Toronto Research Chemicals offers a labelled azoxystrobin, but it's rarely in stock and they'll only make it if you order 20 mg or more (I've checked twice this past year).  They market it through at least one US vendor, who doubles the price, but again, subject to availability.  A researcher at the Univ. of Iowa, Hans-Joachim Lehmler ([email protected]) recently converted a dinitroanisole into  13C6 and 15N2 analogs for metabolism studies using LC-MS/MS and his work was exquisite.  We're able to make identical concentration standards of native and labelled analogs to measure decay/metabolism in plants and see the labelled appearance of one of the metabolites (dinitrophenol at a different retention time) with the shift in the labelled masses.

We've measured azoxystrobin routinely in surface water by LC-MS/MS.  The strobilurins are some of the most sensitive compounds detected by positive-ion ESI MS/MS.  Using direct-injection of 10 ul onto a 150 mm c8 column, we routinely observe

Thank you very much Stephen. Your suggestions are quite insightful to me.