ane soot as measured by FT-IR-PAS. It is apparent that the features in the spectrum are negative-going with re- spect to the baseline. The position of the baseline can be inferred from the spectrum in the 2000-1800 cm -~ region. The spectrum shown here is superimposed on a baseline which slopes toward zero-signal amplitude at lower frequencies, consistent with the overall absorption being associated with the electronic transition of the free electrons in the hexane soot. To obviate this effect, a curvature correction has been applied to the baseline. However, this curvature correction is not sharp enough to generate the negative-going features observed here. The fact that these bands are negative-going is in con- trast with the FT-IR-PAS features observed for all other compounds. When this was first observed it was not clear that the sample examined by FT-IR-PAS was the same as that observed by transmission, ATR, or DRIFT. There could have been sample degradation prior to the PAS measurement, for example.

To establish the fact that the sample was indeed the same as that observed by those other techniques, the following sequence of experiments was performed. A small crystal of NaC1 was run as a reference blank by transmission spectroscopy, with careful attention being paid to reproduction of the exact orientation of the crys- tal blank within the sample chamber. Hexane soot was then deposited on this crystal, and a conventional trans- mission spectrum was obtained of this sample. The top spectrum shown in Fig. 2 was then obtained with use of the blank crystal as a reference. This particular sample was then placed in the photoacoustic cell and an FT-IR- PAS spectrum measured where the reference was a TGS background single beam spectrum. The FT-IR-PAS spectrum so obtained is shown as the middle spectrum in Fig. 2. For purposes of comparison, the bottom spec- trum in Fig. 2 is that obtained by FT-IR-PAS measure- ment of a sample of hexane soot that we prepared by scraping off the soot deposited on a glass substrate, as described above. This bottom spectrum is practically identical to the middle spectrum. This whole procedure was repeated twice. We are able to conclude, therefore, that the spectra measured by transmission and by FT- IR-PAS on the same sample of hexane soot are different.

This difference separates FT-IR-PAS as a measure- ment tool from the other three techniques (transmis- sion, ATR, and DRIFT) which all give similar spectra. The anomalous FT-IR-PAS spectrum must be due either to a spectrometer related artifact or to some chemical characteristic of the hexane soot which is measured only by the FT-IR-PAS method. If it is a spectral artifact it cannot be associated with resonances of the cell itself since these lie well outside the acoustic frequency range of the measurements made with this particular cell and with the interferometer head scanning at 1.25 mm/s. The FT-IR-PAS spectrum is ratioed to the single beam transmission spectrum with no sample in the system. This will be referred to as the DTGS single beam spec- trum.

Since the FT-IR-PAS spectrum is ratioed to the DTGS single beam reference for the spectra reported here, one possible explanation for these negative-going features in the spectrum of the carbon-black is that the triglycine sulfate detector has regions of increased sensitivity which

I I

' lO OO 2000 1500 cm_l

F~G. 2. Soot on small NaC1 chip, 1000 scans, 8 cm -1 resolution (top spectrum); same sample by PAS, 1000 scans, 8 cm -1 resolution (middle spectrum); normal PAS spectrum of hexane soot for comparison (bot- tom spectrum).

coincide with the frequencies observed in the FT-IR- PAS spectrum of the carbon-black. Triglycine sulfate is functioning essentially as a pyroelectric detector. There- fore, if it were to have any regions of increased sensitiv- ity, it is reasonable to conclude that these would be coin- cident with the absorption of triglycine sulfate. In such a case, the increased absorptivity of the material may well result in a slightly larger observed signal than for regions in which it does not absorb.

Triglycine sulfate and its deuterated analog are two sytems which are well studied. Koslovskii e t a l 2 have reported the infrared spectrum of these materials, along with those of the selenate isomorphs, in order to make an assignment of the infrared spectrum. A comparison of the spectra reported by these workers with the FT- IR-PAS spectrum of carbon-black reported here shows that there is an excellent match between the two spectra although the band at 1200 cm -~ reported in this present work appears to be broader than that reported by Kos- lovskii et al. 6

CONCLUSION

We are therefore forced to the conclusion that the only reliable reference for FT-IR-PAS spectroscopy is a well-characterized (preferably hexane soot) carbon-black material. While the acoustic frequency dependence of this reference may not be identical to that observed for a particular sample, the appropriate correction will be a smooth function across the infrared spectrum. Use of a single beam TGS detector spectrum as a reference is quite likely to result in anomalous bands, shoulders, bandwidths, and intensities which will seriously jeop- ardize the exacting work which is being done with the use of FT-IR-PAS.

