gasoline fuels is important for a number of reasons.

Firstly, some contaminants can poison catalytic processes,

leading to poor yields in fuel derivitization processes.

Secondly, some contaminants affect engine performance

and can lead to shortened catalytic converter lifetimes.

Thirdly, some contaminants can lead to undesirable

environmental pollution from exhaust gases.

The analysis of gasoline poses a number of challenges

for the technique of ICP-MS. The volatility of this

hydrocarbon fraction can cause problems with the

retention of a stable plasma. The decomposition of

hydrocarbon materials in the plasma in the absence of

oxygen results in the production of finely divided carbon

particles which can deposit onto the cones, resulting in

downward signal drift and eventually loss of transmission.

The presence of a carbon matrix causes polyatomic

interferences that are not normally observed when

analyzing aqueous samples.

Refractory elements, such as cerium can react with

oxygen-bearing species in an ICP to form metal oxide

(MO+) polyatomic species which can interfere with other

analytes of interest. An example of this is the interference

of isotopes of barium oxide (BaO+) with rare earth

elements such as samarium, gadolinium and europium.

Collision / reaction cells have been used to remove or

reduce many polyatomic species and attempts have been

made at developing methodologies for reactively removing

such refractory metal oxide species. One such method

used oxygen to react the MO+ species to higher oxides

such as MO2

+, leaving the analyte free of interference.1

Unfortunately, since the formation of oxide species can be

undesirable and may cause the loss of analyte sensitivity

(at best) and the formation of new interferences (at

worst), this approach is not universally applicable.

Similarly, the effectiveness of this approach depends upon

the kinetics of the reaction gas and the interfering species.

A preferable situation would be the simultaneous

removal or reduction of all polyatomic species with the

use of a single cell gas. Du and Houk demonstrated the

ability to reduce metal oxides under specific conditions

with the use of helium cell gas.2. Unknowingly, this was

the first reported description of kinetic energy

discrimination (KED). Realizing the significance of this

phenomenon, researchers at Thermo Fisher Scientific

refined their 1998 collision cell and developed a new

collision cell specifically designed to incorporate kinetic

energy discrimination operation. This was released in

2001 in the X Series ICP-MS from Thermo. It has

since been comprehensively re-designed with major

improvements giving Thermo’s 3rd generation collision cell

in the XSeriesII ICP-MS. This design incorporates

a new cell entry lens arrangement that controls the ions

entering the cell by virtue of ion energy, a new post-cell

chicane lens arrangement that aids energy discrimination

and analyte ion transmission under KED conditions in

addition to giving the lowest continuum background of

any ICP-MS, and new electronics to improve analyte

transmission in KED mode.

A convenient method of assessing the effectiveness of

kinetic energy discrimination for removal of metal oxide

species is by measuring the signal intensity ratio of cerium

oxide (CeO+) to cerium (Ce+). Cerium is chosen since it

has a particularly high affinity for combination with

oxygen and cerium oxide has a strong bond that is not

easily dissociated in the plasma (CeO bond enthalpy

= 795 kJ mol-1).