Anal. Chem. 1998, 70, 3028-3032
Analysis of Volatile Organic Compounds in Water and Soil Samples by Purge-and-Membrane Mass Spectrometry Risto Kostiainen,*,†,‡ Tapio Kotiaho,‡ Ismo Mattila,‡ Timo Mansikka,‡ Marja Ojala,‡ and Raimo A. Ketola‡
Department of Pharmacy, Division of Pharmaceutical Chemistry, P.O. Box 56, FIN-00014 University of Helsinki, Helsinki, Finland and VTT Chemical Technology, P.O. Box 1401, FIN-02044 VTT, Finland
A new method, purge-and-membrane mass spectrometry (PAM MS), is introduced for the analysis of volatile organic compounds (VOCs) in water and soil samples. In this method, VOCs are purged from water or soil samples with an inert gas and the stream is directed through a sheet membrane module. The VOCs pervaporate through the membrane directly into the ion source of a mass spectrometer. The limits of detection for nonpolar VOCs such as halogenated hydrocarbons, benzene, toluene, and xylenes were below micrograms per liter in water samples and at low micrograms per kilogram levels in soil samples. The correlation coefficients measured for the compounds studied were typically better than 0.9999 and 0.9975 in water and soil samples, respectively. The relative standard deviations were between 0.5 and 2.0% with water samples and between 4.8 and 14.0% with soil samples. These results demonstrate excellent linearity and repeatability. PAM MS thus provides a highly sensitive, selective, accurate, solvent-free, and rapid analytical method. Tens of samples can be analyzed reliably within 1 h. Contamination of the environment with volatile organic compounds (VOCs) has become an important issue during the past decades, since many VOCs are toxic and may cause serious health risks. The main emission sources of VOCs are industry, traffic, and energy production. Serious local contamination problems often follow from accidents, leakage of petrol and diesel fuel from underground storage tanks, and improper waste treatment. VOCs in soil easily diffuse from the point of emission over wide areas finding their way into groundwater1 and, through construction or sewerage, into households.2,3 The contamination of groundwater is becoming one of the most serious environmental problems. Several water catchments have already had to be closed because of high concentrations of VOCs or other organic contaminants. Intensified research and reconditioning of contaminated areas are needed, as well as continuous monitoring of the quality of drinking water. †
University of Helsinki. VTT Chemical Technology. (1) Mendoza, C. A.; Frind, E. O. Water Resour. Res. 1990, 26, 379. (2) Little, J. C.; Daisey, J. M.; Nazaroff, W. W. Environ. Sci. Technol. 1992, 26, 2058. (3) Kostiainen, R. Atmos. Environ. 1995, 29, 693. ‡
3028 Analytical Chemistry, Vol. 70, No. 14, July 15, 1998
Several methods have been applied to the analysis of VOCs in water4-6 and soil samples.7 The conventional method is the collection of water or soil samples into a headspace vial followed by transfer to a laboratory and analysis by static or dynamic headspace gas chromatography (GC) or GC/mass spectrometry (GC/MS).4,5,8,9 These methods are relatively simple since extensive sample preparation is not required. There are also some disadvantages. The sensitivity of static headspace is limited and the method is thus restricted to samples with relatively high concentrations of VOCs. Dynamic headspace, i.e., purge and trap (P&T), provides high sensitivity but more complicated instrumentation is required. Furthermore, the quantitative analysis of highly volatile organic compounds may be unreliable when Tenax is used as an adsorbent in P&T, because the breakthrough volumes of highly volatile compounds are very large with Tenax.10 While the breakthrough