matrix isolation infrared


ManTech Environmental Technology, Inc., Research Triangle Park, North Carolina 27709. Nancy K. Wilson and Ruth K. Barbour. U.S. Environmental Protecti...
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AMI. Chem. 1992, 6 4 , 292-300

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Evaluation of Gas Chromatography/Matrix Isolation Infrared Spectrometry for the Determination of Semivolatile Organic Compounds in Air Sample Extracts Jeffrey W.Childera* ManTech Environmental Technology, Inc., Research Triangle Park, North Carolina 27709 Nancy K. Wilson and R u t h K. Barbour

US.Environmental Protection Agency, Atmospheric Research and Exposure Assessment Laboratory, Research Triangle Park, North Carolina 27711

The capabllltles of gas chromatography/matrlx isolation Infrared (GC/MIIR) spectrometry for the determlnatlon of semivolatile organk compounds (SVOCs) In environmental alr sample extracts were evaluated. A serles of systematlc experiments, uslng the xylene isomers as test compounds, was conducted to determine the repeatability of the various steps Involved in G W M I I R measurements and to identify parameters which affect the precision In quantltatlve analyses. The repeatabilty of M I I R net absorbance measurements for both single and replicate deposltlons was determlned. The precldon for MIIR net absorbance measurements on single deposltlons was found to be better than 2%, whereas measurements for dally replkate deposnronS had relative standard deviations of less than 4 % . The effect of experlmentai parameters, such as deposltlon tlp posltion and cryogenlc disk tlme resoiutlon, on the overall preclslon Is Illustrated. The detection lhdts and range of linear response of the GC/MIIR method for 0-, m-, and p-xylene were determlned by analyzlng callbration standards ranglng In concentratbnfrom 0.87 to 86.9 ng/bL. The M I I R net absorbance exhlblted a nonhear response at concentrations higher than 52.1 ng/bL, which was most likely due to an Increase In the sample spot size relatlve to the I R beam focus or a decrease In the matrlx-to-solute ratlo to less than what Is acceptable for matrlx W a t h condnhs. The method detectkn hit for the xylene Lsomers was esthnated to be between 1 and 2 ng/pL Injected on-column for routlne measurements. Extensive slgnai averaging was requlred to obtaln IdentHIable spectra at concentrations less than 1 ng/pL. The GCIMIIR method was tested by determlnlng target SVOCs In amblent air sample extracts. The MIIR quantltatlve results were compared to those obtained on the system’s flame lonlzatlon detector (FID). The FID response exhlblted a hlgh blas when unknown compounds coeluted with a target analyte. The ablilty of GCIMIIR to quanth target compounds In the presence of interferents and to dkriminate between coeiuting lsomers is demonstrated. Overall, the GC/MI I R technlque provlded excellent quantltatlve data. However, because of the complexity of the equipment and the tlme requlred for analysis, the current method Is not amenable to routlne measurements.

INTRODUCTION The combination of gas chromatography (GC) and matrix isolation infrared (MIIR) spectrometry (1,2)has been shown to be a powerful technique for identifying target and previously unidentified compounds in complex environmental samples. For example, GC/MIIR has been used to identify polychlorinated biphenyls in a commercial mixture (3),tetrachlorodibenzo-p-dioxins(TCDDs) and tetrachlorodibenzo-

furans in an environmental sample (41,pesticides in groundwater samples (51, and C2-naphthalene isomers in fossil fuel mixtures (6). We have applied GC/MIIR spectrometry to the identification of polycyclic aromatic hydrocarbons (PAHs)in urban air particulate matter (7)and target PAH compounds (8)and alkylbenzene isomers (9) in woodsmoke-impacted air and to the qualitative analysis of a variety of real-world environmental samples (10). We have also reviewed recent advances in the MIIR spectrometry of organic compounds, including the development of GC/MIIR and several early applications of GC/MIIR to difficult analytical problems (11). Although GC/MIIR has been shown to be very useful in qualitative studies, few applications of the technique to quantitative analyses have been reported. Schneider et al. (12) have evaluated the capabilities of GC/MIIR for the quantitative analysis of priority pollutants, and Holloway et al. (5) have reported preliminary results pertaining to the determination of 1,2,3,4-TCDD in environmental samples. Mossoba and co-workers have applied GC/MIIR to the quantitative analysis of actual samples, including the confirmation and determination of 2,3,7,8TCDD in fish extracts (131,ethyl carbamate in alcoholic beverages and foods (141, and fatty acid methyl esters in hydrogenated soybean oil and margarines (15). Mossoba et al. reported a detection limit below 200 pg for 2,3,7,8-TCDD in fish extracts (13) and demonstrated that good agreement between GC/MIIR and more established techniques, such as GC/masa spectrometry (MS) (14) and GC with electron capture detection (13))can be obtained. In most cases, the relative standard deviation (RSD) for replicate analyses was reported to be less than 5% (15). However, these results were obtained by using a rigorous data acquisition protocol with extensive signal averaging (300-1000 scans) and cannot be considered routine measurements. For example, in one study, only one analyte was measured in each sample, and only one actual sample, along with several calibration and quality assurance standards, was analyzed per day (13). Our goal was to evaluate the quantitative capabilities of GC/MIIR for more routine measurements that would allow several target compounds to be determined in representative air sample extracts from large sample sets. Our initial interest was to apply GC/MIIR to the quantitative analysis of an entire class of compounds of environmental interest, such as the PAHs. However, preliminary results in our laboratory exhibited poor precision (>20% RSD) for replicate analyses of standard solutions containing several target PAHs (10). This poor precision was comparable to that reported by Schneider et al. (12)and indicated that systematic problems might exist in GCIMIIR quantitative measurements performed on similar systems. Therefore, a series of experiments was conducted to determine the repeatability of the various steps involved in

