Crude Oil's Polar Portion Yields To MS

A new method makes it easier to analyze asphaltenes, mostly aromatic compounds that make up the heaviest and most polar fraction of crude oil. As oil prices rise, asphaltenes are becoming increasingly important because of their high concentration in heavy oils and tar sands. Laser desorption ionization mass spectrometry usually works well for polar molecules, but the method has given widely varying molecular weight distributions for asphaltenes. Now, Andrew E. Pomerantz of Schlumberger-Doll Research in Cambridge, Mass., Richard N. Zare of Stanford University, and coworkers report that two-step laser MS, in which the desorption and ionization steps are spatially and temporally separated, resolves these problems. The researchers use infrared pulses from a CO₂ laser to desorb neutral species from the sample and then use ultraviolet laser pulses to ionize these desorbed species (J. Am. Chem. Soc., DOI: 10.1021/ja801927v). Unlike laser desorption ionization, the two-step process prevents asphaltene aggregation. The researchers' results show that the asphaltene mass spectrum has a broad maximum near 600 dalton and extends to more than 1,000 Da.