I would like advice on the feasibility of using an ion exchange resin to selectively trap iodide from a matrix containing a large excess of thiosulphate and sodium hydroxide. Can anyone recommend a suitable resin?

This is the tall order! The main problem is that both iodide and thiosulfate have high selectivity for most anion exchange phases (dealing with hydroxide shouldn't be a problem). Before I can make a recommendation, I would need to know what sort of mole ratio of iodide to thiosulfate you expect in your samples. Also, please supply the concentration of thiosulfate and hydroxide that you expect to see in your sample.

Not sure on the exact numbers but hydroxide conc is ~0.5M and thiosulfate ~0.1M. This is a very considerable excess over iodide.

Just a thought: Any chance that a molecular sieve could do this job?

It might help if you made clear what you were trying to accomplish.

I am unaware of any resin which provides a simple method for selectively concentrating iodide in the presence of thiosulfate. About the only option I can think of would be to treat the sample with a silver form cation exchange resin, filter and wash the resin to remove the thiosulfate and then treat the resin/silver iodide precipitate with a reducing agent to liberate the iodide. I'm not sure how practical this would be for trace analysis applications.

Another possibility would be to try using ion exchange selectivity to accomplish your objective. The answer as to whether this will work depends somewhat upon your iodide concentration. If the iodide is also in the molar concentration range, then the task shouldn't be too difficult. On the other hand, if the iodide concentration is in the mM range or lower, the task of accomplishing this separation will be considerably more difficult. Since iodide and thiosulfate are both polarizable anions, they have a number of common characteristics in terms of chromatographic retention. However, you may well be able to make use the differences in valency of iodide and thiosulfate in order to accomplish your task. By utilizing a relatively hydrophilic anion exchange resin in conjunction with high ionic strength eluent, thiosulfate can be made to elute well before iodide. In this case, the objective can be achieved by selecting a concentration which essentially renders thiosulfate unretained under conditions that permit retention of iodide. While a number of columns may be useful for such an application, a good choice would be the Dionex AS16 column.