Gradual desorption

The cause of these discontinuities still seems obscure the number of cases in which they have been found is too great for their attribution to experimental error, but it must be admitted that their detection does, at least in some cases, depend on the particular technique adopted in measuring the adsorption. A possible cause may be that cracks open up, suddenly exposing fresh areas for adsorption if this is the explanation, the discontinuities would be scarcely likely to appear, at any rate at the same pressures, during gradual desorption of the gas. 

Gradual desorption of the immobilised analytes is achieved either by a continuous increase in ionic strength or a change in pH of the mobile phase. Salt gradients,... 

Exposure of a preadsorbed layer of PLL(375)-g[5.6]-PEG-(5) to a solution with a pH value outside of the pH range where adsorption is observed or to a solution with a high ionic strength causes a gradual desorption of the polymer. Therefore, the adsorption of PLL(375)-g[5.6]-PEG(5) and the stability of this adsorbed polymer layer require that the contacting solution be limited to the pH range between the isoelectric point of the surface and the pA/a of the polymer, and to low ionic strengths. 

Note that upon increasing the time of 02 desorption, denoted tHe, the peak A, corresponding to chemisorbed O, decreases much faster than the back-spillover oxygen peak B. Similarly upon increasing the holding time, tH, of positive potential application (UWr=0.2 V) peak A reaches saturation first, followed by a gradual approach to saturation of the backspillover oxygen peak B. 

SFE-GC-MS is particularly useful for (semi)volatile analysis of thermo-labile compounds, which degrade at the higher temperatures used for HS-GC-MS. Vreuls et al. [303] have reported in-vial liquid-liquid extraction with subsequent large-volume on-column injection into GC-MS for the determination of organics in water samples. Automated in-vial LLE-GC-MS requires no sample preparation steps such as filtration or solvent evaporation. On-line SPE-GC-MS has been reported [304], Smart et al. [305] used thermal extraction-gas chromatography-ion trap mass spectrometry (TE-GC-MS) for direct analysis of TLC spots. Scraped-off material was gradually heated, and the analytes were thermally extracted. This thermal desorption method is milder than laser desorption, and allows analysis without extensive decomposition. 

During elution by clean water, in contrast, a well-defined desorption front fails to develop. Instead, the metal desorbs gradually across a broad area. To clear enough lead from the aquifer to meet drinking water standards, the pore fluid in the simulation must be replaced dozens of times, much more often than was required initially to contaminate the aquifer. 

In general, it is observed that after the binding step of proteins to the interface a gradual change of the phase is present (see, e.g., Fig. 10.13 after the a-HSA adsorption step). This effect can be explained by desorption of bound molecules not firmly attached. Given the time span used in these experiments, desorption did not seriously hamper the detection of analytes. 

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