Denuder liquid diffusion

Figure 10.68 SchemaHc depiction of the operation of a liquid diffusion denuder.

Collection Efficiency. Single-Tube Denuders. For the appropriate design of a diffusion-based collection device for an intended application, the ability to estimate the collection efficiency a priori is of considerable help. Although the theoretical soundness of the Gormley-Kennedy equation (equation 1) is not questioned, it is based on the assumption that the uptake probability of the analyte gas at the wall is unity that is, the wall is truly a perfect sink , and every collision results in uptake. This assumption is unrealistic. In recent years, this issue has been reexamined. McMurry and Stolzenburg (42) showed for a liquid-coated denuder how the uptake probability (discussed by the authors in terms of the mass accommodation coefficient ) can be evaluated from collection efficiency measurements. Murphy and Fahey (43) utilized the mathematical solution originally developed for hemodialyzers by Cooney et al. (44) this treatment assumes a constant uptake probability that may be less than unity. To use the Murphy-Fahey approach, however, this probability must be precisely known. 

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... 

The existing systems can be broadly classified into two groups systems that deal with a gaseous analyte, and systems that deal with the analyte in the liquid phase. To date, the first group is composed solely of thermodenuders, devices that rely on thermally cycled sorption-desorption steps. The second group includes wet diffusion denuder-scrubbers and devices that are cyclically sorbent-coated and washed. 

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. 

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