14. M. A. Maxis, C. W. Brown, and D. S. Lavery, Anal. Chem. 55, 1694 (1983).

15. C.W. Brown and R. J. Obremski, Appl. Spectrosc. Reviews 20, 373 (1984).

16. W. Lindberg, J.-A. Persson, and S. Wold, Anal. Chem. 55, 643 (1983).

17. T. Naes and H. Martens, Trends Anal. Chem. 3, 266 (1984). 18. C. W. Brown, R. J. Obremski, and P. Anderson, Appl. Spectrosc.

40, 743 (1986).

19. C. W. Brown, E. A. Bump, and R. J. Obremski, Appl. Spectrosc. 40, 1023 (1986).

20. W. F. McClure, A. Hamid, F. G. Giesbrecht, and W. W. Weeks, Appl. Spectrosc. 38, 322 (1984).

21. E. R. Malinowski and D. G. Howery, Factor Analysis in Chemistry (Wiley, New York, 1980).

22. E. R. Malinowski, Anal. Chem. 49, 612 (1977). 23. E. R. Malinowski, Federation of Analytical Chemistry and Spec-

troscopy Societies, Philadelphia (1985), Paper No. 284.

Performance of a Gas Chromatographic-Matrix Isolation-Fourier Transform Infrared Spectrometer*]-

THOMAS T. HOLLOWAY,$ BILLY J. FAIRLESS, CHARLES E. FREIDLINE, HARRY E. KIMBALL, ROBERT D. KLOEPFER, CHARLES J. WURREY, LALEH A. JONOOBY, and HAROLD G. PALMER Environmental Monitoring and Compliance Branch, Environmental Services Division, U.S, E.P.A., Region VII, 25 Funston Road, Kansas City, Kansas 66115 (T.T.H., B.J.F., C.E.F., H.E.K. R.D.K., C.J.W.); and Computer Science Corporation, 25 Funston Road, Kansas City, Kansas 66115 (L.A.J., H.G.P.)

The unique matrix-isolated (MI) Fourier transform infrared spectra of diethyl ether, propanal, and trifluoromethane are compared with their vapor-phase spectra. The marked increase in resolution seen in the MI spectra is accompanied by substantial frequency shifts from the peaks in the vapor-phase spectra. The spectra were generated with a gas chro- matographic/matrix isolation/Fourier transform infrared spectrometer, with the use of frozen Ar at 13 K as the isolating matrix. The spectra of the 22 isomers of tetrachlorodibenzo-p-dioxin show that each isomer can be distinguished and quantitated at the nanogram level. In addition, preliminary quantitative data were obtained for the pesticides Terbufos ® and Fonofos ®. Finally, a problem with a previously published spectrum of C-13 2,3,7,8-tetrachlorodibenzo-p-dioxin was uncovered, and it was shown that the spectrum was actually that of C-13 2,3,7-trichlorodi- benzo-p-dioxin.

Index Headings: FT-IR; GC/MI/FT-IR; Matrix isolation; Fourier transform infrared; Diethyl ether; Propanal; Trifluoromethane; Tri- and tetrachlorodibenzo-p-dioxin; Terbufos®; Fonofos ® .

I N T R O D U C T I O N

Gas chromatographic /matr ix isolation/Fourier trans- form infrared spectrometer systems have only recently become commercially available. The principles of the ins t rument have been published by Bourne et al. 1 This paper presents some of our experience with one of the first of these inst ruments to be purchased. The Envi-

ronmental Protect ion Agency is ul t imately interested in this ins t rument as a tool for the positive identification and quant i ta t ion of trace substances in the environment. Of special interest is the ins t rument ' s ability to differ- entiate between isomers of complex substances like the chlorinated dibenzo-p-dioxins. These substances have been reported in the news, for several years, as a rec- ognized threat to human and animal life and heal th when distr ibuted in the environment. Significant effort and expense have gone into cleaning up contaminated sites. However, there are major differences in the toxicity of the various chlorinated dibenzo-p-dioxin isomers. Iso- mers with chlorine atoms in the four lateral positions (2,3,7,8) and up to six chlorine atoms in the molecule are the most toxic, and all have LDs0 values ranging from 1 to 100 #g/kg for the most sensitive animal speciesY The

Received 9 September 1987. * This report has been reviewed by the U.S. Environmental Protection

Agency, Region VII, and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the U.S. Environmental Protection Agency, nor does mention of trade names or commercial products constitute endorsement or recom- mendation for use.

