Passive or diffusive sampling is a system that relies on the unassisted molecular diffusion of gaseous agents (analytes) through a diffusive surface onto an adsorbent component. Unlike the active (pumped) sampling, passive samplers dont require electricity (expensive pumps), they have no moving parts, and are very simple to use (no pump operation or calibration). After sampling, the adsorbed analytes are desorbed off the adsorbent by solvent or thermal desorption.
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/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.
Benefits of passive/diffusive sampling:
· Compact, portable, unobtrusive, and inexpensive
· Offers indication of average pollution levels over time periods of 8 hours to weeks/months
· Requires no supervision, is 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)
· Amenable to personal monitoring (breathing zone), indoor air analysis, and outdoor ambient air analysis