SEQUENTIAL CHROMATOGRAM RATIO TECHNIQUE - EVALUATION OF THE EFFECTS OF RETENTION TIME PRECISION, ADSORPTION-ISOTHERM LINEARITY, AND DETECTOR LINEARITY ON QUALITATIVE AND QUANTITATIVE-ANALYSIS

A study of the sequential chromatogram ratio technique was done to characterize and correct for retention time variation, peak-shape change due to concentrations exceeding the adsorption isotherm linear range, and mild detector nonlinearity over a broad concentration range. A sensitive minimization routine was developed to overlay chromatographic peaks from sequential injections using a simple displacement along the time axis. Precise peak alignment to within 0.8 % of the baseline peak width was necessary for qualitative interpretation of the ratio chromatogram. For quantitation of the ratio of injected concentrations for pure-component peaks, time shifts as large as 2.5 % of the base-line peak width caused a negative bias of 1 % or less. A peak-shape analysis method, based on the scaled point-by-point difference of normalized peaks, was adapted as a diagnostic test for peak-shape change and as a means of correcting the ratio value of Injected concentrations. At a concentration 4 times the linear isotherm limit, the ratio chromatogram was distorted, and the estimated correction to the ratio value was +2.3 % of the true value. Thus, restriction of the ratio technique to within the linear isotherm region is recommended. A nonlinear power-law detector model, which preserved the qualitative features of the ratio chromatogram, was easily incorporated into the quantitation method. Detector nonlinearity corrections to the ratio values, equal to +2.0% and +5.8%, were obtained when the true ratios of injected concentrations were 2.0 and 8.0, respectively.