Anal. Chem. 1995, 67,4044-4052
Vacuum Distillation Coupled with Gas Chromatography/Mass Spectrometry for the Analysis of Environmental Samples Michael H. Hiatt
National Exposure Research Laboratory, Characterization Research Division, U.S. Environmental Protection Agency, P.O. Box 93478,Las Vegas, Nevada 89193-3478
A procedure is presented that uses a vacuum distillation/ gas chromatography/mass spectrometry system for analysis of problematic matrices of volatile organic compounds. The procedure compensates for matrix effects and provides both analylical results and confidence intervals from a single sample analysis. Surrogate compounds are used to measure matrix effects relating to boiling point and relative volatility and to provide the information necessary to accurately determine anconcentration. Relative volatility values (a)are experimentally determined for 1 14 organic compounds and are shown to be comparable to gas-water partition coefficients. These compounds include those with boiling points up to 245 "C and gaswater partition coefficients less than 15 000. Multiple samples are tested, and the accuracy of determinations is shown to be within 5%for water, soil, and oil matrices. Method detection limits are below 1 ppb for most anaiytes studied. One of the major objectives of the analytical chemistry research program at the EPA's Characterization Research Division (National Exposure Research Laboratory) in Las Vegas is to broaden the array of pollutants that can be determined with conventional analytical instrumentation. The U.S. Environmental Protection Agency @PA) has developed a vacuum distillation method for determining the concentration of volatile organic compounds (VOCs) in environmental samples' and identifled the relationships controlling analyte recovery and the potential of surrogate-based matrix corrections.2 The purpose of the present study was to incorporate a surrogate-based matrix correction in a general vacuum distillation/gas chromatography/mass spectrometry 0 1 GC/MS) method to be used for routine environmental analyses. At the same time, the list of applicable analytes has been better defined and documented. Suitable compounds would reflect the effects of a matrix on analyte recovery as functions of boiling point @effects) and relative volatility (a-effects). Through the analyses of multiple samples, the ability of the specified surrogates to predict matrix effects is demonstrated. The surrogate prediction routine is simple and provides accurate determination of analyte concentrations in aqueous and mixed-phase samples. The quality assurance needed to document a VD/GC/MS analysis is performed by simply reviewing the surrogate performance in the sample. This (1) Hiatt, M. H.; Youngman, D. R.; Donnelly, J. R Anal. Chem. 1994, 66,905908.
(2) Hiatt, M. H.; Farr C. M. Anal. Chem. 1995, 67, 426-433.
4044 Analytical Chemistry, Vol. 67, No. 22, November 15, 1995
replaces the need for costly matrix spikes or standard addition analyses. A vacuum distillation procedure for determining the relative volatility of analytes as a constant (arvalues) is presented. The arcvalues for 114 compounds are experimentally determined and shown to be comparable to their gas-liquid partition coefficients (K). The arvalues determined in this study are an improvement over previously reported2 a-values (normalized), as the analyst can use published K values to identify potential analytes and estimate their behavior. The boiling point and K value of an analyte govern its performance and determine whether it is suitable for VD/GC/MS analyses. Suitable analytes include compounds with boiling points up to 245 "C and partition coefficients up to 15 OOO, note that this range includes compounds not normally considered as VOCs (e.g., nitrosamines, aniline, and pyridine). Through multiple analyses of various matrices, the accuracy that can be expected for determining an analyte is reported. Method detection limits for VOCs are in the low ppb range for water, soil, and oil matrices. EXPERIMENTAL SECTION Vacuum Distillation Apparatus. The vacuum distiller has been previously described.'t2 In the current study, a Nupro toggle valve (0.172-in. orifice) was used as the sample chamber valve. A vacuum gauge was installed between the cryoloop and the vacuum pump to monitor the integrity of the apparatus under vacuum. The vacuum was considered acceptable at a pressure of 0.5 Torr or less. The condenser column was normally held at 5 "C during vacuum distillations and at 40 "C between distillations. Water was used to replace isopropyl alcohol as the temperature-controlling fluid in the condenser. G U M S Apparatus. A Hewlett-Packard mass spectrometer (Model 5972) and gas chromatograph (HP5890 Series I1 with Model MJSC metal jet separator) with a 60.m x 0.53-mm-i.d., 3.0pm filmthickness VOCOL capillary column (Supelco, Bellefonte, PA) was used for the determination of analytes from the vacuum distillation apparatus. Gas chromatograph operating conditions were 3 min at 10 "C, 50 "C/min ramp to 40 "C; 5 "C/min ramp to 120 "C; 20 "C/min ramp to 220 "C; and isothermal at 220 "C for 3.4 min, resulting in a total run time of 28 min. The jet separator was held at 210 "C and the transfer line at 280 "C. The mass spectrometer was operated at 3.1-s scans of 38-270 amu. The injector was interfaced to the vacuum distillation apparatus by connecting the carrier inlet gas line to the cryoloop valve and back This article not subject to U.S. Copyright. Published 1995 Am. Chem. SOC.
