Mass Spec of Asphaltenes

The Science & Technology Concentrate “Crude Oil’s Polar Portion Yields to MS” appears to indicate this technique of two-step LD-MS has ionized and detected all of the asphaltene (C&EN, May 26, page 38). This is unlikely, because the paper does not allow for mass discrimination, a common problem of new and old mass spectrometric ionization methods. The more numerous small molecules ionize preferentially, and the larger molecules either do not ionize or are not detected in the presence of the small-molecule ions.

Several mass spectrometric ionization methods, including two-step laser MS, appear to agree on the ionic mass ranges of different asphaltenes, and it is suggested that there are no more higher mass molecules in the asphaltenes. This problem can only be resolved by fractionation of the asphaltene to separate the small and large molecules.

If all asphaltenes have the same mass range, why are crude petroleums so different in density and viscosity? In my experience, a lack of high-mass ions may indicate that the ionization method cannot cope with the polydisperse samples, rather than that there is nothing else to ionize.

The article should have been entitled, “MS Yields Again to Crude Oil’s Polar
Portion.”

Alan A. Herod
Cheltenham, England