Molecular characteristics of a Chinese coal analyzed using mass spectrometry with various ionization modes

• Fragments and molecular association may induce more molecules >500 u detected by ESI.
• The mechanisms of assisted-ionization for dopant affect the ionization efficiency.
• Direct APPI mode realized ionization of molecules not detected by APPI with dopants.

A Xinjiang subbituminous coal was directly oxidized in aqueous sodium hypochlorite and sequentially extracted with diethyl ether and ethyl acetate. The extracts were esterified with diazomethane and the esterified products were analyzed using a high performance liquid chromatograph/mass spectrometer (HPLC/MS) equipped with atmospheric pressure photoionization (APPI) or electrospray ionization (ESI) ion source. Toluene and a toluene/anisole mixture (vol/vol = 95:5) were used as dopant for APPI-MS. The relative of organic species with molecular mass over 500 u detected by ESI-MS is much higher than that identified by APPI-MS with and without dopant. The addition of dopant greatly enhanced the ionization efficiency and toluene induced better ionization effects compared to the mixture in both ion signal intensity and number of detected species. However, according to the ionization mechanism of APPI, a small portion of analytes could be well ionized under direct APPI mode rather than dopant-assisted mode.