Zhang Genfa

Continuous liquid-phase fluorometry coupled to a diffusion scrubber for the real-time determination of atmospheric formaldehyde, hydrogen peroxide and sulfur dioxide

Abstract An aqueous scrubber liquid is pumped through a filament-filled narrow bore microporous hydrophobic membrane tube while sample air flows around it. A constant fraction of the analyte gas, dependent on its diffusion coefficient and the effective mass accommodation coefficient in the scrubber liquid and on the diffusion scrubber dimensions and sampling rate, is collected in the liquid. One or more reagents are added to the effluent liquid for specific determination of the gas of interest. The Hantzsch reaction, peroxidase-mediated oxidation of p-hydroxyphenylacetic acid by peroxide, and bisulfite addition to 9-N-acridinylmaleimide, all direct adaptations of previously described continuous flow aqueous analytical procedures, are used for the determination of HCHO, H2O2 and SO2, respectively. A permeation-based SO2 source and previously described Henrys Law-based porous membrane sources of HCHO and H2O2 are integral to the respective instruments for calibration. Programmed inert valves allow the instruments to perform any sequence of zero, calibrate and sample functions. Each instrument occupies a standard two-tier (50×100 cm) laboratory cart and has been successfully field tested. With a filter fluorometer as detector, LODs are 100 pptv HCHO, 30 pptv H2O2 and 175 pptv SO2. There is no significant dependence on relative humidity. Specific interference testing ( SO 2 O 3 for HCHO and H2O2, O 3 H 2 O 2 H 2 SCH 3 SH for SO2) reveals no major interferences.

Intercomparison of six ambient [CH2O] measurement techniques

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.

Intercomparison of near real time monitors of PM2.5 nitrate and sulfate at the U.S. Environmental Protection Agency Atlanta Supersite

[1] Five new instruments for semicontinuous measurements of fine particle (PM2.5) nitrate and sulfate were deployed in the Atlanta Supersite Experiment during an intensive study in August 1999. The instruments measured bulk aerosol chemical composition at rates ranging from every 5 min to once per hour. The techniques included a filter sampling system with automated water extraction and online ion chromatographic (IC) analysis, two systems that directly collected particles into water for IC analysis, and two techniques that converted aerosol nitrate or sulfate either catalytically or by flash vaporization to gaseous products that were measured with gas analyzers. During the one-month study, 15-min integrated nitrate concentrations were low, ranging from about 0.1 to 3.5 μg m -3 with a mean value of 0.5 μg m -3 . Ten-minute integrated sulfate concentrations varied between 0.3 and 40 μg m -3 with a mean of 14 μg m -3 . By the end of the one-month study most instruments were in close agreement, with r-squared values between instrument pairs typically ranging from 0.7 to 0.94. Based on comparison between individual semicontinuous devices and 24-hour integrated filter measurements, most instruments were within 20-30% for nitrate (∼0.1-0.2 μg m -3 ) and 10-15% for sulfate (1-2 leg m -3 ). Within 95% confidence intervals, linear regression fits suggest that no biases existed between the semicontinuous techniques and the 24-hour integrated filter measurements of nitrate and sulfate;, however, for nitrate, the semicontinuous intercomparisons showed significantly less variability than intercomparisons amongst the 24-hour integrated filters.

DETERMINATION OF HYDROGEN PEROXIDE BY PHOTOINDUCED FLUOROGENIC REACTIONS

Abstract 4-Hydroxyproline, 8-hydroxyquinoline, 4-hydroxyquinoline-2-carboxylic acid and 4-hydroxyphenylacetic acid (HPA) react with H2O2 when irradiated with UV light to yield fluorescent products. Sub-micromolar detection limits of H2O2 are possible with HPA. The fluorescent product is the same as that formed in the peroxidase enzyme-mediated H2O2 oxidation of HPA. Several compounds that participate in the photomediated reaction do not react in the enzyme-mediated system. A mechanism for the photomediated reaction involving an aryloxy radical derived from the substrate is suggested. Although the limits of detection for H2O2 do not equal the best achievable with the enzyme-mediated systems, the simplicity of a single-step reaction and an ability to “photodevelop” the product offer a range of novel analytical possibilities.

Transversely illuminated liquid core waveguide based fluorescence detection Fluorometric flow injection determination of aqueous ammonium/ammonia.

The analytical performance of a new type of fluorescence detector, based on a transversely illuminated liquid core waveguide (LCW), has been investigated using the determination of NH(3)/NH(4)(+) as the 1-sulfonatoisoindole. With a very inexpensive combination of a miniature Hg blacklight as an excitation source, a colored plastic sheet as the emission filter, and an integrated blue sensitized photodiode-operational amplifier as the detector (totaling <

A Pulse Amperometric Sensor for the Measurement of Atmospheric Hydrogen Peroxide

100 in hardware cost), we were able to achieve a limit of detection (LOD) of 35 nM (1.6 pmol) NH(3) with a linear dynamic range up to 60 muM NH(3). Details of detector construction and performance are given.

Hematin as a peroxidase substitute in hydrogen peroxide determinations

Gaseous H(2)O(2) is sampled through a Nafion membrane diffusion scrubber while 1 mM HCl is maintained stationary in the scrubber. After a preselected preconcentration time (typically, 5-10 min), a valve is opened to allow the scrubber liquid to flow by gravity over an electrochemical H(2)O(2) sensor for a brief period. The miniature flow-over sensor consists of a Pt/Rh wire working electrode and a Pt wire counter electrode wound respectively on separate segments of a Nafion solid polymer electrolyte tubing supported on a Ag/AgCl wire reference electrode. A simple electronic interface and a personal computer are used to control and record the electrochemical measurement. The liquid phase detection limit for this sensor is ∼30 nM H(2)O(2) in the anodic oxidation mode. For a 9 min gas sample preconcentration period, the LOD (S/N = 3 criterion) is 0.11 ppbv H(2)O(2)(g). Ambient H(2)O(2) data obtained with this instrument were in excellent agreement with those obtained by an established fluorometric technique in a blind intercomparison.