Chapter 19
A Fiber-Optic Spectrochemical-Emission Sensor as a Detector for Volatile Chlorinated Compounds 1
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Κ. Β. Olsen , J. W. Griffin , B. S. Matson , T. C. Kiefer , and C. J. Flynn
Downloaded by UNIV OF ARIZONA on August 6, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0479.ch019
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Pacific Northwest Laboratory, P.O. Box 999, Richland, W A 99352 Department of Physics, Eastern Washington University, Cheney, WA 99004 2
A radio frequency induced helium plasma (RFIHP) detector was designed and tested as a sensor for volatile chlorinated com pounds. The RFIHP detector uses a critical orifice air inlet and an RF-excited sub-atmospheric pressure helium plasma to excite the ambient air sample. The excitation source is coupled to a fiber-optic cable and associated collection optics to monitor the emission intensity of the 837.6-nm emission line of chlorine. The RFIHP detector demonstrated linearity from 0 to 500 ppmv car bon tetrachloride with a correlation coefficient of 0.996 and excellent reproducibility. The detection limit for carbon tetra chloride in air was 5 ppmv. Fluorinated compounds can also be readily analyzed by changing the analytical wavelength to 739.9 nm.
With the heightened concern about environmental issues that developed in the 1970s and 1980s, numerous laws were enacted to protect the human population and the environment from exposure to toxic and hazardous chemicals. Two of the more notable laws enacted by the United States Congress were the Compre hensive Environmental Response, Compensation and Liability Act ( C E R C L A ) , better known as "Superfund," and the Superfund Amendments and Reauthoriza tion Act (SARA). These laws give the President of the United States authority to mandate cleanup of uncontrolled hazardous waste sites that are a threat to public health or the environment. As of November 1989, 1010 sites had been identified and classified as Superfund sites, with 209 additional sites proposed (1). Following identification and classification, the C E R C L A process requires that a Remedial Investigation/Feasibility Study be conducted on all Superfund sites to determine the extent and types of contaminants present in air, water, and soil/sediment samples. Because of the sheer volume of samples sent to the laboratory for analysis, the cost to produce legal, defensible analytical data, the limited number of analytical laboratories available to conduct the analysis, and the S A R A schedule deadlines, a reduction in the number of samples is 0097-6156/92/0479-0326$06.00/0 © 1992 American Chemical Society
In Element-Specific Chromatographic Detection by Atomic Emission Spectroscopy; Uden, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
Downloaded by UNIV OF ARIZONA on August 6, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0479.ch019
19. OLSEN ET AL.
A Fiber-Optic Sensor for Volatile Chlorinated Compounds 327
required. Clearly, the development of new screening methods for use by field teams is needed to decrease the number of samples being sent to the laboratory and to prioritize these samples. The fiber-optic emission sensor offers many attractive features for real-time multipoint environmental field monitoring. These features include the small probe size, the multiplex advantage (i.e., multiple probes with one central detection and data acquisition system), and the potential for fast response. In this study, a sensor was developed that is capable of real-time, in situ moni toring for chlorinated hydrocarbon vapors in the vadose zone (region from ground surface to the water table) at Superfund and hazardous waste sites. A chlorine-specific sensor was selected because a study conducted by Plumb and Pitchford (2) concluded that a high percentage of hazardous waste sites across the country had volatile chlorinated organic compounds contaminating the ground water. Of the 15 compounds most often identified, 10 contain chlorine. This clearly suggests the prevalence of chlorinated hydrocarbon contamination at hazardous waste sites and indicates the need for a detector that could specifically measure chlorine-containing compounds. Plasmas as atomic emission sources (AES) interfaced to gas chromatographs (GCs) and optical spectrometers have demonstrated their usefulness as elementspecific detectors since the first demonstration by McCormack et al. (3) in 1965. Since then, the majority of the work cited in the literature on specific element detectors has been focused on the microwave-induced helium plasma (MIP) as the excitation source of choice for G C - A E S (4-7). The G C - M I P sys tem was found to provide sufficiently low detection limits and elemental specificity for most elements of interest. However, a major drawback of the MIP source was the plasma's inability to remain stable or lit when air was injected into the carrier gas stream. Operating a stable plasma with small but measurable quantities of air without destroying the excitation characteristic of the plasma is a critical requirement for a detector designed to measure total chlorine in vadose-zone air. In 1985, Rice et al. published a study using a lowfrequency, high-voltage, electrodeless discharge-plasma (8). They found that this plasma had detection limits in the picogram range for a multitude of ele ments, including chlorine, phosphorus, sulfur, and mercury. They also found that the plasma was reasonably tolerant to the presence of contaminants and air. Additional advantages included the compactness of the plasma source and low helium consumption. Taking advantage of all the favorable characteristics of that system, we have designed a chlorine-specific detector capable of meas uring volatile organic halogenated hydrocarbon compounds in air. Experimental A schematic diagram of the components used to construct the R F I H P probe is given in Figure 1. The operating parameters of this system are compared with those of the helium discharge-afterglow system reported by Rice et. al (8) (Table I). Major differences of the R F I H P system include using a 6-mm-o.d. χ 0.5-mm-i.d. quartz tube as the plasma chamber, placing a 50-/mi-i.d. χ 10.2-cm-long segment of uncoated capillary column at one end of the plasma tube (used as a critical orifice), operating the plasma chamber at sub-ambient pressure, and viewing the plasma axially. The overall experimental system with specific components used is shown in Figure 2. A schematic of the optical
In Element-Specific Chromatographic Detection by Atomic Emission Spectroscopy; Uden, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
In Element-Specific Chromatographic Detection by Atomic Emission Spectroscopy; Uden, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992. e
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VICI Metronics Dynacalibrator Model 450
Figure 1. Schematic diagram of the radio frequency induced helium plasma detector experimental system. (Reproduced with permission from ref. 9. Copyright 1990 The Society of Photo-Optical Instrumentation Engineers.)
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