Diffusion-controlled passive sampler

Various types of detector tubes have been devised. The NIOSH standard number S-311 employs a tube filled with 420—840 p.m (20/40 mesh) activated charcoal. A known volume of air is passed through the tube by either a handheld or vacuum pump. Carbon disulfide is used as the desorbing solvent and the solution is then analyzed by gc using a flame-ionization detector (88). Other adsorbents such as siUca gel and desorbents such as acetone have been employed. Passive (diffuse samplers) have also been developed. Passive samplers are useful for determining the time-weighted average (TWA) concentration of benzene vapor (89). Passive dosimeters allow permeation or diffusion-controlled mass transport across a membrane or adsorbent bed, ie, activated charcoal. The activated charcoal is removed, extracted with solvent, and analyzed by gc. Passive dosimeters with instant readout capabiUty have also been devised (85). 

An important performance characteristic of passive samplers that operate in the TWA regime is the diffusion barrier that is inserted between the sampled medium and the sorption phase. This barrier is intended to control the rate of mass transfer of analyte molecules to the sorption phase. It is also used to define the selectivity of the sampler and prevent certain classes (e.g., polar or nonpolar compounds) of analytes, molecular sizes, or species from being sequestered. The resistance to mass transfer in a passive sampler is, however, seldom caused by a single barrier (e.g., a polymeric membrane), but equals the sum of the resistances posed by the individual media (e.g., aqueous boundary layer, biofilm, and membrane) through which analyte diffuses from the bulk water phase to the sorption phase.19 The individual resistances are equal to the reciprocal value of their respective mass transfer coefficients and are additive. They are directly proportional to the thickness of the barrier... 

A diffusive/passive sampler is a device which is capable of taking samples of gas or vapour pollutants from the atmosphere at a rate controlled by a physical process such as diffusion through a static air layer or permeation through a membrane, but which does not involve the active movement of air through the sampler (Brown, 1993 Moore, 1987). Compared with methods requiring pumps for active air movement, diffusive samplers are more convenient and have lower associated costs. 

Eatough DJ, Benner CL, Bayona JM, Caka FM, Tang H, Lewis L, Lamb JD, Lee ML, Lewis EA, Hansen LD (1987) Sampling for gas phase nicotine in environmental tobacco smoke with a diffusion denuder and a passive sampler. Proc 1987 EPA/ APCA Int Symp Measurement of toxic and related air pollutants. Air Pollution Control Assoc, Pittsburgh, pp 132-139... 

An alternative method known as passive or diffusive sampling, which does not require a pump or air mover, has gained popularity in recent years. Diffusive samplers operate by allowing the gas and vapor molecules to make their own way to the collection medium by diffusion along a carefully controlled path. The rate of movement, which is a function of the diffusion coefficient of the gas or vapor in air and the path geometry (Figure 1), can be derived from Pick s first law of diffusion ... 

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