Environ. Sci. Technol. 1900, 22, 963-967
Qualitative and Quantitative Environmental Analysis by Capillary Column Gas Chromatography/Lightpipe Fourier Transform Infrared Spectrometryt Donald F. Gurka" and Steven M. Pyle Quality Assurance and Methods Development Division, Environmental Monitoring Systems Laboratory, U.S. Environmental Protection Agency, Las Vegas, Nevada 89 193-3478
rn A new state-of-art commercial gas chromatography/ Fourier transform infrared (GC/FT-IR) lightpipe-containing system has been evaluated for its applicability to qualitative and quantitative environmental analysis of typical environmental contaminants. This system exhibited minimum identifiable quantities, for many compounds, in the 10-50-ng range. On a wide-bore capillary column, quantitation curves generated from chromatogram peak areas were linear over the 10-250-ng range. The mean correlation coefficient for 38 quantitation calibration curves on 24 standards was 0.976. The selectivity of the new system was evaluated with standards, soil, and stillbottom samples. It was demonstrated with 27 standards that no discernible loss in identification selectivity occurred when a narrow-band infrared detector (spectral cutoff 750 cm-l) was used in place of a midband detector (cutoff 700 cm-l). This allows the meaningful utilization of the extra sensitivity associated with narrower frequency range infrared detectors.
Introduction The complexity of many environmental samples may require the use of more than one analytical technique to ensure adequate sample characterization (1). Currently gas chromatography/mass spectrometry (GC/MS) is the method of choice for environmental analysis, and gas chromatography/Fourier transform infrared spectrometry (GC/FT-IR) is the most suitable, alternative spectral technique. However, the past use of GC/FT-IR for trace analysis has suffered from sensitivity in the mid-ppb to low-ppm range ( 2 ) . In addition, the complexity of environmental samples required the use of capillary columns to mitigate the problems associated with frequent chromatographic coelution ( 3 , 4 ) . Since GC/FT-IR sensitivity for typical environmental contaminants was in the hundreds of nanogram range, capillary column quantitation by this technique was precluded by the possibility of column overloading. To obviate this problem, the minimum identifiable quantities (MIQs) of GC/FT-IR needed to be lowered below 100 ng for weak infrared absorbers. The distinction between weak and strong infrared absorbers is important because GC/FT-IR instrumental response factors have been shown to exceed an order of magnitude in range (5). Recent improvements in lightpipe construction (6),the use of small focal area detectors (7), and the minimization of the GC/FT-IR sensitivity loss associated with heating the lightpipe (8) promised to provide the requsite sensitivity for quantitative capillary GC/FT-IR environmental analysis. A prototype GC/ FT-IR interface has been described which exhibits MIQ's in the 20-120-ng range for typical environmental contaminants (9).However, no commercial lightpipe GC/FT-IR system with comparable sensitivity has been reported. 'This work was presented in part at the XXV Colloquium Spectroscopicum Internationale, Toronto, CA, June 23, 1987, Abstract C3.6.
Experimental Section GC/FT-IR Instrumentation. The FT-IR spectrometer used in this study was a Hewlett-Packard (HP) infrared detector (IRD) Model 5965A. The IRD was interfaced to a H P Model 5890 gas chromatograph (GC). Data treatment was carried out with an H P Model 59970C chem station, equipped with a HP 9000 Series 300 computer with 2 megabytes of memory and an 81-megabyte Winchester disk drive. The FT-IR interface contained a narrow-band mercury-cadmium-telluride (MCT) detector with a 750 cm-l spectral cutoff and was purged with liquid nitrogen boiloff. The detector D* (cm Hz112/W a t 1 kHz) was 3.5 X 1O'O. The lightpipe was gold-coated with dimensions of 1mm i.d. by 12 cm. A plot of detector signal intensity versus lightpipe temperature exhibited a slope of 0.01054 V/"C. This corresponds to a loss in signal intensity of one-third between 25 and 300 "C lightpipe temperature. Earlier a loss of more than three-fourths of the signal, over this same temperature range, was reported (IO). A 30 m X 0.32 mm fused silica capillary column coated with a 1-pm DB-5 film was inserted directly into the lightpipe without blocking