EVALUATION OF REFRACTIVE-INDEX ARTIFACTS IN LIQUID-CHROMATOGRAPHIC ABSORBENCY DETECTION - EXTENSION TO NONIDEAL INJECTION AND GRADIENT ELUTION

An anomalous response is often observed in high-sensitivity absorbance detection upon a rapid change in solvent composition. This detector artifact appears to be related to refractive index gradients arising when the injection solvent differs from the mobile phase or during solvent programming. Because this anomalous response is often of the same magnitude as the true solute absorbance, misinterpretation of the acquired chromatogram may result. Characterization of this refractive index artifact has been accomplished by modelling the Z-pattern flow cell as a dynamic lens, with constantly changing refractive index profiles both radial and axial to the direction of flow. A ray-tracing algorithm is utilized to predict the final image diameter and the apparent absorbance caused by the changing refractive index conditions. Initial predictions for an ideal, delta function injection show excellent correlation with the experimentally observed response for a commercially available absorbance detector. Simulations are extended to include the more complex axial profiles commonly encountered in liquid chromatography, including those arising from non-ideal injection conditions as well as solvent programming. Predictions based on this model show good agreement with the shape, direction and magnitude of the observed detector response. In addition to aiding in the characterization of solute peaks, this ray-tracing model has direct implications for the accurate interpretation of system peaks in liquid chromatography. Although investigations described here are limited to the Z-pattern cell, this dynamic lens model may be adapted to the evaluation or improved design of any flow cell of interest. © 1990.