The value of isotope labeling in probing the chemistry underlying mass spectra is well established. The usefulness of this approach and the reliability of conclusions reached thereby can be greatly increased by checking dependence of label retentions in fragment ions on operating conditions. For example, varying the ionizing voltage, source temperature, or repeller potential alters the relative contributions of the two competing processes by which methylcyclopentane loses a methyl radical to form isomeric C5H9+ ions. The ions resulting from the two processes are derived from different parts of the original molecule and so can be distinguished by appropriate labeling.
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