation and helpful discussions.
REFERENCES [1] Current Practice of Gas Chromatography-Mass Spectrometry, (Ed: W. M. A. Neilson). Marcel Dekker, New York, 2001. [2] M. C. McMaster. GC/MS: A Practical Users Guide, (2nd edn.). John Wiley, New Jersey, 2008. [3] NIST/EPA/NIH Mass Spectral Database (NIST11) and NIST Mass Spectral Search Program (version 2.0g). U.S. Dept. of Commerce, Standard Reference Data Program, Gaithersburg, MD, 2011. [4] W. Lindinger, A. Hansel, A. Jordan. Online monitoring of volatile organic compounds at pptv levels by means of proton transfer reaction-mass spectrometry (PTR-MS). Medical applications, food control and environmental research. Int. J. Mass Spectrom. 1998, 173, 191. [5] R. S. Blake, P. S. Monks, A. M. Ellis. Proton transfer reaction mass spectrometry. Chem. Rev. 2009, 109, 861. [6] M. S. Klee, L. M. Blumberg. Theoretical and practical aspects of fast gas chromatography and method translation. J. Chromatogr. Sci. 2002, 40, 234. [7] D. Smith, P. Španěl. Selected ion flow tube mass spectrometry for on-line trace gas analysis. Mass Spectrom. Rev. 2005, 24, 661. [8] C. G. Freeman, M. J. McEwan. Rapid analysis of trace gases in complex mixtures using selected ion flow tube-mass spectrometry. Aust. J. Chem. 2002, 55, 491. [9] P. Španěl, J. R. Cocker, B. Rajan, D. Smith. Validation of the SIFT technique for trace gas analysis of breath using the syringe injection method. Ann. Occupat. Hyg. 1997, 41, 373. [10] J. Kubista, P. Španěl, K. Dryahina, C. Workman, D. Smith. Combined use of gas chromatography and selected ion flow tube mass spectrometry for absolute trace gas quantification. Rapid Commun. Mass Spectrom. 2006, 20, 563. [11] D. B. Milligan, G. J. Francis, B. J. Prince, M. J. McEwan. Demonstration of selected ion flow tube MS in the parts per trillion range. Anal. Chem. 2007, 79, 2537. [12] G. J. Francis, D. B. Milligan, M. J. McEwan. Detection and quantification of chemical warfare agent precursors and surrogates by selected ion flow tube mass spectrometry. Anal. Chem. 2009, 81, 8892. [13] B. J. Prince, D. B. Milligan, M. J. McEwan. Application of selected ion flow tube mass spectrometry to real time atmospheric monitoring. Rapid Commun. Mass Spectrom. 2010, 24, 1763. [14] Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air: Method TO-14A, (2nd edn.), U.S. Environmental Protection Agency, Research Triangle Park, NC, EPA 600/625/R-96/010b, January 1997. [15] Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air: Method TO-15, (2nd edn.), U.S. Environmental Protection Agency, Research Triangle Park, NC, EPA 600/625/R-96-010b, January, 1997. [16] www.syft.com. [17] P. Španěl, D. Smith. Selected ion flow tube studies of the reactions of H3O+, NO+ and O+2 with several aromatic and aliphatic hydrocarbons. Int. J. Mass Spectrom. 1998, 181, 1. [18] Syft Technolgies Ltd Database, 2013.
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The following observations can be made from this comparative study between SIFT-MS and GC/MS undertaken at a laboratory approved to conduct USA EPA TO15[15] compendium methods. The SIFT-MS analysis of certified gas standards in experiment 1 resulted in satisfactory quantitative agreement for most analytes (Table 3) although the instrument had not been pre-calibrated for those compounds – generic gas-phase kinetic data applicable to all SIFT-MS instruments were used. The measured data were then used to make minor adjustment to the kinetic parameters to calibrate the specific SIFT-MS instrument used in the trial. We have found those reagent ionanalyte reactions that are most susceptible to small changes from the published data for ion-molecule reactions (for example, whether the flow tube heater is on or off) are those where association reactions occur (some NO+ reactions) and some 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 confirmed that SIFT-MS offers a viable alternative to GC/MS for monitoring VOCs down to trace levels (pptv) and has the marked benefits of not needing the pre-concentration/ desorption steps that are required by GC/MS for trace level analysis. Further, SIFT-MS is a real-time method and provides results in seconds, it does not require different columns for polar and non-polar analytes, it is much simpler to use and has a linear response over a much wider concentration range than GC/MS. The two techniques are complementary and there are situations where one might be preferred over the other. For example in an analysis of an unknown sample containing a large number of VOCs in the same matrix, GC/MS would be the method of choice as was evidenced in the contaminated soil sample. Nevertheless, in many applications SIFT-MS offers clear advantages over accepted traditional methods for VOC analysis. In particular, SIFT-MS provides unique mass spectrometry based opportunities in real-time ambient air monitoring, because it is robust, easily deployable, easy to use, very sensitive, and no preconcentration and sample preparation is required. We have shown elsewhere[11,12] that it can also analyse down to the pptv level after only 15 min warm up.
Acknowledgements
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[24] P. Španěl, D. Smith. Selected ion flow tube studies of the reactions of H3O+, NO+ and O+2 with several aromatic and aliphatic monosubstituted halocarbons. Int. J. Mass Spectrom. 1999, 189, 213. [25] P. Španěl, D. Smith. Selected ion flow tube studies of the reactions of H3O+, NO+ and O+2 with several amines and some other nitrogen-containing molecules. Int. J. Mass Spectrom. 1998, 176, 203. [26] P. Španěl, D. Smith. Selected ion flow tube studies of the reactions of H3O+, NO+ and O+2 with some organosulphur molecules. Int. J. Mass Spectrom. 1998, 176, 167. [27] D. Smith, T. Wang, P. Španěl. Analysis of ketones by selected ion flow tube mass spectrometry. Rapid Commun. Mass Spectrom. 2003, 17, 2655. [28] P. Španěl, D. Smith. SIFT studies of the reactions of H3O+, NO+ and O+2 with a series of carboxylic acids and esters. Int. J. Mass Spectrom. 1998, 172, 137. [29] T. Wang, P. Španěl, D. Smith. A selected ion flow tube study, SIFT, of the reactions of H3O+, NO+ and O+2 with several Nand O-containing heterocyclic compounds in support of SIFT-MS. Int. J. Mass Spectrom. 2004, 237, 167.
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