Use of carbon-black as a reference material in the near UV and visible ranges has been discussed elsewhere, 7 with the conclusion that some spectral features were ob- served for different carbon-blacks in this spectral region. Others ~ have demonstrated variations in the mid-IR for

APPLIED SPECTROSCOPY 5,55

various carbon-blacks. The conclusion of the present work is that hexane soot is an adequate reference for the mid-IR spectral region. Credence is lent to this re- stricting requirement by the work of Low and Parodi 4 who showed quite clearly that chars, etc., produced from diverse materials such as sucrose, could exhibit substan- tial spectroscopic features when compared with a graph- ite absorber. However, hexane soot produced by the pro- cedure outlined by Keifer e t a l . 1 seems not to exhibit any features in the mid-IR FT-IR-PAS spectrum when analyzed by FT-IR-spectrometers with mirror velocities of 1.25 mm/s or greater. Work on the effect of particle size on FT-IR-PAS spectra has underlined the strength of the assertion that hexane-soot-derived carbon-black is flat. s That work has shown that even with physical mixtures of hexane soot and another material, no fea- tures of the other materials (e.g., p-nitrophenol) are ob- served until that material is present in the mixture at a loading of greater than 80 % by weight of the total mix- ture. Since hexane soot obviously has less functionality

than this, it is the conclusion of this work that hexane soot be proposed as a source compensation reference material for FT-IR-PAS measurements.

ACKNOWLEDGMENTS

The authors gratefully acknowledge complete support of this work by the U.S. Army Research Office under contract #DAAG 29-81-K- 0098.

1. J. R. Keifer, M. Novicky, M. S. Akhter, A. R. Chugtar, and D. M. Smith, SPIE 289, 184 (1981).

2. J. M. O'Reilly and R. A. Mosher, Carbon 21, 47 (1983). 3. W. M. Prest, Jr., and R. A. Mosher, ACS Symposium #200, Col-

loids and Surfaces in Reprographic Technology 11, 225 (1982). 4. M. J. D. Low and G. A. Parodi, Spectrosc. Lett. 13, 663 (1980). 5. S. M. Riseman and E. M. Eyring, Spectrosc. Lett. 14, 163 (1981). 6. L. D. Koslovskii, E. K. Galanov, and L. A. Shuvalov, Opt. Spec-

trosc. 24, 50 (1968). 7. C. H. Lochmuller, R. Rohl, and D. B. Marshall, Anal. Lett. 14, 41

(1981). 8. N. L. Rockley, M. K. Woodard, and M. G. Rockley, Appl. Spec-

trosc. 38, 329 (1984).

Spectrometric Characterization of Metribuzin and its Metabolites*

P. W. ALBRO, C. E. PARKER, t G. D. MARBURY, O. HERNANDEZ, and F. T. CORBIN Laboratory of Environmental Chemistry, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709 (P. W.A, C.E.P., G.D.M., O.H.); and Crop Science Department, North Carolina State University, Raleigh, North Carolina (F.T.C.)

Metribuzin and its three major plant metabolites have been character- ized by ultraviolet, infrared, and proton NMR spectroscopy and mass spectrometry. The reported structures for these compounds have been confirmed, except that deaminated Metribnzin (DA) exists mainly in the enol form rather than the keto form usually reported. Evidence is presented concerning the most probable sites of protonation during ti- tration of these compounds. Whereas diketo Metribuzin (DK) does undergo protonation of the amino group, Metribuzin itself apparently does not. Keto/enol tautomerism as a function of pH greatly complicates interpretation of the spectra. Metribuzin appears to form a poorly sol- uble, hydrogen-bonded dimer (or polymer) when titrated from basic to acidic pH.

Index Headings: Metribuzin; Sencor; DA; DK; DADK; IR spectra; NMR spectra; UV spectra; Mass spectra.

INTRODUCTION

Metribuzin (4-amino-6- (1,1-dimethylethyl)-3-(meth- ylthio)-l,2,4-triazin-5(4H)one; Bay 94337) is a systemic herbicide widely used for weed control in a variety of food crops, including soybeans, potatoes, and tomatoes.

Received 6 September 1983; revision received 16 January 1984. * Paper No. 8823 of the Journal Series of the North Carolina Agric.

Res. Serv., Raleigh, NC 27650. The use of trade names in this pub- lication does not imply endorsement by the North Carolina Agric. Res. Serv. of the products named, nor criticism of similar ones not mentioned.

t Author to whom correspondence should be addressed.

Varietal differences in susceptibility to metribuzin in- jury have been observed, and have been attributed to differences in metribuzin metabolism. 1-4 Three metab- olites have been identified: ~,6 deaminated metribuzin (DA), diketo metribuzin (DK), and deaminated diketo metribuzin (DADK). Although spectral evidence for this identification has not been been published in the open literature, numerous studies on metribuzin and its metabolites 1-3,7-1~ have been published which assume the structures of these compounds to be those shown in Fig. 1. Recent studies in this laboratory for the purpose of determining the pKa values of these four compounds have resulted in the collection of UV spectra at different pH values. 15 Inconsistencies among these spectra led us to suspect that the reported structure of one of the me- tabolites might not be correct as given. This paper re- ports evidence from UV, IR, NMR, and MS on the structures of these four compounds, and should be of assistance in the structure-determination of as yet un- identified metabolites of this herbicide. Additionally, some information is provided concerning the pH-depen- dent structural changes undergone by these compounds.

MATERIALS AND M E T H O D S

Standards and Solvents. Analytical standards of DA, DK, and DADK were obtained from Mobay Chemical Corp., Agricultural Chemicals Division, Kansas City, MO.

558 Volume 38, Number 4, 1984 ooo3-7o2s/84/,so4-o,~5652.oo/o APPLIED SPECTROSCOPY © 1984 Society for Applied Spectroscopy