Fossil fuel characterization using laser desorption mass spectrometry: Applications and limitations

Abstract

traditional gas-phase methods. The application and limitations of these techniques to model compounds and fossil-derived materials is discussed in this study. LD and MALDI mass spectrometry are increasingly used to detect intact molecular species, such as proteins with masses from 1000 to 100,000 amu and beyond.' MALDI is also being used for a few high molecular weight polymers.2 A good example, related to coal-type systems, is the report on lignin mass spectrometry by MALDI.3 Here the mass spectrum shows a wide molecular distribution of several hundred to larger than 16000, with the center of gravity of the distribution around 2600. The results are interpreted in terms of oligomeric lignin molecules. Thus, if there are indeed large molecular species in a polymeric content in fossil fuel extracts, MALDI is an attractive technique. We have previously reported laser desorption of coal extracts that show only low molecular weight ions (<ZOO0 amu).4-6 We compare LD with fast atom bombardment and desorption chemical ionization mass spectrometry, two other soft ionization techniques. All of these techniques produce similar data that differ only in minor details. We have used our two TOF mass spectrometers for coal analysis by MALDI, have carefully analyzed our data and the instrumental conditions and conclude that the bulk of coal material produces only low molecular weight ions (up to 2000 amu). In agreement with these reports, Hanky showed LD and MALDI data of pyridine extracts which produced a distribution of ions between 150 and 1500.7 In addition, field ionization mass spectrometry has found similar pattern in the low mass region.8 Recently, Herod et. aI9-'* have interpreted their LD and MALDI data in terms of high mass species being desorbed. We have presented alternate interpretations for these results.6 In our hands, LD or MALDI of coals and extracts do not show any reproducible ion intensity over mass 2000. The conditions whereby large molecular ions can be desorbed intact are very specialized. This paper will describe the scope and limitations of laser desorption and matrix-assisted laser desorption in time-of-flight mass spectrometers as applied to high molecular weight molecules, such as proteins and polymer systems. EXPERIMENTAL The coals used in this study are the Argonne Premium Coal Samples. The procedures for the pyridine extract of the Argonne Premium Coals have been reported previousl~.~ The laser desorption mass spectra and matrix-assisted laser desorption mass spectra were recorded on a linear time-of-flight mass spectrometer constructed at Argonne and a Kratos Kompact MALDI 111 linear/reflectron time-of-flight mass spectrometer. The spectra were produced by exposing the samples distributed as thin layer on a stainless steel sample holder to laser pulses from either a Nd:YAG or a