546
Anal. Chem. 1982, 5 4 , 546-548
(13) Forster, L. S.; Dudley, D. J . Phys. Chem. 1962, 66, 838-840. (14) Flemlng, a. R.; Knight, A. W. E.; Morris, J. M.;Morrison, R. J. Roblnson, G. W. J . Am. Chem. SOC.1977, 99, 4306-4311. (15) Sadkowskl, P. J.; Fleming, G. R. Chem. Phys. Lett. 1978, 57, 526-529.
s.;
RECEIVED for review July 13, 1981. Accepted November 12,
1981. The authors gratefully acknowledge the support of the National science Foundation (Grant N ~CHE-7915801) . and of an Alfred P. Sloan Fellowship (J.W.B.). This work was performed in partial fulfillment of the requirements of the M.S. degree (E.A.H.) from the Department of Chemistry, University of Colorado, Boulder, CO.
Photoacoustic Infrared Spectroscopy in the Investigation of Bonding Effects in a Formulated Pesticide S. R. Lowry," D. G. Mead, and D. W. Vldrlne Nlcolet Instrument Corporatlon, 5225 Verona Road, Madison, Wisconsin 537 I I
Photoacoustic infrared spectroscopy has been used to study the interactionsof a carbamate insecticlde wlth a clay carrier. The ability of photoacoustic infrared spectroscopy to measure infrared spectra from opaque samples nondestructiveiy Is particularly valuable in thls study where weak bonds mlght be destroyed by sample grindtng. The results of thls study show that the strong N-H stretchlng modes, which appear at approximately 3300 cm-' In the pure insecticlde, are misslng in the subtraction of the N-H group spectrum. Thls suggests that the hydrogen attached to the nltrogen of the carbamate Is forming a reasonably strong bond with the hydrated silicate structure of the clay carrler. Thls lnteractlon may affect the release rate of the pesticide upon appilcation.
Because of the difficulties in applying low levels of potent pesticides evenly over fields, the compounds are frequently formulated onto an inert material in order to facilitate handling. The types of materials commonly used as carriers are clay, sand, or even ground corn cobs. The rate at which the pesticide is released from the carrier plays a crucial role in the overall activity of the compound. Any chemical binding between the pesticide and the carrier could strongly affect the rate of release. Although infrared spectroscopy has proven to be an excellent technique for investigating both chemical bonds and weaker types of bonding, most infrared methods require grinding and mixing of the sample before the spectrum can be obtained. The sample preparation might severely change any subtle interactions between the active compound and the carrier. Several papers have recently appeared in the literature describing the use of photoacoustic spectroscopy in the infrared region (1-4). The technique utilizes the ability of Fourier transform infrared spectrometers to modulate radiation from each infrared wavelength at a different audio frequency. If this modulated energy is focused into a sealed cell, any absorbed energy can be detected as sound when the energy is emitted thermally. The major advantage of photoacoustic infrared spectroscopy is that the technique is nondestructive and the sample need not be transparent to infrared radiation. These advantages suggest that photoacoustic/FT-IR spectroscopy should be very applicable to the study of formulated pesticides. This paper will describe an investigation of 8 systemic carbamate pesticide which has been formulated on clay granules (5). 0003-2700/82/0354-0546$01.25/0
EXPERIMENTAL SECTION The pesticide examined in this work was Thiofanox, a potent systemic and contact carbamate insecticide developed by Diamond Shamrock Corp. The structure is shown below. 0 tH3
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3,3-dimethyl-l-(methylthio)-2-butanone U-[(rnethylamino)carbonyl loxime Technical grade Thiofanox is normally formulated by hot spraying onto granulated diatomaceous earth. The 10% samples in this study were prepared in this manner. The lower formulations were prepared by dissolving weighed portions of Thiofanox in methylene chloride and adding to the granules. The samples were then dried in a rotoevaporator. Samples of 1%,3%, and 7.5% Thiofanox on clay were prepared in this manner. All spectral data were acquired with a Nicolet 7199 FT-IR spectrometer and a plug-in photoacoustic accessory whose design is similar to that described by Blank and Wakefield (6). The samples are run by simply placing the granulated material in the small sample cup, 5 mm diameter by 3 mm depth. The cup is then mounted in the photacoustic cell with an airtight seal. All spectra were acquired at 8 cm-l resolution and ratioed t o R spectrum of carbon black in order t o normalize the intensities at various wavelengths. Because of the need to modulate the infrared wavelengths at frequencies in the audio range, the mirror velocity of the Michehon interferometer was quite slow (0.094 cm/s). This mirror velocity modulates the 1000 cm-l range at about 200 Hz. At least 1000 interferograms were coadded before transformation in order to reduce the noise inherent in solid phase infrared photoacoustic spectroscopy. In many cases, a water spectrum was subtracted from the spectra. This was necessary because water adsorbed on the granules would desorb in the sealed cell during the measurement time.
