Chromatographic selectivity, alteration

Polyamide is available commercially as polyamide-6 (based on e-aminocapro-lactam, Macherey-Nagel) and as polyamide-11 (based on 11-aminoundecanoic acid, Merck). The lipophilic properties of these are different, thus altering their chromatographic selectivity. 

Uf course, the enhancement of chromatographic selectivity by secondary chemical equilibria is neither new nor confined to reversed-phase systems. Most widespread probably has been the exploitation of protonic equilibria by appropriately ati usting the pH of the eluent so that the degree of ionization of the eluite is altered. Generally the ionized and neutral forms of an eluite are retained differently (2( 7. 208). Formation of metal complexes of certain eluites has also been utilized for modulating retention behavior for higher selectivity. 

Figure 3. Identification of thiols In sediments by altering chromatographic selectivity.

It is very interesting to compare the modifying effects observed by Kiselev and Yashin [4] and Bruner et al. [83, 85] for systems with nonvolatile modifiers, i.e. common stationary liquid phases. These modifiers block and deactivate the active sites on graphitized carbon blacks, and also alter the selectivity of the stationetry phase with adsorbent. On the other hand, Lin and Parcher [76, 77] have also observed the same effects with volatile modifiers. However, volatile modifiers offer the significant possibility of continuous, extemeil control of chromatographic selectivity. 

In the course of mixture separation, the composition and properties of both mobile phase (MP) and stationary phase (SP) are purposefully altered by means of introduction of some active components into the MP, which are absorbed by it and then sorbed by the SP (e.g. on a silica gel layer). This procedure enables a new principle of control over chromatographic process to be implemented, which enhances the selectivity of separation. As a possible way of controlling the chromatographic system s properties in TLC, the pH of the mobile phase and sorbent surface may be changed by means of partial air replacement by ammonia (a basic gaseous component) or carbon dioxide (an acidic one). 

Flammable atmospheres can be assessed using portable gas chromatographs or, for selected compounds, by colour indicator tubes. More commonly, use is made of explos-imeters fitted with Pellistors (e.g. platinum wire encased in beads of refractory material). The beads are arranged in a Wheatstone bridge circuit. The flammable gas is oxidized on the heated catalytic element, causing the electrical resistance to alter relative to the reference. Instruments are calibrated for specific compounds in terms of 0—100% of their lower flammable limit. Recalibration or application of correction factors is required for different gases. Points to consider are listed in Table 9.10. 

A potentially more sensitive and selective approach involves reaction of formic acid with a reagent to form a chromophore or fluorophore, followed by chromatographic analysis. A wide variety of alkylating and silylating reagents have been used for this purpose. Two serious drawbacks to this approach are that inorganic salts and/or water interfere with the derivatisation reaction, and these reactions are generally not specific for formic acid or other carboxylic acids. These techniques are prone to errors from adsorption losses, contamination, and decomposition of the components of interest. Enzymic techniques, in contrast, are ideal for the analysis of non-saline water samples, since they are compatible with aqueous media and involve little or no chemical or physical alterations of the sample (e.g., pH, temperature). 

Gel permeation chromatography is the mildest of all chromatographic techniques. It allows to select the buffer conditions during separation so that no alteration of the protein structure or function occurs. 

Hoff and Feit (8) reacted samples in a 2-cm3 hypodermic syringe before injection onto the gas chromatographic column. Reagents were selected either to remove certain functional groups or to alter them to obtain different peaks. Reagents used included metallic sodium, ozone, hydrogen, sulfuric acid, hydroxyl-amine, sodium hydroxide (20%), sodium borohydride (15%), and potassium permanganate (concentrated). 

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