Showing papers by "Purnendu K. Dasgupta published in 1995"

Liquid Droplet. A Renewable Gas Sampling Interface

Abstract: As a gas stream flows across a liquid droplet, the soluble constituents contained in the gas diffuse to and dissolve in the liquid drop. From the viewpoint of analytical chemistry, this droplet is thus a potential sampling interface for such soluble constituents. The problem that needs to be solved relates to the manner of its incorporation in a chemical analysis system. This paper describes the coupling of this sampling interface with a sequential analysis system, demonstration of its applicability for the measurement of parts-per-billion levels of gaseous NH 3 , and a characterization of the performance of this experimental arrangement Various aspects of the behavior of the liquid droplet as a collection interface are theoretically discussed and experimentally examined using NH 3 and SO 2 as test gases.

Continuous Automated Measurement of the Soluble Fraction of Atmospheric Particulate Matter

Abstract: A new approach is introduced for the quantitative collection of aerosol particles to submicrometer size. The design and construction of a continuously operating system is described. Particles are continuously transferred to a liquid stream utilizing condensation of supersaturated vapor to promote particle growth followed by collection that relies on impaction and the thermophoretic effect. The automated system is simple to construct and operate and provides quantitative collection of aerosol particulate matter even at a flow rate of 10 L/min. Individual measurements of aerosol particles and soluble gases are achieved by combining the system with a wetted wall parallel plate diffusion denuder

Continuous Automated Measurement of Gaseous Nitrous and Nitric Acids and Particulate Nitrite and Nitrate

Abstract: An automated measurement system for HONO(g) and HNO 3 (g) along with particulate nitrite and nitrate is described. The system alternately measures the gas phase and aerosol phase constituents. The gases are collected by a parallel plate wet denuder, and the effluent aerosol is collected by steam condensation, followed by impaction/thermophoresis. The fully automated instrument provides separate but quantitative collection of gases and particles at relatively high flow rates and displays very low interference from NO and NO 2 . Coupled to an anion chromatograph, the detection limit estimated for a signal equivalent to that of three times the noise level of the blank is 110 parts per quadrillion (ppq, 10 -15 atm) and 230 ppq of gaseous HONO and HNO 3 , respectively, and 230 and 650 pg/m 3 aerosol nitrite and nitrate, respectively, for a 8-min integrated sample. The instrument operates over prolonged periods with minimal attention. Data are reported for outdoor ambient air measurements and for indoor air with kerosene-fueled space heaters in operation.

A Renewable Liquid Droplet as a Sampler and a Windowless Optical Cell. Automated Sensor for Gaseous Chlorine

Abstract: A droplet of a reagent solution is formed at the tip of a tube centered in a cylindrical chamber through which a gas sample is aspirated. The solution is continuously pumped ; the drop grows, falls, and another drop grows again. The droplet serves not only as a reproducible collector for the sample gas flowing around it but also as a reactor for a chromogenic reaction and as a windowless optical cell. The design and characteristics of this dynamically growing/falling droplet-based gas sensor system are described ; the performance parameters are attractive relative to a static drop. In particular, such systems can be internally calibrated for any humidity effects : at a constant pumping rate, the drop period/frequency is a predictable function of sample relative humidity. The feasibility of the sensor is demonstrated by the fully automated and continuous determination of gaseous chlorine using tetramethylbenzidine solution as a chromogenic collection liquid. At levels relevant to industrial hygiene monitoring, an 18 μL droplet-based sensor equipped with a light-emitting diode photodiode-based detector shows a relative standard deviation of 1.2% (pCl 2 ∼900 ppbv, drop period 1.1 min) while the corresponding blank standard deviation is equivalent to ∼1 ppbv. There appears to be a great potential for such drop-based collectors with in situ photometric or electrochemical signal transducers as automated sensor devices in biphasic measurements (trace gases, solvent extraction, etc.).

Analytical chemistry in a liquid film/droplet

Abstract: The measurement of nitrogen dioxide at the parts-perbillion level is described. The experimental arrangement consists of two optical fibers placed on opposite sides of and in contact with a liquid film (14-57 μL in volume) supported on a U-shaped wire guide and two tubular conduits (one of which constitutes the means for the delivery of the liquid). Light from a green (555 nm) light-emitting diode enters the liquid film, composed of Griess-Saltzman reagent The transmitted light is measured by a referenced photodetection arrangement Sample gas flows past the droplet at a low flow rate (typically 0.10-0.25 L/min). The response is proportional to the sampling period and the analyte concentration. The limit of detection for this nonoptimized arrangement is estimated to be <10 ppb by volume for a 5 min sample. Some unusual characteristics are observed. The initial absorbance, when most of the analyte/reaction product is still near the surface, is higher than that when the content of the droplet is fully mixed. The signal depends on the sample flow rate in a nonmonotonic fashion, first increasing and then decreasing with increasing sampling rate ; the specific chemistry involved in the collection and determination of NO 2 may be responsible.

