Methods Development Using Ion-Pair Chromatography with Suppressed Conductivity Detection

INTRODUCTION Mobile-phase ion chromatography (MPIC) is a technique that combines ion-pair chromatography with suppressed conductivity detection. Ion-pair chromatography is commonly used in combination with UV detection, in which case it is referred to as reversedphase ion-pair chromatography (RPIPC). MPIC has the advantage of using conductivity detection, which serves as a universal detection mode for species likely to be separated by ion-pair chromatography. MPIC is especially well suited to large molecules that carry localized charges (surfactants are a good example), as well as other ions that are not amenable to separation by ion-exchange chromatography (IC). For example, aliphatic quaternary ammonium ions can bind to ion-exchange resins during an IC separation, leading to inefficient peaks or low recovery. Due to the dynamic nature of the ion pair and since organic solvents are usually in the mobile phase, these ions do not adhere to the neutral resin used in MPIC. Also, when ion exchange does not have the required selectivity for a troublesome set of analytes, separation can often be achieved by switching to ion-pair. Suppressed conductivity detection, which has been used with ion-exchange chromatography since 1975, offers the same advantages for MPIC as it does for IC. The suppressor simultaneously lowers background conductivity from the eluent and enhances the analyte conductivity. State-of-the-art suppressors are membrane-based (for high efficiencies), easy to use, and solvent-compatible. Special considerations for suppression of ion-pair mobile phases are discussed in this Technical Note. Detergent and other chemical manufacturers routinely use MPIC for the analysis of compounds such as ionic surfactants or quaternary ammonium compounds. The technique is also of use to cosmetics manufacturers for the analysis of additives such as alkanolamines, and to the pharmaceutical industry for a variety of nonchromophoric, ionic species.