A novel approach to monitor coating amount by short-wavelength near-infrared spectroscopy using a tracer with a long-chain hydrocarbyl group.

Investigation into the use of near-infrared (NIR) as a Process Analytical Technology has been conducted for in-process monitoring of coating amounts for oral pharmaceutical products. However, the low specificity of NIR spectra has made it time consuming and costly to establish quantitative calibration models for commercial production. Here we revealed that long-chain hydrocarbyl group compounds containing saturated hydrocarbon chains, such as cetyl and stearyl, exhibit specific and strong absorption in the short wavelength (SW)-NIR region (800-1,100 nm) with limited interference from peaks corresponding to other components. To simplify the quantitative model, we used cetanol as a model tracer of coating amount to enhance detection sensitivity and analytical precision. The coating amount on crystalline cellulose granules was determined only from the intensity of NIR absorption at a single wavelength, which was attributed to the tracer. The results showed close agreement with quantitative analyses from gas chromatography and measurement of weight gain. In conclusion, we determined coating amount with considerable accuracy from NIR absorption at a single wavelength in the SW-NIR region using the long-chain hydrocarbyl containing compound as a tracer, thereby eliminating the need for complicated statistics.