Table 2. Summary of seven methyl esters (R₂=CH₃): the molecular properties and results of positive-ion APCDI mass spectra obtained by the Q-orbitrap MS.

Molecular properties					Results of positive-ion APCDI mass spectra: m/z, chemical formula, and relative intensities (%) of major ions
Analyte (M)	Molecular mass (nominal Da)	Formula	R1	R2	Protonated molecule, [M+H]+		NH4+ adduct, [M+NH4]+		CH3OH loss from protonated molecule, R1CO+
Methyl acetate	74	C3H6O2	CH3	CH3	75.0439	100	92.0703	29.3	N.D.a
					C3H7O2+		C3H10O2N+
Methyl propionate	89	C4H8O2	C2H5	CH3	89.0595	100	106.086	16.8	57.0334	<1
					C4H9O2+		C4H12O2N+		C3H5O+
Methyl butyrate	102	C5H10O2	C3H7	CH3	103.075	100	120.102	47.5	71.049	1
					C5H11O2+		C5H14O2N+		C4H7O+
Methyl valerate	116	C6H12O2	C4H9	CH3	117.091	100	134.117	13.5	85.0648	<1
					C6H13O2+		C6H16O2N+		C5H9O+
Methyl hexanoate	130	C7H14O2	C5H11	CH3	131.106	100	148.132	27.9	99.0804	<1
					C7H15O2+		C7H18O2N+		C6H11O+
Methyl heptanoate	144	C8H16O2	C6H13	CH3	145.121	100	162.148	11.6	113.096	<1
					C8H17O2+		C8H20O2N+		C7H13O+
Methyl octanoate	158	C9H18O2	C7H15	CH3	159.138	100	176.164	76.7	127.112	<1
					C9H19O2+		C9H22O2N+		C8H15O+

^aNot detected due to detection limit of m/z range.

APCDI, atmospheric pressure corona discharge ionization technique; MS, mass spectrometry.