~f Permission for the publication of Sadtler Standard Spectra (Reg. Trademark) has been granted, and all rights are reserved, by Sadtler Research Laboratories, Division of Bio-Rad Laboratories, Inc.

$ Author to whom correspondence should be sent. FIG. 1. Cryogenic gold disc and deposition tip for the gas chromato- graph effluent.

Volume 42, Number 2, 1988 0oo3.7028/88/42o2.o35952.0o/0 APPLIED SPECTROSCOPY 359 © 1988 Society for Applied Spectroscopy

A 0 . 9 8 ~

0 .B0~

8 . ~ 0 - -

0.60--

0.50--

0.48--

0.30--

0 , 2 0 - -

0.10--

I I - - 1 - 5508 3000 2500 2000

W a v e n u m b e r

I 1S00

m

1000

B

0.d8--

0.58--

0 . 2 0 - -

0.10--

I I I I 4000 3500 3080 2500 2000 1500

Wove numb e r

1000

FIG. 2. Diethyl ether vapor-phase (Copyright, Sadtler Research Laboratories, Division of Bio-Rad Laboratories, Inc., 1983) and matrix-isolated spectra compared. (A) Vapor-phase; (B) matrix-isolated.

highly toxic 2,3,7,8-tetrachlorodibenzo-p-dioxin has LDs0 = 2 ttg/kg. On the other hand, the 1,2,3,4 isomer is basically nontoxic, with an LDso > 1,000,000 ttg/kg (1 g/kg) in guinea pigs2 Clearly, cleaning up environmental traces of the 1,2,3,4 isomer would be a waste of time, effort, and money, but cleaning up traces of the 2,3,7,8 isomer is likely to be critical to the health of those who would otherwise be exposed.

This preliminary study explores the substantial dif- ferences between the solid matrix-isolated spectra and the analogous vapor-phase spectra of some common sub- stances. Also, a comparison of the spectra for the 22 isomers of tetrachlorodibenzo-p-dioxin is shown in chart

form, along with preliminary studies of the reproduc- ibility and accuracy of quantitation techniques.

EXPERIMENTAL

The first part of the instrument consists of a gas chro- matograph (Hewlett-Packard 5890A) outfitted with a 0.32-mm-i.d. x 30-m fused quartz capillary column coat- ed with a bonded methyl-phenyl silicone phase [J&W Scientific Durabond 5 (DB-5)]. The carrier gas is helium with 1% argon. The effluent from the chromatograph is frozen onto a rotating gold disc (Mattson Cryolect ®) held at a temperature of 13 Kelvins and a pressure of about

360 V o l u m e 42, Number 2, 1988

A 0 . 9 0 - -

B. 8~I--

0 . 7 0 - -

g . 6 B - -

0 . 5 0 - -

0. ~.0--

0 . 5 0 - -

0 . 2 8 - -

0 . 1 0 - -

i 35f lg

y J

3000 ' - I - ~ 250~ 2000

Wo. venumber

i i 1580 1000

B

m

p

m

m

B

0 . 5 ~ - -

g . 2 5 - -

0 . 2 0 - -

0 . 1 5 - -

0 . 1 0 - -

0. S 5 - -

0 . 0 0 - -

4~JOS 55EB0 30010 2seo ~eee Isse 10ee

W a v e n u m b e r

FIG. 3. Propanal vapor-phase (Copyright, Sadtler Research Laboratories, Division of Bio-Rad Laboratories, Inc., 1983) and matrix-isolated spectra compared. (A) Vapor-phase; (B) matrix-isolated.

TABLE I. A comparison of the number of peaks and the major frequencies in the matrix-isolated spectrum with the vapor-phase spectrum for diethyl ether, propanal, and trifluoromethane.