can be decreased by using carbon-based adsorbents together with Tenax, the use of these adsorbents may lead to other problems, such as freezing of the cold trap. In addition, significant memory effects may occur with P&T after the analysis of samples containing high concentrations of VOCs. Dynamic or static headspace GC or GC/MS and other fixed laboratory methods require transportation of the sample to laboratory. This is time-consuming and expensive, limiting the number of samples that can be analyzed. Large numbers of samples are nevertheless needed to obtain a reliable pollution profile of a contaminated area; for the amount of VOCs in a contaminated area may differ greatly with the sampling point. For these reasons, rapid, sensitive, and reliable on-site methods are being sought for the analysis of VOCs in a different variety of environmental samples. Numerous on-site analytical methods have indeed been reported,11-13 but most of them lack sensitivity or selectivity. (4) Bellar, T.; Lichtenberg, J. J. Am. Water Works Assoc. 1974, 66, 736. (5) Drozd J.; Novak, J. Chromatogr. 1979, 165, 141. (6) Zhang, Z.; Pawliszyn, J. Anal. Chem. 1993, 65, 1843. (7) Sojak, L.; Kuran, P. J. Chromatogr. A 1996, 733, 119. (8) Voice, T. C.; Kolb, B. Environ. Sci. Technol. 1993, 27, 709. (9) Hewitt, A. D.; Mlyares, P. H.; Legget, D. C.; Jenkins, T. F. Environ. Sci. Technol. 1992, 26, 1932. (10) Brown, R. H.; Purnell, C. J. J. Chromatogr. 1979, 178, 79. (11) Roelant, D.; Purdy, C.; Albers, B. Field Screening Methods for Hazardous Wastes and Toxic Chemicals; Air and Waste Management Association: Pittsburgh, PA, 1993; Vol. 1. (12) Henry, C. Anal. Chem. 1997, 69, 195. S0003-2700(97)00920-7 CCC: $15.00
© 1998 American Chemical Society Published on Web 05/30/1998
Among on-site and on-line methods, membrane inlet mass spectrometry (MIMS) is one of the most suitable and powerful techniques for the analysis of VOCs.14-18 MIMS has been applied successfully to air19-27 and water samples14-18 as well as to the on-line monitoring of industrial wastewaters.21,28 The procedure involves flowing the sample over a membrane, which selectively extracts VOCs from the matrix. The organics then pervaporate through the membrane into the ion source of a mass spectrometer. Sensitivity is below the 1 ppb level, linearity is good, and linear dynamic ranges are of about four decades. MIMS may not be suitable for direct collection of analytes from samples containing solid materials such as soil and sludge, though interference can be greatly reduced by collecting the analyte from headspace to MIMS analysis.29-31 The static headspace-membrane method nevertheless requires equilibration times of several minutes in quantitative work. More rapid analysis can be obtained by coupling dynamic headspace and membrane collection. The technique, called purge and membrane (PAM), was first introduced for the analysis of halogenated VOCs in water samples using electron capture detection (ECD).32 In view of the limited selectivity of PAM ECD, we wish to introduce a new and more selective technique, purge-andmembrane mass spectrometry (PAM MS), for the analysis of individual VOCs in water and soil samples. In a preliminary communication,33 we showed PAM MS to be a highly promising technique. In this paper we will describe it in detail. (13) Kotiaho, T. J. Mass Spectrom. 1996, 31, 1. (14) Lauritsen, F. R.; Kotiaho, T. Rev. Anal. Chem. 1996, 15, 237. (15) Kotiaho, T.; Lauritsen, F. R.; Choudhury, T. K.; Tsao, G. T.; Cooks, R. G. Anal. Chem. 1991, 63, 875. (16) Cooks, R. G.; Kotiaho, T. In Pollution Prevention in Industrial Processes; ACS Symposium series 508; Breen, J. J., Dellarco, M. J., Eds.; American Chemical Society: Washington, DC, 1992; pp 126-154. (17) Degn, H. J. Microbiol. Methods 1992, 15, 185. (18) Wong, P. S. H.; Cooks, R. G.; Cisper, M. E.; Hemberger, P. H. Environ. Sci. Technol. 