-50
0
50
100
150
200
254
Boiling Poira (x)
Figure 1.
Cryoloop trapping efficiencies.
to the injector. The injector inlet temperature was 240 "C, and the inlet pressure was 10 psi. Sample Preparation. Aqueous samples were prepared directly in the 1WmL round bottom flask used in the vacuum distillation of the sample. Moditied samples were prepared by weighing the amount of matrix modifer (i.e., glycerin, salt), adding water, and then adding the analytes of interest. The 2% soap samples were prepared by adding 0.1 mL of soap concentrate (Micro Concentrated Cleaning Solution, International Products Corp., Burlington, NJ) to the water sample. Samples were spiked with a 10-pL methanol solution containing analytes at the concentrations listed in Table 1. Two spike techniques were used to study the matrices not soluble in water. The first technique was to simply spike into a slurry of the sample and 5 mL of water, this is referred to as a waterspike. The second technique, an attempt to maximize matrix contact, is referred to as a vacuum spike. The introduction of analytes to a soil sample contained in a vacuum has been reported to be a more difficult spike to recover and is potentially a more accurate spiking technique compared with the water spike.3 The vacuum spike in this study entailed several steps, the first of which involved weighing the soil in the sample flask and injecting the spike with a syringe onto the material. The flask was then attached to the vacuum distillation apparatus, and the soil plus spike was cryogenically cooled by immersing the flask in a liquid nitrogen bath (-196 "C). When the flask and sample were thoroughly cooled, the apparatus vacuum pump was used to lower the flask pressure to 0.5 Torr. After the air had been removed (3-5 min) and the sample chamber valve closed to isolate the sample, the flask was warmed to 30 "C. After a 1-h equilibration period, the mixture was again cooled cryogenically and removed from the apparatus, 5 mL of water was added, and the sample was reconnected to the apparatus. Three different soils of varying water and organic contents were used in this study. Soils 1and 2 were garden soils that were (3) McDaniel, J. A The Effect of WaterAdded to Soils on the Analysis of Volatile Organic Compounds; Book of Abstracts; Pittsburgh Conference: New Orleans, LA, 1992; Abstract 711. (4) U.S. Environmental Protection Agency. Handbook of RCR4 Ground-Water Monitoring Constituents: Chemical & Physical Properties; Office of Solid Waste: Washington, DC, 1992. (5) Li, J.; Dallas, A J.; Eikens, D. I.; Carr, P. W.; Bergmann, D. L.; Hait, M. J.; Eckert, C. A Anal. Chem. 1993,65, 3212-3218. (6)Vitenberg, A G. J. Chromatogr. 1991,556, 1-24. (7) Snyder, J. R; Dawson, S. A J. Geophys. Res. 1985,90, 3797-3805.
