Hydroxide eluents are mostly used when performing gradient analysis of inorganic anions, organic acids, and other 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.
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 and eluent preparation must be performed quickly to minimize the risk of contamination by carbon dioxide during this stage. The table below contains some examples of hydroxide eluents and recipes for their preparation from saturated solutions of NaOH or KOH. In addition, the table contains information about for which columns these are denoted as standard eluents by their manufacturers.
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.
| Eluent | NaOH* | KOH* | Column** |
|---|---|---|---|
| 10 mM hydroxide | 1.600 g | 2.494 g | – |
| 20 mM hydroxide | 3.200 g | 4.987 g | IonPac AS19 |
| 23 mM hydroxide | 3.680 g | 5.735 g | Shodex IC SI-37 4D |
| 38 mM hydroxide | 6.080 g | 9.476 g | IonPac AS15 |
| 55 mM hydroxide | 8.799 g | 13.71 g | IonPac AS28 |
| 100 mM hydroxide | 16.00 g | 24.94 g | – |
| Recipes for typical carbonate-bicarbonate eluents in 2.0 L ultrapure water (1994 g at 25 °C). *Weight of 50% NaOH solution or 45% KOH solution, stored protected from ambient carbon dioxide. **Indicating the standard eluent for each column as recommended by its manufacturer. Naturally, numerous other eluent compositions and gradients are possible to use with each column. Mentioning of a column do not constitute any endorsement or recommendation but may include a link to examples if available in the Diduco application database. |
|||
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.
To learn more, browse the application examples below, return to the knowledge overview, or contact Diduco to discuss your specific analysis.
I’ll be happy to answer any questions you might have regarding our products and services.
Tobias Jonsson
CEO, PhD in chemistry