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Faster Sampling Rates and High Capacity Offer Advantages

By Kristen L. Schultz

Passive sampling offers advantages over active sampling in that the passive sampling devices do not require an air sampling pump and calibration. Classic passive sampling devices have a two-dimensional geometry with a flat adsorbing surface and are known as axial-type samplers. Since 1990, radial passive sampling devices have emerged that have a 360-degree adsorbing surface. By nature of this radial symmetry, the uptake rates are two to three times higher compared to axial diffusive samplers.

Sampling for Formaldehyde with Radial Passive Sampling Devices

Figure 1. The axial sampler shown on the left has only one diffusive surface. The radial-type sampler on the right has a 360-degree diffusive surface.

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There are certain situations where active sampling is preferred. Air flow across passive samplers has to be sufficient to exchange the analyte concentration in the boundary layer. Active sampling is more forgiving since it continually replaces the air at the face of the sampler. In addition, OSHA recommends using active sampling when monitoring exposures to formaldehyde resulting from the use of formalin solutions. This issue is discussed in detail in OSHA method 1007.
How Passive Sampling Works: Fick’s Law of Diffusion
From Fick’s Law, we know that the sampling rate (Q) is a function of the diffusion coefficient of a given analyte (D) and the geometric constant of the sampler (K): Q = D∙K. The diffusion coefficient (D) always remains constant for a given analyte; therefore, to improve sampling rate (Q), the geometric constant (K) must be improved. K = S/l (see figure 2) where S is diffusive surface and l is the distance between the diffusive and adsorbing surface:

Figure 2. Axial sampling graphic showing diffusive and adsorbing surfaces.

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Q is the sampling rate and has the dimensions of a gaseous flow (if m is expressed in µg, t in minutes, and C in μg/L, Q is expressed in L/min). Therefore, if Q is constant and measured, to calculate the ambient air concentration you need only to quantify the mass of analyte trapped by the adsorbing material and to keep note of the time of exposure of the diffusive sampler.

Where concentration:

To improve the analytical sensitivity, the collected mass m should be increased by enlarging Q. As D is a constant term, one can only try to improve the S/l ratio, namely the geometrical constant of the sampler. Unfortunately, in the common axial symmetry sampler, if S is enlarged, the adsorbing surface A must be enlarged too, in order to keep the two parallel surfaces at a fixed distance. Since the analytes can be recovered from the axial sampler only by solvent extraction, any increase of A leads to a proportional increase of the extraction solvent volume; thus, the improvement of Q is canceled out by the effect of dilution.

The value of distance l could also be reduced, but under the critical value of about 8 mm the diffusion law is no longer valid in the case of low air velocity values, since adsorption rate becomes higher than supplying rate of analyte molecules at the diffusive surface.

When compared to the axial sampler, radial samplers show a much higher diffusive surface without increase of the adsorbing material amount. Even if the adsorbing surface is quite smaller than the diffusive one, each point of the diffusive layer faces the diffusion barrier at the same distance.

Most commercially available passive/diffusive samplers are planar or axial in shape and offer lower sampling rates and limited sampling capacity. As a result, sensitivity can suffer during short-term analysis (due to low sampling rates) or long-term sampling (analyte back-diffusion due to low capacity). A radial coaxial design circumvents these issues by improving the geometry.
Device Design
Two types of radial sampling devices are available: Radiello

^{®}, which contains an adsorbing cartridge inserted into a diffusive body whose porosity controls the rate of diffusion of compounds into the adsorbing surface; and an all-in-one device known as DSD-DNPH where adsorbent is released into the radial diffusive membrane during sampling. The DSD-DNPH is specified in OSHA method 1007 for determination of aldehydes.
The Radiello device uses Florisil

^{®}coated with 2,4-dinitrophenylhydrazine (DNPH) as the adsorbent whereas DSD-DNPH uses silica gel coated with 2,4-DNPH. Aldehydes and ketones diffuse through the diffusive barriers, reacting with DNPH to form stable aldehyde-DNPH derivatives. These DNPH derivatives are then eluted with acetonitrile and analyzed by high performance liquid chromatography (HPLC/UV).
Table 1. Comparison to Axial Samplers: Sampling Rates
Measurement of sampling rates is achieved by generating a test atmosphere from a known concentration of formaldehyde gas into an exposure chamber loaded with passive samplers; active samplers are tested at the same atmospheres. Typically three different concentrations and time periods are tested to determine the sampling rate. For more information, please refer to OSHA method 1007.

Table 2. Comparison to Axial Samplers: Capacity
Analyte capacity is an important specification for passive samplers and should be considered particularly when there is a need for long-term sampling (greater than seven days). When the sampler capacity is reached, the analyte of interest back-diffuses off of the adsorbent and back into the environment, resulting in an underestimation of exposure.

Conclusions
Passive sampling is a reliable and accurate alternative to active sampling in many situations, saving both time and money. There are two styles of devices available, axial and radial. When deciding which device to use, the most important factors are sampling rate and capacity. Radial designs offer faster sampling rates and higher capacity compared to axial samplers.
Table References
1. OSHA Method 1007
2. Assay Technology Technical Insert, 9159-571, 06/2014
3. SKC Publication 1529 Rev 1702
4. DSD-DNPH Application Manual – T708004, MilliporeSigma
5. Radiello Manual – IYP, MilliporeSigma
Acknowledgements/Trademarks
MilliporeSigma is a subsidiary of Merck KGaA, Darmstadt, Germany
Radiello – Instituti Clinici Scientifici Maugeri (ICSM)
Chemdisk – Assay Technology
UMEx – SKC Inc.
Kristen L. Schultz is the franchise product manager for Air Monitoring with MilliporeSigma in Bellefonte, Pa. She can be reached at kristen.schultz@sial.com.

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