37 and 15%water and 21 and 16%organic matter, respectively. Soil 3 was a desert soil containing 3% water and 1%organic matter. Cod liver oil (Squibb Cod Liver Oil Mint Flavored, E. R Squibb & Sons, Inc., Princeton, NJ) was used as the oil matrix. This material was selected as the oil matrix due to its low content of the analytes to be determined. This matrix was assumed to mimic both waste oils and the lipid content of biological samples in its effects on the analyte recovery. Vacuum Distillation Procedure. Prior to a vacuum distillation, the condenser column was cooled to 5 "C. The sample contained in the 1WmL flask (normally at room temperature) was evacuated for 10 min, the water vapors were collected on the condenser column, and the distillate to be analyzed was collected in the cryoloop immersed in liquid nitrogen (-196 "C). The sample chamber valve was closed at the completion of the vacuum distillation and the cryoloop valve switched to allow the GC carrier gas to sweep the cryoloop. The cryoloop's liquid nitrogen bath was removed and replaced with a hot water bath (70-90 "C) to volatilize the distillate. The transfer of the distillate to the GC is completed after 2.5 min. After the sample was vacuum distilled, the condenser column was heated to 40 "C while being evacuated with the vacuum pump for 10 min; this removed the condensed water and potential contaminants. All measurements of analytes were performed by the G U M S analysis of the transfered vapor. RESULTS AND DISCUSSION The a-and #?effectswere demonstrated using GC/MS analyses of a methanol solution containing the vacuum distillatese2In this study, the vacuum distillates were analyzed as vapor transfer between the vacuum distiller and the GUMS. Interfacing the GC/MS and vacuum distiller simplifies analysis but adds another potential source of analyte loss that was evaluated. The next phase of this study was to determine the a-values for the analytes and to select surrogates for measuring the a-and ,%effects. The final phase of this study was the analyses of a variety of matrices to test the GC/MS/VD method. The transfer of vapors from the cryoloop to the GC was investigated for its effect on analyte recoveries. Cryoloop trapping efficiencies were investigated by comparing injections of analytes directly into the GC/MS with the injection of analytes into the vacuum stream just before the cryoloop. Figure 1 shows the cryoloop trapping efficiencies for injections of analytes at a low pressure (0.5 Torr) and at a greater pressure (created by allowing air to be drawn into the cryoloop simultaneous with the injection on the cryoloop). The injection with air simulates the pressure within the cryoloop at the initiation of a vacuum distillation (sample flask is at atmospheric pressure), and the low-pressure injection simulates the cryoloop internal pressure later in a vacuum distillation cycle when the air has been evacuated. The plot of analyte recoveries versus boiling point (Figure 1) shows that trapping efficiencies vary closely in relation to an analyte's boiling point. The most volatile analytes are trapped least effectively for both injection pressures. The analyte efficiencies improve with increasing boiling point up to 220 "C. Analytes with boiling points above 220 "C also demonstrate lower efficiencies and most likely reflect a less efficient transfer from the cryoloop to the GC. The trapping efficiency drops when air is bled into the cryoloop. This indicates that efficiency drops with increasing pressure (mass transfer), and, therefore, during a vacuum distillation, the efficiency of the cryotrap will increase as air is evacuated Analytical Chemistty, Vol. 67, No. 22, November 15, 1995
4045
Table 1. Relative Volatility Values (an)
arvalue compound
SUrP
bpb,4(“C)
Volatile Gases -30 24 -13 12 -24 4
dichlorodifluoromethane trichlorofluoromethane vinyl chloride chloroethane chloromethane bromomethane
a9
ded
80 80 80 80 80 80
0.07 0.20 0.48 1.01 1.37 1.82
0.02 0.02 0.06 0.02 0.07 0.12
40 40 25 40 40 100 40 40 9 100 40 40 40 9 40 40 9 40 40 26 40 25 40 40 40 25 25 40 40 40 40 25 25 40 24 40 21 40 40 40 40 40 40 40 25 40 40 40 26 40 20 40 40 100 40 100
