We next aimed to exploit RuAAC for untargeted analysis of internal alkynes in biological samples, naively hoping that peaks uniquely found in RuAAC-treated biological extracts would correspond to putative internal alkynes. Instead, we found that RuAAC-derivatized samples contained a large number of chromatographically resolved species whose isotopic pattern was characteristic of a mono-metallic ruthenium complex. These are easily identified because ruthenium has seven stable isotopes with a characteristic intensity distribution (Figure ). Thus, either during the course of the reaction, or during the 1-minute exposure of the reaction to protic solvents during fast UPLC analysis, the Cp ⋆ RuCl(cod) catalyst forms a variety of distinct, chromatographically resolvable molecules. We are able to tentatively identify two of these peaks on the basis of exact mass and isotopic pattern matching (Figure and). Kinetic profiling of the derivatization reaction revealed that peak b, a putative de-chlorinated catalyst ion ((Ru C 10 H 15 C 8 H 12 ) + (m/z = 345.1121, rt = 2.98 min))