Consistency in Circle Cell FT-IR Analysis of Aqueous Solutions

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Abstract
The CIRCLE CELL®, based on the cylindrical internal reflection technique (CIR), has become a popular FT-IR accessory because it allows for easy collection of IR spectra from aqueous solutions. In this paper we describe a detailed study of the quality of quantification possible with the use of aqueous acetone mixtures over a concentration range of 0.01 to 90% (w/w). During a period of four months, the method's sensitivity, accuracy, precision, and time stability were evaluated. With the use of a Nicolet 60SX FT-IR, an MCT-A detector, 500 scans, and a 300-s sampling delay, five sets of triplicate runs, approximately one month apart, were made at each of 10 concentrations. Our results indicate the following conclusions: (1) quantification by peak height gives better precision, whereas peak area gives a more linear response and correlation with Beer's law over a large concentration range; (2) water bands produce spectral artifacts during subtraction, (3) repositioning of the CIRCLE CELL® optical bench between runs and the micro-boat sampling cell between samples critically affects the quantification results; (4) deviations from Beer's law are caused by chemical effects and insufficient resolution errors; and (5) the reliable sensitivity limit is 0.05% w/w. These findings suggest that precise quantitative analysis of aqueous solutions is now a practical method with the use of FT-IR spectroscopy.