The Applicability Of Passive Sampling For Chemical Exposure Assessment

Passive (diffusive) sampling is a useful tool for monitoring chemical exposure, because of its simplicity to use in the field, both for monitoring chemical compounds in ambient and indoor air and for assessment of occupational exposure. The sampling rates of diffusive samplers are much dependent on the sampler construction, but are normally lower than with active (pumped) sampling. This can be seen both as an asset and a limitation, depending on the question to be answered by the sampling procedure. When assessing occupational exposure, the levels of compounds to be collected are often high enough to use diffusive sampling also for short-time measurements. The passive sampler is convenient because there is no necessity for a pump and hence much less interference with a worker's ability to perform his/her work as normal. This makes the passive samplers very attractive for personal exposure assessment. In ambient and in non-occupational indoor air, the concentrations of various volatile organic compounds (VOCs) are usually very low, often in levels of mg m~3 or even ng m~3. To be able to passively collect a sufficient mass of the compound(s) for analysis of interest, longer sampling times are needed than what is attainable with active sampling. This can give a better description of the average composition of the ambient or indoor air than one or a few hours sampling time, which is common when performing pumped sampling. However, if the aim is to monitor fast, dynamic processes in ambient or indoor air, active sampling is preferred.

Comprehensive Analytical Chemistry 48

R. Greenwood, G. Mills and B. Vrana (Editors)

Volume 48 ISSN: 0166-526X DOI: 10.1016/S0166-526X(06)48003-X

© 2007 Elsevier B.V. All rights reserved. 57

For many years, passive samplers have been considered to offer an attractive, cost-effective alternative to active sampling [1-3]. Kauppinen has addressed the importance of cost-effective measurements and survey strategies as general recommendations for health surveillance [4]. Nothstein et al. compared the cost effectiveness as a function of the number of annual samplers, for five passive samplers and one active sampler (pumped charcoal tubes) [5]. Including costs for validations, sampling equipment and labour, the calculations indicated that, in general, the unit cost was lower for a passive sampler than for an active sampler. If the passive sampler is thermally desorbed and thereby can be reused, the price per analysis is even lower. As the costs for performing the measurements are reduced, more samples are allowed to be taken, thereby giving a better description of the exposure situation at, e.g., a workplace.

Passive samplers are user-friendly devices that can normally be operated by the user, thus enabling self-assessment of exposure (SAE) [6-9].

3.2 PASSIVE SAMPLING, BASIC THEORY

Passive sampling uses the principles of mass transport across a diffusion layer. Samplers utilising this concept received an increasing interest since the early 1970s, when the first mathematical treatment of the principles was published [10]. It was an attempt to identify and codify the factors controlling uptake rate, in the application of Fick's law of diffusion. These fundamental laws were stated by the German physiologist Adolf Eugene Fick in 1855 [11]. Further details on the theory of passive sampling are given in Chapters 2 and 6 of this book; here only the very simplest and most basic equations are given.

Fick's first law describes the diffusive flux (J), or rate of diffusion dn/dt, of a solute across an area A, as

dt ox where dn is the amount of solute crossing an area A in time dt, and ôc/ôx is the concentration gradient of the solute. D, the diffusion coefficient, gets the unit m2 s-1.

For diffusion through a tube, the concentration gradient within the tube falls linearly (Fig. 3.1), and is given by

dx L

Fig. 3.1. Diffusion through tubes. Diffusion from concentration C to Co through a tube with length L.

Fig. 3.1. Diffusion through tubes. Diffusion from concentration C to Co through a tube with length L.

Diffusive I sampler

" L Collector

Fig. 3.2. Diffusion through tubes. Diffusion from concentration C to Co — 0 through a tube-type diffusive sampler with length L and cross-sectional area A.

where C0 — concentration at the interface of the sorbent (g cm-3), C — external concentration being sampled (g cm-3) and L — length of diffusion path (cm).

The integrated Fick's first law then describes the rate of flow through the tube with

where m — mass transported (ng) and t — time (s).

A diffusive sampler has a collector (Fig. 3.2). The collector consists of an adsorbent (or a chemosorbent, i.e., a reagent coated on an adsorbent or filter that reacts with the compound to be sampled, forming a stabile derivative). If the concentration on the collector surface is zero (C0 — 0), Fick's law is reduced to m — daC (3.4)

The expression DA/L has the unit of cm3 s-1 and represents what can be considered the sampling rate of the system (comparable to the pumped sampling rate in active sampling). As A and L are physical parameters associated with the sampler construction, the sampling rate is constant for a certain analyte and diffusive sampler (except for some minor and often negligible effects of temperature and pressure). The diffusion coefficient can be theoretically calculated and predicted

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