ENHANCED POSSIBILITIES FOR IDENTIFICATION USING SERIES-COUPLED CAPILLARY GAS-CHROMATOGRAPHIC COLUMNS .2. RETENTION INDEXES AS AN IDENTIFICATION TOOL IN SELECTIVITY TUNING

For calculating retention (tR) times from retention indices in series-coupled column systems with different flow-rates in the individual columns, the dependence of the dead time and of the parameters a and b of the log tR versus carbon number plots for n-alkanes on the pressure of the medium is discussed. It is shown that the intercept a of the log tR versus carbon number plots is composed additively of an easily determinable flow-independent term and a flow-dependent term. On this bases, a method was developed for the precalculation of the retention times of any substances from their retention indices under the condition of selectivity tuning by varying the flow-rates in a system of series-coupled GC columns. The method allows statements on the presence or absence of compounds of interest by comparing calculated and measured system retention times. By repeating the identification algorithm at different pressure settings and, consequentl, flow-rates, the peak shifts expected in selectivity tuning can be pursued on the one hand; on the other, a degree of certainty about the positive identification of compounds can be achieved owing to the feasible avoidance of peak overlapping, which is not possible by means of the classical retention index concept. © 1990.