4.5.2 Indirect Photometry

Indirect photometry utilizes a monitoring ion that is UV-active to trace the presence of a non UV-active compound. An ion of the running buffer is chosen, generally to have a high UV absorbance intensity, and its absorbance is monitored. When the sample zone passes through the detector, it will be accompanied by a corresponding drop in the UV absorption due to the monitoring ion.

A very simplified explanation for this phenomenon is that the monitoring ion is displaced by an analyte with the same charge based on electroneutrality. However, the displacement ratio can be greater than one and analytes with charges opposite to the monitoring ion can be detected. This can be better explained by a closer analysis of the Kohlrausch Regulation Function.