Which parameter is used in mass spectrometry to identify a compound?

• A. Ion mass-to-charge ratio
• B. Molecular size
• C. Absorption spectrum
• D. Retention time

Answer: (A)

In mass spectrometry, the identifying parameter is the mass-to-charge ratio of ions (m/z). The instrument ionizes the sample and separates ions based on their mass relative to their charge, producing a spectrum that serves as a fingerprint for the compound. The molecular ion peak and characteristic fragment ions, along with their relative intensities, uniquely help identify the substance by matching to libraries or calculating a molecular formula. Absorption spectra and retention time are relevant in other techniques (UV-Vis spectroscopy and chromatography, respectively) and do not define the identity in mass spectrometry. While molecular weight can be inferred from the molecular ion, what truly distinguishes the compound in MS is its m/z-based fingerprint.