0003-2700/92/0364-0292$03.00/00 1992 Amerlcan Chemical Society

ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992

GC/MIIR measurements and to identify parameters which affect the precision in GC/MIIR quantitative analyses. The repeatability of several steps, including acquiring MIIR spectra,measuring the net MIIR absorbance, and normalizing the spectra of the target compounds to that of an internal standard, was determined for both single and replicate depositions. The effect of experimental parameters, such as deposition tip position and cryogenic disk time resolution, on the overall precision was also investigated. The detection limits and range of linear response of the GC/MIIR method for o-, m-, and p-xylene were determined by analyzing calibration standards ranging in concentration from 0.87 to 86.9 ng/pL. The method was tested by determining target semivolatile organic compounds (SVOCs) in ambient air sample extracts. When possible, the Gc/MIIR results were compared to those obtained by the system's flame ionization detector (FID). In these studies, o-, m-, and p-xylene, common airborne pollutants, were chosen as representative test compounds for the following reasons. They are present in relatively high concentrations in the semivolatile fraction of most air sample extracts. A deuterated internal standard is commercially available and is separated Chromatographicallyfrom the target analyta, which allows both the FID and Mmt data to be used for quantitation. The m- and p-xylene isomers are typically difficult to separate chromatographically and to distinguish by GC/MS but can be distinguished by their unique MIIR spectra The o-xylene isomer is well separated from the other isomers, which allows the system's FID to be used for an independent comparison to the MIIR results. The compounds elute relatively quickly from the chromatograph, which enables several replicate analyses to be performed in one day. EXPERIMENTAL SECTION

Instrumentation. All GC/MIIR data were collected on a Mattson Instruments (Madison, WI) Model 3800 Cryolect system (2). The 3800 Cryolect system used in this study consists of a Mattson Instruments Sirius 100 Fourier transform IR spectrometer and Starlab data system, a Hewlett-Packard (HP) 5890A capillary GC system (Avondale,PA), and a Mattson Instruments matrix-isolation cryogenic module. The GC instrument was equipped with an HP 19245A on-column injector, an FID, an HP 3392A integrator, and an HP 19405A sampler/event control unit. The analytical column was a 30-m X 0.25-mm4.d. DB-5 fused-silica capillary with a 0.25-pm film thickness (J & W Scientific, F o h m , CA). The carrier gas was 1.01% argon in helium (National Welders, Raleigh, NC) and had a flow rate of 1.64 cm3/min. Under these conditions, the matrix-to-solute (M/S) ratio was approximately 370 to 1for a GC peak with a full width at half-height (fwhh) of 5 s containing 17.4 ng of a xylene isomer with a molecular weight of 106. The effluent from the analytical column was split 20% was directed to the FID, and the remaining 80% was directed through an open-split cross (16) and then through a heated, fused-silica transfer l i e to the cryogenic disk. The splitter was made from a stainless steel Valco ZT.5 tee (Valco Instruments Co., Inc., Houston, TX). The arms of the splitter were made from deactivated fused silica: a 30-cm length of 0.150-mm4.d. tubing extended to the FID, and a 45-cm length of 0.250-mm4.d. tubing extended to the open-split cross. The splitter and open-split crow were mounted directly in the GC oven. The transfer line, a 60-cm length of 0.150-mm-i.d. deactivated fused silica, was enclosed in an insulated, heated stainless steel conduit. During a set time interval at the beginning of the GC run, a 60-cm3/min flow of helium was directed across the opensplit interface to divert the solvent. This flow was reduced to 0.1 cm3/min during matrix deposition. The draw through the transfer line was 1.4 cm3/min. The supply gases for the column inlet and the open-split cross were each passed through separate Supelco (Bellefonte,PA) Series 2-3800 heated gas purifiers and Supelco 2-3900 OM-1 oxygen/moisture indicator tubes. The FID heating block and the deposition tip heating block were held at 300 "C,whereas the transfer line conduit was held at 250 "C. The cryogenicdisk was maintained at 14 K throughout

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Table I. Parameters for MIIR Absorbance Measurements analyte

baseline range, cm-'

MIIR peak mas, cm-'

p-xylene-dlo p-xylene m-xylene o-xylene

2286-2265 810-785 785-760 760-720

2278 797 770 744

matrix deposition and MIIR spectral acquisition. The pressure in the cryogenic chamber was 2 X 10"' Pa. The GC temperature program was as follows: initial temperature of 40 "C, held for 0.2 min and then increased at 2 .C/min to 100 "C. The cryogenic disk was rotated at the default setting of 50 pm/s during sample deposition. Under these conditions, a GC peak with a fwhh of 5 s should deposit in a spot on the cryogenic disk with a diameter of 250 pm. The MIIR spectra were acquired after the GC separation was completed. Single-beam sample spectra were ratioed to singlebeam background spectra collected with the cryogenic disk positioned 12 s before or after the GC peak maximum and plotted as absorbance files. Unless otherwise noted, the MIIR spectra were derived from 128 coadded, double-sided, 8192-point interferograms. A triangular apodization function and a zero-filling factor of 2 were applied to the coadded interferograms prior to the fast Fourier transform. This resulted in MIIR spectra with a nominal resolution of 4 cm-'. The iris aperture was set to the minimum value during spectral acquisition and optical alignment. The detector is a HgCdTe (Infrared Associates, Inc., Cranbury, NJ) with a 0.05- X 0.05-cm element and a D* of 6.442 X 1O'O cm Hz112/Wat 10 kHz. The phase-sensitive infrared reconstructed (PSIR) chromatogram was collected with a cryogenic disk time resolution of 0.6 s with eight scans coadded at each time increment. The spectrometer bench and IR beam path were purged with the boil-off from a liquid nitrogen tank. The MIIR net absorbance was measured directly by using the quantitative software package supplied with the Cryolect system. No baseline correction was applied to the spectralfiles prior to measuring the net absorbance. The parameters used to measure the peak height of specific bands in MIIR spectra of the target compounds are given in Table I. Prior to initiating the quantitative studies, the GC/MIIR system was aligned by following the procedure described by Mossoba et al. (13). Preliminary alignment of the system was accomplished by using a beam expander-collimator in conjunction with a HeNe alignment laser. The beam expander-collimator provided a visible beam that filled the parabolic focusing and collecting mirrors on the GC/MIIR system and allowed the optical alignment of the system to be visually observed. After the system was optically aligned, the diameter of the IR beam was estimated to be 367 pm by measuring the peak-to-peak voltage of the interferogram with the IR beam focused on the bare disk and then on the 343-pm alignment hole. The detector output voltage when the beam was focused over the alignment hole was 14% of that measured on the bare disk. The diameter of the IR beam in this system is similar to that (375 pm) measured by Mossoba et al. (13). The width of the matrix was estimated visually to be approximately half the diameter of the alignment hole, or 172 pm. This matrix is significantly narrower than that (300 pm) reported by Bourne and Croasmun (16) and what is generally assumed to be the caee in commercial GC/MIIR instruments. Although the performance of this system has been empirically optimized in its present configuration, it does not meet the requirements for optimum sensitivity in that the sample spot size, the diameter of the focused IR beam, and the size of the detector element are not matched. Description of Sample Extracts. The extracts analyzed in this study were archived samples from a field study conducted to determine the impact of residential wood combustion on air quality. Each sample was colleded over a 12-h period during the heating season from Nov 1986 to Feb 1987 in Boise, ID, with a PMIomedium-flow sampler. The samplers were equipped with a Teflon-coated fiberglass filter followed by a cartridge containing approximately 200 g of XAD-2 sorbent resin and had a flow rate of 0.113 m3/min (4 cfm). The SVOCs collected on the XAD-2 resin were extracted with dichloromethanefollowed with methanol

