Reversed-phase systems for the analysis of catecholamines and related compounds by high-performance liquid...

Phase systems using alkyl-modified silica as an absorbent, used as much and as a support for dynamically coated ion exchangers, were investigated for their capability in separating catecholamines and related compounds. Simple reversed-phase adsorption chromatography with C8-bonded silica is not able to separate these compounds very well because of (i) the very small retention of the more basic compounds in circumstances where the acidic compounds are well separated, (ii) bad peak shapes and (iii) low column efficiences, although the last drawback can be circumvented by the addition of inorganic anions to the eluent. The addition of a dynamically coated cation exchanger, sodium dodecylsulphate (SDS), to the eluent not only brings about drastic changes in the selectivity, but also makes available an additional degree of freedom for influencing the selectivity. The retention of the basic solutes increases upon addition of SDS and the retention becomes inversely proportional to the counter ion (Na+) concentration. Further, it was found that columns previously loaded with SDS can be used with SDS-free eluents when a pre-column, loaded with SDS, is used or with eluents containing a very small amount of SDS (less than 0.001%, w/v). These SDS-coated phase systems behave similarly to phase systems containing SDS in the eluent and show a better column stability and UV background.