General theory of indirect detection in chromatography

Indirect detection is used in liquid chromatography to detect sample components that more or less lack detectable properties. A detectable component, the probe, is added to the mobile phase and the sample containing the components is injected. Every solute, even those not normally detectable, will give rise to detectable peaks due to the presence of the probe. The theory currently used to describe this situation is different for different adsorption isotherm models and also restrictions have to be imposed on the adsorption isotherm parameters. We present a general theory that describes both the retention times and areas of the detected peaks without making any a priori adsorption isotherm assumptions. Our general theory is compared with the current theory and validated by computer simulations. An alternative quantification method is suggested based on the measurement of the relative areas of the sample components without the requirement of having standard curves of known solutions of the sample components. Using this approach, the new theory is able to predict the solute concentrations in cases when the current theory did not, i.e., when the saturation capacities of the sample components and the probe differed from each other. In addition, a new application is proposed: the estimation of adsorption isotherm parameters.