Compound ~ Diethyl ether Propanal Trifluoromethane

MI VP A MI VP A MI VP A

Number of distinct resolved peaks >2% of principal peak 26 9 17 Number of peaks including shoulders >2% prin. pk. 30 15 15

Frequency ~ ~ ~ A (wavenumbers)

M~or peak 1 2985 2992 -7 M~or peak 2 2870 2872 -2 M~or peak 3 1388 1395 -7 M~or peak 4 1354 1354 0 M~or peak 5 1127 1135 -8 Major peak 6 1046 1038 +8 M~or peak 7 1447 1450 - 3

22 11 11 5 2 3 31 18 13 6 4 2

2986 2993 - 7 3054 3034 - 2 0 2948 2978 -30 1377 1377 0 2840 2812 +28 1134 1152 +18 2740 2720 +20 1145 1152 +7 1731 1760 -29 696 701 +5 1698 1745 -47 - - - - - -

A P P L I E D S P E C T R O S C O P Y 361

A 8 . 9 8 - -

8 . 8 8 - -

8 . 7 8 - -

8 , 6 8 - -

8 . 5 8 - -

8 . a 8 - -

8 . 5 8 - -

8 . 2 8 - -

8 . 1 8 - -

I 3588

I 5888

[ I 2588 2888

W a v e n u m b e r

I . ) I l

1588 1888

V-

+I

B 0 . 1 8 - -

0.88"

8+ 8 6 - -

8 . 0 4 - -

8 . 0 2 - -

8.88--- I [ I I I

a808 558g 5808 2588 2888 1588

W a v e n u m b e r

I 1800

Fro. 4. Trifluoromethane vapor-phase (Copyright, Sadtler Research Laboratories, Division of Bio-Rad Laboratories, Inc., 1983) and matrix- isolated spectra compared. (A) Vapor-phase; (B) matrix-isolated.

1 x 10 -5 Torr. At this temperature the separated com- ponents are trapped in a solid matrix of frozen Ar, which serves to isolate the molecules from each other. Figure 1 shows a photomicrograph of the deposition tip posi- tioned to deposit the matrix on the gold disc, and the thin layer of frozen Ar deposited. One then obtains Fou- rier transform infrared spectra by passing wide-spectrum IR radiation from the interferometer (Mattson Sirius 100) into the matrix, reflecting it off the gold disc and back through the matrix, and then focusing it on a Hg- Cd-Te detector. Most of the MI/FT-IR spectra shown in this paper were generated by time averaging of 32 scans .

C O M P A R I S O N OF MATRIX-ISOLATED AND VAPOR-PHASE IR SPECTRA

Vapors of three simple compounds--diethyl ether, propanal, and trifluoromethane--were injected into the gas chromatograph in about 1.0-5.0 #L quantities in or- der to obtain appropriate amounts of the substances to give good spectra. Figure 2 shows the matrix-isolated spectrum of diethyl ether and, above it, on the same frequency scale and a normalized absorbance scale, the corresponding published vapor-phase spectrum (Sadt- ler). There are some notable differences between the spectra:

3 6 2 Volume 42, Number 2, 1988

A 8.1B--

8 . 8 8 - -

8 . 8 6 - -

8 . 8 4 - -

0.02--

8 . 8 g - -

4888 I I I I I I

3588 3Bg8 2588 288g 158 B 188~I

W a v e n u m b e r

FIG. 5 .

B } , .8 - -

1 . 6 - -

1 , a - -

1 . 2 - -

l . g - -

8 . 8 - -

8 . 6 - -

8 . 4 - -

B . 2 - -

488

A I I I I "]

3588 3808 258g 2BSB 158g [888

W ~ v e n u m 6 e r

B

m

B

T r i f l u o r o m e t h a n e s p e c t r a u n d e r d i f f e r e n t e x t e n t s o f m a t r i x i so lat ion: (A) w i t h a s m a l l a m o u n t injected; (B) w i t h a larger a m o u n t in jec ted .

1. There are 15 peaks, including shoulders, in the vapor- phase spectrum, but these are resolved into at least 26 distinct peaks in the matrix-isolated spectrum.

2. As shown in Table I, most of the major peaks are shifted slightly in frequency.

Figure 3 shows a similar set of spectra for propanal. For propanal the differences are:

1. There are indications of about 18 peaks in the vapor- phase spectrum, but there are 22 distinct peaks and several more discernible shoulders in the matrix-iso- lated spectrum.

