Feasibility of Remote Detection of Water Pollutantsand Oil Slicks by LaserExcited Raman Spectroscopy Mark Ahmadjian and Chris W. Brown’ D e p a r t m e n t of Chemistry, University of Rhode Island, Kingston, R. I. 02881
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A simple, inexpensive optical system has been constructed for obtaining Raman spectra of samples located remotely from the instrument. The capabilities of this system are demonstrated by obtaining spectra of dilute solutions of NO3- and of oil films on the surface of water with the samples located 21 ft from the instrument. During the past few years considerable emphasis has been placed on the development of analytical methods to automatically monitor the chemical composition of our natural waterways. Most of the current methods require that samples be collected and analyzed in a laboratory, all of which consumes considerable time and manpower. Thus, there is an obvious need for instrumentation to do the analysis automatically without having to remove samples from their environment, and we have been exploring the feasibility of using Raman spectroscopy to fulfill this need. Previously, we have shown that it is possible to detect inorganic anions such as NO3-, so42-, Po43-, and C032- in the range of 25-75 ppm by laser-excited Raman spectroscopy using conventional instrumentation (Baldwin and Brown, 1972). In a similar study Bradley and Frenzel (1970) detected benzene in water a t 50 ppm. Thus, it is possible to detect both inorganic and organic pollutants a t low concentrations by laser-excited Raman spectroscopy using conventional instrumentation (Baldwin and Brown, 1972). In a similar study Bradley and solutions of anions and of oil films on the surface of water.
Experimental Raman spectra were measured on a Spex Industries Model 1401 double monochromator using photon-counting detection and a CRL Model 52A argon-ion laser emitting I(-600 mW power a t the laser). a t 4880 or 5145 ! A schematic of the optical system for the remote detection of spectra is shown in Figure 1. The collimated laser beam is reflected by the plane mirrors M 1 (30 mm diam), Mz (12 mm diam), and M3 (102 X 127 mm) and is focused by lens L1 (113 mm diam and 165 mm F.L.) onto the sample. The scattered light is collected by the same lens, over a large angle, and is returned to the monochromator as a large collimated beam. Just prior to the monochromator this large beam is focused by lens LZ (same di-
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Environmental Science & Technology
Results and Discussion Initially, the remote optical system was aligned and tested using cc14 as the sample. The spectrum of CC14 measured with the remote system is shown in Figure 2. From this spectrum it was possible to make a rough comparison of the sensitivity of the remote system compared to spectra of samples measured in the spectrometer. The overall sensitivity of the remote system, taking slit width and band intensities into consideration, is about 1ilo that of conventional sampling optics. However, the spectrum of cC14 demonstrates that good spectra can be obtained by the remote system. After obtaining the spectrum of CC14 with the remote system, we changed the sample to 1M NaN03, and a good spectrum was obtained. The concentration of N a N 0 3 was reduced to one-half and the spectrum recorded again. This process of reducing the concentration and recording the spectrum was continued until the minimum detectable concentration was reached. The spectrum of 300 ppm NO3- is shown in Figure 3. The only band observable a t this concentration is VI, the symmetric stretching vibration, a t 1051 cm-1. At present, the minimum detectable level for NO3- with the remote optical system is -150 ppm. The desired level of detection is 45 ppm, which is the upper limit specified for drinking water (U.S. Public Health Service, 1962). As an additional demonstration of the remote detection system, we have used it to detect =2 grade oil (household heating grade) on the surface of water. The spectrum of the oil in the C-H stretching region is shown in Figure 4. First, we obtained a spectrum of the oil sample contained in a capillary tube in the spectrometer (spectrum a ) , then on the surface of water with the sample located in the spectrometer (spectrum b ) , and finally on the surface of water by the remote system (spectrum c ) . Oils are weak Raman scatterers, they give a high fluorescent background, and they tend to vaporize in the laser beam.
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Schematic of the optical system used for obtaining Raman spectra of samples located 21 f t from instrument
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mensions as L1). The image formed by this lens is then focused onto the entrance slits of the monochromator by a Rexator E1.7 camera lens L3 (55 mm F.L.). The latter lens is mounted in an X Y Z translator to allow for maximum adjustment. To obtain a reasonable distance ( 2 1 f t ) in our laboratory, two mirrors (102 x 127 mm) were placed between M2 and L2 to reflect the scattered light into the spectrometer. These are not included in the optical diagram, since they were use$ only for convenience. With the exception of the camera lens, all of the mirrors and lenses were obtained from Edmond Scientific Co. and were relatively inexpensive. The trick to the optical arrangement is that the laser beam has a very small diameter (-1 mm), whereas the collimated scattered beam has a large diameter (-13 cm). The geometry of the optical system takes advantage of these facts so that both beams traverse the same optical path between the instrument and the sample.
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RAMAN SPECTRUM OF CCI,
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Figure 3. Raman band of 300 ppm. of N o s - in water obtained with remote system
Therefore, it is difficult to measure their Raman spectra even under ideal conditions. However, the problem of vaporizing is reduced by having the oil on the surface of water, since the water dissipates the heat. The spectra in Figure 4 clearly demonstrate that the spectrum of oil measured remotely is comparable to those taken in the sample compartment. There are several advantages to this remote optical system. Since the laser beam and the scattered beam are both collimated over most of the light path, it is possible to extend the light path to almost any realistic distance. Furthermore, by enclosing the optical path in a light-tight pipe, such as 4-in. plastic plumbing pipe, interferences
from daylight or other external lights can be avoided. This, latter advantage is made possible by the fact that the laser and scattered beams traverse the same path to and from the sample. Raman spectroscopy shows considerable promise of becoming a valuable method for detecting chemical water pollutants. The present work suggests that the method can be used to detect pollutants in situ from a remote location, and that it is ideally suited for direct detection of pollutants with the spectrometer located on shore or in a boat. L i t e r a t u r e Cited Baldwin, S. F., Brown, C. W., W a t e r R e s . , 6,1601 (1972). Bradley, F. B., Frenzel, C. A . , i b i d . , 4, 125 (1970). U.S. Public Health Service, “Public Health Service Drinking Water Standards,” Publication No. 956. Government Printing Office, Washington, D.C., 1962.
Receiced for review N o c e m b e r 2, 1972. A c c e p t e d J a n u a y 22, 1972. T h i s u o r k was supported by t h e Office of W a t e r Resources Research, D e p a r t m e n t of t h e Interior. T h e R a m a n i n s t r u m e n t o tion x a s purchased w i t h m a t c h i n g f u n d s f r o m t h e N a t i o n a l Science F o u n d a t i o n a n d t h e Lrniuersity of R h o d e Island.
Volume 7, Number 5, May 1973
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