Octanesulphonic acid

A rapid and simple assay for the analysis of OTC, TC, CTC, and DXC in milk was established using an on-line sample cleanup. Coexisting substances in milk were removed by the precolumn head-cut-loading method, and TCs were rapidly loaded on an analytical column after changing the eluent (ion-pair mobile phase containing octanesulphonic acid). This method could be carried out after deproteinization with TFA only. The authors reported recoveries ranging from 80% to 104% (49). 

HPLC conditions column 2xICE-ASl 300 X 7.9 mm conductivity detection solvent = 2 mM octanesulphonic acid in 2% propan-2-ol flow rate 0.5 ml/min. 

The mobile phase for reversed-phase HPLC of peptides often contains additives ( pairing agents ) that improve the resolution of components. These are frequently strong acids (octanesulphonic acid or phosphoric acid) and act, together with control of the pH of the aqueous part of the mobile phase, by fine-tuned interactions with the basic and acidic groupings that may be present in the peptides (Figure 4.15). 

The k values are proportional to the counter-ion concentration, so that the retention times are not only affected by the type but also by the concentration of the ion-pair reagent. Under certain circumstances, two reagents instead of one may be used for optimizing a separation problem, e.g. a mixture of pentane- and octanesulphonic acid. Of course, retention times can also be adjusted by changing the amount of organic solvent in the mobile phase and all optimization procedures described in Chapter 10 can be used, including the addition of competing compounds. 

Fig. 13.3 Separation of UV-inactive anionic and cationic compounds. (Reproduced by permission of Elsevier Science Publishers BV from M. Denkert etal., J. Chromatogr., 218, 31 (1981).) Anions detected before the system peak produce positive peaks and cations produce negative peaks, whereas the reverse applies for compounds following the system peak. Conditions column, 10 cm x 3.2 mm i.d. stationary phase, riBondapak Phenyl, 10rim mobile phase, 0.5 ml min 4x 10 " M naphthalene-2-sulphonate in 0.05M phosphoric acid UV detector, 254nm. Peaks 1 = butylsulphate 2 = pentylamine 3 = hexanesulphonate 4 = heptylamine 5 = octanesulphonate 6 = octylsulphate S = system peak.

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