the path beam. Helium carrier flow was 33 cm/s, and no makeup gas was used. The GC was ramped from 35 to 280 OC at 10 "C/min. One-microliter splitless injections were used with a Hewlett-Packard 7673A autoinjector for quantitation and identification limit studies and for sample analysis. Quantitation was accomplished by using the integrated Gram-Schmidt (GS) peak areas. GC/FT-IR data of 8-cm-l resolution was collected a t 3 scans/s to magnetic disk. A modified version of the U.S. EPA vapor-phase FT-IR spectral library (3045 spectra) was used for identification purposes. GC/MS Instrumentation. The mass spectrometer was a Hewlett-Packard 5970A mass selective detector (MSD) utilizing a 40-megabyte Winchester disk drive and a Hewlett-Packard 9816 desk top computer. The MSD was scanned from 50 to 800 amu at a nominal 1-amu resolution. The probability based matching (PBM) software (11) and the 42 261 National Bureau of Standards mass spectral library were used. The PBM search strategy was AU + A = 3, number of ions = 15, tilt option = full, and range option = scan range. Preparation of Standard Solutions. Standard solutions at a concentration of 1 pg/pL were obtained from the US.EPA Standards Repository. Four dilutions were made for each quantitation calibration curve, and each dilution was analyzed in duplicate. To minimize the effect of instrumental drift on sample quantitation, quantitations were performed immediately after the preparation of Calibration curves. The high precision of the duplicate data indicated that the injection technique was reproducible. Results Evaluation of System for Quantitation. Very little has been reported concerning the capability of lightpipe GC/FT-IR systems for on-the-fly quantitation. Sparks et al. (12) and Lam et al. (13) reported quantitative GramSchmidt (GS) data for a few compounds and Cooper and
Not subject to US. Copyright. Published 1988 by the American Chemical Society
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300-
Table I. GC/FT-IR Quantitation Parameters and Identification Limits
01
C 0
n
VI
;
200-
compound N-nitrosodimethylamine N-nitrosodipropylamine bis(2-chloroethyl) ether 10
20
30
Time, min.
Figure 1. Gram-Schmidt chromatogram of 10 ng each of 12-cornponent base-neutral extractables mixture.
bis(2-chloroisopropyl) ether
bis(2-ch1oroethoxy)methane dimethyl phthalate
Taylor reported capillary GC/FT-IR calibration curves for a few strongly absorbing compounds, at a lightpipe temperature (230"C) too low for environmental analysis (14). Only five low-boiling compounds were identifiable at levels below 100 ng, suggesting that real sample capillary column quantitation, without column overloading, was not possible for this system. To truly test the capability of a modern GC/FT-IR system for trace-level quantitation, standard solutions were prepared of 24 commonly monitored environmental contaminants. The standards were chosen to represent strong and weak infrared absorbers exhibiting a wide range of boiling points. Concentrations were chosen to span the range from near the analyte identification limit to the column overloading limit (ca. 250 ng). Correlation coefficients were generally greater than 0.98 for these four concentration-level (each level analyzed in duplicate) calibration curves. The results are shown in Table I. Identification limits for strong infrared absorbers (e.g., esters, aryl ethers, N-nitrosoamines) are around 5-10 ng, while those for medium-strength absorbers (e.g., chlorinated benzenes) are about 25 ng, and those of weak absorbers (e.g., polynuclear aromatics) are about 50 ng or greater. This is an order of magnitude greater sensitivity for these compounds over that reported for an older commercial GC/FT-IR interface (2). A GS reconstructed chromatogram curve for 12 neutral extractables is shown in Figure 1. The signal/noise (S/N) ratio for the least intense chromatogram peak (di-n-octyl phthalate) was about 4:l. FT-IR spectra for 5 ng of Nnitrosodimethylamine, 5 ng of 4-chlorophenyl phenyl ether, 5 ng, of dibutyl phthalate, and 25 ng of phenanthrene are shown in Figure 2. Selectivity of Narrow-Band MCT Detector. The cryogenically cooled MCT detectors used in modern GC/FT-IR analysis are marketed with a variety of lowfrequency spectral cutoffs (15). These different cutoffs are achieved by varying the ratio of Cd-Te and Hg-Te semiconductors in the detector element (16), and detector sensitivity