Volatile ion pair reagents (e.g., heptafluorobutyric acid and tetrabutyl ammonium) have been added either to the sample vial or to the mobile phase to improve the chromatography of ionic species, such as paraquat and diquat. The introduction of an ion pair reagent into the mobile phase increases the interactions between the quaternary ammonium compounds and the C18 stationary phase, providing the necessary retention and resolution.

The type and quantity of ion pair reagent added has to be a compromise between improvement in separation and retention and minimizing the suppression observed in ES.

Hydrophilic interaction chromatography (HILIC) has been explored for the determination of paraquat and diquat by MS/MS without the need for ion-pairing reagents. HILIC separates compounds by passing a hydrophobic or mostly organic mobile phase across a neutral hydrophilic stationary phase, causing solutes to elute in order of increasing hydrophilicity.

Although the chromatography behavior on HILIC is not as good as that observed using the ion pair systems, the MS sensitivity using the HILIC mobile phase was claimed to be significantly greater.

Reference: Analysis of Pesticides in Food and Environmental Samples, CRC Press, 2008, edited by José L. Tadeo