RESULTS AND DISCUSSION Figure 1 shows the measured photoacoustic spectra of carbon black powder, water vapor, and a sample of Agsorb, a granulated clay. The carbon black curve closely approximates that of a black body absorber and gives a good measure of the energy available at each frequency. This spectrum is similar to a single beam transmittance spectrum. The ratio of a photoacoustic spectrum to one from carbon black powder yields a spectrum whose intensities are normalized to the total 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982
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Figure 1. (A) Single beam spectrum of carbon black. (B) Vapor phase spectrum of water. (C) Slngle beam spectrum of clay granules.
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Figure 3. (A) Spectrum of 10% Thlofanox on Agsorb. (8) Spectrum of blank Agsorb. (C) Subtraction result showing differences.
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Figure 2. (A) Ratloed spectrum of clay. (B) Water spectrum. (C)
Subtraction result showing the elimination of water. energy present. Figure 2 shows the ratioed spectra from the blank carrier and the water. The spectrum of the blank carrier shows a major problem with any hydrated sample in a photoacoustic experiment. Once the cell is sealed, small amounts of water will desorlb into the transmitting gas, and the resulting spectrum is dominated by the stronger signal from the vapor-phase molecules. The last spectrum in Figure 2 was obtained by subtracting the water spectrum from the blank carrier. The major peaks in the difference spectrum correspond to a hydrated silicate structure. The strongest band a t approximately 1020 cm-l can be assigned to the Si-0 vibrations. The peaks in the 0-H stretch region (3800-3000 cm-l) and the strlong peak a t 1630 cm-‘ indicate that substantial amounts of water are adsorbed on the clay. The broad peak at approximately 3600 cm-l indicates that several different types of hydrogen bonds are present. The top spectrum in Figure 3 is the water-corrected spectrum from a sample of 10% Thiofanox formulated on the Agsorb. Although the spectrum is still dominated by the
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Figure 4. (A) Thlofanox on granules by subtraction. (E) Transmission spectrum of reference Thiofanox in absorbance mode.
Agsorb peaks, the peaks corresponding to the C-H stretch of the tert-butyl group and the C-0 stretch of the carbonyl are clearly present. The second spectrum in Figure 3 shows the water-corrected blank carrier. A comparison of the 0-H[ stretch regions in the two spectra shows that the total absorbance is less in the spectrum from formulated sample. Thiii difference suggests that the presence of the Thiofanox may hinder water adsorption on the clay. The third spectrum in Figure 3 is the subtraction result produced by zeroing out the strong silicate peak. The negative peaks in the 3600-cm-I region show that the absorbance difference was real and not, simply due to differences in total intensity of the spectra Figure 4 shows the difference spectrum from the previous, figure and an absorbance spectra of the Thiofanox taken in transmittance mode. Although the two spectra are quite similar, two significant differences can be observed. The first
548
ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982 9
Figure 5. Spectra for quantitative analysis: (A) 1% Thiofanox, (6) 3% Thiofanox, (C) 7.5% Thiofanox, (D) 10% Thiofanox.