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I

7.50

A

2

8.00

8.50

9.00

9.50

Retention Time (min) Flgure 1. Expanded portions of (A) an FID Chromatogram and (e) a PSIR chromatogram of a standard mixture containing 17.4 ng/pL of the xylene isomers with 19 ng/& of ~ - x y b n e - dadded , ~ as an internal standard. Peak identification: (1) p-xylene-d,o, 7.96 min; (2) m- and p-xylene, 8.18 min; (3) o-xylene, 9.17 min.

by using a continuous-flow technique. Extracts were paired and combined to provide composite samples representing a given sampling period, either nighttime or daytime, at a particular site. Therefore, each extract represents a total air volume sampled of approximately 160 m3. Sample collection and extraction are presented in more detail elsewhere (17,18).Selected dichloromethane extracts of air samples from an outdoor site, an indoor environment, and an indoor environment with a wood-fueled stove were analyzed in this study. The indoor air samples were diluted by a factor of 2 with respect to the outdoor samples prior to analysis. Calibration standards with concentrationsof 0.87,1.74, 5.21, 10.4,17.4,34.7,52.1,69.4,and 86.9 ng/pL of each xylene isomer were prepared with 19 ng/pL p-xylene-dIoadded as an internal standard to each solution. Neat samples of 0-, m-,and p-xylene and p-xylenedlowere used as received from Aldrich (Milwaukee, WI). Solutions were prepared in pesticide grade dichloromethane (Fisher Scientific, Fair Lawn, NJ).

RESULTS AND DISCUSSION A series of systematic experiments was performed to determine the repeatability of MIIR net absorbance measurements and the precision of daily replicate depositions and analyses. A solution containing 17.4 ng/pL of each xylene isomer, with 19 ng/pL of p-xylene-dlo added as an internal standard, was used as a test sample in most of these experiments. As is shown later, this concentration falls slightly below the midpoint of the linear range of the technique, is representative of concentrations found in ambient air sample extracts, and is therefore considered a fair and good test of the system. Expanded portions of representative FID and PSIR chromatograms of this test mixture are shown in Figure 1. Under the chromatographicconditions described in the Experimental Section, baseline resolution of the internal standard and the m-end p-xylene isomer pair is achieved. In addition, o-xylene is well separated chromatographicallyfrom the other isomers, which allows the FID response to be used to independently confirm the MIIR results. The retention times recorded by

the integrator for the FID response and those measured by the PSIR method were synchronized by using the software provided with the instrument. In general, the FID retention times and the PSIR peak maxima matched fairly well throughout the chromatogram, showing only minor discrepancies. For example, if the retention times of the FID and the PSIR peak maxima for p-xylene-dIo were synchronized, the retention time for o-xylene might be off by f0.6 s in the PSIR chromatogram. Theae discrepancies were not systematic and often varied from run to run. As is shown later in this report, any discrepancies between the FID retention timea and the PSIR peak maxima can be compensated for by using a disk-swey protocol that collects MIIR spectra at discrete time intervals before and after the GC peak maxima. Examination of the apparent peak shape as measured by the PSIR chromatogram is a good check of the overall performance of the GC/MIIR system. For example, poor optical alignment has been shown to cause distorted peak shapes (13). The chromatographic peak shape and effective resolution of the PSIR chromatogram are determined by several factors, including the chromatographic efficiency of the analytical column and the transfer efficiency of the splitter and the open-split cross; the rotational speed of the cryogenic disk during deposition; the number of MIIR files collected per time increment on the disk, which is determined by the disk resolution parameter in the PSIR program; possible spreading of the effluent and matrix as they are deposited onto the surface of the cryogenic disk; and the diameter of the IR beam and the area of the detector element. In these experiments, the chromatographic resolution is degraded slightly and the peak widths are slightly broader in the PSIR chromatogram as compared to the FID trace. For example, the fwhhs for p-xylene-dlo, the m-and p-xylene isomer pair, and o-xylene are 2.5,4.9, and 2.5 s, respectively, as measured from the FID chromatogram, whereas the fwhh is 4.9 s for each compound as measured on the PSIR chromatogram. The fwhh observed in this study is similar to that (5.2 s) found by Mossoba et al. (13)for subnanogram depositions of 2,3,7,8-TCDD. Judging from the fwhhs, peak shapes, and chromatographic separation exhibited in the FID trace, the efficiency of the analytical column and splitter is not the limitingfactor in theae experiments. In addition, Bourne and Croasmun (16)have shown that peak broadening and tailing are minimized with the type of open-split interface used in this system. This is consistent with the PSIR chromatogram shown in Figure lB, in which the peak shapes are very symmetrical and do not exhibit tailing. The disk resolution parameter was set at the highest resolution (0.6 s) allowed by the sptem’s software and most likely did not contribute to the degradation of chromatographic resolution. On this GC/MIIR system, the matrix is wider (15%) than the inner diameter of the tubing used for the deposition tip, which indicates that the effluent spreads slightly during the deposition process. If the effluent spreads evenly in both directions along the length of the deposition, a GC peak with a fwhh of 2.5 s will deposit in a spot with a diameter of only 143 pm. Therefore, spreading should not cause significant broadening in the apparent peak width as measured by the PSIR method. Bourne and Croasmun (16) have shown that the effective peak widths measured by GC/MIIR systems are narrower when higher disk rotational speeds are used during matrix deposition. At higher disk rotational speeds,the chromatographic peak is deposited over a larger area. For the very narrow peaks characteristic of high-resolution capillary columns, this would enable the deposition spot size to more closely match the focus of the IR beam. On this particular instrument, the disk rotational speed is set a t a fixed value of 50 pm/s by the software and cannot be changed, so the effect of the disk rotational speed could

ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992

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0.025

A

I15 mAbs

.

