Dense-gas fractionation and MALDI characterization of carbonaceous pitches

The combination of multistage, dense-gas extraction (DGE) and matrix-assisted, laser desorption/ionization (MALDI) mass spectrometry (MS) was investigated for the fractionation and characterization of an oligomeric model pitch synthesized via the thermal polymerization of anthracene. Our approach was shown to be effective both for elucidating the molecular properties of the anthracene pitch and for generating narrow molecular weight (MW), oligomeric cuts that can serve as MW standards. DGE was used to fractionate 10- to 15-g charges of anthracene pitch into relatively pure, gram- and decigram-sized cuts containing 99+% monomers, 90+% dimers, and 80+% trimers, respectively. The multistage, semi-batch DGE process was carried out in two steps with toluene being used as the dense-gas solvent at both subcritical and supercritical conditions. In the first step, lower-MW species were stripped off as the overhead; for the second step, the remaining residue was extracted, yielding the desired oligomeric cuts as overhead. Computer process simulation was used to guide the experimental work and demonstrated the effect of column temperature gradient on the liquid reflux ratio and, as a result, product purity. MALDI-MS played a critical role in guiding the selection of DGE operating conditions, yielding absolute MW information on the fractions that were produced.