Diffusion scrubbers

HONO Because of its importance as an OH source by photolysis at dawn, particularly in polluted areas, there have been a number of measurements reported for HONO. The two methods used most commonly have been DOAS and denuder methods. In addition, diffusion scrubber and photofragmentation—laser-induced fluorescence methods have been developed, although they have not seen widespread use. 

There are some methods that are specific to HCHO. For example, the Hantzsch reaction of HCHO, collected with a diffusion scrubber, with ammonium acetate, acetic acid, and acetylacetone to form diacetyldihydrolutidine, which is measured using its fluorescence at 470 nm, has been applied to air measurements (Dasgupta et al., 1988, 1990 Kleindienst et al., 1988a,b Lawson et al., 1990 Khare et al., 1997). Reaction with 1,3-cyclohexanedione and ammonium acetate to form a dihydropyridine derivative that is measured by fluorescence has been used in conjunction with a diffusion scrubber (Fan and Dasgupta, 1994). Enzymatic methods have been used in which formaldehyde dehydrogenase catalyzes the oxidation of HCHO to HCOOH in the presence of -nicotinamide adenine dinucleotide, NAD+, which is reduced to NADH. The latter is measured by fluorescence at 450 nm (Lazrus et al., 1988 Ho and Richards, 1990). 

In another intercomparison using TDLS, several DNPH methods, the 1,3-cyclohexanedione diffusion scrubber, and the enzymatic method were compared using both spiked samples and ambient air. The TDLS was used as a standard for comparison. For ambient air measurements, results obtained with the 1,3-cyclohexanedione diffusion scrubber were about 30% higher than those obtained with TDLS, whereas results for the enzymatic method were about 35% lower. The DNPH cartridge measurements were quite variable, which may... 

Dasgupta, P. K., S. Dong, H. Hwang, H.-C. Yang, and Z. Genfa, Continuous Liquid-Phase Fluorometry Coupled to a Diffusion Scrubber for the Real-Time Determination of Atmospheric Formaldehyde, Hydrogen Peroxide, and Sulfur Dioxide, Atmos. Environ., 22, 949-964 (1988). 

Lindgren, P. F., and P. K. Dasgupta, Measurement of Atmospheric Sulfur Dioxide by Diffusion Scrubber Coupled Ion Chromatography, Anal. Chem., 61, 19-24 (1989). 

When the denuder active surface is an inert porous membrane such that the analyte molecule must diffuse across the pores to be trapped by an absorber liquid, only a fraction of the membrane surface is porous, and the pores may also be tortuous. Consequently, collision at the membrane surface is not synonymous with uptake. Corsi et al. (45) developed a numerical solution for the collection efficiency observed for such a membrane-based diffusion denuder, hereinafter referred to as a diffusion scrubber (DS). Both groups of researchers dealing with the issue of less than unity uptake probability reached the conclusion that this value must be very much less than... 

With porous membrane DS devices of this geometry and for thin membranes with low-tortuosity pores (i.e., where the diffusion distance within the pores is very small compared to the radial diffusion distance in the DS), good predictions for the collection efficiencies can be obtained if the nominal X and dfd0 values are both multiplied by the fraction of the surface that is porous. For example, with a diffusion scrubber based on such a membrane tube (L = 40 cm, dQ = 0.5 cm, d = 0.045 cm, fractional porosity 0.4), the corrected X values for H20.2 as sample gas are 0.32, 0.16, 0.11, and 0.08, respectively for Q = 0.5, 1.0, 1.5, and 2.0 L/min, and the corrected djda value is 0.036. The collection efficiencies predicted from Figure 1 (interpolating between dfdQ values of 0.02 and 0.05) are in good agreement with... 

Diffusion scrubbers are membrane-based denuders in which the sample air flows on one side of a membrane and a suitable scrubber liquid flows on the other side. The analyte gases of interest are collected in the scrubber liquid, and the effluent is subjected to analysis. The simplest geometry is that of a conventional single-tube denuder. Air is sampled through a tubular membrane while the scrubber liquid is pumped in a countercurrent fashion through an external jacket tube surrounding the membrane tube. 

Figure 4. Diffusion scrubber device for H202 removal from air, consisting of the following A, Teflon tubing adaptor By Teflon Swagelok union C, 6-mm o.d. glass tube D, air outlet E, porous Teflon tube (2 mm i.d. x 30 cm, 70% porosity) F, 8-mm i.d. glass tube G, scrubber solution with concentric Teflon displacement rod H, air inlet and /, Teflon tube connection to porous Teflon tube. Cross section inset a, 8-mm o.d. (6-mm i.d.) glass tube b, air stream c, porous Teflon tube d, H20 scrubber solution and e, Teflon displacement rod. (Reproduced from reference 53. Copyright 1986 American Chemical Society.)...

Diffusion Scrubber Coupled-Ion Chromatography. Over the past decade, ion chromatography (IC) has become the technique of choice for the determination of anions. Even with conventional sorbent-coated de-nuders for acid gases, the actual determination is most commonly conducted by IC. The recognition of this fact led to the coupled DS--IC system (96). The system is shown schematically in Figure 9. Items A through H constitute... 

Figure 9. Configuration of the DS-IC system A, clean air input B, mass-flow controller C, permeation device chamber D and H, vents E, needle valve-rotameter F, needle valve G, mass-flow meter I, diffusion scrubber Jy scrubber liquid reservoir K, needle valve-rotameter L, suction pump M, injection valve Ny peristaltic pump O, eluent flow F, downstream chromatographic components and Q, sample loop. (Reproduced from reference 96.

Snated filter, e.g., a glass fiber mat impregnated with a mercurous salt ition and a humectant (29). Although detectabilities well below 100 pptv were obtainable, batch to batch filter performance showed marked variability and impregnated filters could not be stored over a month-long period without loss of performance. We therefore carried out the desired reaction using a diffusion scrubber air was sampled through a porous hydrophobic membrane tube while a dilute mercurous nitrate solution was circulated on the outside of the membrane tube. Mercury liberated by reaction at the gas-liquid interface in the pores is carried by the air stream to the detector. 

Figure 1. (a) Diffusion scrubber used for the ( translation system, (b) Diffusion scrubber used for the Hg-translation system. 

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