2. Table I shows the rather remarkable frequency shifts

of the major peaks in the two spectra. Note that the shifts are not all in the same direction. The unresolved

9 1

7 0 3

6 4 FIG. 6. Structure and numbering system of dibenzo-p-dioxin.

A P P L I E D S P E C T R O S C O P Y 3 6 3

TCDD ISOMER"

1 3 6 8

1 3 7 9

1 3 6 9

1 3 7 8

1 2 4 8

1 2 4 7

1 4 6 9

1 2 4 6

1 2 4 9

1 2 7 8

1 4 7 8

1 2 6 8

1 2 3 6

1 2 3 4

1 2 6 9

1 2 3 7

1 2 3 8

2 3 7 8

1 2 3 9

1 2 7 9

1 2 6 7

1 2 8 9

LABELED 18OMER8

D-13 2378

C I -37 2378

W A V E N U M B E R

1500 1400 1300 1200 1100 1000

, h i iii , j

,i II I II =

I~ I

I I

d i l l h i

,It, I I

Ii ,

ii I i

,l i i

Ill J I

I , IJ i ,

i I i i

I I

I ,I I I L

i I I I i

i I it

9 0 0 800 700

I

"L IST~ D Y ICRI AS NG RET EN'I rOb TII~ E C N ~-5 CO .UM I.

I iI

I I tll i ~1

FIG. 7. Chart of the spectral lines of the 22 TCDD isomers as deter- mined by GC/MI/FT-IR. Frequency and relative intensity are indi- cated.

carbonyl doublet of the vapor-phase spectrum be- comes 2 distinct peaks in the matrix-isolated spec- trum, and these are strongly shifted.

Figure 4 shows the two spectra of the very simple molecule trifluoromethane. 1. The matrix-isolated spectrum is better resolved, with

more peaks than the vapor-phase spectrum. The C-H stretch at 3054 cm -1 is extremely weak but has also been included in Table I.

2. The frequency shifts between the two spectra are shown in Table I.

There are some fundamental similarities and differ- ences between the molecules in the matrix-isolated (MI) and vapor-phase (VP) samples which may have an effect on the spectra:

1. In both cases most of the molecules are isolated from each other, so that there should be little effect from intermolecular forces. On this basis alone, one would expect the spectra to be the same.

2. The MI samples are locked in a solid matrix of pre- sumably inert Ar atoms. No molecular rotations can occur in this matrix, while the VP molecules are free to rotate.

I I I I 8 ,828- -

f l .868--

8. ~58--

8. 848--

8.83f l - -

8,82B--

8 . 8 1 0 - ~

8. 888-- I f I I

~888

FIG. 8.

I 3588 58fl0 25~0 2fl0fl 15Bfl 10Off

Waven~tmber

Matrix-isolated spectrum of 2,3,7,8-TCDD.

3. The VP spectra are obtained on molecules at a tem- perature of about 300 K. The MI molecules are at 13 K, very near the absolute zero. Consequently, one would expect nearly all the MI moelcules to be in the vibrational ground state.

Figure 5 shows matrix-isolated spectra of trifluoro- methane at a concentration that is low enough to allow one to expect a good degree of isolation of the molecules and with an amount injected that is large enough to prevent the Ar from continuing to isolate the molecules from each other. The spectra clearly indicate that the resulting intermolecular attractions broaden the peaks dramatically as the concentration increases. A less ob- vious but very important observation is that the fre- quencies of the peaks shift with concentration.

A feature which complicates the interpretation of MI spectra is the ubiquitous presence of water in several forms (including matrix-isolated, vapor, and ice) and of solid carbon dioxide. Peaks due to these species are al- ways in the same regions of the spectrum and have been removed from the matrix-isolated spectra in Figs. 2-4 to facilitate easy comparison with the vapor-phase spectra. Removal of these extraneous peaks with simultaneous baseline correction is done with the use of the "gbascor" program included as a part of the Expert-IR software package (Mattson).

A second artifact in many low-level MI spectra is the presence of an interference pattern in the baseline due to the reflection-refraction of light at each end of the sample path. This is the same effect used to calibrate conventional liquid cells for IR work, and allows the estimation of the thickness of the solid Ar layer on the disc. Values of the Ar layer thickness calculated from various good spectra in our lab range from 0.012 to 0.015 mm. This pattern can be readily seen in the baseline of the MI spectrum of trifluoromethane from 800 cm -I to 2200 cm -1 (Fig. 5A).

