Several hundred chemical analytes (contaminants) are collectible on activated carbon (charcoal) and recoverable for laboratory analysis. Since experimental validation for each analyte following criteria set by NIOSH and CEN is costly, there is a need to describe a method for validation by analogy from analytes which have been exhaustively tested to chemically similar analytes which have not been tested.
The Uptake (Sampling) Rate of a particular analyte by a diffusive monitor has been shown to be described by Fick's Law according to the equation:
Uptake (Sampling) Rate = (AD)/(L) … where …
D = Analyte Diffusion Co-efficient(cm2/min);
A = Cross-Sectional Area of Sampler Orifice(cm2);
L = Path Length of Sampler Orifice(cm)
While (A) and (L) can be measured, and the Uptake Rate calculated from a literature value of (D), diffusion co-efficient measurements may be subject to experimental artifacts which prevent diffusion co-efficients measured in one system from being translatable to other systems. Accordingly, it is usually more accurate to extrapolate diffusion parameters from direct measurements in the system being used and known physical-chemical laws.
Small molecules travel faster than large ones. Diffusion co-efficients for different analytes have been shown to be a function of molecular dimensions. Specifically, the two-dimensional (Fickean) diffusion rate will be directly proportional to the average cross-sectional area presented by the molecule in the plane perpendicular to the direction of diffusion. Since molecules rotate freely in the vapor phase, the average cross-sectional area presented by a vapor molecule corresponds closely to a parameter which may be called the Molecular Cross-Section.
Molecular Cross Section (MCS) = (Molecular Volume) 2/3
A plot of 1/MCS vs Uptake Rate for chemically similar analytes follows a linear relationship with slope equal to "k" and a high correlation co-efficient. Uptake Rates for analytes which have not been measured experimentally can be estimated with good accuracy by extrapolation from data for chemically similar analytes.
(Uptake Rate) = k [ 1/(MCS) ]
Estimation Procedure
(1) Determine Uptake Rate for a series of Analytes as specified in NIOSH/CEN/ANSI documents.
(2) Compute the average of the factor "k" for a series of chemically similar analytes.
(3) Determine the Molecular Volume using "Molecular Modeling Pro" computer software or other valid method.
(4) Calculate Uptake Rate for Analyte as follows:
Uptake Rate = k/(Molecular Volume)2/3
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For the purpose of this calculation, analytes are currently grouped into the following chemically similar groups:
aldehydes ... polar/oxygenated organics ... volatile non-polar organics ... semi-volatile non-polar organics