Measurement of Gases by a Suppressed Conductometric Capillary Electrophoresis Separation System

Abstract: This paper describes the direct measurement of soluble ionogenic atmospheric gases by a suppressed conductometric capillary electrophoresis separation system (SuCCESS). A small circular wire loop is incorporated at the sampling end of a fused silica capillary located immediately at the tip in the same plane as the capillary. When the loop is dipped into a solution and withdrawn, a liquid film is formed on it. The film is in fluid communication with the capillary and acts as a microreservoir. When the film end is lifted relative to the destination side, all or part of the film contents can be injected into the capillary. To perform gas sampling, a series of automated operations are conducted with a commercial CE instrument modified in a minor fashion : the film-bearing loop is lowered into a sample chamber, and air is sampled for a preset period of time at a preselected flow rate (typically 1 min at 100 cm 3 /min). The capillary is then lifted to introduce an aliquot from the film for analysis and then dipped into the running electrolyte source vial, and electrophoresis is commenced. Under the above sampling conditions, 1 ppb SO 2 can be detected. The system should be applicable for use with other detection modes and nonaqueous electrolytes.

Apparatus and method for flow injection analysis

Abstract: Method and apparatus for flow injection analysis (FIA) using an electroosmotic pump. The apparatus includes: an electroosmotic pump having a grounding joint; a sample injection valve for introducing a sample into a carrier stream which valve is in fluid communication with the grounding joint by way of a conduit; a dispersion coil in fluid communication with the sample injection valve; and a detector which is in fluid communication with the dispersion coil. The grounding joint couples the electroosmotic pumping system and the FIA system but electrically isolates them. Generally, the method utilizes one fluid which is electroosmotically pumped to propel a carrier for flow injection analysis at a controllable flow rate. More specifically, the method includes the steps of: adding a sample to a liquid carrier stream to form a sample zone in the carrier stream; flowing a liquid pumping stream by electroosmosis; and connecting the pumping stream with the carrier stream to propel the carrier stream.

Two-dimensional conductometric detection in ion chromatography. Analyte identification, quantitation of very weak acid anions, and universal calibration

Abstract: A universal and sensitive detection method for anion chromatography is described. Following suppressed conductometric detection carried out with an NaOH eluent and an electrical suppressor, the eluite is transported through a microscale electrodialytic NaOH generator and a second conductivity detector. While the first detector responds primarily to strong acid anions, the second detector records a decrease in the NaOH background signal, regardless of the pK a of the analyte. Routine detection of very weak acids at the low micromolar level is possible. The dual detection method not only constitutes the most generally applicable approach to IC by combining the detection merits of both suppressed and single-column IC, it provides possibilities of peak identification beyond retention characteristics. Area ratio values from the two detector signals provide unique signatures for each analyte ion. It is possible to estimate the limiting equivalent conductance of an unknown eluite and the analyte concentration without specific calibration. The method is also effective in diagnosing the occurrence of coelution.

Apparatus for capillary electrophoresis having an auxiliary electroosmotic pump

Abstract: Apparatus for capillary electrophoresis having an auxiliary electroosmotic pump. Following a conventional capillary electrophoresis system having a single power supply, a separation capillary, and an optical detector, a conductive membrane connects a second capillary to a second power supply. The second capillary and second power supply act together as an auxiliary electroosmotic pump capable of augmenting or inhibiting the bulk electroosmotic flow in the separation capillary. The apparatus can be used to optimize the stacking profile of the sample and, thus, improve the separation efficiencies for charged solutes.

Artifact peroxides produced during cryogenic sampling of ambient air

Abstract: Peroxides were found to be produced as artifacts during cryogenic sampling with Horibe traps. Cryogenic trap sampling was compared to collection with a wet effluent diffusion denuder and a Nafion membrane diffusion denuder. Hydrogen peroxide and hydroxymethyl hydroperoxide measured in the cryogenic trap samples were significantly higher. In comparison, no evidence of artifact methyl hydroperoxide production was found. The amount of artifact H2O2 and HMHP produced increased with decreasing trap temperature. Spiking ambient air with ethene or isoprene showed that these hydrocarbons, in the presence of ozone, can be responsible for the artifact production of peroxides. Our results clearly suggest that the peroxide data obtained by cryogenic sampling and reported in the literature should be interpreted with caution.

Fast voltammetric sensors for the measurement of soil water activity

Abstract: Small inexpensive soil water sensors and the necessary electronics package for fast voltammetry (l04 V/s scan rate) are described. The sensor consists of a blunt ended hypodermic needle cathode containing a concentric PTFE insulating tube and therein, a Pt-wire anode. The sensing film is formed by dip-coating a 1:7 blend of a perfluorosulfonate and a polyestersulfonate polymer at the tip of the needle. Atop this sensing film is a composite protective film of cellulose acetate and nylon that inhibits the passage of ionic or low MW constituents and allows the transport of water. The sensor is sheathed in a PTFE tube that protrudes beyond the sensor tip and the protrusion gap is filled with a paste of talc and BaSO4 which serves as a water conduit between the soil and the sensor proper. The sensor is capable of making reliable moisture measurements between the wilting point and the field capacity of the soil sample; the response behavior consists of the combination of two nearly linear segments, the sensor output increasing steeply between the wilting point and ca. 60% of the field capacity and then much more gradually to the field capacity. The sensors have been field tested. No effect on sensor performance is observed even after burial under soil for several months.