Liquid cells of constant path length with high density polyethylene windows for the ir region are generally difficult to obtain. Most commercially available polyethylene cells are sufficient only for survey spectra and cannot be used for careful measurements during long or repeated scans since repeatability and adequate absorbance measurements are difficult to obtain. Moreover, the measurement of air sensitive solutions is often unsuccessful because air leaks develop and because determining the actual cell thickness is not easy.
Andreas T. Tsatsas and William M. Risen, "Far Infrared Sealed Liquid Cell with Polyethylene Windows," Appl. Spectrosc. 24, 383-384 (1970)