LASER-BASED DYNAMIC SURFACE-TENSION DETECTION FOR LIQUID-CHROMATOGRAPHY BY PROBING A REPEATING DROP RADIUS

Modifications to the design of the time-based dynamic surface tension detection (DSTD) system are reported. Both a theoretical and experimental evaluation of the modifications are presented to demonstrate a reduction in flow-rate fluctuation derived systematic noise. The instrument-based limit of detection in relative surface tension change for the improved volume-based DSTD is determined to be 0.15% (3 x root-mean-square noise) as compared to the 0.45% level achieved and previously reported for the time-based method. The radius-based limit of detection far the drop is determined to be 0.5 mu m, and may be limited by the vibrational stability of the drop. Detector selectivity is shown to remain unchanged after the modifications as only the observed noise levels are affected. Limits of detection for a variety of commercially available surfactant solutions are presented, along with those for a number of common organic and ionic chromatographic mobile phase modifiers, in order to demonstrate the enhanced selectivity of the DSTD for surface active species. The utility of the volume-based DSTD is demonstrated for the detection of trace intermediate to high molecular mass impurities in a high concentration low molecular mass surface active polymer matrix by comparing the response of refractive index and volume-based dynamic surface tension detection of a Carbowax sample analyzed using size-exclusion chromatography.