Determination of Band Broadening in SEC by Analyzing Narrow Standards with Molar Mass-Sensitive Detectors

G. MEIRA, M. YOSSEN, J. VEGA

Bratislava

Conferencia; 20th Bratislava International Conference on Macromolecules, ?Advanced Polymeric Materials? (APM 2006); 2006

International Union of Pure and Applied Chemistry (IUPAC)

A theoretical method was proposed for estimating the (asymmetrical and non-uniform) band broadening function (BBF) in size exclusion chromatography (SEC). The following data is required: a) the concentration- and molar mass chromatograms from a set of narrow standards; and b) the molar mass calibration logM(V). In the narrow range of each standard, the BBF is assumed uniform but skewed. Such uniform BBFs are estimated through a nonlinear optimization procedure that compares one of the measured chromatograms with its theoretical prediction based on the other. The BBF can be assumed arbitrary or represented by an exponentially-modified Gaussian (EMG). The EMG is defined by 2 parameters: the standard deviation s, and the exponential decay t. The global non-uniform BBF covering the complete fractionation range is obtained by interpolation between the uniform BBFs. In each analysis, the molar mass chromatograms must be corrected for the inter-detector volume (IDV), prior to estimating the uniform BBF. The IDV is generally unknown, especially for detectors are in parallel. For detectors in series, an upper limit of the IDV is given by the shift between peaks of the LS and DR chromatograms. The theoretical method was extended to simultaneously estimate the BBF and the IDV. From the BBF, the BB-corrected chromatograms are calculated, and the ratio between the BB-corrected chromatograms provides an unbiased estimate of logM(V). Thus, the best IDV-BBF solution was chosen as that which better reproduced the (a priori known) calibration. The extended method was validated with 2 SEC experiments involving different chromatographs, detectors, and column sets. The precise determination of the IDV seems vital for an adequate estimation of the BBF, and this problem is particularly serious when the detectors are in parallel. The non-uniform BBFs resulted skewed, and somewhat narrower in the mid-range of elution volumes. At present, no explanation can be given to this last observation.