Mass difference ±	Origin
0.984016	OH ↔ NH2, e.g. de-amidiation, CHNO compounds
1.99566322	F ↔ OH, halogen exchange with hydroxy group (typically -F + OH)
2.01565	± 2H, opening or forming of double bond
4.955395	Na+↔ NH4+, salt adduct
7.00467078	F ↔ CN, halogen exchange with cyano group
8.96577868	Cl ↔ CN, halogen exchange with cyano group
13.979265	O ↔ 2H, e.g. Oxidation follwed by H2O elimination
14.01565	± CH2, alkane chains, waxes, fatty acids, methylation
14.01565	-[C3H6ON] ↔ -[C2H4ON], acrylamide versus iodoacetamide in cysteine alkylation (gels)
15.973937	Na+↔ K+, salt adduct
15.977156	S ↔ O, sulfur compounds
15.994915	± O, e.g. oxidation/reduction
17.026549	± NH3, ammonium adduct/neutral ammonium loss
17.026549	NH4+ ↔ H+, salt adduct
17.96611268	Cl ↔ OH, halogen exchange with hydroxy group (typically -Cl + OH)
17.99057822	F ↔ H, halogen exchange
18.010565	± H2O, water addition/loss
18.994581	CN ↔ COOH, nitrile compounds
20.929332	K+↔ NH4+, salt adduct
21.981944	Na+↔ H+, salt adduct
24.995249	CN ↔ H, nitrile compounds
27.010899	± HCN, nitrile compounds
27.994915	± CO
28.006148	- 2N, nitrogen loss, e.g. azido compounds (N2)
28.0313	± C2H4, natural alkane chains such as fatty acids
29.97418	NO2 ↔ NH2, nitro compounds
29.997989	-NO, nitroso compounds
31.972071	± S, sulfur compounds
31.98983	± 2O, oxygen loss
33.021464	- NH2OH, loss from hydroxamic acids
33.96102768	Cl ↔ H, halogen exchange
33.987721	± H2S, sulfur compounds
37.955881	K+↔ H+, salt adduct
37.989162	2CN ↔ 2COOH, nitrile compounds
40.0313	+(C3H6O - H2O), acetone condensation after dehydration
42.010565	± COCH2
42.04695	± C3H6, propylation
43.005814	± CONH (wrong calc. in ref.)
43.94948442	79Br ↔ Cl, halogen exchange (typically -Br +Cl)
43.98983	± CO2
45.94743792	81Br ↔ Cl, halogen exchange (typically -Br +Cl)
47.966986	± SO, sulfur compounds
52.9152631	79Br ↔ CN, halogen exchange with cyano group
54.9132166	81Br ↔ CN, halogen exchange with cyano group
56.0626	± C4H8, butylation
58.00548	± CO2CH2
58.041865	+C3H6O, acetone condensation
61.9155971	79Br ↔ OH, halogen exchange with hydroxy group (typically -Br + OH)
63.9135506	81Br ↔ OH, halogen exchange with hydroxy group (typically -Br + OH)
63.961901	± SO2, sulfur compounds
63.998286	-CH3SOH, specific loss from singly oxidized methionine residues
77.9105121	79Br ↔ H, halogen exchange
79.9084656	81Br ↔ H, halogen exchange
79.956816	± SO3, sulfur compounds
91.93562072	I ↔ Cl, halogen exchange (typically -I +Cl)
97.967381	± H2SO4, sulfur compounds
97.976897	± H3PO4, phosphorous compounds
100.9013994	I ↔ CN, halogen exchange with cyano group
109.9017334	I ↔ OH, halogen exchange with hydroxy group (typically -I + OH)
125.8966484	I ↔ H, halogen exchange
146.05791	± [Deoxy-Hexose-H2O, C6O4H10], e.g. Fucose
162.052825	± [Hexose-H2O, C6O5H10], e.g. Glucose, Galactose, Mannose, Fructose
164.068475	± [Deoxy-Hexose-H2O, C6O5H12], e.g. Fucose
176.03209	± [Glucuronic acid-H2O, C6O6H8]
180.06339	± [Hexose, C6O6H12], e.g. Glucose, Galactose, Mannose, Fructose
194.042655	± [Glucuronic acid, C6O7H10]
203.079374	± [HexNAc-H2O, C8O5NH13], N-acetylhexoseamine
206.05791	+ sinapic acid - H2O, (C11H12O5-H2O), MALDI-matrix for proteins, possible photo- or condensation adduct
221.089939	± [HexNAc, C8O6NH15], N-acetylhexoseamine
289.073243	± [Glutathione-H2O,C10O5N3SH15]
291.095419	± [Neu5Ac-H2O, C11O8NH17], N-acetylneuraminic acid, sialic acid, NANA
305.068158	± [Glutathione+O-H2O, C10O6N3SH15]
307.083808	± [Glutathione, C10O6N3SH17]
309.105984	± [Neu5Ac, C11O9NH19], N-acetylneuraminic acid, sialic acid, NANA
324.10565	± [Sucrose-H2O, C12O10H20]
342.116215	± [Sucrose, C12O11H22]
484.171011	+Tetraphenyl-tetramethyl-trisiloxane, (C28H32O2Si3), ( from silicon-based diffusion pump oil (DC7040, Dow corning)
546.186661	+Pentaphenyl-trimethyl-trisiloxane, (C33H34O2Si3), ( from silicon-based diffusion pump oil (DC7050, Dow corning)