Eluant salicylic acid

Salicylic acid and Its metabolite were separated by two methods. The first was thin layer chromatography on cellulose with BAW solvent as for the In vivo metabolism studies. A quicker separation was achieved with a polyamide column. The entire 400 pL from an individual assay was placed on top of a 0.8 x 2.0 cm column packed with Polyamide-6 (Accurate Chemical and Scientific Corp.). The salicylic acid metabolite was eluted with 6 mL water but salicylic acid was retained. 3a70B scintillation fluid (Research Product International Corp.) was used to determine the radioactive content of the entire 6 mL of eluant. Separation of salicylic acid and its metabolite by polyamide column chromatography was verified by thin layer chromatography. 

Figure 9.20 Chromatograms of standard mixture of analgesics, column and conditions detailed in text. Eluant 40% methanol/60% 1 % HOAc. Detection (a) 235 nm (b) 254 nm. Peaks (1) paracetamol (2) caffeine (3) salicylamide (4) aspirin (5) phenacetin (6) salicylic acid.

There are a variety of organic acids which can be used as eluants. As an example, the chromatogram obtained with a nicotinic acid eluant is shown in Fig. 3-103. Nicotinic acid is most suitable for the separation of monovalent anions. In contrast to other monocarboxylic acids, it is characterized by a good solubility in water and a low equivalent conductance. In addition, its propensity to adsorb at the stationary phase is very low, which prevents the formation of the usual system peak in most cases. However, divalent anions are strongly retained under these chromatographic conditions. For a rapid elution of divalent anions, salicylic acid or trimesic acid eluants are more suitable. 

It is imperative to consider the kind of counter ion in an ion-pair reagent for selecting the appropriate detection method. If suppressed conductivity detection is applied, the ion-pair reagent is used in its hydroxide form. With direct conductivity detection, salicylate is preferred as the counter ion for tetraalkylammonium cations [19, 20], since these salts exhibit a lower background conductivity in aqueous solution. According to Wheals [21], eluants such as cetyltrimethylammonium bromide in combination with citric acid at pH 5.5 have proved suitable for UV-, RI-, and amperometric detection, as well as for direct conductivity detection. This example is an impressive illustration of the versatility of ion-pair chromatography. 

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