Diffusion-based samplers

WELL-DEFINED DIFFUSION TUBE Fig. 13-4. Static sampler based on the diffusion principle. 

Prichard, H.M. and K. Marien, A Passive Diffusion Rn-222 Sampler Based on Activated- Carbon Adsorption, Health Physics 48 797-803 (1985). 

In an effort to optimize the solvent-containing passive sampler design, Zabik (1988) and Huckins (1988) evaluated the organic contaminant permeability and solvent compatibility of several candidate nonporous polymeric membranes (Huckins et al., 2002a). The membranes included LDPE, polypropylene (PP), polyvinyl chloride, polyacetate, and silicone, specifically medical grade silicone (silastic). Solvents used were hexane, ethyl acetate, dichloromethane, isooctane, etc. With the exception of silastic, membranes were <120- um thick. Because silicone has the greatest free volume of all the nonporous polymers, thicker membranes were used. Although there are a number of definitions of polymer free volume based on various mathematical treatments of the diffusion process, free volume can be viewed as the free space within the polymer matrix available for solute diffusion. 

The evaluation of diffusion samplers reported in this paper was initiated as an internal project at Arthur D. Little, Inc., to develop for ourselves a base of data which would help us to judge the utility of diffusion samplers in our work. Diffusion samplers were purchased from the two firms active in the marketplace when the field study began (early 1979), Abcor and 3M. A program was devised wherein, during each of approximately 50 surveys being undertaken in plastic fabrication plants, four to six diffusion samplers would be exposed, each adjacent to a charcoal tube personal sample. At the conclusion of the field sampling, a data base of over 100 pairs of samples (each pair including a diffusion sample and a charcoal tube sample) had been compiled for each of the two diffusion samplers. 

Based on the research of Dr. E. D. Palmes (1) at New York University s Institute of Environmental Medicine and on additional study supported by a contract from the United States Bureau of Mines, a unique personal sampler has been designed to passively collect N0X. This is accomplished by way of molecular diffusion and subsequent trapping of the molecules onto a matrix coated with triethanolamine (TEA) at the closed end of the sampler. Constructed of polypropylene, the tubular sampler shown in Figure 1 is small, lightweight, unbreakable and can be easily worn in the breathing zone of the employee whose exposure is to be monitored. No pumping mechanism is required. The components of N0X diffuse at constant, known rates towards the sealed end of... 

Johnson, N. D. Evaluation of a Diffusion Denuder Based Gas/Particle Sampler for Chlorinated Organic Compounds, Presented at the 1986 EPA/APCA Symposium on Measurement of Toxic and Related Air Pollutants, Research Triangle Park, NC, 1986. 

The first protocols developed for evaluation of the performance of diffusive samplers were based on workplace applications. The European standard EN838 1995 (EN (1995)) is an example. This approach has been adapted to provide a protocol for the evaluation of the performance of diffusive samplers for ambient air monitoring (EN, 2004a). It describes a series of tests that enable a calculation of the measurement uncertainty. The key sampler related factors assessed are ... 

Available data based on limited exposure tests in chambers and comparisons with pumped sampling methods in the laboratory and field show the techniques can be used to determine mean concentrations of VCX3s over periods of a day to several weeks for fixed site and personal monitoring. Problems of poor recovery at the desorption stage and possible losses by back diffusion means the investigator needs to consider carefully the choice of sampler and, in the case of thermally desorbable tubes, the optimum sorbent for the investigation. 

The in situ pore water sampler also called pore water equilibrator or dialyzer is based on the principle that given enough time, a contained quantity of water in the sampler and adjacent pore water will diffuse and equilibrate through a dialysis membrane, or other materials such as porous Teflon, with the surrounding water and its dissolved solutes. The in situ equilibrator can either be removed from the soil to collect pore water or the pore water can be collected through an attached tubing while the device remains in the sediment. 

There are a number of techniques that can be used in the field. These include electrochemical sensors for gases such as O2 and SO2 and diffusive samplers containing immobilized reagents that produce a visible color change with visual detection on exposure to a specific chemical. Passive diffusion tubes can also be used for analyte preconcentration. Subsequent laboratory analysis is usually undertaken by thermal desorption coupled with GC. This approach is particularly useful for trace organic compounds such as polyaromatic hydrocarbons (PAHs) and VOCs. Spec-trometric techniques such as Fourier transform infrared (FTIR) spectrometry, correlation spectrometry, and the laser based LIDAR (light detection... 

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