Uncoupling the effects of convection and diffusion on refractive index gradient detection in high-temperature liquid chromatography

Abstract

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.

A Microscale-Molecular Weight Sensor: Probing Molecular Diffusion between Adjacent Laminar Flows by Refractive Index Gradient Detection

“…A RI-based detector originally developed by Pawliszyn provided very sensitive measurements of the RIG created by the axial concentration gradient of an analyte as it passes through a capillary. − This system probed the RIG by measuring the deflection angle of a laser beam. RIG detection has also been developed where the radial concentration gradient was probed. − The data were found to be dependent on the diffusion coefficient of the analyte, and thus the molecular weight, when the appropriate FIA sample introduction conditions were met. , The scale of this previous molecular weight sensor system was significantly larger compared to the μ-MWS dimensions we report here. Furthermore, the time scale for the molecular weight determination was longer, ∼90 s 31,32 versus 3 s with the μ-MWS described here.…”

“…Refractive index (RI) detection has been widely used in traditional-scale HPLC and CE for many years, − and more recently, RI detection has been developed for microscale analysis. − This universal technique has been widely used to analyze polymers, sugar, proteins, and other analytes that do not appreciably absorb or fluoresce. A RI-based detector originally developed by Pawliszyn provided very sensitive measurements of the RIG created by the axial concentration gradient of an analyte as it passes through a capillary. − This system probed the RIG by measuring the deflection angle of a laser beam.…”

Detection Modes with High Temperature Liquid Chromatography—A Review

“…However they are not extremely sensitive and are traditionally plagued by baseline drift and noise level much higher than other detectors. A refractive index based detector able to work with thermal gradient applications in liquid chromatography has been implemented about 10 years ago (164). The conclusions of the study were that liquid chromatography at high temperature with the refractive index detector may be used without loss of sensitivity for analytes if high flow rates are used, or if the eluent is well cooled at a constant temperature before entering the refractive index detection system.…”

Noncontact and Noninvasive Monitoring of Gas Diffusion from Aqueous Solution to Aprotic Solvent Using the Optical Beam Deflection Method

“…The optical beam deflection (OBD) method, , based on the mirage effect, is a well-known noncontact and noninvasive analytical method, which can be applied to a physical or chemical system where a refractive index gradient is produced. Since the refractive index of a substance is a function of temperature, it has been used for probing a temperature gradient, which is generated by either the nonradiative relaxation of photon energy absorbed by molecules , or reaction heat released from a chemical reaction. − It also has been used for probing a concentration gradient generated in an electrode/solution interface, − flow injection systems, − and capillary electrophoresis. , The concentration gradient-induced OBD method has been used for evaluating the diffusion coefficients of some electrolytes in aqueous solutions. , The transient signal of the concentration gradient-induced OBD has been used for separating large and small species, such as sucrose and poly(acrylic acid), since they have different diffusion coefficients …”

“…Since the refractive index of a substance is a function of temperature, it has been used for probing a temperature gradient, which is generated by either the nonradiative relaxation of photon energy absorbed by molecules 14,15 or reaction heat released from a chemical reaction. [16][17][18][19][20][21] It also has been used for probing a concentration gradient generated in an electrode/ solution interface, [22][23][24][25][26] flow injection systems, [27][28][29] and capillary electrophoresis. 30,31 The concentration gradient-induced OBD method has been used for evaluating the diffusion coefficients of some electrolytes in aqueous solutions.…”