Imprecision in Circle Cell FT-IR Analysis of Aqueous Solutions

Factors which produce imprecision during quantitative analysis of aqueous solutions using the CIRCLE CELL® have been investigated. The FT-IR spectra of aqueous acetone solutions have been recorded over the concentration range 0.01 to 90% (w/w). Using a micro-boat sampling cell, fixed in position within the CIRCLE CELL® optical bench, we have found that the uncertainty in repositioning the micro-boat sampling cell, within the CIRCLE CELL® optical bench, was the most important factor causing imprecision during quantitative analysis of aqueous solutions, especially as the detection limit was approached. At low concentrations, quantification using peak height values was more precise than that using integrated peak area values. Integrated peak area values, however, gave a more linear response and better correlation with Beer's law over a large concentration range. The reliable detection limit was found to be 0.05% (w/w).