Monday, September 22, 2003

Ion Exchange Chromatography

Want to hear about what I did today? Here's a sample...

Chromatography is the art of separating chemicals. Fundamentally, any chromatigraphic technique uses two phases, a stationary phase and mobile phase, inside a long column. The stationary phase is usually a sort of resin or coating that interacts with the analytes that are present in the mobile phase. Depending on the level of interaction, the analytes will stick around in the column for different amounts of time. (Note that an important feature of the mobile phase is that it cannot interact with the stationary phase.) After exiting the column, a detector will 'see' the analytes pass through and will produce a signal that is recorded by a computer. Thus you can produce a graph of signal vs. time, and you will see 'peaks' on the chromatogram that will represent the amount of analyte flowing through the system and the time it took to travel through the system. Because every compound will have a unique retention time, you can use chromatography to assist in understanding the composition of unknown samples. Amazing.

There are so many ways to do this it's unthinkable - from gas chromatography/mass spectroscopy (my specialty) to liquid to size exclusion to gel permeation... to ion exchange, which is what I learned about today. Here's an excerpt from the introduction of my IEC report...

Ion exchange chromatography (IEC) is a specialized analytical technique whereby ions are separated and detected by attractions between oppositely charged ions. A subset of IEC techniques is simply called ion chromatography (IC), during which two ion exchange processes are used. Our objective in this lab was to analyze the electrolyte content of Rolla tap water and the fluoride content of Crest toothpaste.

The primary differences between ion chromatography and liquid chromatography are an immobilized ion stationary phase and an anion regenerant. During the first stage of IC, the analyte anions go through an “anion exchanger,” and are separated from their respective cations. The second stage is a high capacity cation exchanger that protonates the mobile phase (eluent) counter ions, making them less conductive so the detector doesn’t see them. This second step is usually called suppression, and is done in a suppression column. After the second step completes, the suppression column will have lost a good number of its hydronium ions. Hence, an ion exchange chromatograph uses an anion regenerant (a strong acid like H2SO4) to replace those ions. As implied earlier, the detector is an electrical conductivity detector, and is particularly sensitive to ions. When a substance with an electrical conductivity different then the background passes through, the computer records a peak.


Hmm, I guess you need to understand liquid chromatography so you can figure that garbage out. Oh well, maybe that can be for another time.

Until then,