1995, 29, 215. (19) Collins, G. G.; Utley, D. Chem. Ind. 1972, 15, 84. (20) LaPack, M. A.; Tou, J. C.; Enke, C. G. Anal. Chem. 1990, 62, 1265. (21) LaPack, M. A.; Tou, J. C.; Enke, C. G. Anal. Chem. 1991, 63, 1631. (22) Cisper, M. E.; Gill, C. G.; Townsend, L. E.; Hemberger, P. H. Anal. Chem. 1995, 67, 1413. (23) Ketola, R. A.; Ojala, M.; Sorsa, H.; Kotiaho, T.; Kostiainen, R. Anal. Chim. Acta 1997, 349, 359. (24) Ketola, R. A.; Mansikka, T.; Ojala, M.; Kotiaho, T.; Kostiainen, R. Anal. Chem. 1997, 69, 4536. (25) Cisper, M. E.; Garrett, A. W.; Cameron, D.; Hemberger P. H. Anal. Chem. 1996, 68, 2097. (26) Gordon, S. M.; Callahan, P. J.; Kenny, D. V.; Pleil, J. D. Rapid Commun. Mass Spectrom. 1996, 10, 1038. (27) Lloyd, D.; Thomas, K.; Price, D.; O’Neil, B.; Oliver, K.; Williams, T. N. J. Microbiol. Methods 1996, 25, 145. (28) Ketola, R. A.; Honkanen, T. Mansikka, T.; Kostiainen, R. Kotiaho, T. Komppa, V.; Wickstro¨m, K. Vahervuori, H. Development of Membrane Inlet Mass Spectrometric Method for On-line Analysis of Industrial Wastewaters. 44th Conference on Mass Spectrometry and Allied Topics, Portland OR, 1996; presentation. (29) Yang, M. J.; Harms, S.; Luo, Y. Z.; Pawliszyn, J. Anal. Chem. 1994, 66, 1339. (30) Wenhu, D.; Kuangnan, C.; Jianli, L.; Zhenying, D. Mass Spectrosc. (Tokyo) 1987, 35, 122. (31) Stetter, J. R.; Cao, Z. Anal. Chem. 1990, 62, 182. (32) Kostiainen, R.; Kotiaho, T.; Ketola, R. A.; Virkki, V. Chromatographia 1995, 41, 34. (33) Mansikka, T.; Ketola, R.; Ojala, M.; Kotiaho, T.; Kostiainen R. Analysis of Volatile Organic Compounds from Solid Samples by MIMS 44th ASMS Conference on Mass Spectrometry and Allied Topics, Portland, OR, 1996.
Figure 1. Experimental setup for purge-and-membrane mass spectrometry of water samples.
Figure 2. Experimental setup for purge-and-membrane mass spectrometry of soil samples.
EXPERIMENTAL SECTION The experimental setup for water samples is presented in Figure 1 and that for soil samples in Figure 2. The vial used for the water samples was modified from a commercially available 20-mL purge vial (Tekmar). A water sample (5 mL) was purged at room temperature with nitrogen (99.9%) at 60 mL min-1. The nitrogen stream containing the purged analytes was directed through a sheet membrane inlet as described earlier.23 The purge vessel used for soil samples was a standard 20-mL headspace vial. Glass beads with diameter of 4 mm were put on the bottom of the vial. Soil sample (5 g) was weighed on top of the beads, and the vial was sealed with septum. The vial was heated to 80 °C in a water bath. The soil sample was purged with nitrogen (99.9%) at 100 mL min-1. The nitrogen flow was directed to the bottom of the vial with a stainless steel needle (Figure 2), and the purged analytes were directed through the membrane module. The material of the sheet membrane was dimethylpolysiloxane (SSP-M100, Specialty Silicone Products Inc., Ballston Spa NY) with a thickness of 25 µm and contact area of 28 mm2. Temperature of the inlet was maintained at 70 °C. The molecules diffusing through the membrane were directed straight to the ion source of the mass spectrometer, which was a Balzers QMG 421C quadrupole instrument with a mass range of 1-500 amu. The instrument was equipped with an open cross-beam electron impact (70 eV) ion source (Balzers Aktiengesellschaft, Balzers, Liechtenstein). Detection limits (signal-to-noise ratio 3:1) and linear dynamic ranges of the test compounds were recorded with selected ion monitoring mode (SIM). The stock solutions of test compounds and internal standard (fluorotoluene) were made by weighing 1 g of compound and dissolving it in 100 mL of methanol (Mallinckrodt Specialty Chemicals Co., Paris, KE, nanograde purity). Further dilutions of the stock solutions were made with methanol. The standard and internal standard solutions (50 µL each) were spiked into a pure soil sample. The sample was carefully mixed, and the vial Analytical Chemistry, Vol. 70, No. 14, July 15, 1998
