Can a Mass Spectrometrist Be Comfortable with Ion Chromatography?

[john-e-madden]

I’ve been working in the field of Ion Chromatography (IC) for more than 20 years, but in the last 12 months I’ve been spending much of my time discovering the field of Mass Spectrometry (MS). As I talk to more and more Mass Spectrometrists, I discover they are just as apprehensive of IC as Ion Chromatographers are of MS. The main reason for their apprehension is the use of strong acids and bases as eluents in IC. It is often considered a statement of fact that potassium hydroxide and sulfuric acid (or methanesulfonic acid) eluents cannot be interfaced with an MS. However, in the case of IC, this doesn’t necessarily hold true.

It is true that you cannot pump a solution of potassium hydroxide or methanesulfonic acid (MSA) directly into an MS. While IC systems do use strong acids and bases as eluents, the suppressor device commonly used in IC eliminates these to produce a stream of pure water. Pure water is of course easily interfaced with a MS.

The role of a suppressor in an IC system dates back to the very emergence of IC as a technique. It was well known that acids and bases could be used to elute ionic species from an ion exchange stationary phase. The problem resided with trying to detect the ions of interest as most small ions are non-chromophoric, thus excluding the use of an absorbance detector. It was the development of the suppressor that enabled the interfacing of an Ion Exchange Chromatography system with a conductivity detector. By suppressing the eluent to pure water, the ions of interest could be easily detected against a background of almost non-conductive water.

A suppressor is essentially an ion-exchange device that exchanges anions with hydroxide, or cations with hydronium. The earliest versions of suppressors were packed bed suppressors and had more in common with separator columns than what we think of today as a modern suppressor. Modern suppressors use ion exchange membranes and electrolysis reactions to work, but the principle is still the same. A solution of potassium hydroxide is pumped through an anion suppressor; the potassium ions are removed and replaced with hydronium ions. The net effect is that a stream of potassium hydroxide solution becomes a stream of pure water. For a cation suppressor, methanesulfonate ions are replaced with hydroxide ions; the result is the same.

An additional benefit of this process is that the ions of interest, which usually take the form of the eluent during the ion exchange process (i.e., potassium or methanesulfonate), are converted to their hydronium or hydroxide form. In suppressed conductivity this is advantageous because these forms of ions are usually the most conductive, thus enhancing sensitivity. In an IC-MS system this effect is also advantageous because the hydronium and hydroxide forms of ions are the easiest to ionize, thus also improving sensitivity. Additionally, because all ions are present in the same form, matrix effects from the sample are minimized, improving precision and reproducibility. Sample matrix effects often plague LC-MS systems, but IC-MS systems are far less prone to them.

The result of all this is that an IC system that uses suppressed conductivity detection is inherently very easy to interface to a modern MS instrument and offers many advantages when analyzing polar ionic compounds. As more and more Mass Spectrometrists learn the advantages of IC as a front-end, they are losing their apprehension for IC and instead gaining a respect for it. The separation of organic acids, phosphonates, sugar phosphates, ionic pesticides and amines from each other and the matrix ions is often a trivial matter for an IC system. With the wide range of high capacity ion exchange columns offered under the Thermo Scientific™ Dionex™ IonPac™ brand, almost any ion exchange separation can be accomplished. Thanks to the suppressor, and a few short How-to videos, interfacing this to an MS is a relatively simple matter.