Passive (Diffusive) Sampling Overview

Passive or Diffusive Sampling Techniques and Devices

Passive or diffusive sampling relies on the unassisted molecular diffusion of gaseous agents (analytes) through a diffusive surface onto an adsorbent. Unlike active (pumped) sampling, passive samplers require no electricity (expensive pumps), have no moving parts, and are simple to use (no pump operation or calibration). After sampling, the adsorbed analytes are desorbed off the adsorbent by solvent or thermal desorption.

Benefits of Passive/Diffusive Sampling:

• Compact, portable, unobtrusive, and inexpensive
• Offer an indication of average pollution levels over time periods of 8 hours to weeks or months
• Require no supervision, noiseless, and can be used in hazardous environments
• Low cost allows for sampling at multiple locations (e.g., for highlighting pollution "hotspots"; or determining long term data trends in a specific geographical area)
• Suitable for personal monitoring (breathing zone), indoor air analysis, and outdoor ambient air analysis

How Do Passive Samplers Work?

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 where S is diffusive surface and l is the distance between the diffusive and adsorbing surface.

Most commercially available passive or 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.

Like active sampling devices, passive sampling devices are also packed with a wide range of adsorbent media to selectively collect these contaminants at low levels over a period of time from 15 minutes up to 3 weeks. We offer a selection of radial passive sampling devices, Radiello^® and DSD, that provide up to 2-3× faster sampling rates and greater sensitivity compared to badge style devices.