It is often pointed out that one of the key challenges faced by direct MS techniques (of which Selected Ion Flow Tube Mass Spectrometry – SIFT-MS is one) is their ability to differentiate between isomers. Since isomeric species have identical mass, MS techniques – even high resolution time-of-flight instruments – provide no utility in of themselves to solve this issue. The problem of differentiating isomers can only be addressed by the ionization process that is employed ahead of the mass spectrometer.
In this regard, SIFT-MS performs far in advance of other techniques due to its unique multi-reagent ionization system. Syft’s instruments employ eight positively and negatively charged reagent ions whose varied ionization chemistry enables the discrimination of many complex isomeric mixtures. This has been acutely demonstrated by the Syft development team recently with the independent quantitation of ethylbenzene and xylene from within a mixture of aromatic hydrocarbons.
Ethylbenzene and xylene are alkylated benzenes with the same molecular formula (C8H10) and in most cases identical chemical behaviour. Both have acute toxicity, but different concentration thresholds at which they pose a danger to health. Traditionally, chromatographic separation has been required to distinguish between these compounds but even this approach can pose difficulties due to the similarity between the isomers.
Researchers at Syft Technologies have shown that these compounds can be measured independently using the OH– reagent ion. The reaction of OH– with xylene forms the product ion C8H9– with a mass to charge ratio of m/z=105. On the other hand OH– reacts with ethylbenzene through a two-step process to form the product ion HO2– with mass to charge ratio of m/z=33. These product-ion structures have been verified by calculating optimized geometries and energies using the B3LYP D3/6-31++G(d,p) method (D.L. Crittenden, University of Canterbury).
This work illustrates the power of Syft’s multi-reagent system to achieve selectivity at a level that heretofore has only been achieved though chromatography. The discrimination of ethylbenzene and xylene will be valuable in many industries, perhaps most notably, for the real-time quantitation of ethylbenzene and xylenes in the interior of new vehicles.
For more information on SIFT-MS and its ability to resolve isomers in real time, browse our website or contact us.