Mass Lost	Fragment	Suggests
15	•CH₃	Methyl group present
17	•OH	Alcohol
18	H₂O	Alcohol (dehydration in the mass spectrometer)
28	CO or C₂H₄	Carbonyl compound (loss of CO) or an ethyl/alkene fragment
29	•CHO or •C₂H₅	Aldehyde (loss of CHO) or an ethyl group
43	C₃H₇⁺ or CH₃CO⁺ (acylium)	Propyl fragment, or a methyl ketone (acylium ion is often a strong, diagnostic peak)
45	•COOH	Carboxylic acid
57	C₄H₉⁺ or C₂H₅CO⁺ (acylium)	Butyl fragment, or an ethyl ketone
77	C₆H₅⁺ (phenyl cation)	Monosubstituted benzene ring

Acylium ions (R–C≡O⁺, the same cationic species that drives Friedel-Crafts acylation in Appendix C) are common, stabilized fragments from ketones and aldehydes, which is why losses corresponding to R–CO⁺ (43, 57, and similar) are frequently prominent peaks.

The molecular ion (M⁺) — the unfragmented, ionized starting molecule — gives the molecular weight directly and is the anchor point for interpreting every fragment loss above it (Chapter 20).

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