Design Of The Permeation Passive Sampler

Permeation passive samplers are most often badge type, although tubetype designs are also used. The badge design is dedicated by the need to have a large surface area of the membrane in order to achieve a high effective sampling rate. Air velocity has virtually no effect on the sampling rate of permeation passive samplers because permeability constants of analytes are several orders of magnitude lower than their diffusion coefficients in air (the resistance to mass transfer in the permeation membrane (1M) is much higher than that in the stagnant boundary layer of air (1B) at the surface of the sampler ds/Da ^ =DeK). Consequently, analyte depletion in the vicinity of the sampler does not occur even in stagnant air. An example of badge-type permeation passive sampler using a bed of granulated sorbent as the collecting medium is a sampler designed at Gdansk University of Technology (GUT) [12,13]. The design is shown in Fig. 4.2. The GUT sampler is machined of polyamide. The sampler is equipped with PDMS membrane of 50-mm thickness (the sampling area of the permeation passive sampler is around 6 cm2). Active carbon is used as the receiving phase (~300mg of active carbon; 40-60 mesh, specific surface area of 1500 m2g-1).

Fig. 4.2. Design of GUT permeation passive samplers. Key: 1, screw cap; 2, protective screen mount; 3, PDMS membrane of 50 mm thickness; 4, protective stainless steel screen; 5, washer; 6, main body; 7, O-ring; 8, plug; 9, opening for a screw-in holder; 10, set screw; 11, active carbon; 12, glass wool.

Fig. 4.2. Design of GUT permeation passive samplers. Key: 1, screw cap; 2, protective screen mount; 3, PDMS membrane of 50 mm thickness; 4, protective stainless steel screen; 5, washer; 6, main body; 7, O-ring; 8, plug; 9, opening for a screw-in holder; 10, set screw; 11, active carbon; 12, glass wool.

4.4 CALIBRATION OF GUT PERMEATION PASSIVE SAMPLERS

In order to relate the amount of analyte collected by a passive sampler to its TWA concentration in the air, the calibration constant of the sampler for a given analyte must be known. In addition, all parameters affecting the uncertainty of the final result such as temperature, humidity and air velocity should be defined. However, the uncertainty of determination of the analyte concentration by permeation passive samplers is affected mainly by the uncertainty in determination of calibration constants k. Parameters affecting the determination of the analyte concentration by permeation passive samplers are illustrated as a cause-and-effect diagram in Fig. 4.3 [14-16].

Consequently, calibration of the samplers is very important, as it determines the accuracy and reliability of further measurement results [12,13,17].

The membrane of the permeation passive sampler has a well-defined surface area, which is mandatory for quantitative measurements. The calibration constants k for target analytes can be determined from Eq. (4.4) by exposing the samplers to known, constant concentrations of these analytes in a standard gas mixture for known periods of time [12,13]. Each sampler has to be calibrated individually for each individual target compound, with replicate experiments carried out for each exposure time [9,12,16,18,19]. Assuming that the permeation rate of a given analyte through the membrane remains constant for a constant analyte concentration in the standard gas calibration mixture, the plots of the amount of analyte trapped by collecting medium vs. exposure time should be linear. The calibration constants k can be found from the reciprocals of the slopes of these lines (plotted individually for each compound) divided by the mean concentration of analyte in the gas calibration mixture. Examples of the calibration constants determined in model experiments for selected volatile organics belonging to four homologous series (n-alkanes, aromatics, re-alcohols and acetic acid esters of re-alcohols) are presented in Table 4.1.

4.5 DETERMINATION OF THE CALIBRATION CONSTANTS OF GUT PERMEATION PASSIVE SAMPLERS WITH SILICONE MEMBRANES BASED ON PHYSICO-CHEMICAL PROPERTIES OF THE ANALYTES

Air may contain a large number of organic compounds, and their composition may change over time [20-23]. Consequently, the need to k k

texp massanaij,te

Fig. 4.3. The cause-and-effect diagram for the parameters affecting the determination of the analyte concentration in air by permeation passive samplers. k: calibration constant of the permeation passive sampler; massanalyte: mass of the analyte trapped on the sorption bed of the passive sampler, determined chromatographically; texp: sampler exposure time; massst: chromatographically determined mass of analyte standards; flow: flow rate of the standard gas mixture; tsorpt: exposure time of the passive sampler in the calibration chamber; RSDresults: relative standard deviation of the determination of analyte mass trapped on the sorption bed (depends on the uncertainty of the injection volume (Vinj)), uncertainty of carbon disulfide volume (VCS2) and the calibration of the GC-FID system (cal) (depends on the uncertainty of the injection volume (Vinj), uncertainty of the volume of the standard in the calibration mixture (Vst), uncertainty of the dilution of the calibration mixture (Vdill) and relative standard deviation of standard injection (RSDst)). Uncertainty of the mass of the analyte trapped by the passive samplers depends on the uncertainty of the calibration of the GC-FID system (cal) and the relative standard deviation of determination of the mass of the analyte trapped by the sorption bed (RSDresults). Reproduced from Ref. [16] with permission from Elsevier.

calibrate permeation passive samplers for each individual target compound is the single biggest obstacle in the widespread adoption of these samplers for air sampling. Experimental determination of the individual calibration constants k of permeation passive samplers is time-consuming and costly.

Equation (4.9) indicates that permeability of a compound through a polymeric membrane depends on the solubility and diffusivity of the chemical in the membrane material [10,13,16]. Since diffusivity in the PDMS polymer depends only weakly on the structure of a compound,

0 0

Post a comment