B

8.00

4OW

3500

3000

ZSOO

2000

1500

1000

Wavenumber FIgm 2. Representathre MIIR spectra of (A) p-xylenedlo,15 ng on disk, (B) m- and p-xylene, 14 ng each,and (C) 0-xylene, 14 ng, from a standard mixture. The asterisk denotes the absorption bands used for quantltatbn.

not be determined. In these experiments, the slight peak broadening as measured by the PSIR method can most likely be attributed to the fact that the IR beam focus and detector element are larger than the sample spot size for a narrow (2.5 s) GC peak. Nevertheless, peak broadening is minimal and baseline resolution of the internal standard and the m- and p-xylene isomer pair is still achieved in the PSIR chromatogramThe MIIR spectra of the xylene isomers and the deuterated internal standard are shown in Figure 2. The MIIR spectrum of p-xylene-dlo exhibits characteristic shifts in the C-H stretching frequencies to lower energy upon deuteration (Figure 2A). The C-D stretching frequency at 2278 cm-' was used for quantitation. This band was used because,although it is lower in intensity than other bands in this spectrum, it falls in a region of the spectrum with a high signal-to-noise (SIN) ratio and no interferenta. The stronger out-of-plane C-D deformation is shifted below the cutoff frequency of the detector and therefore could not be used for quantitation. Because these are relative, and not absolute, measurements, the use of different vibrational modes, which have different intrinsic absorbances, to quantify the deuterated internal standard and the native analytes does not pose a problem in this study. The internal standard is used to compensate for run-to-run and day-to-day variations in the response of the GC/MIIR system, which depend on several factors, such as injection volume, chromatographic performance, transfer efficiency of the analyte through the splitter and open-split interface, deposition efficiency onto the cryogenic disk, and optical alignment. Although similar physical characteristics between the internal standard and analytes are highly desirable, equivalent intrinsic absorbances are not required. No detectable spectral overlap between the internal standard and the m- and pxylene isomer pair was observed (compare Figure 2A,B), which is consistent with the baseline separation exhibited in the PSIR chromatogram. The native xylene isomers can be characterized by the out-of-plane C-H deformation vibrations in the 1000-650-cm-' region. The frequencies of these vibrations are determined by the number of adjacent hydrogen atoms remaining on the aromatic ring (19). Thus, each alkylated-benzene positional isomer has a unique MIIR spectrum. As shown in Figure 2, 0 - , m-, and p-xylene exhibit strong bands at 744,770, and 797 cm-', respectively. These distinct bands allow the xylene isomers to be separated and distinguished by MIIR spectrometry, even though, as is the case with m- and p-xylene, they appear to coelute chromatographically.

8.05

8.10

8.15

8.20

8.25

8.30

8.35

8.40

Retention Time (mln)

Figure 3. Profile of m-xylene ( 0 )and p-xylene (A)deposited on the cryogenic disk pbtted as net absorbance versus retention time. The dashed vertical line indicates the position of the FID peak maximum.

The coeluting m-and p-xylene isomers can be completely resolved by acquiring MIIR spectra at 0.6-5 intervals across the chromatographic peak and then measuring the MIIR net absorbance for each isomer at each time increment. As shown in Figure 3, the two isomers do not exactly coelute. In fact, neither isomer actually elutes at the peak maximum, as determined by both the FID and the PSIR chromatograms. In this case, acquiring MIIR spectra at the retention time of the GC peak maximum would result in obtaining spectra at the trailing edge of the m-xylene peak and at the leading edge of the pxylene peak. Therefore, to ascertain the true retention time of each isomer, a spectral survey of the cryogenic disk must be performed at a relatively high time resolution. Although these two isomers are usually separated by 1.2 8, this interval is not constant and can change slightly from run to run. Thus, this disk-survey protocol is required for the determination of coeluting compounds and for accurate and precise quantitative measurements of single compounds, such as o-xylene, where acquiring MIIR spectra off the actual peak maximum by as little as 0.6 s can change the net absorbance by as much as 3%. As mentioned before, if the retention time of one peak in the PSIR chromatogram is synchronized to the retention time measured by the FID, often the other peaks in the chromatogram are not exactly synchronized. This is especially true of long chromatographic runs. If this protocol is not followed, the location of the deposition on the disk might not be determined exactly, which can contribute to errors occurring in replicate depositions. Repeatability of MIIR Net Absorbance Measurements. The repeatability of the MIIR net absorbance measurement and the effect of repeated cryogenic disk movement on the precision of the measurement were determined by analyzing single depositions of the test solution. One deposition was analyzed each day over a 5-day period for this set of experiments. Prior to acquiring MIIR spectra of the deposited teat compounds, the cryogenic disk position was synchronized with the FID retention times by using the PSIR chromatogram. A spectral survey of the deposition was conducted to ascertain the exact retention time of the m-and p-xylene isomers. The depositions were analyzed by four different methods. For this discussion, these methods are defined as static, sequential, exaggerated, and survey. Static Method. This series of experiments was designed to determine the repeatability of acquiring an MIIR spectrum and measuring ita net absorbance with minimal repositioning of the disk. To do this, a background spectrum of 128 madded scans was taken at a disk position corresponding to 12 s before the p-xylene-dlopeak maximum. The disk was then moved to the position of the p-xylene-dlopeak maximum, and 10

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Table 11. Average Daily Percent RSDs of Replicate MIIR Absorbance Measurements for a Single Depositiona method

n

static sequential exaggerated survey

5 5 5 5

p-xylene-dlo 0.48 0.82 1.12 0.89

(0.16) (0.17) (0.31) (0.35)

m-xylene

p-xylene

o-xylene

0.93 (0.33) 1.67 (0.54) 1.58 (0.73) 2.06 (0.53)

1.15 (0.42) 1.87 (0.56) 2.40 (0.61) 2.29 (0.47)

1.08 (0.31) 1.37 (0.29) 1.62 (0.50) 1.58 (0.55)

normalized to p-xylene-dlo m-xylene p-xylene o-xylene 1.84 (0.70) 1.66 (0.51) 1.94 (0.83)

1.78 (0.54) 2.53 (0.50) 2.04 (0.57)

1.57 (0.41) 1.90 (0.42) 1.98 (0.84)

Values in parentheses are standard deviations of daily averages.

