Analysis of heterogeneous fluorescence decays. Distribution of pyrene derivatives in an octadecylsilane layer in capillary electrochromatography

The distribution of solute molecules in the stationary phase in capillary e lectrochromatography (CEQ has been investigated with time-resolved fluoresc ence in the frequency domain. The analysis of fluorescence decay poses a ch allenging problem for the complex decay kinetics of heterogeneous systems s uch as the C-18 stationary phase. The nonlinear least-squares (NLLS) method selects the decay model by minimizing the chi (2) value. The chi (2) crite rion, in conjunction with the requirement that the residues should be rando mly distributed around zero, frequently leads to a feasible set of multiple decay models that can all fit the data satisfactorily. The maximum entropy method (MEM) further chooses a unique model from the group of feasible one s by maximizing the Shannon-Jaynes entropy. The unique model, however, is n ot necessarily the most probable one. In this paper, the best model for the fluorescence decays of solute molecules is selected with NLLS using the ch i (2) statistics, the stability of the fit, and the consistency within repl icate experiments. In addition, the recovered lifetime parameters of the tr ue model should display the same trend as the fluorescence decay profiles w hen an experimental condition is varied. Using these criteria, a Gaussian d istribution of fluorescence lifetimes satisfactorily fits the data under al l experimental conditions. An additional minor component with a discrete li fetime is attributed to the systematic errors in the measurements. The dist ribution is a manifestation of an ensemble of heterogeneous microenvironmen ts in the stationary phase of CEC. MEM is not suitable for the modeling of CEC data because of its inaccuracy in recovering broad fluorescence lifetim e distributions and its lack of consistency in the replicate measurements i n the studies of high-voltage effects.