Molar Range Detection Based on Sideband Differential Absorption Spectroscopy with a Concentrated Reference

Conventional absorption spectroscopy (CAS) with a blank reference has only a slight capacity to detect high concentrations at characteristic wavelengths owing to the corresponding large molar absorption coefficient (ε) on the scale of 10³ or 10⁴ cm^-1 M^-1. To monitor concentrated analytes as high as the molar range in a plating bath and on a chemical production line, we propose a new approach using sideband differential absorption spectroscopy (SDAS). SDAS is obtained by subtracting the absorption spectra of the samples, A(λ,C_x), from that of a reference containing a concentrated standard analyte, A(λ,C_ref>C_x), resulting in concave spectra with peaks at the sideband of conventional spectra with generally low ε values on the scale of 100 cm^-1 M^-1 or less. The negative absorbance changes linearly with the sample concentration at a certain peak wavelength, obeying Lambert-Beer's law. In this work, SDAS was obtained and verified using inorganic and organic substances, such as chromate potassium, rhodamine B, and paracetamol.