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was sealed with a septum. Water standards were prepared by diluting the stock solutions with deionized water (Milli-Q) and adding 100 µL of internal standard to 50 mL of water standard. The test compounds were trichloroethene [79-01-6] 99.5%, benzene [71-43-2] 99.5%, tetrachloroethene [127-18-4] 99%, o-xylene [95-47-6] 99.8%, toluene [108-88-3] 99.5%, ethyl acetate [14178-6] 99%, diisopropyl ether [108-20-3] 98.5%, phenol [10895-2] 99.5%, and butyl acetate [123-86-4] 99%, from Merck (Darmstadt, Germany); 1,1,1-trichloroethane [71-55-6] 99%, 1,4dichlorobenzene [106-46-7] 98%, and methyl-tert-butyl ether (MTBE) [1634-04-4] 99.5% from Fluka Chemie AG (Buchs, Switzerland); 1,2-dichloroethene (mixture of isomers) [156-592], [156-60-5] 98% from Aldrich Chemie (Steinheim, Germany); dichloromethane [75-09-2] 99.5% from Mallinckrodt Baker, Inc. (Paris, Kentucky); m-fluorotoluene [352-70-5] from Sigma Chemical Co. (St. Louis, MO); and ethanol [64-17-5] 99.5% from Primalco Oy (Rajama¨ki, Finland). RESULTS AND DISCUSSION The configuration of the experimental setup for water (Figure 1) and soil samples (Figure 2) was designed to be simple and easy to use. A switching valve was installed between the nitrogen supply and purge vessel to avoid overpressurizing the gas lines and obtaining too strong a flow rate at the beginning of the purge step. The operating parameters in the analysis of water samples were chosen on the basis of an earlier study.34 The flow rate of 60 mL min-1 was found to be suitable. The sample was not heated, to keep the method as simple as possible. It is clear, however, that purge times for more polar compounds can be decreased by heating the sample to 40-50 °C. The purge vessel used in the analysis of soil samples was a disposable 20-mL headspace vial. The same vial can also be used for sampling. The glass beads were put on the bottom of the vial, and the purge flow was directed through the beads as a means of spreading the flow of nitrogen through the whole sample. It is not clear, however, that nitrogen even then interacts with all parts of the sample because soil tends to be nonhomogeneous. Heating of the sample was therefore carried out to assist desorption of VOCs from soil particles. The beads and headspace vials are both disposable and memory effects caused by the vial are eliminated. The vial volume of 20 mL was found to be suitable for a 5-g soil sample. The dead volume, about 15 mL, inside the vial was rapidly flushed with nitrogen at a flow rate of 100 mL min-1. The sheet membrane inlet that was used and the most important instrumental parameters, i.e., temperature, thickness, flow rate, and area of membrane, had been studied in detail with air samples in our earlier work.23,24 The membrane was polymethylsilicone membrane with a thickness of 25 µm, which allows response times of a few seconds and thus very rapid analysis. As suggested in the earlier study,23 the membrane was heated to 70 °C in order to decrease response times. Sensitivity is also dependent on the membrane material: polymethylsilicone membrane gives good sensitivity for nonpolar compounds and less for polar compounds.35 (34) Kostiainen, R. Chromatographia 1994, 38, 709. (35) Lauritsen, F. R.; Choudhury, T. K.; Dejarme, L. E.; R. G. Cooks, R. G. Anal. Chim. Acta 1992, 266, 1.
