We are working on HPLC analysis of torsemide in human plasma using furosemide as ISTD. Samples are extracted on SPE, final elution is done with 40% MeCN or 70% MeOH (no buffer). Final eluate is evaporated at45 degrees celsius under a stream of nitrogen for 20 min.

We got inconsistent results from run to run. It is found that furosemide is extremely light-sensitive, which is not highlighted in most literature. Does anyone know if torsemide is also light-sensitive? How stable are they at 45 degrees celsius at pH 5 in aq. MeCN? What would be good ISTD candidates for torsemide?

Thank you very much for your info.