Table 111. Percent Relative Standard Deviations of Replicate Injections“ n daily composite

5 35

normalized FID m-, p-xylene o-xylene 1.13 (0.65) 1.25

1.26 (0.35) 1.28

MIIR normalized to FID p-xylene-dlo m-xylene p-xylene 3.44 (1.48) 4.26

4.35 (2.48) 4.89

3.58 (2.73) 4.58

o-xylene 6.29 (2.59) 6.47

normalized MIIR m-xylene p-xylene o-xylene 2.30 (0.88) 2.91

2.37 (0.58) 2.66

3.77 (1.22) 3.85

Values in parentheses are standard deviations of daily averages.

replicate spectra of 128 scans each were taken at that disk position. Each sample spectrum was then ratioed to the original background spectrum, and 10 absorbance files were generated. This procedure was repeated for each xylene isomer. The average daily RSDs for these measurements over a 5-day period are presented in Table I1 under the heading “static”. The MIIR net absorbance for m-, p-, and o-xylene could be measured to within approximately 1% RSD, whereas the average RSD for p-xylene-dlo was significantly less at approximately 0.5%. The enhanced precision observed for replicate measurements of p-xylene-d,,, as compared to the measurement precision for the native xylene isomers, can be explained in part by the lower peak-to-peak noise over the baseline range used for quantitation. For the concentrations used in these experiments, the SIN ratio for p-xylene-dlowas approximately 50 to 1,whereas the SIN ratios for the native xylene isomers were between 30 and 20 to 1. These results indicate that errors associated with the MIIR net absorbance measurement are small and did not contribute significantly to the poor precision chiuacteristic of the technique in previous studies (10). Sequential Method. In these experiments, the precision of the system in acquiring several MIIR sample and background spectra in sequence and in normalizing the analyte spectra to that of the internal standard was determined. In this method, MIIR spectra, along with the appropriate background spectra, were obtained of each test compound in sequence, by starting with p-xylened,,, then repositioning the disk to obtain spectra of the m- and p-xylene isomer pair, and then proceeding to o-xylene. This is the normal method that would be used during routine qualitative analyses. This sequence was repeated 10 times for the same deposition without synchronizingretention times between replicate analyses. The average daily RSDs for the MIIR net absorbance measurements under these conditions are presented in Table I1 under the heading “sequential”. The average daily RSDs for each test compound are slightly higher than those obtained under the static method. This is most likely due to the necessity to acquire a new background spectrum for each sample spectrum and to the small errors involved in repeated disk movement. However, the average daily RSDs are still lower than 2% for each test compound. In addition, normalizing the MIIR net absorbance of m-, p-, and o-xylene to that of the intemal standard did not significantly affect the precision. Exaggerated Method. The contribution of disk movement to the overall precision of net absorbance measurements was further investigated by introducing an exaggerated disk movement to the sequential spectral acquisition process. The MIIR spectra of the test compounds were obtained on the

same deposition in sequence as before, but the last spectral acquisition in each sequence was followed by a disk movement corresponding to a retention time of 45 min. This additional disk movement is representative of the latest GC retention time anticipated in most analyses of air sample extracts. This sequence was repeated 10 times without synchronizing between replicate analyses. The average daily RSDs for the MIIR net absorbance measurements under these conditions are presented in Table I1 under the heading “exaggerated”. The average daily RSDs for each test compound were between 1.5 and 2.5% and did not change significantly from those obtained by the sequential method. This indicates that the disk-tracking mechanism in this particular instrument is functioning properly and that repeated disk movement does not degrade the precision of replicate MIIR net absorbance measurements. This is in contrast to previous studies by Mossoba et al. (13)where a backlash of approximately 50 pm was observed during positioning of the cryogenic disk, which contributed to errors of up to 20% for replicate measurements on a single deposition. Survey Method. In the survey method, MIIR spectra were acquired at specific disk positions before, after, and including the actual chromatographic peak maxima. For p-xylene-dIo and o-xylene, MIIR spectra were acquired at the chromatographic peak maxima and a t 1.2 and 0.6 s before and after the maxima. For m- and p-xylene, MIIR spectra were acquired at the chromatographic peak maximum and at 2.4,1.8, 1.2, and 0.6 s before and after the maximum. This sequence was repeated 10 times on the same deposition without synchronizing between replicate analyses. The average daily RsDs for the MUR net absorbance measurements under these conditions are presented in Table I1 under the heading *survey”. The average daily RSDs for each test compound did not change significantly from those obtained with either the sequential or the exaggerated methods and were between 1.5 and 2.3%. Although the survey method ensures that the peak maximum is measured accurately, no improvement in precision is realized for replicate analyses of a single deposition. However, this method should improve the precision for replicate depositions, where repeatability in determining the actual peak maximum is important. Repeatability of Replicate Depositions. To determine the repeatability of combined spectral acquisition and matrix deposition, seven replicate injections of the 17.4 ng/pL xylene standard solution were made per day over a 5-day period. Each deposition was completely analyzed before the next injection was made. The PSIR chromatogram was used to synchronize the cryogenic disk position with the FID retention times for each deposition. The survey method was used to

ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992

obtain MIIR spectra of each xylene isomer and the internal standard. The initial step of this series of experiments was to determine the precision of the normalized FID peak area for the replicate injections. As shown in Table 111,the average daily RSDs for the FID peak area of each target compound normalized to the FID peak area of the internal standard were approximately 1.25%. The RSDs from composite measurements of the normalized peak areas over the 5-day test period (a total of 35 measurements per analyte) were also approximately 1.25%. This indicates that the chromatographic system, including the injector, analytical column, splitter, and FID, were functioning properly and were stable over a 5-day period. The next step was to normalize the MIIR net absorbance for each test compound to its respective FID peak area. If the FID peak area is assumed to give an accurate representation of the amount of material injected into the system, the MIIR net absorbance of each test compound should be related to its respective FID peak area. Also, because of the excellent precision characteristic of the FID peak area and the MIIR net absorbance measurements, most of the error associated with this normalization can be attributed to the various steps of the matrix deposition process. These steps include the transfer efficiency of the open-split interface, sample delivery through the transfer line, and the actual deposition of the matrix. The average daily RSDs for the MIIR net absorbance normalized to the FID peak area were between 3.44 and 6.29%. The RSDs from composite measurements over the 5-day period were not significantly higher. These results indicate that the errors associated with the matrix deposition process under carefully controlled conditions are relatively small and that the MIIR system was stable over the 5-day test period. When the MIIR net absorbance of the xylene isomers was normalized to the net absorbance of the internal standard, the average daily RSDs were slightly lower (2.30-3.77%) than when the net absorbances were normalized to the FID peak areas. This indicates that any slight variations in sample delivery to the cryogenic disk were compensated for by the use of an internal standard. In these experiments, the replicate analyses for o-xylene exhibited slightly poorer precision than did those for m-and p-xylene. One possible explanation for this is that the m- and p-xylene elute more closely to the internal standard than does o-xylene. Therefore, slight changes in the delivery or deposition of m-and p-xylene more closely mimic those of the internal standard. The precision characteristic of these replicate measurements is significantly better than that obtained previously in our laboratory, which ranged from 10 to 15% for the same test compounds (20). Several factors, such as improved optical alignment, use of higher time resolution on the cryogenic disk, and careful control of the deposition tip position, most likely contributed to this improvement. . The use of the optical alignment method described by Mossoba et al. (13)improved the response of the system by approximately 33% in terms of MIIR net absorbance per nanogram of analyte injected on-column. However, no significant improvement in the precision of the MIIR net absorbance measurement for either single or replicate depositions was observed after optical alignment. Other factors that could have contributed to the improved precision include using a higher time resolution in the PSIR chromatogram to synchronize the cryogenic disk position and FID retention time and using the survey method to obtain MIIR spectra of the deposited test compounds. The use of these procedures ensured a more accurate measurement of the actual peak maximum and most likely improved the precision of replicate depositions. However, the RSDs for daily

105

297

.

100 C 95 -

90

-

-

25

50

-

I

75 0

75

100

125

150

Tip Position (pm)

Flgure 4. Effect of deposition tip position on normalized net absorbance of o-xylene relathre to the FID peak area.

replicate depositions using these procedures were still in the 7-8% range. The one factor that had the most significant effect on the precision of replicate depositions was careful control of the deposition tip position with respect to the surface of the cryogenic disk. The position of the deposition tip is controlled by a micrometer that is graduated in increments of 0.001 in. (25.4 pm). Manual adjustments of the tip position relative to the cryogenic disk can be made by observing the deposition tip through a binocular microscope and a small observation mirror in the cryogenic chamber. During routine analyses, the deposition tip is approximately 125 pm from the cryogenic disk. When this is the case, the reflection of the tip from the cryogenic disk slightly overlaps with the reflection of the outer wall of the tip in the observation mirror. This optimum tip position is consistent with findings of previous studies where the maximum absorbance in both GC/MIIR (1)and direct deposition GC/IR (21) experiments was obtained when the distance between the collection substrate and the deposition tip was less than or equal to the inner diameter of the deposition tip. To illustrate the effect of the tip position on the precision of daily replicate depositions, a series of injections of the 17.4 ng/pL standard solution were made with the deposition tip moved 0.001 in. further away from the disk for each deposition. The MIIR net absorbance of o-xylene normalized to its FID peak area, relative to the normalized response with the tip in the optimum position, was then plotted versus the deposition tip position. As shown in Figure 4, the relative MIIR net absorbance decreases dramatically (more than 13%) when the tip is moved only 0.001 in. away from the optimum position but does not change significantly with additional movements away from the disk. The precision of these measurements is less than that exhibited by other replicate measurements because the tip position was set at the beginning of each run and was not adjusted during the run. In this GC/MIIR system, if the cryogenic disk does not rotate perfectly concentrically, the distance between the cryogenic disk and the deposition tip can change during the run if the tip position is not monitored closely. Adjustments in the disk-rotating assembly can be made to minimize this tracking error, but it is difficult to reduce the error to less than 0.003 in. from one quadrant of the disk to another. In the experiments performed to evaluate the precision of replicate injections and to generate the calibration curves, the tip position was carefully monitored and adjusted as warranted during each deposition. Generation of Calibration Curves. The detection limits and range of linear response of the GC/MIIR method for the xylene isomers were determined by analyzing standard solutions of these compounds ranging in concentration from 0.87

298

ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992

Table IV. FID and MIIR Regression Results detection anal*

method

m-, p-xylene

FID

o-xylene m-xylene

FID MIIR MIIR MIIR

p-xylene o-xylene

slope

0.0593 0.0609 0.0935 0.0984 0.1686

intercept

-0.0108 -0.0102

0.0866 0.0992 0.1830

correlation coeff (9) 0.9999 0.9997 0.9996 0.9998 0.9991

I* -V

16t

0

-

D

20

40

BO

80

Concentratlon (ng/pL)

Figure 5. Calibration curves for o-xylene (.), p-xylene (O), and m-xylene (A). The sdld line represents the linear least-squares fit, and the dashed line depicts an extrapolation of that line.

to 86.9 ng/pL, with 19 ng/pL pxylenedlo added as an internal standard to each solution. The complete set of calibration standards was analyzed each day over a 5-day period. The FID calibration curves were linear throughout the concentration range (plots not shown) and had correlation coefficients greater than 0.999 (see Table IV). This verified that factors associated with the chromatographicsystem, such as injection technique and the splitter union, allowed quantitative results to be obtained over the target concentration range. However, calibration curves for the normalized MIIR net absorbance (relative to the net absorbance of the internal standard) plotted versus concentration were not linear over the entire concentration range (see Figure 5). The nonaalized net absorbance values for concentrations greater than 52.1 ng/pL fell well below the extrapolated linear regression line (dashed line in Figure 5). The net absorbance values for each xylene isomer in standard solutions with a concentration greater than 52.1 ng/pL were less than 0.2 absorbance units. Therefore, the MIIR data for the calibration set would be expected to obey Beer's law and exhibit a linear response. Linear relationships have been reported for concentration ranges over 2 orders of magnitude for conventional MI deposition (22) and over less than l order of magnitude at much lower concentrationson commercial GC/MIIR systems (5,13). Possible explanations for the nonlinear response include an increase in the sample spot size deposited on the disk with increasing concentration and a corresponding decrease in the M/S ratio to a value below what is required for matrix isolation conditions. The fwhh measurements from the PSIR chromatogram are relatively constant throughout the concentration range (4.38-4.86 8). However, the peak base width increased with an increase in concentration. For example, the peak base width for the 10.4 ng/pL standard is 9.6 s, whereas that for the 69.4 and 86.9 ng/pL standards is 16.8 s. This indicates that the deposition spot size is not constant over the concentration range and varies from 480 to 840 pm in total length. Therefore, the relative amount of analyte in a given deposition that is in the focus of the IR beam decreases with increasing concentration, which would result in a lower relative net absorbance and a negative deviation from linearity. A similar,

hh

C

A

ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992 Table

V.