concerns the negative peaks in the 0-H, N-H stretch regions in the difference spectrum. While the negative peaks in the 3600 cm-l region indicate a reduction in hydration, the lack of a positive peak at 3400 cm-l suggests that the hydrogen on the N-H group of the carbamate must be forming a strong bond with the carrier. A second difference in the two spectra occurs in the lo00 cm-l region. The absorbance spectrum from the neat Thiofanox has a strong peak a t approximately 950 cm-' which is very weak in the subtraction spectrum of the formulated sample. This peak can probably be attributed to the -C-N-0- structure of the carbamate, While this difference does suggest some interaction with the silicate, the intensity may be due to saturation in the dominant Si-0 peak in the region. Although the subtraction result indicates that the material is binding with the carrier, the type of binding is not proven. A sample of the formulated pesticide was extracted with chloroform in order to provide evidence for actual chemical bond formation. If a reaction is actually taking place, some amount of the Thiofanox would be changed or permanently bonded to the silicate substitute. The spectrum from the extracted material closely matched the reference spectrum from Thiofanox and no peaks corresponding to organic material were observed in the spectrum from the extracted granules. This result clearly demonstrates that the Thiofanox is not reacting with the clay and any interactions are reversible. The results described above strongly suggest that the Thiofanox is forming a hydrogen bond with the carrier and that this bond formation decreases the sites available for hydration. In order to better understand this effect, we performed a series of experiments on samples containing reduced amounts of pesticide. Figure 5 shows the subtraction
results for samples of 1%, 3%, 7.5%, and 10% Thiofanox on the agsorb carriers. Several effects can be observed as the amount of Thiofanox is reduced. First, the subtraction results have improved in the 1000 cm-' region. This indicated that even in a spectral area with potential saturation problems, the peaks can be nulled when the sample and reference are similar (7, 8). Second, the lack of a negative peak at 1630 cm-l in the lower concentration samples indicates that more hydration sites are available, and the degree of hydration is approaching that of the bland carrier. Finally, the drastically reduced N-H stretch peak in spectra A and B suggests that most of the carbamate molecules are binding with the clay, while in the 10% sample a significant number of molecules are not forming a strong hydrogen bond. An attempt to quantitate the percent Thiofanox based on the intensity of the carbonyl peak was not totally successful. Although the intensity of the peak did decrease with the concentration, the magnitude of the peak is the high concentration samples was relatively smaller than expected. Two possible explanations for this effect are sample degradation or limited penetration depth in the samples. Because of the requirement for a modulated signal, the information measured in photoacoustic spectroscopy is mainly from the surface of the sample. The effective depth of measurement is a function of both sample absorbance and the modulation frequency. If the Thiofanox layer is sufficiently thick, significant amounts of signal may be lost. Although quantitative photoacoustic should be feasible (9), a better understanding of the spectroscopic variables involved in the photoacoustic experiment is necessary before the technique can be used with confidence. In these experiments we have utilized photoacoustic spectroscopy to ensure that no changes in the spectra are caused by sample preparation methods. The results demonstrate that a measurable interaction is ocurring between the carbamate and the clay structure. Extraction studies indicate that the bond is weak and reversible.
ACKNOWLEDGMENT The assistance of C.L. Gray from Diamond Shamrock Corp. is gratefully acknowledged.
LITERATURE CITED (1) Rockley, M. G. Chem. Phys. Lett. 1979, 68, 455. (2) Vidrine, D. W. Appl. Spectrosc. 1980, 314, 34. (3) Mead, D. G.; Lowry, S. R.; Vdrine, D. W.; Mattson, D. R.; Perkowitz, S. Ed. I n Fourth International Conference on Infrared and Millimeter Waves, Miami, FL, Dec 10-15, 1979, p. 231. (4) Mead, D. G.: Lowry, S. R.; Anderson, C. R. Int. J . InfraredMi//imeter Waves 1981, 2 , 23. (5) Lowry, S. R.; Gray, C. L. I n "Pesticide Analytical Methodology"; Harvey. J., Jr., Zweip, G.. Eds.; American Chemical Society: Washington, DC, 1980, p 2 9 8 (6) Blank, R. E.; Wakefleid, T. Anal. Chem. 1979, 51, 50. (7) Krishnan, K.; Hili, S. L.; Witek, H.; Knecht, S.Proc. SOC.Photo-Opt. Instrum. Eng. 1981, 289, 96-98. (8) VanEvery, K. W.; Hamudeh, I.M.; Griffiths, P. R. Proc. SOC.PhotoOot. Instrum. €nu. 1981. 289, 114-117. (9) Rkckiey, M. G.; Eavis, D. M.; Richardson, H. H. Appl. Spectrosc. 1981, 35, 165.
RECEIVED for review August 3, 1981. Accepted December 11, 1981.