ENVIRONMENTAL SAMPLES

T e t r a c h l o r o d i b e n z o - p - d i o x i n s . Figure 6 shows the structure of dibenzo-p-dioxin and the numbering sys- tems used to designate the various substituted isomers.

364 Volume 42, Number 2, 1988

A 6 .5O- -

6, 25- -

6. t S - -

6 , 1 6 - -

0.06 I I I I I I I 1766 1606 1566 1466 1568 1266 1166 |666 966 896

~..A. I

706

B 6.635

6 .636 - -

0 ,025 - -

6 . 626 - -

6 . 615 - -

f l ,616- -

0 ,665 - -

6 .669 - -

1760 1600 1560 1466 1.~00 12fl6 1106 1600 90fl 600 706

W~venumber Wavenumber

C

6 . 6 6 4 6 - -

0 .0030 - -

A b s o r 0 .6626 - - b

n c e

FIG. 9.

6. 6 6 6 6 - - ,

I [ I I I I I [ ] 1"~00 1666 1566 1466 1566 1266 1 1 6 g 1966 9fl6 866 766

D

6 .6616 - -

6 .6665 - -

6 . 6 6 6 6 - -

I / I I I I I I I I 1)66 1666 1566 t400 150B 1266 1160 166B 966 866 700

W6venamber W ~ v e n u m b e r

Spectra of 1,2,3,4-TCDD at (A) 20.0 ng (32 SCANS), (B) 2.5 ng (128 SCANS), (C) 0.30 ng (640 SCANS), and (D) 0.156 ng (5000 SCANS).

Tetrachlorodibenzo-p-dioxin (TCDD) alone has 22 iso- mers. The GC/MI/FT-IR method is capable of distin- guishing each of these isomers separately, even at the nanogram level. Figure 7 is a composite relative inten- sity-frequency chart of the spectra of the 22 isomers. The data for this chart come from our own spectra as well as those of Brasch. 4 Spectra taken from standards contain- ing more than one isomer were differentiated with the use of the interpretive techniques of Grainger and Gel- baum2 The spectra are presented only from 1700 cm -1 and below, since there are no detectable peaks above 1700 cm-L The aromatic C-H stretch expected is apparently deactivated by the four chlorine atoms on the ring. The principal peak in each of the isomers ranges between 1430 and 1500 cm-L

Figure 8 displays the spectrum of the highly toxic 2,3,7,8-TCDD as an example of the spectra obtained.

SENSITIVITY, LINEARITY, AND DETECTION LIMITS

In addition to identification of a specific "dioxin" iso- mer in the environment, the concentration must also be determined. Preliminary studies have demonstrated the feasibility of quantitation of the TCDD isomers. Figure 9 compares the spectrum of 1,2,3,4-TCDD at four dif- ferent concentrations ranging from 0.156 to 20.0 ng. The spectrum is still identifiable even at 0.156 ng. At the lower concentrations, the quality of the spectrum improves if a larger number of FT-IR scans are used. The detection

APPLIED SPECTROSCOPY 365

0.300-

0.200-

i 0"1111

ng injected

Fro. 10. Beer ' s law plot of the absorbance vs. a m o u n t injected of 1,2,3,4-TCDD.

limit is clearly below 0.156 ng, but the number of scans needed to generate usable spectra makes this value close to the practical limit.

Figure 10 presents a graph of the absorbance of the principal peak vs. concentration of 1,2,3,4-TCDD. It can be seen that Beer's law is obeyed over this entire range. Data are taken from Table II, with the average absor- bances plotted against concentration of the standard.

REPRODUCIBILITY

A reasonable degree of reproducibility of absorbance for a given concentration is necessary for the quantitative use of an analytical method. Table II shows the absor- bances obtained for a series of multiple injections at several concentrations. Even at the lowest concentration (156 picograms) the absorbance is adequately reprodu- cible. In order that a high-quality spectrum could be obtained at 156 picograms, the FT-IR was run at 5000 scans instead of the usual 32 scans.

