Carbonate-bicarbonate eluents are common to use when performing routine analysis of anions using ion chromatography (IC) with suppressed conductivity detection, most often in isocratic mode. These eluents contain bicarbonate (HCO3–) and carbonate (CO32-) ions, plus sometimes also hydroxide (OH–), at concentrations and ratios optimized to give suitable retention and selectivity for the studied anions on the column in use.
As all liquid chromatography mobile phases, carbonate-bicarbonate eluents should be prepared from ultrapure water with 18.2 Mohm·cm resistivity. The table below contains some examples of carbonate-bicarbonate eluents and recipes for their preparation from pure anhydrous sodium salts. In addition, the table contains information about for which columns these are denoted as standard eluents by their manufacturers. It would also be possible to prepare stronger stock concentrate solutions and then dilute them to prepare the final eluent just before use, provided that they have not been contaminated by the bottle or environment during storage. An alternative approach is to purchase ready-made concentrate solutions of sodium bicarbonate and sodium carbonate in IC quality and dilute them to the desired concentrations before use.
Eluent | NaHCO3 | Na2CO3 | NaOH* | Column** |
---|---|---|---|---|
1.7 mM bicarbonate 1.8 mM carbonate | 0.286 g | 0.382 g | – | Hamilton PRP-X110 ICSep AN1 ICSep AN300 IonPac AS4A-SC Shodex IC SI-90 4E |
1.0 mM bicarbonate 3.2 mM carbonate | 0.168 g | 0.678 g | – | Metrosep A Supp 5 Shodex IC SI-50 4E |
1.0 mM bicarbonate 3.5 mM carbonate | 0.168 g | 0.742 g | – | ICSep AN2 IonPac AS14 |
3.6 mM carbonate | – | 0.763 g | – | Shodex IC SI-35 4D Shodex IC SI-52 4E |
1.4 mM bicarbonate 4.5 mM carbonate | 0.235 g | 0.954 g | – | IonPac AS22 |
5.0 mM bicarbonate 5.0 mM carbonate | 0.840 g | 1.060 g | – | Metrosep A Supp 10 |
7.5 mM carbonate 0.75 mM hydroxide | – | 1.590 g | 0.120 g | Metrosep A Supp 16 |
Recipes for typical carbonate-bicarbonate eluents in 2.0 L ultrapure water (1994 g at 25 °C). *Weight of 50% NaOH solution, stored protected from ambient carbon dioxide. **Indicating the standard eluent for each column as recommended by its manufacturer. Naturally, numerous other eluent compositions 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. |
Note that all low-concentration high-pH eluents are sensitive to contamination by ambient carbon dioxide since this gas will form carbonate in solution, which thus change the properties of the eluent and make it stronger and reduce retention. This is most critical for hydroxide eluents, but also weak carbonate-bicarbonate suffers from this phenomenon. 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 or contact Diduco to discuss your specific analysis.
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