Comprehensive characterization of natural organic matter by MALDI- and ESI-Fourier transform ion cyclotron resonance mass spectrometry.
Natural organic matter (NOM) is a complex and non-uniform mixture of organic compounds which plays an important role in environmental processes. Due to the complexity, it is challenging to obtain fully detailed structural information about NOM. Although Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) has been demonstrated to be a powerful tool for providing molecular information about NOM, multiple ionization methods are needed for comprehensive characterization of NOM at the molecular level considering the ionizing selectivity of different ionization methods. This paper reports the first use of matrix assisted laser desorption/ionization (MALDI) method coupled with FT-ICR-MS for molecular characterization of NOM within a mass range of 200-800 Da. The mass spectral data obtained by MALDI were systematically compared with data generated by electrospray ionization (ESI). It showed that complementary molecular information about NOM which could not be detected by ESI, were provided by MALDI. More unsaturated and aromatic constituents of NOM with lower O/C ratio (O/C ratio<0.5) were preferentially ionized in MALDI negative mode, whereas more polar constituents of NOM with higher O/C ratio were preferentially ionized in ESI negative mode. Molecular anions of NOM appearing at even m/z in MALDI negative ion mode were detected. The results show that NOM molecules with aromatic structures, moderate O/C ratio (0.7>O/C ratio>0.25) and lower H/C ratio were liable to form molecular anions at even m/z, whereas those with higher H/C ratio are more likely to form deprotonated ions at odd m/z. It is speculated that almost half of the NOM molecules identified by MALDI may be aromatic or condensed aromatic compounds with special groups which are liable to absorb electron from other molecules to generate free radical anions during MALDI ionization.
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Science, Chinese Academy of Sciences, P.O. Box 2871, 18 Shuangqing Road, Haidian District, Beijing 100085, China.
Dong
D
Cao
Department of Endocrinology and Rheumatology, Navy General Hospital, PLA, 6 Fucheng Road, Haidian District, Beijing 100048, China.
Huogao
H
Huang
Central Lab Navy General Hospital, PLA, 6 Fucheng Road, Haidian District, Beijing 100048, China.
Ming
M
Hu
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Science, Chinese Academy of Sciences, P.O. Box 2871, 18 Shuangqing Road, Haidian District, Beijing 100085, China.
Lin
L
Cui
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Science, Chinese Academy of Sciences, P.O. Box 2871, 18 Shuangqing Road, Haidian District, Beijing 100085, China.
Fanglan
F
Geng
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Science, Chinese Academy of Sciences, P.O. Box 2871, 18 Shuangqing Road, Haidian District, Beijing 100085, China.
Ziyu
Z
Rao
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Science, Chinese Academy of Sciences, P.O. Box 2871, 18 Shuangqing Road, Haidian District, Beijing 100085, China.
Hongyun
H
Niu
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Science, Chinese Academy of Sciences, P.O. Box 2871, 18 Shuangqing Road, Haidian District, Beijing 100085, China.
Yaqi
Y
Cai
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Science, Chinese Academy of Sciences, P.O. Box 2871, 18 Shuangqing Road, Haidian District, Beijing 100085, China. Electronic address: yhkang@rcees.ac.cn.
Yuehui
Y
Kang
D015057:1-Naphthylamine / Q000031:analogs & derivatives / Q000032:analysis / Q000737:chemistry; D005583:Fourier Analysis; D009930:Organic Chemicals / Q000032:analysis* / Q000737:chemistry; D021241:Spectrometry, Mass, Electrospray Ionization / Q000295:instrumentation / Q000379:methods*; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization / Q000295:instrumentation / Q000379:methods*
D009930:Organic Chemicals; C418992:1,8-bis(dimethylamino)naphthalene; D015057:1-Naphthylamine
Fourier transform ion cyclotron resonance mass spectrometry; Matrix assisted laser desorption/ionization; Molecular characterization; Natural organic matter