200

Quantitative Analysis of Ambient A i r Sample Extractsa

concentration of analyte in extract, naIuL

FID sample

n

m-,p-xylene

outdoor indoor indoor with wood-fueled stove

3 4 3

33.21 (0.09) 102.85 (1.28) 105.57 (1.66)

MIIR o-xylene

m-,p-xylene

12.02 (0.09) 33.04 (0.68) 37.74 (0.90) 108.85 (0.98) 51.36 (0.44) 100.15 (1.84)

m-xylene

p-xylene

o-xylene

23.59 (0.58) 77.38 (1.02) 73.65 (1.90)

9.45 (0.15) 31.48 (0.42) 26.50 (0.33)

10.87 (0.26) 35.36 (0.70) 43.99 (0.61)

OValues in parentheses are standard deviations of replicate measurementa.

1400

1200

1000

800

Wavenumber

Flguro 7. Representative MIIR spectra of 695 pg of o-xylene reference standard obtained by coaddlng (A) 128 scans, (B) 512 scans, and (C) 2048 scans.

o-xylene can be obtained. The total time required (19min and 22 s) to signal average for 2048 scans was deemed too long for routine measurements in our laboratory. However, for special cases in which identification of a specific compound is desired, the fact that the analytes remain frozen on the disk allows the use of signal averaging to provide improved detection limits. This is one of the most significant advantages that GC/MIIRand other cryogenic deposition techniques have over the more common on-the-fly GC/IR spectrometry with a light pipe. For example, with light pipe GC/IR methods only 8-10 scans could be coadded for a GC peak with a fwhh of 2.5 s. This inability to perform more extensive signal averaging contributes to the relatively poor sensitivity of light pipe GC/IRspectrometryas compared to cryogenic deposition techniques. For example, the minimum identifiable quantities of environmentally important compounds required for analysis by light pipe GC/IR methods typically range from 10 to 50 ng (25). Other advantages and disadvantages of GC/MIIR 1

as compared to light pipe GC/IR spectrometry have been discussed in detail by Schneider et al. (26). Analysis of Ambient Air Sample Extracts. The quantitative GC/MIIR methods established during this evaluative study were tested on actual ambient air sample extracts. A representative FID chromatogram of the outdoor air sample extract is shown in Figure 8. The major componenta of the extract are alkylated aromatics, including toluene, ethylbenzene,xylenes, ethyltoluenes, and trimethylbenzenes. Compounds tentatively identified in the indoor air samples that were not detected in the outdoor sample included (R)-(+)-limonene, p-dichlorobenzene, and several normal aldehydes and alkanes. The concentrations of specific xylenes in each extract were determined by the FID peak area and by the MIIR net absorbance on the GC/MIIR system. Quality assurance reference standards were analyzed daily to verify that the calibration curve had not shifted. A summary of the results is The Gc/MIm 'Ompared given i x ~ with the FID resulta for m- and p-xylene in each extract, with the result usually agreeing to within 5% with no evidence of bias. However, the GC/MIIR results for o-xylene appeared to exhibit a low bias, especially for the two indoor air samples. Examination of the MIIR spectra a t the retention time expected for o-xylene in each of the extracts revealed that an unidentified compound with an absorption band at 1730 cm-' coeluted with o-xylene in the two indoor air samples (see Figure 9). This compound contributes to the FID peak area attributed to 0-xylene, but is not measured as part of the MIIR net absorbance. Therefore, in this case, the presence of the coeluting compound causes the FID response to be artificially high.

''

CONCLUSIONS The G c j M I I R technique provided very precise quantitative results. The repeatability of MIIR net absorbance measurements for replicate analyses was characterizedby average RSDs of less than 4%. While this was not as good as the precision exhibited by the FID peak area, it is comparable to

34

J

Retention Time (min) 8. FID chromatogam of an extract from an outdoor air sample collected on an XAD2 carbidge. Tentathm peak identlficatkns: (1) toluene, 4.82 mln; (2) ethylbenzene, 7.82 min; (3)p-xylenad,,, 7.94 mln; (4) m- and p-xylene, 8.16 mln; (5) o-xylene, 9.17 mln; (6) n-propylbenzene, 12.34 min; (7) 3+thyttoluene, 12.80 min; (8) hthyttoluene, 12.89 mln; (9) 1,3,5-trlmethylbenzene, 13.20 mln; (10) 2-ethykoluene, 13.84 mln; (1 1) 1,2,44rImethylbenzene, 14.68 min; (12) 1,2,3-trimethyIbenzene, 16.46 mln; (13) naphthalene, 27.30 min.

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ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992

IA

/I

benzene, 103-65-1;3-ethyltoluene,620-14-4;4-ethyltoluene,62296-8; 1,3,54rimethylbenzene,108-67-8;2-ethyltoluene,611-14-3; 1,2,3-trimethylbenzene,526-73-8; naphthalene, 91-20-3.

