Novel Method for Sampling Oil Spills and for Measuring Infrared Spectra of Oil Samples Chris W . Brown,’ Patricia F. Lynch, and Mark Ahmadjian Department of Chemistry. University of Rhode Island, Kingston, R. I . 02881
During the past year we have used infrared spectrometry to monitor oil spills which occurred along the New England Coast and to test simulated oil slicks in the laboratory. Much of our sampling has been done by dipping a AgCl window into the water containing the spill and measuring the spectrum of the oil that adhered to the window. However, after weathering, both in the laboratory and on the sea, for a period of time, it is difficult to separate the oil from the water. The water and lighter oils form emulsions, whereas the water is occluded in the heavier oils. In both cases, it is almost impossible to obtain an adequate spectrum. We have tried chemical extraction, vacuum distillation, and freeze drying to eliminate the water. All were unsuccessful and usually added or removed chemical components. Recently, we discovered the method discussed in this paper. This method reduces the water content to negligible amounts and also provides a new way to measure infrared spectra. Samples are collected by dipping A1 foil into water containing the oil or by smearing the oil on A1 foil. The oil adheres to the foil, whereas the water is repelled and will eventually drain off. In this way, most of the water is eliminated from the samples. The foil also serves as an excellent “sample cell” for oil samples. To eliminate the need for removing the oil from the foil after the water drains off, the spectrum can be measured by replacing one of the plane (flat) mirrors in the sample optical path with the A1 foil. All of the spectra were recorded on a Perkin-Elmer Model 521 infrared spectrometer. This instrument has only one small plane mirror in the sample optical path, but this mirror is inconvenient to reach and adjust. Since we did not want to disturb the permanent optical alignment of the instrument, we did not use this mirror. Instead, we found it more convenient to use a Wilks Model 45 Universal Micro Sampling System (any infrared specular reflection attachment could be used). This sampling system is designed to obtain spectra of micro samples and attenuated total reflection spectra. However, in the present work we modified the system and used it for a different purpose. The optical diagram for the microsampling system, shown in Figure 1, is placed in the sampling compartment of the instrument so that the light from the source is reflected by mirror MI into the beam condenser portion of the system (M3 and M4). Mirror Mg reflects the light back into the monochromator. For our experiments with oil on A1 foil, we machined a plate from 78-inch A1 sheet to the exact size and shape of the plane mirror M2, and we replaced M2 with this plate, which serves as a backing for the A1 foil. To obtain a spectrum of an oil sample on A1 foil, the foil is placed on the backing plate with its shiny surface toward the light beam--i.e., the foil is used as a reflector surface replacing mirror M2. All mirrors including the A1 plate ( a t Mz) are adjusted in the prescribed manner to give a maximum transmitted signal a t 2000 cm-I ( a frequency at which the oil does not absorb), and a spectrum is recorded in the usual manner.
The capabilities of this method to remove water and to provide good spectra are demonstrated in Figures 2 and 3. Two spectra of the same field sample are shown in Figure 2: spectrum a was obtained by the A1 foil technique, and spectrum b by conventional transmission spectrometry using AgCl windows. Both samples are of comparable
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Figure 1. Optical diagram of the Wilks Micro Sampling System. Mirror M 2 is replaced with the AI foil and backing plate
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Figure 2. Infrared spectra of a field sample of oil in the 0 - H and C-H stretching region Spectrum a , AI foil technique: spectrum b, conventional transmission
Author t o whom correspondence should be addressed
s p e c t r u m using AgCl w i n d o w s
Figure 3. Three spectra in the 650 to 1200 cm-’ region using the AI foil technique.
tions; however, the HzO band a t -3400 cm-1 is almost absent from the spectrum obtained by the A1 foil technique. Our method of identifying petroleum samples is based on a fingerprint of bands between 650 and 1200 cm ( I ) . Spectra, in this region, of oil samples obtained by the A1 foil technique are shown in Figure 3. Spectrum a is of a sample from a known origin and spectrum b is of the same sample after weathering in the open sea for several days. The similarity shows that spectra obtained by this technique are suitable for identification. Spectrum c is of the same sample as is shown in Figure 2. Using conventional transmission spectrometry the spectrum of this sample was of very poor quality due to the presence of water, but on A1 foil (spectrum c) the quality of the spectrum compared favorably with a and b. It should be emphasized that this method gives transmission and not reflection spectra. The light is transmitted through the oil film (or absorbed by it), reflected by the A1 foil, and transmitted back through the oil film to the next mirror in the optical system. Thus, the light passes through the oil twice ( i e . , the effective sample path length is doubled), and the absorption of each band is increased.
Spectrum a. sample of known origin; spectrum b, same sample after weathering on the sea for several days; spectrum c. sample that originally contained excessive water (shown in Figure 1)
Received for review June 25, 1973. Accepted September 24. 1973.
thicknesses as can be seen by comparing the intensity of the band a t -2900 cm due to the C-H stretching vibra-
(1) P. F. Lynch and C. W. Brown, Environ. Sci. Techno/., in press
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A N A L Y T I C A L C H E M I S T R Y , VOL. 46,
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