Detection of Chlorinated Hydrocarbons in Aqueous Surfactant Solutions by Near-IR Raman Spectroscopy XUELONG SHI, STEPHEN J. PARUS, KURT D. PENNELL, and MICHAEL D. MORRIS* Department of Chemistry (X.S., S.J.P., M.D.M.) and Department of Civil and Environmental Engineering (K.D.P.), University of Michigan, Ann Arbor, Michigan 48109-1055 Near-IR Raman spectroscopy is used to detect chlorinated hydrocarbons under sorfactant-enhanced soluhilization conditions. The Raman bands of tetrachloroethylene (PCE) and 1,2-dichlorobenzene (DCB) in micelle solutions could be observed in the presence of humie acid when a 784- nm diode laser was used. With 532- or 632.8-nm excitation, humic acid fluorescence obscured the Raman signals. For quantification, the inte- grated area of the carbon-chlorine stretch mode (PCE) or the phenyl ring-breathing mode (DCB) was used. Test results for samples with unknown concentrations based on linear calibration curves were in agree- ment with results from an accepted gas chromatography method. De- tection limits were calculated to be 240 ppm for tetrachloroethylene and 500 ppm for 1,2-dichlorobenzene. Our study has shown the feasibility of this technique for field applications. Index Headings: Raman spectroscopy; Diode laser; Chlorinated hydro- carbons; Surfactant solutions; Environmental analysis. INTRODUCTION The migration and fate of nonaqueous-phase liquid (NAPL) organic contaminants in the subsurface have been the subject of intensive investigation in recent years.~ Of particular concern are sites contaminated with one class of nonaqueous-phase liquid organic contaminants, chlo- rinated solvents, which, because of their high densities and low viscosities, are not typically confined to the un- saturated ground-water zone. 2 To prevent further migration of these contaminants, and to allow safe use of already contaminated areas, con- taminated sites require immediate remedial action. One promising approach is surfactant-enhanced aquifer re- mediation (SEAR). 3-6 This technique is based on the am- phipathic property of the surfactant and can be carried out by injecting aqueous surfactant solutions into con- taminated areas. These solutions contain surfactant con- centrations sufficient to cause the formation of micelles, which are responsible for the enhanced solubilization of organic liquids in aqueous phase. The successful field implementation of this remediation technique requires an accurate and portable analytical tool to assist in as- sessment of NAPL recovery efficiency and to identify NAPL compositions and locations. Analytical techniques currently available for determi- nation of organic concentrations in surfactant/organic mixtures are not well suited for in situ or on site mea- surements. Gas chromatography has excellent detection limits but usually requires relatively long instrument time and sample preparation steps. 7,8 Infrared absorption mea- surement is difficult or impossible in aqueous systems because of strong water background absorption. 9 Received 28 December 1994; accepted 20 April 1995. * Author to whom correspondence should be sent. Raman spectroscopy, on the other hand, appears quite promising for detecting those organics because they are strong Raman scatterers and water is a weak Raman scat- terer. ~° Sampling systems for Raman spectroscopy are generally simpler than those needed for infrared spec- troscopy. Most importantly for field applications, in the visible or near-infrared regions, glass containers and glass optics can be used in Raman spectroscopy instead of fragile and expensive crystal optics used in infrared ab- sorption spectroscopy. The solubilities of chlorinated hydrocarbons in pure water are too low (150 ppm for tetrachloroethylene, for example) for normal Raman spectroscopy, and surface- enhanced Raman spectroscopy is often used for direct detection. ~,~2 However, because of the enhanced solu- bilities of organics in surfactant solutions, efficient re- mediation procedures can concentrate the compounds to such concentration levels that normal Raman spectros- copy is adequate to detect those organics. For monitoring remediation processes, low detection limits are less im- portant than the existence of compact, reliable field-de- ployable instrumentation. The simplicity and portability of modern Raman in- strumentation, along with the minimal sample prepara- tion requirements, make the technique attractive. But practical applications of the technique under surfactant- enhanced solubilization conditions will be possible only if the surfactant and humic acid do not give strong flu- orescence background and interfere with the analyses. We have investigated the fluorescence background from sur- factant and humic acid using different excitation sources (532-nm Nd:YAG, 632.8-nm He-Ne, and 784-nm diode laser). It was found that, in the presence of humic acid, the Raman signals of chlorinated hydrocarbons can be observed with 784-nm diode laser excitation, but they are obscured in the large fluorescence background with visible laser excitation. In addition, calibration and quan- tification procedures have been developed, and the results of Raman spectroscopic measurements have been veri- fied by comparison with those from accepted analytical techniques. EXPERIMENTAL Apparatus. Raman spectra were obtained with locally constructed systems. The excitation source was a diode pumped Nd:YAG operated at 532 nm (Adlas, Stow, MA), a 632.8-nm He-Ne (MWK Industries, Corona, CA) or a 784-nm semiconductor diode (Model 5412-H 1, Spectra Diode Labs, San Jose, CA). In each case, the laser power at the sample was about 25 roW. Small glass containers were used to hold the samples. Excitation and back-scat- 1146 Volume 49, Number 8, 1 9 9 5 0003-702S/95/4908-114652.00/0 APPLIED SPECTROSCOPY © 1995 Society for Applied Spectroscopy