The spring of 1974 was a very busy time for Elo Hansen and me. Amidst the usual teaching load, I was preparing to leave with my family for a year-long assignment in Brazil, where I was to work as a radiochemist for the International Atomic Energy Agency. At the same time, Elo and I had two projects in progress: an “enzyme stirrer” and an “air gap electrode.” The latter device we conceived, designed, patented and sold to a Swiss company (for an amount that was sufficient to fund a single ski trip). To improve its speed of response, we rigged the electrode within a flow cell that was then connected upstream to two reservoirs, one with ammonium chloride and the other with sodium hydroxide. We switched between the streams by pinch valves. We were unable to decide which was the slowest component of this clumsy design, until we replaced the valves with a syringe. Then we simply injected ammonium chloride into sodium hydroxide with a hypodermic needle between the flow cell and the pump. This provided an immediate answer to the slow response rate, but it also astonished us when we repeated the experiment with different concentrations of the analyte. The repeatability of sharp peaks was better than about 5%. The problem now became a lack of time – I was about to leave for a year. We asked the Swiss Company to give us yet another pump (It was a unique design that provided a smooth – but not quite pulseless – flow), that I could pick up on my way to Brazil. We had just time for one quick experiment with a dye using colorimetric detection – and that was all.
I had to leave, while Elo continued with the “enzyme stirrer,” collaborating with Jerry Guilbault on the development of novel enzymatic assays. To Elo’s everlasting credit, however, he somehow found time to file a Danish patent application (Dan. Pat. Appl. No. 4846/74, Sept. 1974, subsequent US. Pat. No. 4,022.575.)

We had a feeling that we had stumbled upon something good, but it did not take the highest priority since Elo was departing for his perennial summer retreat on the island of Hirsholm and I was heading to Brazil for “diplomatic service.”

Apparatus in which FIA began, the Technical University of Denmark, spring 1974
1- peristaltic pump (Zellweger, Uster) 2- Air Gap electrode,
3- Potentiometer (Radiometer, Copenhagen),
4- Chart recorder (Radiometer Copenhagen).
Reagent bottles with ammonium chloride, water and sodium hydroxide. Flow rate 14 ml/min, injected volume 1 ml.