Use of polarized ATR measurements to simplify referencing in continuous sample monitoring by absorption spectroscopy

Abstract

Using the difference in the effective path lengths of perpendicular- and parallel-polarized light in attenuated total reflectance (ATR) spectroscopy, we demonstrated acquisition of infrared (IR) absorption spectra without needing periodic measurements of a standard reference such as an empty optical cell. With a liquid sample constantly in contact with the ATR internal reflection element (IRE), we used the perpendicular-polarized signal, which has lower effective path length, as the reference signal, and the parallel-polarized signal, as the sample signal. To demonstrate the validity of this method and the need for periodic measurements of a standard reference in the conventional method, we measured unpolarized and polarized ATR spectra of air and isooctane using a Fourier transform infrared (FTIR) spectrometer on two consecutive days. For the conventional method, we used the unpolarized signals of air and isooctane to calculate the absorbance and showed that there were some artifacts with baseline curvature when the air signal from day 1 was used with the isooctane signal from day 2, but not when we used the air and isooctane signals from the same day. The artifacts were due to some inevitable drifts in the instrument between days; hence, the air signal must be measured as often as the instrument drift demands. With the proposed method, we showed that the absorption spectra could be approximately reproduced between days without any measurements of air. The proposed method merits consideration for process analytical technology (PAT) applications for example of continuous flow through an ATR cell where the introduction of a new substance (e.g., air or vacuum) for periodic baseline measurements is non-trivial.