New and Emerging Analytical Techniques for the Detection of Organic Contaminants in Water
Detailed chemical analysis of water is a prerequisite for assuring the safety of water supplies. Limitations in our ability to identify contaminants in water also limit our ability to assure water quality and to assess environmental impacts. Recent improvements in analytical techniques have expanded the "analytical window," that is, the compound range that is amenable to specific identification. Using these improved tools, much has been learned about the occurrence, fate, and transport of organic chemicals in the environment. The boundaries of the analytical window represent the "analytical frontiers" and are continuously expanded. These boundaries are defined loosely by the contaminant concentration and chemical properties, such as molecular weight, polarity, chemical lability, and structural complexity.
In typical natural water samples, the mass of compounds that is within the analytical window (i.e., that can be specified in terms of both a specific structure and concentration) is small compared to the total organic carbon. Using coupled gas chromatography/mass spectroscopy (GC/MS) in conjunction with derivatization, the specifically identified compounds accounted for less than 12 percent (or < 1 mg/L) of the organic carbon in different groundwater samples (Reinhard et al., 1994). For 80 percent (or 2 to 10 mg/L) of the total organic carbon, this fraction typically remains uncharacterized or only in aggregate form (e.g., in terms of average chemical properties, functional group content, or size distribution) (Fujita et al., 1996).
Even though good mass spectra are obtained from most of the contaminants that are detected, most remain unidentified and/or unquantified because of a lack of reference spectra and/or reference compounds. The information that can be deduced from the mass spectra may be enough to propose structures using spectral determinations. However, verification and quantification of the proposed structures are often impossible because of a lack of reference spectra.
Since most of the organic carbon in environmental samples is not amenable to specific identification, aggregate properties (or group parameters) are often used as indicators of general water quality. Included in this category are such parameters as elemental composition, functionality, acidity, metal binding properties, total halogen content, ultraviolet (UV) absorption, fluorescence properties, and molecular size distribution. Although very important, analysis of these parameters is not discussed in detail here.
Analytical procedures consist of several interdependent operations carefully tuned to provide maximum efficiency. Some of the analytical operations that may apply. The sensitivity of the method defines the lower limit of the sample size that must be processed. The sample is processed to preserve, recover, isolate, and/or concentrate the analytes to produce extracts that are compatible with subsequent instrumental analyses. The processes that lead to a concentrate that can be analyzed using instrumental methods are called sample preparation. Derivatization is a microanalytical technique that serves one or several of the following purposes: increase recovery from the aquatic matrix, facilitate separation from other organic constituents, and/or improve identification or detection. Because reference compounds for most contaminants are not available, synthesis is often necessary for structure verification and the development of rapid detection and quantification procedures.
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Journal of Chromatography & Separation Techniques