Systematic investigations of peak distortions due to additives in supercritical fluid chromatography

The impact of eluent components added to improve separation performance in supercritical fluid chromatography was systematically, and fundamentally, investigated. The model system comprised basic pharmaceuticals as solutes and eluents containing an amine (i.e., triethylamine, diethylamine, or isopropylamine) as additive with MeOH as the co-solvent. First, an analytical-scale study was performed, systematically investigating the impact of the additives/co-solvent on solute peak shapes and retentions, using a design of experiments approach; here, the total additive concentration in the eluent ranged between 0.021 and 0.105 % (v/v) and the MeOH fraction in the eluent between 16 and 26 % (v/v). The co-solvent fraction was found to be the most efficient tool for adjusting retentions, whereas the additive fraction was the prime tool for improving column efficiency and peak analytical performance. Next, the impacts of the amine additives on the shapes of the so-called overloaded solute elution profiles were investigated. Two principal types of preparative peak deformations appeared and were investigated in depth, analyzed using computer simulation with mechanistic modeling. The first type of deformation was due to the solute eluting too close to the additive perturbation peak, resulting in severe peak deformation caused by co-elution. The second type of deformation was also due to additive-solute interactions, but here the amine additives acted as kosmotropic agents, promoting the multilayer adsorption to the stationary phase of solutes with bulkier aryl groups.