Analyte dissociation

Apparent Chemical Deviations Apparent deviations from Beer s law- arise when an analyte dissociates, associates, or reacts with a solvent to produce a prttduct with a different iihsorpti<>n spectrum than the analyte. A common example of this behavior is found with aqueous solutions of acid-base indicators. For example, the color change iissociated with a typical iitdicalor tlln arises from. shifts in the equilibrium... 

If the pollutant is charged and exhibits UV-absorbing properties, the CZE mode is readily recommended. Eor basic pollutants, moderately low to low pH buffers are indicated and the analyte migrates coelectroosmotically as a cation (protonated species) whereas for acidic pollutants, high pH buffers will promote the analyte dissociation and it migrates counterelectroosmotically as an anion. In both cases, buffer pH and concentration are the variables to optimize before the addition of any modifiers is considered. 

The pKa or acid dissociation constant is a measure of the strength of an acid in solution. A larger value for the pKa tells us that the analyte dissociates to a lesser extent and is therefore a weak acid. For basic analytes, the reverse applies, in that a higher value for the pKa means a stronger base. 

An interesting application area for anion exchange chromatography is the analysis of herbicides based on phenoxycarboxylic acids and their derivatives. Generally, these compounds were separated at chemically modified silicas using a methanol/water mixture as an eluant to which acetic acid was added to suppress analyte dissociation. The list of herbicides that may be analyzed with anion exchange chromatography comprises ... 

Figure 14 Time evolution of SPR signal upon the capture (association phase) and release of target analyte (dissociation phase) from a hydrogel film with immobilized ligand molecules. Binding of prostate specific antigen to respective antibodies coupled in a dextran-based hydrogel that is probed with an LRSP mode is presented.

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