In situ time-resolved fluorescence spectroscopy for capillary electrochromatography (CEC) is described in the frequency domain. Fluorescence decay of the solute molecules is collected directly in the packed stationary phase of the CEC capillary. The fluorescence lifetime profile of the solute molecules reveals the microenvironments they experience in the C18 chromatographic interface. A quartz flow cell and experimental optimization of the signal-to-noise ratio are described that enable the collection of high-quality decay data and subsequent calculation of fluorescence lifetime profiles of the solute molecules. The distribution of pyrene (PY), 1-pyrenemethanol (PY-MeOH), and 1-pyrenebutanol (PYBuOH) into the C18 stationary phase and the solute-C18 phase interactions are probed, under separation conditions for CEC. All three molecules display a Gaussian distribution of lifetimes, consistent with an ensemble of heterogeneous microenvironments in the C18 stationary phase. The least polar molecule PY diffuses deeply into and interacts extensively with the C18 phase, experiencing high hydrophobicity and significant heterogeneity of microenvironments. The retention order of PY-MeOH, PY-BuOH, and PY in CEC is determined by their interactions with the stationary phase, revealed by their fluorescence lifetime distributions. [References: 53]
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