Trace enrichment on a metal-loaded thiol stationary phase in liquid chromatography: effect of analyte structure and pH value on the (de)sorption behaviour.
In order to obtain more information on the (de)sorption processes on a metal-loaded thiol phase, various compounds containing heterocyclic nitrogen atoms have been studied. Adenine and adenosine, which contain no carbonyl groups in a position ortho to the ring nitrogen, were chosen to study the role of the binding donor atoms and, therefore, of the 'minimum' structure required for efficient pH-dependent (de)sorption. The results indicate that a single heterocyclic nitrogen atom, preferably with a high pKa value, is sufficient to provide pH-dependent (de)sorption of chelating phases loaded with soft metal ions, such as Ag(I) or Hg(II), which can profitably be used in an on-line combination with column liquid chromatography. Barbiturates were chosen on the basis of their structural similarity with the earlier studied (halogenated) uracil derivatives. The (de)sorption behaviour of the barbiturates was studied as a function of the pH, the metal ion [Cu(II), Ag(I), Hg(II), Pd(II)] bound to the stationary phase and the type of stationary phase. The combined data show that the pKa value of an analyte and the stability constant of the complex formed with the metal ion sorbed on the stationary phase give a reliable indication of the optimum (de)sorption pH. Application of this principle is demonstrated by the determination of four barbiturates in plasma.