CN- secondary ions form by recombination as demonstrated using multi-isotope mass spectrometry of 13C- and 15N-labeled polyglycine

We have studied the mechanism of formation CN- secondary ions under Cs+ primary ion bombardment. We have synthesized C-13 and N-15 labeled polyglycine samples with the distance between the two labels and the local atomic environment of the C-13 label systematically varied. We have measured four masses in parallel: C-12, C-13, and two Of (CN)-C-12-N-14, (CN)-C-13-N-14, (CN)-C-12-N-15, and (CN)-C-13-N-15. We have calculated the C-13/C-12 isotope ratio, and the different combinations of the CN isotope ratios ((CN)-C-27/(CN)-C-26, (CN)-C-28/(CN)-C-27, and (CN)-C-28/(CN)-C-26). We have measured a high (CN-)-C-13-N-15 secondary ion current from the C-13 and N-15 labeled polyglycines, even when the C-13 and N-15 labels are separated. By comparing the magnitude of the varied combinations of isotope ratios among the samples with different labeling positions, we conclude the following: CN- formation is in large fraction due to recombination of C and N; the C=O double bond decreases the extent of CN- formation compared to the case where carbon is singly bonded to two hydrogen atoms; and double-labeling with C-13 and N-15 allows us to detect with high sensitivity the molecular ion (CN-)-C-13-N-15.