A significant limitation to the use of ATR/FT-IR analysis for aqueous solutions is the relatively high bulk concentration of analyte that is required. One approach to improving the detection of an analyte is to incorporate a thin polymeric phase at the surface of the ATR element. The purpose of the polymeric phase is to extract the analyte of interest and concentrate it within the depth of penetration of the evanescent wave. With the use of a very high molecular weight polyvinyl chloride phase, the time necessary to reach equilibrium for a 0.05% (v/v) nitrobenzene in a 1.5% (w/v) methanol/water solution was over 60 min. A study was undertaken to determine whether incorporating a chloroparaffin plasticizer into the polymeric phase would reduce the time required to reach the maximum level of absorbance achieved for the analyte. Specifically, the ability of phases which are mixtures of PVC and chloparaffin plasticizers to concentrate the analyte from aqueous solutions has been investigated. The results indicate that incorporating chloroparaffin into a PVC phase reduces the time required to reach the maximum absorbance for the analyte in an aqueous solution containing 1.5% methanol by approximately 45%. The addition of chloroparaffin also results in an increase in the magnitude of absorbance observed for the analytes investigated in this study.
Marc C. Ertan-Lamontagne, Kimberly A. Parthum, W. Rudolph Seitz, and Sterling A. Tomellini, "Evaluation of Chloroparaffin/PVC Phases for ATR/FT-IR Analyses," Appl. Spectrosc. 48, 1539-1544 (1994)