decreases with lower cutoff frequencies. Previously, it had been assumed that the higher sensitivity associated with higher frequency cutoff detectors would be associated with lower selectivity (15). Accordingly, the selectivity of a high-frequency-cutoff MCT detector (cutoff 750 cm-l) was determined by injecting 27 typical environmental standards into the GC/ FT-IR system, and the library search results are shown in Table 11. Seventeen of the 18 analytes, whose reference spectra are contained in the search library, were properly identified on the first search hit. Search hits with the correct functionality were obtained for the nine standards whose reference spectra were not in the search library. 964
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diethyl phthalate di-n-butyl phthalate butyl benzyl phthalate ethyl hexyl phthalate di-n-octyl phthalate 4-chlorophenyl phenyl ether 4-bromophenyl phenyl ether pyrene naphthalene acenaphthylene acenaphthene fluorene phenanthrene anthracene fluoranthene 1,4-dichlorobenzene 1,2,4-trichlorobenzene 1,2,4,5-tetrachlorobenzene
concn range and identification correlation vmaa! limit, ngn coeffb cm-' 10-100 10-250 10-100 10-250 10-100 10-250 10-100 10-250 10-100 10-250 10-100 10-250 10-50 10-50 10-100 10-250 10-100 10-250 10-100 10-250 10-100 10-250 10-50 10-50 10-100 10-250 50-250 25-250 50-250 25-250 50-250 50-250 50-250 50-250 50-250 25-250 25-250 25-250
0.993 0.989 0.999 0.990 0.993 0.966 0.976 0.958 0.964 0.925 1.000 0.996 0.996 0.998 0.994 0.989 0.990 0.970 0.990 0.980 0.959 0.968 0.993 0.998 0.989 0.993 0.850 0.979 0.998 0.995 0.996 0.888 0.967 0.910 0.978 0.994 0.993 0.996
1483 1482 1125 1092 1085 1281 1274 1273 1272 1271 1273 1244 1242 840 781 769 786 1451 808 3063 773 1091 1460 1441
nLower e.nd of range is at or near the identification limit (MI$). GS peak areas used for quantitation. *Determined at four concentration levels. Each level analyzed in duplicate.
This selectivity is remarkable because the search algorithm was stressed with low S/N analyte spectra, generated by the analysis of standard concentration levels, which were close to the analyte identification limit. It thus appears that the extra sensitivity of this narrow-band MCT detector is not accompanied by loss of identification selectivity. However, selectivity problems may be experienced with higher cutoff frequency detectors (>750 cm-l) or for the detector used in this study, with small or highly symmetrical molecules having intense fundamental vibrations below the cutoff frequency (e.g., polynuclear aromatics or highly halogenated alkanes). GC/FT-IR Analysis of a Soil Extract. The spectral selectivity of this system was further tested by analyzing a soil extract obtained from a chemical dump site. The GC/FT-IR selectivity was confirmed by reanalyzing the same extract with the MSD. The infrared and mass spectral chromatograms of this extract are shown in Figure 3. While the FT-IR detected 48 analytes, the MSD recorded 110. The first GC/FT-IR library search fit for 24 of the most abundant soil extract components is shown in Table 111. MSD molecular weights agreed with the FT-IR identifications in every case. IRD search fits exceeded 739 while the MSD fits exceeded 65 for 23 of the 25 analytes. This strongly suggests that the validity of the
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0 5129
(dt
(b)
Wavenumbers Figure 2. GCIFT-IR spectra of (a) 5 ng of N-nitrosodimethylamine, (b) 5 ng of 4-chlorophenyi phenyl ether, (c) 5 ng of di-n-butyl phthalate, and (d) 25 ng of phenanthrene. 2.0
0.c 4000
3000
1000
2000
I
IO1
0 4000
5
1'0
1'5
20
25
3000
2000
1000
30
Time (Min.) Flgure 3. Gram-Schmidt chromatogram (a) and MSD reconstructed ion chromatogram (b) of soil extract.
search results from a directly linked gas chromatography/Fourier transform infrared/mass spectrometry (GC/IRD/MSD) system may be inferred from a comparison of the two spectral search results. In addition to its isomeric discrimination ability, the IRD could distinguish between benzylic and aromatic chlorine. This information was not obtainable from the MSD library search results. As a test of the sample quantitation ability of this system, three of the extract analytes were quantitated. The
Wavenumbers Figure 4. GC/FT-IR spectra of soil extract components: (a) 31.4 nglpL 1,4-dichlorobenzene, (b) 174 ng/pL 1,2,4-trichlorobenzene, (c) 73.1 ng/KL 1,2,4,5-tetrachIorobenzene.
