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radiello® – Fast and Efficient Diffusive Sampling for Thermal Desorption

By: Klaus Buckendahl, Reporter EU Volume 27

Klaus Buckendahl

klaus.buckendahl@sial.com

For most diffusive/passive air sampling systems, the collected samples are desorbed via chemical desorption. A typical example of a desorption solvent is carbon disulfi de (CS2) for the desorption of volatile organic compounds (VOC) / BTEX from activated charcoal. Due to its health implications and pungent odor, many users would prefer not to work with CS2 if possible.

In addition to cartridges for solvent desorption, the Radiello diffusive sampling line offers two cartridges for thermal desorption of VOC/BTEX and phenolic compounds (see Table 1 for compound listing). After sampling, the adsorbent cartridge is transferred into an empty thermal desorption tube with ID >4.8 mm (see Figure 3) that is then heated in a thermal desorber. As a result, all of the retained compounds are thermally desorbed and directly transferred into a GC instrument. Unlike solvent desorption, no dilution is involved.

Figure 3. Placing cartridge into TD tube

Because of their radial design (see Figure 1), the Radiello samplers provide significant increases in sampling rates relative to other diffusive samplers that are commercially available. Such devices for thermal desorption often have diffusion caps installed, and typical sampling rates are in the range of 1-3 mL/min, making them more suitable for longer term measurements. In contrast, sampling rates for Radiello samplers are considerably higher (often by an order of magnitude, see Table 1).Such sampling rates are more on par with sampling rates observed with active sampling followed by thermal desorption (e.g. NOISH 2549, VOC Screening, recommends 10-50 mL/min) Faster sampling rates allow for shorter sampling times. Also, unlike active sampling, passive sampling requires no expensive pumps that need to be calibrated. This also permits simpler handling in hazardous environments.

Figure 1. Axial and radial sampler design

VOC/BTEX

The material used in the adsorbing cartridge (RAD145 for analyzing VOC/ BTEX by thermo desorption/GC) is graphitized carbon (Carbograph 4). Because of graphitised carbon’s limitation in capacity, there is a greater risk of back diffusion. To circumvent this issue, a modified yellow diffusive body was developed to house the cartridge during sampling. The yellow diffusive body, due to its smaller pore size and thicker diffusive wall, provides a diffusive path of 150 mm while the standard white diffusive body (used with chemical desorption cartridge RAD130) has a path length of 18 mm. By this extension of the diffusive path length, the sampling rates and therefore the amount of analyte reaching the adsorbent cartridge are reduced, allowing for longer exposure times while greatly reducing the risk of back diffusion. Unlike chemical desoprtion, there is no dilution effect in thermal desorption. The complete amount of collected analyte is transferred onto the GC system, thereby increasing sensitivity.

Figure 2. Yellow diffusive body (RAD 1202)

Phenols Sampler for TD

Tenax TA is used for the adsorption and thermal desorption of phenolic compounds (RAD147). The Tenax TA cartridge requires a white diffusive body.

Please note: The diffusive bodies are not interchangeable between cartridges. The presented sampling rates are only valid for the listed adsorbing cartridge and diffusive body combination.

Please refer to table 1 for a complete listing of sampling rates for VOCs and Phenolic compounds.

The European Commissions LIFE project “ARTEMIDE (High Temporal Resolution Monitoring of VOCs by Diffusive Sampling)” which focused on benzene, MTBE and 1,3-butadiene, employed radiello® samplers for thermal desorption. This project and was recently recognized as one of the 24 best LIFE projects in 2004/2005 funded by the European Commission.

The radiello® product portfolio consists of adsorbing cartridges, diffusive bodies, ready-to-use samplers, standards for calibration and accessories, such as a shelter for outdoor measurements.

For more information on passive diffusive air sampling and the complete line of radiello® products, please request your free copy of the radiello® overview brochure (IXV), the radiello®CD (IXW) containing all information and instructions for these products, or visit our Web Site at www.sigma-aldrich.com/radiello

Table 1 Compounds and sampling rates at 25°C (Q298)for Radiello samplers suitable for thermal desorption

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