IDENTIFICATION OF COMPOUNDS GIVING A SPECTRUM

The instrument described includes microprocessor control of the instrument as well as data workup soft- ware. Included is a spectral search system which can match up an experimental spectrum to a library spec- trum by several different procedures. The results of our studies have emphasized the need for a library of matrix- isolated spectra for spectral matching. Trials using va- por-phase spectra for comparison gave very poor results.

T A B L E II. Reproducibility of the absorbance of the principal peak in 1,2,3,4-TCDD during mult iple injections.

Absorbance at 1433 cm -1 Concen- Rel. std. t ra t ion 1 2 3 Mean dev. (%)

20.0 ng 0.306 0.282 0.284 0.291 4.6 2.5 ng 0.032 0.035 0.038 0.035 8.6 0.3 ng 0.0037 0.0043 0.0039 0.0040 7.6 0.156 ng 0.0021 0.0029 - - 0.0025 23

The reason for this lack of success is undoubtedly the difference in resolution and the frequency shifts shown above. The development of a library of matrix-isolated spectra requires a reliable set of standard compounds.

RESOLUTION OF A PROBLEM USING THE AVAILABLE IDENTIFICATION TECHNIQUES

Three techniques now available for environmental identification and quantitation of environmentally in- teresting compounds which provide complementary in- formation for organic molecules are gas chromatography, matrix isolated Fourier transform infrared spectrometry, and mass spectrometry.

A problem which was resolved in our laboratory illus- trates the usefulness of employing these techniques to- gether. Figure 11 shows the gas chromatogram we ob- tained for a commercial standard of C-13 isotopically substituted 2,3,7,8-TCDD. The standard shows 5 major peaks and many small ones. The major peak eluted at 14.4 rain. However, under the same chromatographic conditions, the naturally abundant compound eluted at about 17.5 rain. Since the only difference between the C-13 substituted and natural 2,3,7,8-TCDD is a few per- cent in the mass, it seemed that the C-13 compound should elute at almost the same time as the natural one. But the compound eluting at 14.4 min matched a pub- lished spectrum of C-13 2,3,7,8-TCDD 6 which we had in our library. The 17.58-min peak gave a spectrum re- markably close to the published spectrum, but it did not match in about 10 minor peaks. A GC/MS spectrum showed that the 14.4-min peak was trichlorodibenzo-p- dioxin rather than the tetrachlorodibenzo-p-dioxin of interest (see Fig. 12 for a comparison of the two infrared spectra). The closeness of the match suggests that the compound appearing at 14.4 min is 2,3,7-trichlorodiben- zo-p-dioxin. The authors of the published spectrum had used the same commercial standard that we used, and

FIG. 11. Gas ch roma tog ram of a commercia l ly available (?-13 2,3,7,8-TCDD s tandard .

366 Volume 42, Number 2, 1988

A 0 . 3 0 - -

0.25--

0 . 2 0 - -

0 . 1 5 - -

0 . 1 0 - -

0.05--~~ 0 . 0 0 - -

I I 1700 1600 1500

I I 1400 1308 I I I I 1200 1100 1800 900 800 700

W a v e n u m b e r

B

F I G . 1 2 .

0 . 1 4 - -

0 . 1 2 - -

0 . 1 0 - -

8 . 0 8 - -

0 . 0 6 - -

0 . 0 0 - -

0.02--

I [ I 1780 1600 1500 1480

Comparison spectra of (A) C-13

I I I I 1300 1200 1100 1000 900 808 700

W a v e n u m b e r

2,3,?-TRICDD and (B) C-13 2,3,7,8-TCDD.

had made the usual assumption that the major compo- nent was the compound on the bottle label. However, it was not, and the three techniques together alerted us to the problem and allowed us to resolve it. The correct spectrum for the C-13 2,3,7,8-tetrachlorodibenzo-p-di- oxin is the one labelled C-13 2,3,7,8-TCDD in Fig. 12, rather than the one previously published. 6

A PESTICIDE DECOMPOSITION STUDY: FONOFOS ® AND TERBUFOS ®

Two related pesticides, Fonofos ® and Terbufos ®, showed similar but significantly different spectra. The spectra are shown in Fig. 13 and the structures of the two compounds are shown in Fig. 14. The P-O-C vibra-

tions at 900-1050 cm -1 are nearly the same in both spec- tra, but Fonofos ® shows additional peaks at about 3100 cm -1 and 1600 cm -1, and below 800 cm -1, which are typ- ical of the aromatic substituent.

