Showing papers by "Purnendu K. Dasgupta published in 1997"

Intercomparison of six ambient [CH2O] measurement techniques

Abstract: From May 29 to June 3, 1995 a blind intercomparison of six ambient formaldehyde measurement techniques took place at a field site near the National Center for Atmospheric Research in Boulder, Colorado. The continuous measurement methods intercompared were tunable diode laser absorption spectroscopy, (TDLAS); coil/2,4-dinitrophenylhydrazine, (CDNPH); 1,3-cyclohexanedione-diffusion scrubber (CHDDS); and the coil enzyme method (CENZ). In addition, two different cartridge methods were compared: silica gel-2,4-dinitrophenylhydrazine (DPNH) systems and a C-18-DNPH system. The intercomparison was conducted with spiked zero air (part 1) and ambient air (part 2). The CH2O standards for part 1 were calibrated by several independent methods and delivered to participants via a common glass manifold with potential trace gas interferants common to ambient air (O3, SO2, NO2, isoprene, H2O). The TDLAS system was used to confirm the absolute accuracy of the standards and served as a mission reference for part 1. The ambient phase lasted 44 hours with all participants sampling from a common glass tower. Differences between the ambient [CH2O] observed by the TDLAS and the other continuous methods were significant in some cases. For matched ambient measurement times the average ratios (±1σ) [CH2O]measured/[CH2O]TDLAS were: 0.89±0.12 (CDNPH); 1.30±0.02 (CHDDS); 0.63±0.03 (CENZ). The methods showed similar variations but different absolute values and the divergences appeared to result largely from calibration differences (no gas phase standards were used by groups other than NCAR). When the regressions of the participant [CH2O] values versus the TDLAS values, (measured in part 1), were used to normalize all of the results to the common gas phase standards of the NCAR group, the average ratios (±1σ), [CH2O]corrected/[CH2O]TDLAS for the first measurement period were much closer to unity: 1.04±0.14 (CDNPH), 1.00±0.11 (CHDDS), and 0.82±0.08 (CENZ). With the continuous methods used here, no unequivocal interferences were seen when SO2, NO2, O3, and isoprene impurities were added to prepared mixtures or when these were present in ambient air. The measurements with the C-18 DNPH (no O3 scrubber) and silica gel DNPH cartridges (with O3 scrubber) showed a reasonable correlation with the TDLAS measurements, although the results from the silica cartridges were about a factor of two below the standards in the spike experiments and about 35% below in the ambient measurements. Using the NCAR gas-phase spike data to calibrate the response of the silica gel cartridges in the ambient studies, the results are the same within statistical uncertainty. When the same gas phase calibration was used with the C-18 cartridges, the results showed a positive bias of about 35%, presumably reflecting a positive ozone interference in this case (no ozone scrubber used). The silica DNPH cartridge results from the second participant were highly scattered and showed no significant correlation with the TDLAS measurements.

Capillary ion chromatography with on-line high-pressure electrodialytic NaOH eluent production and gradient generation

Abstract: A small inexpensive system is described that allows high-performance suppressed anion chromatography on a capillary scale. A fully computer-controlled stepper motor-driven syringe-type dispenser, equipped with a 500 μL-capacity glass syringe is capable of pumping at pressures up to 1000 psi when equipped with an appropriate inlet check valve. Fused-silica capillary columns ∼50 cm in length and 180 μm i.d., packed in-house with a commercial packing, provide excellent performance, significantly exceeding the efficiencies observed for the same packing in commercially available 2 mm bore format. The system operates with a pressure drop of <800 psi at a flow rate of 2 μL/min. The system utilizes a novel electrodialytic NaOH eluent generator that is deployed on the high-pressure side of the pump and thus requires no special measures for electrolytic gas removal. This device permits both isocratic and gradient operation with excellent eluent purity; the NaOH concentration is generated linearly with applied curre...

Measurement of atmospheric formaldehyde using a drop collector and in-situ colorimetry

Abstract: A sensitive and affordable approach is described for the in-situ measurement of ambient formaldehyde. Air is sampled around a 100 microliter aqueous drop containing 3-methyl-2-benzothiazoline hydrazone. After a desired period of sampling (typ. 5 min) and a waiting period of 10 min for the reaction to be completed, a second reagent (FeCl3) is added to the drop by means of a conjoined conduit. A blue product is formed and is read after an additional 10 min of reaction by a fiber-optic/light emitting diode based photodetector. A fresh drop is then formed and the process begins anew. As demonstrated here, the limit of detection is ∼6.25 fig m−3 HCHO but can be significantly improved by using longer sampling times and a sampling rate higher than 100 ml min−1 used in most of this work. This is the first example of a chromogenic drop sensor that utilizes sequential reagent addition.