3030 Analytical Chemistry, Vol. 70, No. 14, July 15, 1998
Figure 3. Analysis of toluene (1.0 µg/L, m/z 91), 1,4-dichlorobenzene (20 µg/L, m/z 149), and ethyl acetate (300 µg/L, m/z 70) in a water sample by PAM MS.
Water Samples. The purge-and-membrane procedure for the analysis of water samples can be divided into a purge step and a membrane extraction step. The sensitivity of PAM MS is dependent on the recovery (R) of VOCs from water in the purge step (eqs 1 and 2)36 and on the diffusion of the purged compound through the silicone membrane (eq 3; Fick’s first law).
R ) (noi,w - ni,w(t))/noi,w ) 1 - exp
(
)
-Ft Vg + nwRT/Hi,w
(1)
where noi,w and ni,w(t) are the numbers of moles of component i in the sample before and after purging, F is the gas flow rate, t is the purge time, Vg is the volume gaseous phase, nw is the moles of water sampled, R is the gas constant (R ) 8314.4 mL kPa mol-1 K-1), T is the absolute temperature (K), and Hi,w ) Henry’s coefficient (kPa).
Hi,w ) pi/xi,w ) pi0/xi,w(satd) ) pi0yi,w
(2)
where pi is the vapor pressure, xi,w is the mole fraction i in aqueous sample, pi0 is the vapor pressure of pure i, xi,w(satd) is the mole fractional solubility of i in water, and yi,w is the activity coefficient.
Iss ) ADS(Ps/L)
(3)
where Iss is the steady-state flow through the membrane, A is the membrane surface area, D is the diffusion constant, S is the solubility constant, Ps is the vapor pressure of the analyte on the sample side of the membrane, and L is the thickness of the membrane. Figure 3 shows the analysis of toluene, 1,4-diclorobenzene, and ethyl acetate by PAM MS. The signals rapidly increase and then decrease more slowly in good agreement with eq 1, which indicates that the amount of sample molecules in the gas phase is highest at the beginning of the purge step. After achieving a maximum, the signal begins to weaken, reflecting the decrease in concentration of the analyte in the water sample, and so the (36) Curves, J.; Noy, T.; Cramers, C.; Rijks, J. J. Chromatogr. 1984, 289, 171.
Table 1. Limits of Detection (LODs), Relative Standard Deviations (RSDs) with and without Internal Standard, Correlation Coefficients, and Signal Rise Times Measured with Spiked Water Samples by PAM MS RSD (%) compound
(µg/L)
+ int stdb
- int std
corr coeff
rise timec (s)
toluene o-xylene 1,4-dichlorobenzene dichloromethane trichloroethene MTBE diisopropyl ether ethyl acetate butyl acetate phenol ethanol
0.1 0.2 2 0.3 0.3 10 40 60 4 700 1000
1.2 0.5 1.5 2.3 0.6 1.1 0.4 3.3 2.3 nm nm
5.5 0.9 0.5 2.1 8.2 2.3 5.3 4.8 6.8 nm nm
0.999 94 0.999 90 0.999 91 0.999 60 0.999 93 0.999 74 0.999 77 0.993 98 0.999 34 nm nm
10 12 19 8 11 23 17 16 20 200 30
LODa
a LODs are defined as signal-to-noise ratio 3:1. b Fluorotoluene was used as the internal standard. c Rise time is defined as a signal rise time from 10 to 90%. nm, not measured.
