Probes sampling efficiency

Calibration is necessary to allow correlation between collected dialysis concentrations to external sample concentrations surrounding the microdialysis probe. Extraction efficiency (EE) is used to relate the dialysis concentration to the sample concentration. The steady-state EE equation is shown in equation (6.1), where Coutiet is the analyte concentration exiting the microdialysis probe, Ci iet is the analyte concentration entering the microdialysis probe, CtiSSue> is the analyte tissue concentration far away from the probe, Qd is the perfusion fluid flow rate and Rd, Rm, Re, and Rt are a series of mass transport resistances for the dialysate, membrane, external... 

If Ciniet has a value of zero, then equation (6.1) reduces to what is commonly called relative recovery and denoted in the literature as RR. The terms EE and RR are frequently interchanged and sometimes the general word, recovery, is used to describe the sampling efficiency of the microdialysis probe. It is important to note that the equivalence of EE versus RR under certain conditions such as with highly regulated concentrations of neurotransmitters has been debated in the literature 46... 

The value of ( used here corresponds to 10 3 of its value in room temperature aqueous solutions. [80] On the one hand, using such weak friction improves the sampling efficiency in the simulations and does not affect equilibrium structural properties. On the other hand, the dynamical properties that we observe may be different from those probed by SM-FRET techniques, which would not be able to resolve conformational dynamics on such fast time scales. Thus, the relevance of the following analysis of dynamical properties relies on the assumption that increasing the friction will not significantly alter our main conclusions. It is interesting to note in this context that the folding mechanism in similar models has been observed to be relatively insensitive to the value of the friction coefficient. [81]... 

The preceding discussion shows that the sampling efficiency for thin L-shaped probes is a function of two parameters the deviation from the isokinetic conditions and the response of the particles to the deflection of the fluid streamlines upstream of the sampler. The deviation from the isokinetic conditions is a function of the velocity ratio (U/Uq), whereas the particle response is a function of the ratio of particle inertia to fluid drag. This ratio in a dimensionless form is known as the particle inertia parameter, the Stokes number, or the Barth number (K), defined as ... 

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 11. Effect of the probe relative wall thickness on the sampling efficiency. (Reproduced with permission from reference 46. Copyright 1985.)...

Figure 13 illustrates that the sampling efficiency appears to be independent of the probe relative wall thickness at sampling velocities equal to and higher than the upstream local velocity. This observation contrasts with the results obtained with the sand particles shown in Figure 11. This difference can be explained as follows In the presence of a blunt probe, the fluid streamlines deflect ahead of the probe nozzle even at a velocity ratio U/Uq =...

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. 

One type of probe is a hand-held stainless-steel hollow auger, which has soil-air vent holes drilled into the shaft near the bottom and is fitted with a gas sample port near the top (Lovell, 1979). Another type of hollow probe has a straight shaft with a gas sample port at the top and soil-air vent holes at the bottom this probe is pounded into the soil by means of a captive hammer that slides up and down the shaft to either drive the probe into or remove it from the soil (Dyck, 1972 Chemical Projects Limited, Toronto, Ontario, Canada, written common., 1972 Lovell, 1979). Both the hollow auger and hollow hammer-probe are efficient dynamic soil-gas samplers in light soils. Neither sampler works well in stony or hard, compacted soils, or in soils containing layers of caliche attempts to use probes in these soils may either bend the probe or disturb the soil to the extent that the soil-gas sample is diluted by atmospheric air. Soil gas can not be sampled in wet soils with these probes. 

Figure 13 shows the sampling efficiency versus the velocity ratio for L-shaped probes having a tip angle 6 of 18° and probe relative wall thicknesses of 0.4, 0.8, and 1.2. The 0.08-mm sand at 6.3 vol% discharge solids concentration, and 2.63 m/s bulk velocity was used in these experiments. At this tip angle, the increase of C C0 at U U0= 1 is eliminated. 

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