If you are talking about chemicals for macroetching stainless steel welds, then Aqua Regia etchant would be used for stainless steel welds. It must be mixed from 1 part nitric acid (HNO3) with 3 parts hydrochloric acid (HCl) by volume. This must be done by a person experienced in mixing acids under a fume hood. If no one in your company is experienced and/or the proper safety equipment is not available, then it would be advisable to have a local metallurgical testing company perform the work.
Having said that, I thought a "prequalified" welding procedure was one that required no testing. What code are you working to?
As a chemical engineer, I don't understand why do you need to make a chemical test on the weld.
As far as I know, welds are tested only by means of physical tests: bending, tensile, impact, metallurgical structure etc.
You can determine the approximate chemical composition of the welded metal knowing the chemical composition of the base metal and of the electrode (or rod, if you're using TIG) and the dilution, which is a function of the welding method you're using (SMAW, MIG, TIG, FCAW).
Giovanni S. Crisi
Sao Paulo - Brazil