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To date there have been no systematic, quantitative investigations of the effect of sample preparation on the matrix-assisted laser desorption/ionization time-of-flight (MALDI) mass spectrometry response for polydisperse systems. To this end, the interrelationships between sample preparation, analyte molecular weight distribution (MWD) and solubility, and signal response were investigated for mixtures of alkylated polycyclic aromatic hydrocarbon (PAH) oligomers, the constituents of petroleum pitch that serve as precursors for advanced carbon materials. These PAH oligomers served as a useful analyte system for study, as their solvent solubilities decrease significantly with each increasing oligomeric unit. Molecular weight standards consisting of relatively pure dimer and trimer cuts of the starting M-50 petroleum pitch were produced using a dense-gas/supercritical extraction (DGE/SCE) technique and were then used to produce oligomeric mixtures of well-defined composition for study. Both traditional, solvent-based and newer, solvent-free sample preparation methods were evaluated, and their effects on both homogeneity and signal response were determined. While solvent-free sample preparation methods produced homogeneous samples and reproducible results regardless of the MWD of the analyte, solvent-based samples that contained more than one oligomeric cut produced non-homogeneous samples and poor reproducibilities. The differing solubilities of dimer, trimer, and tetramer oligomers in a given solvent (e.g., CS(2) or toluene) were found to be the cause of the inhomogeneities observed in solvent-based sample preparation. A quantitative analysis study performed with dimer/trimer mixtures over a wide range of compositions via solvent-free sample preparation indicates that linear, reproducible calibration curves can be generated and used to calculate the molecular composition of unknown dimer/trimer mixtures with confidence.

Dense-gas/supercritical extraction (DGE/SCE) was used for the fractionation of a representative petroleum pitch, M-50, into its oligomeric constituents using toluene as the extractive solvent. A small, pilot-scale column was operated in the semicontinuous/semi-batch mode under a linear positive temperature gradient of 380À330 °C from the top to the bottom of the column and over a pressure range of 15À75 bar. This DGE column was used to produce high-purity monomer, dimer, and trimer fractions of M-50 suitable for use as molecular standards for petroleum pitches and for polycyclic aromatic hydrocarbon (PAH) oligomers. During the experimental runs, matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) was used to monitor in real time the progress of the separation (and oligomeric purity) by performing rapid analyses of the molecular-weight distributions (MWDs) of the overhead fractions being collected. These real-time analyses provided us with the ability to fine-tune in situ the operating conditions according to the separation desired. The separation of petroleum pitches and other heavy fossil fuels into narrow molecular-weight fractions by semi-continuous DGE has proven to be an invaluable first step in the isolation and structural characterization of the individual species present in these multi-component, poorly defined systems. Furthermore, these molecular standards are also suitable for quantitative analysis.

Quantitative analysis of partially soluble and insoluble polydisperse materials is challenging due to the lack of both appropriate standards and reliable analytical techniques. To this end, matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) incorporating a solvent-free sample preparation technique was investigated for the quantitative analysis of partially soluble, polydisperse, polycyclic aromatic hydrocarbon (PAH) oligomers. Molecular weight standards consisting of narrow molecular weight dimer and trimer oligomers of the starting M-50 petroleum pitch were produced using both dense-gas/supercritical extraction (DGE/SCE) and preparative-scale, gel permeation chromatography (GPC). The validity of a MALDI-based, quantitative analysis technique using solvent-free sample preparation was first demonstrated by applying the method of standard addition to a pitch of known composition. The standard addition method was then applied to the quantitative analysis of two insoluble petroleum pitch fractions of unknown oligomeric compositions, with both the dimer and trimer compositions of these fractions being accurately determined. To our knowledge, this study represents the first successful MALDI application of solvent-free quantitative analysis to insoluble, polydisperse materials.