Sorption sample volume

To be an acceptable substitute for wet collectors and to satisfy the NIOSH criterion for acceptable methods, a sorbent material must have a demonstrated sorption capacity for the analyte that is adequate for sampling a reasonable volume of workplace air at an established rate. Typically, a sample volume of at least 12 L (1 h at 0.2 L/min) is desirable. 

If sorption is 50% efficient but desorption is 100% efficient, the recovery measured is 50%> and it is impossible to know whether sorption or desorption was inefficient or if reduced recovery was produced by a combination of both. Therefore, method development requires either optimizing sorption while controlling desorption, or vice versa using an iterative approach [67,72], Alternatively, a statistical factorial design can be used to determine and optimize quickly variables important to SPE [110]. Using either approach, it is important to consider the major factors influencing retention, including sample pH, sample volume, and sorbent mass. 

Selected data published by Patsias and Papadopoulou-Mourkidou [114] illustrate sorption s dependence on sample volume (Figure 2.36). Their research pursues development of an automated online SPE-HPLC methodology for analysis of substituted anilines and phenols. Recovery (%) was measured for numerous compounds on various polymeric sorbents, but the only data presented here are those in which a styrene-divinylbenzene polymeric sorbent was used for analysis of aniline, phenol, 4-nitroaniline, and 4-nitrophenol. Aqueous sample volumes of 5, 10, 25, 50, 75, 100, 125, and 150 mL were acidified to pH 3 before SPE. 

Figure 2.36. Dependence of SPE sorption on sample volume. Graphic based on selected data from Ref. 114.

Increasing the amount of sorbent will increase the sample volume that can be passed through the sorbent before breakthrough. The dependence of sorption on sorbent mass is illustrated (Figure 2.37) for SPE recovery from a 50-mL sample volume (72 ppb) in which two Cg columns, each contain-... 

Inject a small sample volume, circumventing the column by connecting the sampler to the detector. Without the column, only one peak will appear. Make a note of its area. Now connect the column and inject the same sample volume. Depending on the column selectivity, 1, 2,3 or more peaks will appear. Add all the peak areas and compare the sum to the peak area obtained from the run without column. If the difference is less than 5-8%, irreversible sorption in the stationary phase is negligible. 

The variables that affect SBSE extraction performance are similar to the ones affecting SPME, namely, extraction time, extraction temperature (mainly for headspace mode, it should be noted however that the sorption process on the PDMS coating is not favored at high temperatures), selection of modifiers to increase the extraction efficiency (sueh as small amounts of organic solvents or high contents of salts), pH, stirring, and sample volume. 

Values of the uptake at saturation, of butane, carbon dioxide and nitrogen, by a sample of carbon, expressed as a volume of liquid v,. The carbon had been "burnt off" to different extents by heating in oxygen at 500°C on a sorption... 

A procedure for characterizing the rates of the volume change of gels has not been uniformly adopted. Often, the kinetics are simply presented as empirical sorption/desorption curves without quantitative analysis. In other cases, only the time required for a sample of given dimensions to reach a certain percentage of equilibrium is cited. One means of reducing sorption/desorption curves to empirical parameters is to fit the first 60% of the sorption curve to the empirical expression [119,141]... 

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