The Environmental Protection Agency was concerned about (1) determining their persistence in the environ- ment, and (2) differentiating between these two pesti- cides in order to settle a dispute over pesticides found in groundwater samples. These concerns led to a decom- position study, using the GC/MI/FT-IR to follow any decrease in concentration of the pesticides with time. In Table III the Relative Weight Response (R.W.R.) values from the gas chromatograph are compared with the ab- sorptivities for the principal peak of a 20-ng/tLL Fono- fos ® standard in hexane for replicate injections. From

A P P L I E D S P E C T R O S C O P Y 3 6 7

0.90--

0 .80- -

0 .70- -

0 .60- -

0 .50 - -

0 . a 0 - -

0 .50 - -

0 . 2 0 ~

0.10

A 7 5500 3000

I I 2500 2000

W a v e n ~ m b e r

t l! 1580 1000

0 .90- -

0 .80- -

0 .70- -

0 .60- -

0 .50 - -

0 .a0 - -

0 .30- -

0 .20--

0 .10- -

I f 3500 3000

FIG. 13. Matrix- isolated spectra

Table III it can be seen that the reproducibility of the IR peak intensity is about the same as that from the areas given by the flame ionization detector of the gas chromatograph.

When Fonofos ® was dissolved in deionized water un- der laboratory conditions and the hexane extracts were injected, very little change in concentration with time was seen over a 2-week period. When dissolved in river water and allowed contact with normal environmental conditions outside, the concentrations of the pesticides decreased rapidly and were easily followed by the GC/ MI/FT-IR. For example, an 8-ng/ttL Terbufos ® solution decomposed to undetectable levels in 11 days. By con- trast, Fonofos ® decomposed at a much slower rate, even in the river water. An approximately 20-ng/ttL sample

[ I 1 2500 2000 1500 1000

Wavenumber

of (A): Fonofos ® and (B) Terbufos ®.

decomposed to about half that value in a month's time. The presence of the aromatic ring on Fonofos ® appar- ently stabilizes it considerably.

T A B L E III. Comparison of the R.W.R. values and absorptivities for a 20 ng//~L Fonofos ® in hexane standard for replicate injections.

Absorptivi ty at

Injection 1 0 2 5 c m -1 9 5 4 c m -~ R . W . R .

1 2 .98 x 10 3 2 .90 × 10 -3 4 8 7 2 2 .93 x 10 3 2 .93 x 10 -3 5 1 8 3 3 .15 x 10 3 3 .13 × 10 -3 497 Average 3 .02 x 10 -3 2 . 9 9 x 10 -3 501

Rel. S t d . D e v . 2 . 7 % 3 . 0 % 2 . 2 %

368 Volume 42, Number 2, 1988

A°

CH3CH2-O \ ~ S

be a valuable tool for solving some difficult environmen- tal problems. Because of the large number of spectra tha t are generated from each real sample, and the con- straints on the ins t rument (for example, the gold disc has to be warmed up each day to remove deposited com- pounds), one can realistically run and analyze data for about 3-4 samples per day using two operators to cover 12 h of operation. I t will be necessary to prioritize sam- ples tha t need the capabilities of this instrument.

The high resolution of the peaks in the very cold ma- trix-isolated state should also be of theoretical interest.

B.

S CH3 CH3CH2-O~ - -S-CH2-S- -C- -CH 3

/ I CH3 CH2-O" CH 3

FIG. 14. Structural formulas of (A) Fonofos ® and (B) Terbufos ®.

C O N C L U S I O N S

Prel iminary studies have shown tha t the GC/MI /FT- IR technique is useful for the identification and quan- t i tat ion of specific isomers at nanogram and subnano- gram levels. Therefore the G C / M I / F T - I R method will

ACKNOWLEDGMENT Support of C.J.W. by the E.P.A. Distinguished Visiting Scientist

program (Grant CR-813621010) is gratefully acknowledged.

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