A Multiple Parallel Plate Wetted Screen Diffusion Denuder for High-Flow Air Sampling Applications

Abstract: Diffusion denuders are commonly used today for air sampling applications where it is necessary to discriminate between gaseous- and aerosol-phase analytes. In some applications, such as isotopic analysis, it is necessary to collect a sufficient analyte mass necessitating high sampling rates. Wetted denuders are preferred in many applications because they can be readily coupled to automated continuous analysis systems. We describe here a compact (5 cm diameter, 30 cm length) diffusion denuder with 11 individually wetted parallel screens (with an wetted area in excess of 0.1 m2) that allow operation at high flow rates. The effluent liquid is collected at the bottom from two V-shaped grooves. With SO2 as the test gas, ∼95% is collected even at a flow rate of 50 L/min with very low particle losses.

Oil Field Hydrogen Sulfide in Texas: Emission Estimates and Fate

Abstract: Hydrogen sulfide is released into the atmosphere during oil recovery operations. However, little is quantitatively known concerning the total sulfur flux due to these fugitive emis sions. A mobile laboratory, equipped to measure atmospheric gases and meteorological parameters, was used to collect quantitative data in the vicinity of various oil field operations. Fugitive emissions of H2S and soil sulfate levels were studied. Concentrations of atmospheric sulfur gases (SO2, H2S, mercaptans) were measured in the oil-producing regions of several west Texas counties. Hydrogen sulfide was by far the dominant reduced sulfur gas in all locations. Except in the vicinity of refining/processing plants, mercaptan levels were below 200 pptv. Sulfur dioxide levels were also very low, typically below 200 pptv. In all locations, a strong diurnal pattern of the ambient H2S concentration was observed. Specific sources, the flux, and the fate of the H2S emitted in oil field operations was studied. Crude oil storage tanks w...

A Falling Drop for Sample Injection in Capillary Zone Electrophoresis

Abstract: The inlet of a capillary is joined to an open stream of buffer formed on an inclined narrow surface, and a constant voltage is applied to the capillary A drop of sample falls directly to the capillary inlet and then is washed away quickly Thus, a short sample plug with limited dilution by the buffer stream and limited fronting or tailing is injected in electrokinetic mode No moving parts are involved, and no adjustment of voltage is necessary in the injection process The method is simple, reproducible, easy to automate and should prove to be a valuable complement to the present injection techniques for capillary zone electrophoresis

Electromigration injection from a microreservoir-electrode in capillary separation systems

Abstract: A capillary electrophoresis system (10) comprising: a separation capillary (20) with a first distal tip (30) and a second distal tip (140); a source vessel (50) containing a solution (40); a microreservoir-electrode (59) comprising a wire loop; a power source (60) connected to the microreservoir-electrode by wire (57); a control system (200); a detector (90); and a final destination vessel (160) containing electrolyte (150) and a ground electrode (155).

Electrochemical sensing of gases based on liquid collection interfaces

Abstract: Although many electrochemical gas sensors have been reported, electrochemical gas sensors based on liquid collection constitute a smaller subset. Minimally, a liquid interface based electrochemical gas sensor is composed of two electrodes and an ion conducting electrolyte. There is a large number of possible arrangements of these parts, and many choices exist for their composition and preparation methods. This results in a diverse and rich technology now available for gas sensing. The measurement of some analyte gases of interest, notably ozone, nitrogen oxides, hydrogen peroxide, formaldehyde, ammonia, sulfur dioxide and hydrogen sulfide are specifically discussed. Finally, the recent reviews that are likely to be the most relevant to the further development of electrochemical detection approaches for gases with a liquid collection interface are cited and discussed.

Chromatography on Water−Ice

Abstract: Ice contains a thin film of water on its surface, the thickness ranging from nanometer to micrometer levels, depending on the bulk temperature and the composition of the solution used in making the ice. It follows that water held on the surface of an ice particle should endow a column packed with microparticulate ice the properties of a chromatographic column containing a pellicular packing. We demonstrate here the normal phase separation of two dyes on a column packed with millimeter-size ice particles, held at -18 °C.