Showing papers by "Purnendu K. Dasgupta published in 2019"

Carbonic Acid Eluent Ion Chromatography

Abstract: Alkali metals, amines and alkanolamines are separated on a poly(butadiene)-maleic acid on silica stationary phase using a carbonic acid (H2CO3*) eluent with and without a mineral acid. The H2CO3* eluent is prepared in situ by high pressure permeative introduction of gaseous CO2 through thin membranes supported upon a porous steel disk. The permeation flux and thus the eluent concentration is controlled by varying the applied CO2 pressure. This novel frit-supported membrane device tolerates much higher liquid and gas pressures than Teflon AF capillaries, permitting [H2CO3*] exceeding 0.53 M and attaining a pH of 3.3. Silicone was presently preferred over Teflon AF, both as planar membranes, as mechanical properties of the latter change as large amounts of CO2 dissolve in it. After separation, the CO2 can be efficiently removed via another gas permeable membrane device permitting detection of the eluting bicarbonate salt conductometrically in a background of nearly pure water. Most analytes are more sensiti...

Time-of-Sight Liquid Flow Measurements in the Low Nanoliters per Minute Scale.

Abstract: We describe an affordable and robust measurement technique applicable to nanoscale liquid flow. The approach can provide good precision (<1% RSD) in the 1.5-15 nL/min flow range. The motion of a conductive/nonconductive immiscible segmental interface in a capillary is followed by an admittance detector. The conductive marker segment, e.g., a salt solution, is protected on both sides from the principal flow stream by immiscible guard segments, typically a fluorocarbon (FC) liquid, of significantly greater impedance. Fluorosilylation of the capillary ensures no other liquid film between the FC segments and the wall (perfect piston). A given interface/marker can typically be used only once in interface/front tracking systems. We overcome this by putting the sensor capillary in a valved configuration where the flow direction in the sensor is reversed before the guard/marker segments escape. Several strategies are possible to interpret flow rate from the sensor output, including the rate of the interface movement. During the measurement process, a change in this rate of movement can be detected in <1 s. Small temperature variations in the 25-35 °C range did not affect sensor behavior.

Attenuation Coefficients of Tubular Conduits for Liquid Phase Absorbance Measurement: Shot Noise Limited Optimum Path Length.

Abstract: We trace the history of liquid core waveguides (LCWs, also called liquid core optical fibers) and the role Teflon AF (TAF) has played in their development. We show that, in any shot noise limited situation, the optimum signal-to-noise ratio (S/N) occurs at a path length of 1/αa{ln[1 + 2(αa/αb)]}, approximately 2/αb under most conditions, αa and αb being the light attenuation coefficient due to the analyte and the background, respectively. The analysis shows that LCW length should be selected depending on the applicable αb value. An overly long LCW may exhibit a lower signal-to-noise ratio. Water-filled TAF-clad fused-silica (FS) tubes show the lowest attenuation across the wavelength range. Nevertheless, except at λ ≥ 600 nm, the observed αb values far exceed those reported for pure water: it appears that both impurities in the water and waveguide losses are involved. In examining the attenuation in various water-filled tubes, we find that the transmission of air-surrounded FS tubes is second only to TAF-clad FS tubes and is better than that of TAF tubes or externally mirrored FS tubes. Surprisingly, except for a window centered at ∼250 nm, light transmission in a water-filled poly(tetrafluoroethylene) (PTFE) tube is worse than in poly(ether ether ketone) (PEEK) tubing. Light transmission in PTFE tubes improves with increasing wall thickness.

Direct Photothermal Measurement of Optical Absorption in a Flow System.

Abstract: We describe here a simple photothermal detection scheme in a flow stream based on the temperature difference upstream and downstream of the point of illumination. We use a single, two-junction 25 μm diameter thermocouple to measure the temperature change. The baseline standard deviation in the dark is ∼0.001 °C that increases up to 0.0016 °C depending on the illumination source. We demonstrate the detection of several chromatographically separated dyes both with a 1.5 mm and a 0.1 mm i.d. detection cell, respectively, with a white LED and a solid-state laser source. With an inexpensive 660 nm, 19 mW laser as the light source, the estimated detection limit for methylene blue (MB) was 30 nM, corresponding to 120 amol in the illuminated volume. The dimerization constant of MB and the quantum efficiency of the monomer was determined.