decrease in concentration in the gas phase, as the purging continues. The time needed for complete purging of a compound from a water sample varies. Equation 1 shows that nonpolar and volatile compounds with large Henry’s coefficients are purged faster than polar compounds or those with high boiling points with small coefficients. This is nicely demonstrated in Figure 3, which shows significantly longer purge times for more polar ethyl acetate and less volatile 1,4-dichlorobenzene than for nonpolar and volatile toluene. More detailed data are presented in Table 1. In real analysis, complete purging is not necessary. The sample can be removed when the signal has achieved a maximum, which even with polar compounds takes less than a few minutes. In this case, the analysis time strongly depends on the diffusion of the analyte through the membrane. The time needed for the signal to increase from 10 to 90% of the maximum signal, i.e., rise time, is determined by eq 4, where l is the membrane thickness
t10-90% ) 0.237 l2/D
(4)
and D is the diffusion constant The rise times for the compounds studied were typically between 10 and 20 s and shorter for nonpolar than polar compounds (Table 1). The rise times are longer than those for the air samples presented in our previous study,23 partly due to the dead volume of the purge vial, which leads to slightly increased rise times. This is not significant in real analysis, however. The sensitivity of PAM MS varies with the compound. The limits of detection (LODs) for the nonpolar compounds ranged from 0.1 to 0.3 µg/L but increased significantly with increasing polarity and decreasing volatility of the compound (Table 1). This is in good agreement with eq 1, since the Henry’s coefficients are large for nonpolar and volatile compounds. Furthermore, with silicone membrane the permeability (DS; see eq 3) of polar compounds is lower than that of nonpolar compounds, explaining the partly reduced sensitivity with polar compounds.37 The sensitivity of PAM-MS in the analysis of nonpolar compounds is
Figure 4. Analysis of toluene (m/z 92) and xylenes (m/z 106) in an authentic soil sample by PAM MS with fluorotoluene (20 mg/kg, m/z 109) used as internal standard.
at the level obtained with conventional MIMS38 and better than required in the EPA guidelines for drinking water.39 The reproducibility of PAM MS was tested with six standard samples where fluorotoluene served as internal standard. The relative standard deviations (RSDs) were between 0.4 and 3.3% (Table 1), indicating extremely good reproducibility. The RSDs measured without internal standard were somewhat worse than those obtained with internal standard. The linearity of the method was tested with six samples (typically within one decade) using the internal standard. The correlation coefficients were typically better than 0.999, indicating excellent linearity of the method. The results show that the method can be used in applications where high accuracy of measurements is required. Soil Samples. Figure 4 depicts the analysis of VOCs in an authentic soil sample by PAM MS where fluorotoluene is used as an internal standard spiked to the sample. The signals of internal standard (m/z 109) and analytes (toluene m/z 92 and xylenes m/z 106) strengthened rapidly, achieving a maximum within 13 s. The signal of the internal standard decreased more rapidly than that of the analytes, indicating that, in the authentic sample, the VOCs were more tightly bound than the spiked internal standard. The time needed for the complete desorption of VOCs from a soil sample depends on the gas/sample matrix partition coefficient, which in turn is affected by the polarity and volatility of the compounds and the quality of the soil matrix. The adsorption of VOCs in soil is a sum of adsorption (i) on the mineral grains, (ii) on the surface of the water layer on mineral grains, (iii) on the organic material, and (iv) on the micropores of particles.40,41 Furthermore, VOCs are partly dissolved in water film on the particles. All these factors affect the process of desorption from soil. In principle, if the concentration of the analyte is the same in two different soil samples, the respective peak areas recorded by PAM MS should be the same and only the times needed for (37) LaPack, M. A.; Tou, J. C.; McGuffin, V. L.; Enke, C. G. J. Membr. Sci. 1994, 86, 263. (38) Ketola, R. A.; Virkki, V. T.; Ojala, M.; Komppa, V.; Kotiaho, T. Talanta 1997, 44, 373. (39) Pontius, F. W. J.-Am. Water Works Assoc. 1992, 84, 36. (40) Farrell, J.; Reinhard, M. Environ. Sci. Technol. 1994, 28, 53. (41) Farrell, J.; Reinhard, M. Environ. Sci. Technol. 1994, 28, 63.
