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.

Illustration of hydroxide eluent


To ensure that liquid chromatography analysis methods meet regulatory demands of retention repeatability as dictated by pharmacopoeia and the ICH guidelines, it is important to protect weakly buffered eluents from ambient carbon dioxide. The formation of carbonate species will otherwise change the pH and strength of the eluent, thus shifting retention.
Suppressed ion chromatography with conductivity detection is a powerful and sensitive technique to analyse charged ionic species, especially anions of strong acids. The chemically regenerated membrane suppressor Xenoic® XAMS can enable analysis of such anions down to levels of a few microgram per litre (part per billion, ppb).
Official quality control method for the first-generation bisphosphonate clodronate, which inhibits bone resorption and is used to treat bone weakness or pain caused by cancer that has spread to the skeleton. This analysis was performed according to the European Pharmacopoeia monograph method which is used to determine related substances that could be present as impurities in the active ingredient.
Gradient elution is a powerful tool when aiming to separate many compounds having a wide range of different chemical characteristics. Hydroxide gradients are the most common in ion chromatography since they can be converted into pure water with nearly zero conductivity.
Separation of inorganic anion standards at eluent conditions recommended by the column manufacturer.
Separation of inorganic anion standards at eluent conditions recommended by the column manufacturer.

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