Diffusive uptake rates

The expression DA/L has units of cm3 s 1 and represents the diffusive uptake rate of the sampler under ideal conditions. 

For indoor environments the effects of changes in temperature and pressure on diffusive uptake rate will be insignificant compared with other sources of error. High humidity can affect the sorption capacity of hydrophilic sorbents such as charcoal. This will reduce the time before saturation of the sorbent occurs. 

A diffusive end cap which contains a stainless steel mesh screen can also be used and this can prevent air movement within the diffusive air gap when exposed to high air velocities (Brown, 1993). Evaluation of the sampler for workplace monitoring found that the diffusive uptake rate of the sampler was not influenced by air velocities as low as 0.007ms-1. 

The tube type samplers have lower diffusive uptake rates and are therefore less prone to starvation effects. In consequence they require longer exposure periods to collect the same mass of analyte. The occurrence of reverse diffusion depends on the analyte-sorbent interaction, but for weaker adsorbents such as Tenax TA this does occur with more volatile compounds for example, compounds more volatile than toluene. 

Figure 3.1 Experimentally determined diffusive uptake rates for toluene using different exposure periods and the Perkin-Elmer type sampler packed with Tenax TA.

The badge type samplers have higher diffusive uptake rates and, because strong sorbents are used, are not prone to reverse diffusion effects. They require solvent desorption (which is not easily automated) and the use of toxic solvents, and recovery of some compounds is poor. Contaminants in the solvent can reduce sensitivity as does the dilution effect typically 2 ml of solvent is used to desorb and 1 pi (i.e. 0.05 % of the collected mass of analyte) is used for GC analysis. 

The most widely used badge type sampler for indoor air studies has been the OVM 3500. This is a circular badge with a 1-cm diffusion length containing a charcoal wafer. Desorption of VOCs is carried out within the monitor itself by the addition of carbon disulfide. Exposure periods applied have ranged from 24 h to 3 weeks. The diffusive uptake rates reported by the manufacturer for 8-h exposure periods are about 30 ml/ min, but actual values are compound specific. For the monitoring of hexane in the workplace, the diffusive uptake rate is not significantly affected by ambient air movement, provided that there is a minimum air velocity of about 0.1 m/s (HSE, 1992). 

Brown et al. (1992) measured toluene, xylenes, decane and the TVOC concentrations in the air of 100 homes using the Perkin-Elmer tube packed with Tenax TA and a 4 week exposure period. Investigations of the influence of exposure time on net diffusive uptake rate have shown that for a more volatile VOC, such as benzene, the uptake rate is lower for 6 weeks than for 3 weeks exposure periods. There was no effect of exposure period on uptake for the less volatile compounds such as undecane. Brown and Crump (1993) further examined the influence of exposure period by exposing the tube samplers for periods of up to 7 months. No further decline in uptake rate with exposure period occurred for benzene after 5 months and no significant decline in the rate for undecane occurred until exposure periods exceeded 5 months. 

Diffusive uptake rates for six VOCs were determined experimentally by Brown et al. 

Table 1.5-1 also summarises the results for the 28-day exposure periods reported previously. It should be noted that different VOC mixtures and concentrations were used in the two experiments however, the results show some differences in the diffusive sampling rates for 14 and 28 days, the greatest change being for benzene, which is the most volatile of the compounds studied. Figure 1.5-1 illustrates the change in effective diffusive uptake rate with exposure period for toluene by combining the data reported previously with the data in Table 1.5-1. This shows that it is important to use the diffusive...

Table 1.5-1. Results of an experiment to measure 14 days diffusive uptake rates for 9 VOCs using the Perkin-Elmer tube with Tenax TA. (RSD = relative standard deviation).

---