Sampling efficiency, effect

Results. Various solvent mixtures were tested for extraction efficiency. The test sample was a bone-dry sediment reference material containing 24.6 ppm of Arochlor 1242. This reference material is a real sediment from New Bedford Harbor which was homogenized and carefully assayed for PCB s by the Cincinnati EPA facility. Figure 3 shows recovery of 1242 using (1) hexane alone, (2) hexane and water (1 1), (3) hexane, water, and ethyl ether, (4) ethyl ether and water, (5) ethyl ether, water, and methanol, (6) methanol and hexane (1 1), and (7) water, methanol, and hexane (1 4 5). This last combination appears to give the best recovery. When added in this order to a dry sample, the effect of the water is to wet the sample, thus permitting extraction by methanol. The extracted PCB is partitioned almost exclusively into the hexane from the aqueous methanol. Final recovery is calculated from initial weight and hexane volume. 

In multicomponent systems A"0 can be written as a sum of the individual absorption coefficients A ot = 2TA , where each AT,(A ) depends in a different way on the wavelength. If one or more of the components are fluorescent, their excitation spectra are mutually attenuated by absorption filters of the other compounds. This effect is included in Eqs. (8.27) and (8.28) so that examples like that of Figure 8.4 can be quantified. The two fluorescent components are monomeric an aggregated pyrene, Mi and Mn. The fluorescence spectra of these species are clearly different from each other but the absorption spectra overlap strongly. Thus the excitation spectrum of the minority component M is totally distorted by the Mi filter (absorption maxima of Mi appear as a minima in the excitation spectrum ofM see Figure 8.4, top). In transparent samples this effect can be reduced by dilution. However, this method is not very efficient in scattering media as can be seen by solving Eqs. (8.27 and 8.28) for bSd — 0. Only the limit d 0 will produce the desired relation where fluorescence intensity and absorption coefficient of the fluorophore are linearly proportional to each other in a multicomponent system. 

In addition to these chemical artifacts, physical artifacts can also occur. For example, the problems of particle bounce (e.g., see Wedding et al., 1986) and reentrainment in impactors were discussed earlier. In addition, air turbulence is known to have a significant effect on the overall sampling efficiency of particle inlets (e.g., Wiener et al., 1988 Francois et al., 1995). 

SFE manifests its best advantages when extracting analytes from solid and semisolid rather than liquid samples. A primary limitation in extracting analytes from liquid sample matrices is the mechanical difficulty of retaining the liquid matrix in the extraction vessel. To extract a liquid sample by SFE successfully, analysts must first mix it with a solid material, such as diatomaceous earth or alumina, so that the sample is no longer free-flowing. Control of sample matrix effects is critical in SFE to limit coextractives, moderate the influence of moisture, and improve the efficiency of the extraction. Recent studies have shown that the addition of both inert and active sorbents to the sample matrix can improve the efficiency of SFE (153). 

The matrix appears to control extractability in a wide variety of samples, the effect of water content in the matrix has been reported (101,102). Onuska and Terry reported decreases in extraction efficiency with wet and dry sediment, differences in weight to weight percent water were 0.3% to 19.8%. McNally and Wheeler suggested the opposite increased extraction efficiencies with the addition of water as... 

Ahnoff M, Wurzer A et al. (2003) Characterization of serum albumin and lysoPCs as major contributors to plasma sample matrix effects on electrospray ionization efficiency, Presented at the 2009 ASMS conference, Montreal, Canada... 

Mills et al. performed extensive investigations into the photocatalytic degradation of 4-chlorophenol. These included studies on the effects of different titania samples [102], effects of annealing temperature on the photocatalytic efficiency of titania [ 103] and a mechanistic study of the decomposition process. The rate of chlorophenol destruction was found to drop when using titania photo catalysts that had been heated above 600 °C. This was believed to be due to a build up of the rutile phase and a reduction of surface area following heat treatment above these temperatures. A number of intermediates were reported including 4-chlorocatechol, hydroquinone, benzoquinone and 4-chlororesorcinol [104],... 

Serious errors in measuring solids concentration arise as a result of improper sampling. The effectiveness of sampling devices is usually expressed as the ratio of the measured solids concentration, C, to the upstream local solids concentration, Cq. The concentration ratio (C/Cq) is also known as the aspiration coefficient (25), separation coefficient (26), or sampling efficiency (27). 

Figure 6. Effect of particle size on the sampling efficiency of side-wall sampling. (Reproduced with permission from reference 40. Copyright 1985.)...

The effect of particle bouncing on the sampling efficiency of thick L-shaped probes was first noted in gas-solid systems by Whitely and Reed (44). They found that sampling efficiency for thick L-shaped probes was higher than unity at U/Uq = 1. To estimate the sampling efficiency due to particle bouncing at the isokinetic velocity, Belyaev and Levin (25) and Yoshida et al. (45) proposed an empirical equation. This equation can be written in a slightly different form as... 

Figure 13, from Nasr-El-Din et al. (47), shows the effect of the probe relative wall thickness on the sampling efficiency for polystyrene particles of... 

Din and Shook 46). Figure 14 compares the calculated sampling efficiency for a thick probe having a probe relative wall thickness of 0.8, considering the inertial effect alone and with the particle bouncing effect, with the experimental measurements. Clearly, the agreement is much better when both effects are considered. 

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