spectra of these analytes (1,4-dichlorobenzene, 1,2,4-trichlorobenzene, and 1,2,4,5-tetrachlorobenzene)are shown in Figure 4. The extract concentrations of these analytes Environ. Sci. Technol., Vol. 22, No. 8, 1988
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Table 11. Narrow-Band MCT Selectivity with Environmental Standards retention time, min 3.31 7.44 7.90 8.64 8.95 10.40 10.72 10.85 13.12 14.80 14.80 15.20 16.58 16.60 16.76 17.90 19.00 19.20 20.86 22.10 22.60 24.70 25.87 26.02 26.40 28.30 29.10
compound
amount injected, ng
IRD library search hit"
5 25 25 25 10 25 25 25 25 25 5 25 25 5 5 5 25 25 5 50 50 25 250 250 25 25 250
1 1 1 (di-n-propyl ether) 1 1 1 1 1 1 1 2 (phenanthrene) 1 (4-bromophenyl phenyl ether) 1 1 1 1 (naphthalene) (3-methylphenanthrene) (di-n-propyl phthalate) (phenanthrene) (3-methylbiphenyl) 1 1 (anthracene)
N-nitrosodimethylamine bis(2-chloroethy1)ether 1,4-dichlorobenzene bis(2-chloroisopropyl) ether N-nitrosodipropylamine
bis(2-ch1oroethoxy)methane 1,2,4-trichlorobenzene naphthalene 1,2,4,5-tetrachlorobenzene acenaphthylene dimethyl phthalate acenaphthalene fluorene diethyl phthalate 4-chlorophenyl phenyl ether 4-bromophenyl phenyl ether phenanthrene anthracene di-n-butyl phthalate fluoranthene pyrene butyl benzyl phthalate benzo [ a ]anthracene chrysene di-n-octyl phthalate bis(2-ethylhexyl) phthalate benzo[b]fluoranthene
"Parentheses indicate compound not in library. Best match is indicated.
Table 111. MSD Molecular Weight Confirmed GC/FT-IR Results
time, min 3.73 4.40 5.11 5.40 6.91 6.97 7.11 7.91 8.32 9.36 10.33 10.80 10.90 11.20 11.30 11.41 11.95 12.18 12.63 12.85 13.15 13.73 13.82 14.17
15.80
first GC/FT-IR library search hit toluene tetrachloroethylene chlorobenzene 1,3-xylene 2-chlorotoluene benzaldehyde 4-chlorotoluene 1,4-dichlorobenzene 1,2-dichlorobenzene methyl benzoate 3,4-dichlorotoluene 1,2,4-trichlorobenzene 2-chlorobenzyl chloride 4-chlorot.mzy1 chloride 1,2,3-trichlorobenzene hexachlorobutadiene 4-chloromethyl benzoate 2-chloromethyl benzoate 2,4,5-trichlorotoluene 2,4,6-trichlorotoluene 1,2,4,5-tetrachlorobenzene butyl benzoate 1,2,3,4-tetrachlorobenzene diphenyl ether pentachlorobenzene
library search fit IRD" MSDb 890 914 820 739 981 972 930 894 80 912 873 955 880 903 807 831 822 290 950 917 937 902 902 920 780
89 89 89 79 95 78 62 97 93 20 89 86 84 96 95 81 65 81 95 86 94 78 94 63
89
"Maximum fit = 1000. bMaximum fit = 100.
were 31.4, 174, and 73.1 ng/pL, respectively. Relative Number of Detections with MSD, IRD, and 1982 GC/FT-IR Interface. Although the detection selectivity (number of detections leading to useful information) is more important to environmental analysis than the absolute number of detections, it is of interest to compare the number of real sample detections obtainable from mass and FT-IR spectrometers. Past environmental studies, utilizing both MS and FT-IR, usually indicated 968
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a greater number of mass spectrometer detections. However, samples containing a large number of oxygen-containing analytes (e.g., organophosphorus pesticides) can yield a greater number of FT-IR detections (5, 17). To test the relative number of detections, a herbicide still-bottom extract was analyzed on the MSD, and on an old commercial GC/FT-IR interface (reported in ref 17), as well as on the new IRD. Injections of identical amounts of extract to each system yielded 47 MSD detections, 7 (old) GC/FT-IR system detections, and 56 IRD (excluding solvent and column bleed) detections. See Figure 5 for the still-bottom chromatograms. IRD data indicated that over 85% of the detected analytes were chloro aromatics. Tentative IRD identifications for tetrachloroethylene, 4-chlorophenol, 2,4- and 2,6-dichlorophenol, 2,4-dichloroanisole, and benzyl alcohol were obtained. As expected, the majority of IRD detections yielded analyte functionality, not tentative identifications. The infrared information revealed that the majority of the analytes were chlorinated phenols, chlorinated benzyl alcohols, and chlorinated cyclic and acyclic ethers.
