Fishing with Ion Chromatography Suppressors

I actually started using ion chromatography (IC) – of all things – to measure sulfate. Many people, including Hamish Small, say that IC became a commercial success primarily because it could measure sulfate. And that’s true. Sulfate is a compound with no optical handle and before IC, people were measuring it with the barium chloranilate method, which took a lot of effort. And although ion chromatographs of that generation didn’t come with autosamplers, the fact that you could measure sulfate in parts-per-million concentrations within 15–20 minutes was wonderful. Importantly, the whole concept and execution of IC depends on very understandable and exploitable chemistry.

One thing that really attracted my curiosity in IC was the suppressor. The suppressor was both the unique differentiator that made IC possible in that form, but also the weakest link in the chain. Indeed, suppressed IC suffered from i) the need for suppressor column regeneration, ii) variable retention of weak acids (as a function of suppressor column condition), and iii) suppressor column induced band broadening.

I was using Nafion tubes in air sampling work as dryers, and their properties piqued my interest. I knew that Nafion (a sulfonated fluoropolymer made by DuPont) was a cation exchange material and I wondered if, instead of using packed suppressor columns, I could use a Nafion fiber and pump chromatographic eluent through it. Bathing the fiber in acid would allow continuous exchange.

I also knew the mass transfer equations applicable to denuders, which are used to capture gas molecules while transmitting the aerosol, from my work in atmospheric sampling. It’s not intuitive, but the equation states that mass transfer to the wall is independent of the diameter of the tube given the same overall volume flow rate. That truth seemed to be of great significance to me. After all, band dispersion in chromatography depends acutely on the diameter of the tube. Therefore, the solution to improving suppression columns in this regard was simply to use tubes with a smaller diameter.

At the time, the tubes available were about 0.9mm in diameter. I had just moved to Texas Tech University, and I was talking to DuPont about the fibers. I asked if they could make 0.3 mm tubes. Yes, they could, but agreement would be needed from “upstairs,” so I wrote a letter to Jack Kirkland. Six months later, I received a reply (I still have the letter) stating that DuPont was independently interested in the project. Jack suggested that I contact The Dow Chemical Company; Stevens and Small from Dow had just published a paper on using sulfonated polyethylene fibers as suppressors. Ironically, they were using exactly the same diameter that I wanted.

However, it turns out that sulfonated polyethylene isn’t a great ion exchange membrane. I knew that Nafion would be far better – and yet DuPont would not make what I wanted. I don’t know the full story, but in any case, like all good assistant professors, I was concerned about the safety of my idea. So I decided to go solo and attempt to make what I needed myself. I got up one morning and suddenly realized that I knew how to do it. I put fishing line inside a Nafion tube, coiled it and then boiled it in water, thermosetting the fishing line. The process produced beautiful coils and, more importantly, created suppressors with ten times less dispersion. My first two papers as an assistant professor featured the “annular helical suppressor” and were published simultaneously in Analytical Chemistry in 1984 (1, 2).

I take great pleasure in stating that, despite failing to pursue my idea with the manufacturer, I was able to take an alternative route by patenting the process of putting DuPont fishing line inside DuPont Nafion tubes. What’s more amazing is that someone is still selling those suppressors – SeQuant in Sweden! Those two papers also kicked off an excellent relationship with Dow and then Dionex – but that’s another story.

The moral: if you have an idea you believe in, don’t be afraid to fly solo or go fishing.