Diffusive samplers diffusion coefficient

Divine, C.E. and McCray, J.E. 2004, Estimation of membrane diffusion coefficients and equilibration times for low-density polyethylene passive diffusion samplers. Environ. Sci. Technol. 38 1849-... 

J = diffusion flux (g/s), D = air diffusion coefficient (cm2s 1), A = cross sectional area of the sampler (cm2) and dc/dx = the concentration gradient of the compound across the stagnant air gap. 

Gibson, L.T., Cooksey, B.G., Littlejohn, D. andTennent, N.H. (1997a) Determination of experimental diffusion coefficients of acetic acid and formic add vapours in air using a passive sampler. Analytica Chimica Acta, 341, 1-10. 

Since no pump is used to pull air into the sampler, the sampling rate is dependent on the diffusion coefficient for each compound collected. The sampling rate for a specific compound is expressed in the equation ... 

PIMs have shown considerable potential for passive sampling of specific contaminants in waters [87]. In this application, low membrane diffusion coefficients can be an advantage. Passive samplers are generally left immersed in a river, lake, or contaminated water site for several days, and a PIM can slowly and selectively accumulate and transport the analyte to the receiver phase within the passive sampler. 

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 ... 

The uptake rate of the sampler is given by the expression D AIL), which has units (cm s ) dimensionally equivalent to a volume flow rate. The geometry of the sampler is known from measurement, but very few diffusion coefficients have been measured. They can be calculated, assuming certain rules for calculating molecular size, but these need to be correlated with experimental determinations of uptake rates from atmospheres of known concentration. This gives a correction factor that depends on the chemical makeup of the molecule, and which can then be used to adjust the calculated values of other, similar molecules. 

The diffusion coefficient is affected by temperature and pressure, but so too is the concentration. The mass collected by the sampler is therefore proportional to the square root of the absolute temperature and independent of pressure. The temperature effect should be less than 0.2%°C , and is normally ignored, since the variation from the measured uptake rate should be less than 5% from room temperature to 0°C and from room temperature to 40°C. 

Kotrappa, P, Bhantin, D.P., Dhandayutham, R. (1975). Diffusion sampler useful for measuring diffusion coefficients and unattached fraction of radon and thoron decay products. Health Phys. 29, 155—162. 

Diffusive samplers, also called diffusive monitors, passive samplers or passive monitors, are utilized for sampling without the need for an air mover, that is, without a pump. Manmade diffusive sampling operates by allowing gas or vapor molecules to diffuse through a defined volume of still air or through a polymer membrane, until they reach a sorbent bed. The principles of uptake are to consider that the passive sampling medium is uniform and porous and that it traps PAHs from the atmosphere by gaseous diffusion, and sorption. The mass collected is a function of the external concentration and the diffusion coefficient of the molecules. The diffusion coefficient varies in a known manner with temperature and pressure, and so the result can be corrected for these parameters [107]. 

Figures 3 and 4 display the paired results obtained with Ab-cor and 3M diffusion-type samplers, respectively. These results were analyzed through use of the "t" test for paired samples and the calculation of correlation coefficients and regression equations, with the results of these analyses shown in Table I. A statistically significant correlation is seen between the data set for each type of diffusion sampler and the corresponding tube/ pump sample data set, and the "t" test fails to refute the null hypothesis that there is no significant systematic difference between each of the diffusion sampler data set and the corresponding tube/pump data set.

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... 

The substance-specific kinetic constants, kx and k2, and partition coefficient Ksw (see Equations 3.1 and 3.2) can be determined in two ways. In theory, kinetic parameters characterizing the uptake of analytes can be estimated using semiempirical correlations employing mass transfer coefficients, physicochemical properties (mainly diffusivities and permeabilities in various media), and hydro-dynamic parameters.38 39 However, because of the complexity of the flow of water around passive sampling devices (usually nonstreamlined objects) during field exposures, it is difficult to estimate uptake parameters from first principles. In most cases, laboratory experiments are needed for the calibration of both equilibrium and kinetic samplers. 

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