0.63 0.64 0.86 0.88 1.31 1.34 1.37 1.43 1.51 2.29 2.3 2.34 2.75 3.5 3.55 3.6 3.83 3.88 3.91 3.92 4.12 4.28 2.43 5.34 5.54 6.14 6.27 6.39 6.87 6.07 7.89 7.93 8.05 10.1 11.1 10.9 11 11.6 12.3 14.1 15.4 18.7 19.2 19.6 20 23.4 23.9 24.9 26 26.2 26.6 26.7 30.3 33.3 33.6 33.8
0.07 0.02 0.06 0.03 0.04 0.45 0.18 0.03 0.04 0.43 0.46 0.09 0.05 0.21 0.27 0.12 0.07 0.12 0.11 0.27 0.08 0.09 0.04 0.07 0.09 0.2 0.17 0.09 0.36 0.24 0.73 0.59 0.7 1.6 1.9 0.2 0.1 0.6 0.6 0.7 0.4 0.9 1.4 1.4
1.88 1.91 2.08 2.5 2.72 3.03 3.75 4.04 4.5
0.08 0.04 0.06 0.07 0.05 0.06 0.18 0.17 0.4
concnC(ppb)
Kd
Volatiles 1,l-dichloroethene carbon tetrachloride hexafluorobenzene 1,l-dichloropropene l,l,l-trichloroethane allyl chloride 2,2-dichloropropane tetrachloroethene pentafluorobenzene iodomethane
a
a
trans-1,2-dichloroethene trichloroethene isopropylbenzene fluorobenzene benzene ethylbenzene 1,4difluorobenzene toluene m,p-xylenes benzene-d6 1,l-dichloroethane toluene-& n-propylbenzene cis-1,2-dichloroethene o-xylene o-xylene-& chlorobenzene-& chloroform styrene chlorobenzene bromobenzene bromobenzeneds 4-bromo-1-fluorobenzene methylene chloride methylene chloride& 1,2-dichloropropane 1,2-dichloropropane-ds l , l ,1,2-tetrachloroethane bromodichloromethane trans-l,3-dichloropropene bromochloromethane 1,Z-dichloroethane dibromochloromethane cis-1,3-dichloropropene 1,2-dichloroethane-d4 bromoform dibromomethane 1,3-dichloropropane l,Z-dibromoethane& l,l,Z-trichloroethane 1,1,2-tnchloroethane-d3 1,Z-dibromoethane 1,1,2,2-tetrachloroethane cis-1,4-dichloro-2-butene 1,2,3-trichloropropane truns-1.4-dichloro-2-butene n-butylbenzene sec-butylbenzene hexachlorobutadiene p-isopropyltoluene telt-butylbenzene decafluorobiphenyl 1,3,5-trimethylbenzene 2-chlorotoluene 1,2,4-trimethylbenzene
a a C
P a a+P
P C
C C
37 76 82 104 74 45 69 121 85 42 48 87 152 85 80 136 88 111 138 79 57 111 159 60 144 143 131 62 145 132 156 155 152 40 40 96 95 130 90 112 68 84 120
a
a C
B
104 84 150 97 120 131 114 112 132 146 152 157 156 Neutral Semivolatiles 183 173 215 183 169 206 165 159 169
4046 Analytical Chemistry, Vol. 67, No. 22, November 15, 1995
40 40 40 40 40 25 40 40 40
1.415 1.55j
2.205
4.365 3.28j 3.935 4.4 2.49j 5.1l5 5.1 5.855
9.335
20.23j 20
1.65j 2.25j 3.5Z5
2.4 1.7 1.9 1.7 2.4 0.7 2 2.8 8.1 2.9 7.4
Table 1 (Continued) awalue
sue
compound khlorotoluene 1,3-dichlorobenzene 1,4-dichlorobenzene 1,2,4-trichlorobenzene 1,2-dichlorobenzene 1,2,4trichlorobenzened~ 1,2-dichlorobenzene-d4 1,2,3-trichlorobenzene pentachloroethane naphthalene naphthalene-& 1,2-dibromo-3chloropropane
P P
C
35
diethyl ether ethyl methacrylate methyl methacrylate methacrylonitrile acrolein 4-methyl-2-pentanone 2-hexanone ethyl acetate-2-I3C acrylonitrile acetophenoneds isobutanol tetrahydrofuran acetonitrile acetone acetone& 2-butanone propionitrile 1,Cdioxane-de 1,4-dioxane
a C
a a
N-nitrosodimethylamine N-nitrosomethylethylamine N-nitrosodi-n-propylamine N-nitro sodiethylamine
aniline o-toluidine
P
1-methylnaphthalenedl0 2-methylnaphthalene 2-picoline pyridine pyridine45 N-nitrosodibutylamine
a
bpb34("C)
concnC(ppb)
Neutral Semivolatiles 162 40 173 40 174 40 214 40 180 40 213 25 181 24 218 40 100 162 218 40 217 25 196 40 Soluble Volatiles 80 117 100 101 100 90 100 53 200 117 100 128 100 77 250 78 100 202 100 108 100 66 n/a 82 100 56 100 57 490 80 100 97 100 101 240 101 100 Basic Semivolatiles 154 500 165 500 206 500 177 500 184 500 200 500 Marginal 241 100 245 500 100 129 116 100 115 100 240 500
Kd
a? 4.78 5.72 6.14 7.73 7.86 7.88 8.03 11.3 13.2 16.7 18 38.9
1806 1506
12007 5806 6006
3806 5800 57506
15000
34.9 48.4 71.4 102.9 116.8 119.9 131.1 150 161 161 175 456 545 600 600 770 1420 5800 6200
de$ 0.43 0.73 0.84 1.22 1.19 1.19 1.23 1.6 3.3 2.2 3.7 4.9 5.7 2.8 4.1 2.4 1 8.4 2.1 32 20 156 67 103 32 110 320 700
129 1900 2400 4900 13700 15200
37.3 800 2000 2200 2300 2100
67 67 6800 13100 15000 21000
17 5200 600 5000
a Surrogate type: a,a-surrogate , P-surrogate; and c, check surrogate. * Boiling point of analyte. Concentration of analyte solutions used to determine a-values. d Partition coe cient of analyte between headspace and water at 20 "C. e Three to four replicates. ,Standard deviation, lo.