Conclusions A lightpipe GC/FT-IR system, with low nanogram sensitivity, is now commercially available. This system is over 20 times more sensitive over that reported for a 1982 vintage, commercial capillary GC/FT-IR system (2). This sensitivity improvement, coupled with the demonstrated GC/FT-IR quantitative capability, indicates that low nanogram identification and quantitation confirmation may be achieved with a directly linked GC/FT-IR/MSD SYStem. Such a system was recently exhibited at the 1987 Pittsburgh Conference (18). Utilization of the optical microscope GC/FT-IR system reported by Griffiths et al. (19),in conjunction with a mass spectrometer, may lower this capability to the subnanogram level. Preliminary results suggest that a 750-cm-' cutoff MCT detector may be used for environmental analysis without a significant loss in selectivity. The universality of this
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dibutyl phthalate, 84-74-2;butyl benzyl phthalate, 85-68-7; ethyl hexyl phthalate, 117-81-7;dioctyl phthalate, 117-84-0; 4-chlorophenyl phenyl ether, 7005-72-3; 4-bromophenyl phenyl ether, 101-55-3;pyrene, 129-00-0;naphthalene, 91-20-3; acenaphthene, 83-32-9; fluorene, 86-73-7;phenanthrene, 85-01-8; acenaphthylene, 208-96-8; anthracene, 120-12-7; fluoranthene, 206-44-0; 1,4-dichlorobenzene, 106-46-7; 1,2,4-trichlorobenzene, 120-82-1; 1,2,4,5-tetrachlorobenzene,95-94-3.
Literature Cited (1) Gurka, D. F.; Betowski, L. D. Anal. Chem. 1982, 54, 1819-1824. (2) Gurka, D. F. Appl. Spectrosc. 1985, 39, 827-833. (3) Rosenthal, D. Anal. Chem. 1982,54, 63-66. (4) Davis, M.; Giddings, J. C. Anal. Chem. 1983,55,418-424. (5) Gurka, D.F.; Hiatt, M.; Titus, R. Anal. Chem. 1984,56, 1102-1110. (6) Yang,P. W. J.; Ethridge, E. L.; Lane, J. L; Griffiths, P. R. Appl. Spectrosc. 1984, 38, 813-816. (7) Yang, P. W. J.; Griffiths, P. R. Appl. Spectrosc. 1984, 38, 816-821. (8) Brown, R.S.; Cooper, J. R.; Wilkins, C. L. Anal. Chem. 1985, 57,2275-2279. (9) Gurka, D. F.; Titus, R.; Griffiths, P. R.; Henry, D.; Giorgetti, A. Anal. Chem. 1987,59, 2362-2369. (10) Gurka, D. F.; Laska, P. R.; Titus, R. J. Chromatogr. Sci. 1982,20, 145-154. (11) McLafferty, F. W.; Stauffer, D. B. J . Chem. Inf. Comput. Sci. 1985, 25, 245-252. (12) Sparks, D. T.; Lam, R. B.; Isenhour, T. L. Anal. Chem. 1982, 54, 1922-1926. (13) Lam, R.B.; Sparks, D. T.; Isenhour, T. L. Anal. Chem. 1982, 54, 1927-1931. (14) Cooper, J. R.; Taylor, L. T. Anal. Chem. 1984, 56, 1989-1993. (15) Griffiths, P. R.; de Haseth, J. A.; Azarraga, L. V. Anal. Chem. 1983,55, 1361A-1387A. (16) Griffiths, P.R.;de Haseth, J. A. Fourier Transform Infrared Spectrometry:Wilev-Interscience: New York, 1986: T) 215. (17) Gurka, D. F.: Titus; R. Anal. Chem. 1983,58, 2189-2194. (18) Borman, S.Anal. Chem. 1987,59, 769A-774A. (19) Fuoco, R.;Shafer, K. H.; Griffiths, P. R. Anal. Chem. 1986, 58, 3249-3254.
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Flgure 5. Reconstructed chromatograms from herbiclde stlll-boii&m extract analysis by (a) mass selective detector, (b) vintage 1982 commercial GC/FT-IR, and (c) new commercial GC/FT-IR.
conclusion awaits the results of more exhaustive studies with standards and real environmental samples. Registry No. N-Nitrosodimethylamine, 62-75-9; N-nitrosodipropylamine, 621-64-7; bis(2-chloroethyl) ether, 111-44-4; bis(2-chloroisopropyl)ether, 3963832-9; bis(2-~hloroethoxy)methane, 111-91-1;dimethyl phthalate, 131-11-3;diethyl phthalate, 84-66-2;
Received for review August 7,1987. Accepted January 12,1988. Although the research described in this paper has been funded wholly or in part by the US.Environmental Protection Agency, it has not been subjected to Agency review and therefore does not necessarily reflect the views 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.
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