REFERENCES B

4000

1 " " I " " l " ' 3500 3000

2500

2000

1500

1000

Wavenumber Figure 9. MIIR spectra of components eluting at the retention time expected for o-xylene in extracts from (A) the outdoor air sample, (B) the Indoor air sample, and (C) the indoor air sample from a residence with a wood-fueled stove. that of routine GC/MS measurements performed in our laboratory and is more than acceptable for environmental analyses. The MDL of the GC/MIIR technique for routine measurements was determined to be between 1and 2 ng/cLL injected on-column for the xylene isomers. In general, although the sensitivity will vary for different compound classes, the MDL of G€/MTLR is similar to that achieved with GC/MS in the full-scan mode, which allows the two techniques to be used in a complementary way. Although the MDL is adequate for determining the m a t concentrated SVOCs, lower detection limits are required to detect other SVOCs of interest, such as target PAH compounds, which are often 1 or 2 orders of magnitude less concentrated in air sample extracts. Extensive signal averaging was required to obtain identifiable spectra of subnanogram quantities of the test compounds deposited on the cryogenic disk. Although careful optical alignment improved the sensitivity of the GC/MIIR system, any further gain in sensitivity would necessitate redesigning the focusing optics and matching the deposition spot size, the IR beam focus, and the detector element. Detection limits below 50 pg have been reported for a direct deposition GC/IR interface which uses an optical arrangement based on an IR microscope (27). However, it should be noted that rigorous quantitation has not been demonstrated to date with the direct deposition interface. Nevertheless, a similar design should be pursued for use in GC/MIIR systems to improve the MDL for environmental compounds. In general, we have found that, although the GC/h4ITR method is capable of providing excellent quantitative data, the technique using the current configuration is not amenable to routine determinations of target compounds in environmental samples. The complexity and cost of the equipment, the expertise required to operate the instrument and interpret the data, and the time required for analysis currently prohibit the use of the technique for routine measurements. As such, the technique is best used as a research tool to aid in resolving difficult analytical problems. Registry No. Toluene, 108-88-3;ethylbenzene, 100-41-4;p xylene, 106-42-3;m-xylene, 108-38-3;o-xylene, 95-47-6; propyl-

Reedy, G. T.; Ettinger, D. G.; Schneider, J. F.; Bourne, S. Anal. Chem. 1985, 57,1802-1809. Bourne, S.; Reedy, G.; Coffey, P.; Mattson, D. Am. Lab. 1984, 16, 90-101. .. . Schneider, J. F.; Reedy, G. T.; Ettinger, D. G. J. Chromatogr. sei. 1985. 23. 49-53. Brasch, J. W. Prmeedlngs of the 1987 EPAIAPCA Symposlum on Measurement of Toxic and Related Pollutants; Air Pollution Control Association: Pittsburgh, 1987; pp 42-58. Holioway, T. T.; Fairless, B. J.; Freidline, C. E.; Kimbaii, H. E.; Kloepfer, R. D.; Wurrey, C. J.; Jonooby, L. A,; Palmer, H. 0. Appl. Spectrosc. 1988, 42. 359-369. Schneider, J. F.; Raphaellan, L. A.; Boparai, A. S.; Hansen, M. C.; Erickson, M. D. J. Chromatogr. Scl. 1989. 27,592-595. Childers, J. W.; Wilson, N. K.; Barbour, R. K. Appl. Spectrosc. 1989, 43, 1344-1349. Childers, J. W.; Wilson, N. K.; Barbour, R. K. Proceedings ofthe 1988 €PA IAPCA Symposium on Measurement of Toxic and Related Alr Pollutants ; Air Pollutlon Control Assoclation: Pittsburgh, 1988 pp 15-20. Childers, J. W.; Wilson, N. K.; Barbour, R. K. Proceedings of the l989 €PA I A 8 WMA International Symposium : Measurement of Toxic and Related A t Pollutants; Air and Waste Management Association: Pktsburgh, 1989; pp 885-870. Childers, J. W.; Wilson, N. K.; Barbour, R. K. Appllcatkn of Oes ChromatographylMaMw-IsolatlonInfrared Spectromeby to the Analysis of Envlronmental Air Samples ; EPA180013-9010997; US. Environmental Protection Agency: Research Triangle Park, NC, 1990. Wilson, N. K.; Childers, J. W. Appl. Spectrosc. Rev. 1989, 25, 1-81. Schneider, J. F.; Schneider. K. R.; Spiro, S. E.; Bbrma, D. R.; Sytsma, L. F. Appl. S ~ ~ C ~ ~ 1981, O S C45, . 568-571. Mossoba, M. M.; Niemann, R. A.; Chen, J. T. Anal. Chem. 1989, 61, 1878- 1685. Mossoba, M. M.; Chen, J. T.; Brumiey, W. C.; Page, S. W. Anal. Chem. 1988. 60,945-948. ' IMossoba, M. M.; McDonald, R. E.; Chen, J. T.; Armstrong, D. J.; Page, S. W. J. Agric. FOodChem. 1990, 38,88-92. Bourne, S.;Croasmun, W. R. Anal. Chem. 1988, 60,2172-2174. Highsmith, V. R.; Rodes. C. E.; Zweidinger, R. B.; Lewtas, J.; Wisbhh, A.; Hardy, R. J. P r m d l n g s of the 1988 EPAIAPCA Symposlum on Measuement of Toxic and Related Alr Pollutants; Air Pollution Control Association: Pktsburgh, 1988; pp 804-813. Merill, R. G., Jr.; Zweidinger, R. B.; Dorsey. J. A.; Mae, R. F.; Koinis, T. X. Prmeedlngs of the 1986 EPA IAPCA Symposlum on Measurement of Toxic and Related A t Pol/utants; Air Pollution Control AssocC ation: Pittsburgh, 1988; pp 821-827. Bellamy, L. J. The Inffared Spectra of Complex Molecules. 3rd ed.; Chapman and Hall: New York, 1975; Voi. 1, pp 87-89. Wllson, N. K.; Barbour, R. K.; Childers, J. W. 1990 Pittsburgh Conference and Exposltion on Analytical Chemistry and Applied Spectroscopy, New York, 1990; Paper 447. Griffiths, P. R.; Henry, D. E. Prog. Anal. Spectrosc. 1986, 9 , 455-482. Stout, P. J.; MamantOV, G. Appl. S P ~ C ~ O S1987, C . 41, 1048-1052. Fuoco, R.; Shafer, K. H.; Grifflths, P. R. Anal. Chem. 1986, 58, 3249-3254. Hembree, D. M.; Garrison, A. A.; Crocombe, R. A,; Yokby, R. A.; Wehry, E. L.; Mamantov, G. Anal. Chem. 1981, 53, 1783-1788. Gurka, D. F.; Pyle, S. M. €nvlron. Sci. Technol. 1988, 22,983-987. Schneider, J. F.; Demirglan, J. C.; Stlckler, J. C. J. Chromatogr. Sci. 1088, 24,330-335. Bourne. S.; Haefner, A. M.; Norton, K. L.; Grlfflths, P. R. Anal. Chem. 1990, 62,2448-2452.

RECEIVED for review July 18,1991. Accepted Odober 28,1991. Although the research described in this article has been funded by the United States Environmental Protection Agency through Contract 68-DO-0106 to ManTech Environmental Technology, Inc., it has not been subjected to Agency review and therefore does not necessarily reflect the view of the Agency, and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.