DETERMINATION OF CHLORINATED HYDROCARBONS IN WATER BY FIBEROPTIC EVANESCENT-WAVE SPECTROSCOPY AND PARTIAL LEAST-SQUARES REGRESSION

An application of the multivariate calibration technique of partial le ast-squares (PLS) regression to near-infrared spectra of a fiber-optic sensor based on the evanescent wave principle is presented. The sensi ng element consists of a quartz glass fiber with a silicone cladding w hich enriches nonpolar water contaminants. Due to the interaction of t he extracted molecules with the part of the light which is transmitted in the evanescent wave zone of the cladding, absorbance spectra of th e contaminants can be collected. In view of a sensor application for i n-situ environmental analysis, aqueous solutions of chlorinated hydroc arbon solvents (CHS), which often can be found as major water contamin ants, have been measured. PLS regression was applied to three sets of CHS samples, representing typical features of NIR evanescent wave spec tral data. These are, e.g., strong overlapping of the absorption bands of different CHS components, peak distortions due to temperature vari ations between reference and sample measurement and noisy data at anal yte concentrations near to the limit of detection, respectively. For t richloroethene and 1,1-dichloroethene, where the calibration model was built for samples within a small concentration range of 1-9 mg l-1, s atisfactory prediction results could be obtained with a relatively sma ll root-mean-square error of 0.3 mg l-1 compared to analytical referen ce measurements. In contrast to this, for a three component system of dichloromethane, trichloromethane and trichloroethene with strongly ov erlapping absorption bands, where samples over a very broad concentrat ion range from 3-4940 mg l-1 were included in the PLS model, the predi ction accuracy decreased enormously and for some samples strong deviat ions between real and predicted data occurred. Nevertheless, applying multivariate calibration to this difficult system with similar spectra l features and huge differences in the concentration of the species al lowed an acceptable spectral distinction and at least a semi-quantitat ive determination of the CHS species.