4
from the sample flask. The loss of analyte occuring at the cryoloop is not easily distinguished from losses due to analyte condensation, and therefore these cryoloop losses are included as a component of ,%effects. The boiling point-condensation relationship previously described2 is minimized in this study a s both the samples and standard solutions are vacuum distilled (previous study did not vacuum distill standard solutions) using the same vacuum distillation conditions. Because analyte condensation on the condenser, cryoloop trapping efficiencies, and the cryoloopto-GC transfer are essentially the same for samples and standards, there is a normalization of the &effects. The ,&surrogates (surrogates to measure boiling point effects) are now used to rectify any variation of boiling point effects between the analyses of standard solutions
and the analyses of samples (including the cryoloopto-GC transfer). The effects produced by varying the condenser column operating temperature were evaluated by comparing analyte responses obtained from direct GC injection of analytes with those from the vacuum distillation of analytes using different condenser temperatures. Figure 2 presents the relationship of analyte recovery to condenser temperature using the surrogates naphthalene& and ethyl a ~ e t a t e Z - ~a s~representative C analytes. The recoveries of both analytes are maximized when the condenser column is between -6 and 10 "C. As these compounds have greatly different boiling points, the ,&effects (at the condenser) appear to be at a minimum when the condenser column temperature is between -6 and 10 "C. The minimizing of the B-effect Analytical Chemistry, Vol. 67, No. 22, November 15, 7995
4047
loo
,
7
I
70
-20
I
1
I
I
I
-10
0
10
20
I
30
I 40
Condenrer Temperamre cc)
Figure 2. Analyte recovery versus condenser column temperature.
-20
IO
0
10
20
30
Cdumn Temperalure 6 )
Figure 3. Water collected on condenser column versus condenser column temperature.
enhances the response of the higher boiling analytes and simplifies the correction of such effects. The amount of water being collected on the condenser column was also a consideration. Preparation of the condenser column between distillations is made simpler if the amount of condensed water is minimized. Water collected as a function of condenser column temperature (Figure 3) shows that the amount of water collected decreases .as the condenser column temperature is increased. A condenser operating temperature between -6 and 10 "C minimizes analyte condensation, and one between 5 and 10 "C minimizes water collected. The operating temperature of 5 "C was used in this study to allow for some temperature fluctuations with minimal impact on analyte recoveries. Determination of Relative Volatility Values. The recovery of an analyte in the absence of observable p-effects depends on its relative volatility. Using the same operating conditions to perform each distillation within a set (a series of lbmin vacuum distillations required to completely evaporate a 5mL sample) makes the influence of p-effects (within experimental variation) consistent. With the response of analytes comparable between distillations in a set, the rate of removing analytes by distillation (a-effects) can be measured. Therefore, the recovery of an analyte corresponding to its relative volatility is calculated to be its response in the initial vacuum distillation divided by the sum of responses for the set. 4040 Analytical Chemisfty, Vol. 67, No. 22, November 15, 1995
A cylindrical flask (15" i.d., &m length) replaced the standard 10GmL round bottom flask for this part of the study. The cylindrical flask produced a constant sample surface (2.7 cm2) in the flask and a more reproducible distillation rate of water (
