Topic: hydroxide eluent

Hydroxide eluents are mostly used when performing gradient analysis of anions using suppressed ion chromatography (IC). These eluents have the advantage of turning into water in the suppressor, thus resulting in very low background conductivity.

Eluent preparation

Hydroxide eluents cannot be prepared from solid pellets since these contain a significant amount of carbonate species on the surface and must instead be diluted from carbonate-free concentrated solution that have been stored protected from contamination by ambient carbon dioxide. Suitable sources are saturated 50% NaOH and 45% KOH (consult analysis certificates for exact concentration), or alternatively, the IC quality concentrate solutions available (typically around 0.1 M). The water used for hydroxide eluents must be freshly prepared ultrapure water with 18.2 Mohm·cm resistivity. Eluent preparation must also be performed quickly to minimize the risk of contamination by carbon dioxide during this stage.

An alternative approach is to use so-called eluent generators that create hydroxide ions from splitting water and complementing them with cations from a reagent cartridge to manufacture the eluent. This method typically produces clean eluents but require special equipment and repeated replacement of the reagent cartridges.

Protecting eluents from carbon dioxide

Hydroxide solutions are extremely sensitive to contamination by ambient carbon dioxide since this will form carbonate in solution, which thus change the properties of the eluent and make it stronger and reduce retention. The effect can be avoided by protecting the eluent using an adsorbent (such as in Xenoic® EQAX-TC1 Trap Cartridge) that removes carbon dioxide from the incoming air during eluent withdrawal. The traditional approach was to sheet the eluent bottle with helium or nitrogen gas at low pressure, but this requires a more elaborate setup and larger investments to achieve similar results, and is not sustainable since helium is a non-renewable natural resource that easily escapes the atmosphere when released.

Learn more

To learn more, browse the application examples below or contact Diduco to discuss your specific analysis.