Analysis of Petroleum Samples by DRC-ICP-MS

The analysis of petroleum products (i.e. gasoline, oil, and diesel) presents a challenge for ICP-MS. Owing to the nature of these samples and their associated highly organic solvents, a number of analytical challenges are
encountered. First, the high degree of volatility of the samples increases the vapour loading and causes destabilisation of the plasma. This is observed in the form of decreased precision as well as the intermittent shutdown of the plasma. Second, these samples are rich in carbon, which is poorly ionised in the plasma due to its high first ionisation potential (11.26 eV). As a result, non-ionized carbon deposits in its elemental form and clogs the interface cones. Also, the addition of oxygen to the plasma (to burn the excess carbon) can have the adverse effect of destabilising the plasma by continuously changing its overall impedance.

Finally, the high levels of carbon lead to a number of carbon-based polyatomic interferences on several analytes of interest. Fortunately, these challenges can be overcome. The ELAN® ICP-MS family of instruments from PerkinElmer (USA) features a 40 MHz free-running RF generator. This generator utilises electronic tuning to compensate for the small shift of RF frequency brought about by the impedance change in the plasma. Unlike 27 MHz fixed frequency generators, this compensation is virtually instantaneous because there are no moving parts. As a result, the ELAN offers a very robust plasma capable of remaining lit even when highly volatile samples are aspirated.

The use of a Peltier-cooled spray chamber reduces the amount of matrix vapors reaching the plasma. This means that the sample introduction system delivers a higher analyte/matrix ratio to the plasma. Finally, the use
of a Dynamic Reaction Cell™ (DRC™) offers a superior way to eliminate carbon-based polyatomic interferences without sacrificing analyte sensitivity. This is accomplished by introducing a highly reactive gas that reacts
preferentially with the polyatomic ions but not with the ions of interest. This work demonstrates the ability of the ELAN DRC II to analyse petroleum samples which have been diluted with kerosene.