Design and characterization of a dynamic surface tension detector (DSTD) for use with flow injection analysis and highperformance liquid chromatography is reported. The DSTD optically measures the repeating drop rate at the end of a capillary tube connected to the terminating end of the flow system. Surface-active analytes cause a significant decrease in the drop volume that is readily measured. The selectivity
A model based on Polseullle flow Is expanded upon to describe the coupled effects of diffusion and convection on the sensitivity of the radial refractive Index gradient (RIG) measurement with the Z-conflguratlon flow cell for high-performance liquid chromatography (HPLC) detection. Diffusion and convection are shown to affect the radial concentration profile probed by RIG measurements. These effects were characterized by first examining the response for a polymer series with a diffusion coefficient range of 10-5 to 10-T cm* 2 34567/s at room temperature for a range of flow rates to determine the relative contribution of convection and diffusion as a function of analyte residence time within the flow cell. Experimentally, It was determined that the flow cell dependent convection contribution to the change In the radial concentration gradient was 43.34 ± 4.26 pm. Diffusion Is shown to appreciably affect RIG sensitivity when the translational migration of an analyte within the flow cell, as predicted by the Stokes-Elnsteln equation, Is greater than 20 pm. Diffusion and convection effects were then examined at a single, well-characterized flow rate over a range of temperatures from 25 to 125 °C. Sensitivity for analytes In the convection-dominated response region remained constant, while those In the diffusion-sensitive response area demonstrated a temperature dependence. The presence of diffusionand convection-dominated sensitivity response regions and the existence of a smooth transition between the two regions suggested that the effects of diffusion and convection were coupled but could be readily uncoupled. Since translational diffusion Increases with temperature, this temperature dependence is consistent with the proposed model. Lastly, the effect of temperature on relative baseline position and noise was examined. The flow rate dependent slope of the deflected angle for RIG baseline measurements was shown to have a temperature dependence, as the formation of a thermal lens within the flow cell caused a constant deflection of the probe beam. Baseline noise levels increased less than a factor of 3 over the 100-deg temperature range examined, thus the RIG detector Is compatible with hlgh-temperature and thermal gradient microbore HPLC.
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