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Characterization of humic substances by matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry
Author(s) -
Mugo Samuel M.,
Bottaro Christina S.
Publication year - 2004
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.1635
Subject(s) - chemistry , analyte , mass spectrometry , analytical chemistry (journal) , matrix (chemical analysis) , sample preparation , mass spectrum , matrix assisted laser desorption/ionization , desorption , ionization , humic acid , matrix assisted laser desorption electrospray ionization , chromatography , ion suppression in liquid chromatography–mass spectrometry , ion , tandem mass spectrometry , electron ionization , organic chemistry , fertilizer , adsorption
Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOFMS) and laser desorption/ionization (LDI‐)TOFMS have been used to characterize Suwannee River humic substances, obtained from the International Humic Substances Society (IHSS), and Armadale soil fulvic acid (ASFA). An array of MALDI matrices were tested for use with humic substances, including α ‐cyano‐4‐hydroxycinammic acid (CHCA), 2‐(4‐hydroxyphenylazo)benzoic acid (HABA), 2,5‐dihydroxybenzoic acid (DHBA), sinapinic acid, dithranol and norharmane. DHBA yielded the best results, exhibiting superior ionization efficiency, low noise, broad applicability to the analytes of interest, and most importantly producing an abundance of high mass ions, the highest observed being m/z 1848. A number of sample preparation modes were investigated; the overlayer method improved sample/matrix homogeneity and hence shot‐to‐shot reproducibility. The choice of the matrix, mass ratio of analyte to matrix, and the sample preparation protocol, were found to be the most critical factors governing the quality of the mass spectra. Matrix suppression was greatly enhanced by ensuring good mixing of matrix and analyte in the solid phase, proper optimization of the matrix/analyte ratio, and optimizing delayed extraction to ensure complete matrix–analyte reaction in the plume before ions are moved to the flight tube. A number of common features, in particular specific ions which could not be attributed to the matrices or to contaminants, were present in the spectra of all the humic substances, regardless of origin or operational definition. Additionally, a prominent repeating pattern of peaks separated by 55, 114 and 169 Da was clearly observed in both LDI and MALDI, suggesting that the humic compounds studied here may have quasi‐polymeric or oligomeric features. Copyright © 2004 John Wiley & Sons, Ltd.