Determination of fatty acid methyl esters by two-color two-photon resonance-enhanced femtosecond ionization mass spectrometry

A standard sample mixture containing thirty-seven fatty acid methyl esters (FAMEs) was measured by femtosecond laser ionization mass spectrometry. FAME molecules with double bonds were efficiently ionized via resonance-enhanced two-photon ionization by absorbing the first photon at 206 nm at the edge of the absorption band of the pi ->pi* transition and subsequently ionized by absorbing the second photon at 257 nm. The intensity of the molecular radical ion was enhanced significantly using this two-color ionization scheme, which minimizes the excess energy in the ionized state, when compared with electron ionization mass spectrometry and vacuumultraviolet photoionization mass spectrometry. This approach was then used for the reliable identification of FAMEs contained in an actual sample of biofuel.