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Table 2. Limits of Detection (LODs), Relative Standard Deviations (RSDs) with and without Internal Standard, Correlation Coefficients, and Signal Rise Times Measured with Spiked Soil Samples by PAM MS RSD (%) compound
(µg/kg)
+ int stdb
- int std
corr coeff
rise timec (s)
benzene toluene o-xylene 1,2-dichloroethene 1,1,1-trichloroethane trichloroethene tetrachloroethene MTBE
4 1 1 7 20 8 5 15
7.9 5.0 10.4 4.8 14.0 9.0 12.4 13.3
49.7 57.8 37.8 35.4 21.5 41.4 38.2 19.0
0.997 87 0.998 75 0.998 82 0.997 54 1.000 00 0.998 91 0.999 89 0.999 96
5 6 6 6 7 8 8 9
LODa
a LODs are defined as signal-to-noise ratio 3:1. b Fluorotoluene was used as the internal standard. c Rise time is defined as a signal rise time from 10 to 90%.
nonpolar compounds with the silicone membrane is higher than that of polar compounds.37 For these reasons, better LODs are expected for nonpolar compounds than for polar compounds. Still, the LODs measured with spiked samples were 3-4 orders of magnitude below the guidelines for VOCs in soil. The limit values for most common VOCs are between 10 and 50 mg/kg.43 The reproducibility of the PAM MS method for soil was determined with six spiked samples where fluorotoluene provided the internal standard. The relative standard deviations were better than 14% (Table 2), indicating a good reproducibility. The relative standard deviation without the internal standard was 19-58%, showing that the use of internal standard is necessary. The reproducibility is clearly worse than for water samples, due no doubt to the more complicated desorption process. The linearity of the method was tested with seven samples in which the concentrations varied between 0.1 and 100 mg/kg. Correlation coefficients better than 0.9975 indicated good linearity of the method.
complete purging of the analyte should differ. It follows that, in principle, the quantitative results are independent of the matrix. In practice, as shown in Figure 4, the signals of the analytes decrease slowly after the main desorption of the analyte, indicating that the residual amounts of analytes in soil are very tightly bound and long purge times are required for the complete desorption. The maximum signal is obtained within a few minutes and the quantitation can also be done on the basis of peak height, but in this case the quantitative result is dependent on the matrix. The LODs were between 1 and 20 µg/kg (Table 2), indicating high sensitivity of the method. Since the measurements were made with spiked samples and VOCs are more tightly bound to the matrix in authentic samples than in spiked samples (see Figure 4), the LODs represent the most favorable case, however. The LODs are dependent on the adsorption factors of VOCs in soil (see above) and on the polarity of the VOCs. It has been shown that polar compounds are more tighly adsorbed on the soil matrix than are nonpolar compounds.42 Moreover, the permeability of
CONCLUSIONS We have shown PAM MS to be a very fast, sensitive, accurate, and simple method for the analysis of VOCs in water and soil samples. The method can easily be adapted for on-site analysis, where a rapid screening of large numbers of samples is needed. The extremely good repeatability in the analysis of VOCs in water samples also makes the method very suitable for tasks where high accuracy is required. In addition, the method can be applied for the monitoring of desorption processes from various kinds of materials. The disadvantage of the method is limited selectivity, since no chromatographic separation is used. However, the Solver program developed at VTT for deconvolution of the identity and quantity of the individual compounds from a mixture mass spectrum has proved to be a powerful tool for solving this problem.23,24,39 We expect that use of the PAM MS method together with the Solver program will allow very rapid and reliable analysis of complex mixtures.
(42) Goss, K.-U. Environ Sci. Technol. 1992, 26, 2287. (43) Moen, J. E. T.; Cornet, J. P.; Evers, C. W. A. Soil Protection and Remedial Actions: Criteria for Decision Making and Standardization of Requirements. In Proc. 1st TNO Conf. Contaminated Soil, Utrecht 11.-15.11, 1985, 1986.
Received for review August 21, 1997. Accepted April 20, 1998.
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AC9709209
