Question

Collision and reaction cells are used in ICP-MS to resolve interferences from the element(s) of interest. Briefly discuss the similarities and differences between collision and reaction cells.

Answer 1

Collision or Reaction Cells are means of removing spectral interferences in quadrupole ICP-MS. However, there is still a debate about their routine use in many applications, because CRCs are operated in two distinctly different modes:

• Reaction mode uses specific reaction gases to remove known, reactive interferences from each analyte isotope.

• Collision mode uses a non-reactive gas and a process called kinetic energy discrimination (KED) to selectively attenuate all polyatomic interferences based on their size.

KED exploits the fact that all polyatomic ions are larger than analyte ions of the same mass, so they collide with the cell gas more often as they pass through the cell, emerging with lower residual energy. These low energy ions are excluded from the ion beam by a bias voltage at the cell exit. Reaction mode can remove some interferences very efficiently, as the reactions occur quickly .

KED by contrast requires the ions to undergo many collisions before the difference in residual energy between analyte and polyatomic ions is large enough to allow them to be separated.

Effective collision mode operation requires:

• A cell that operates with a light cell gas (helium) at high cell gas pressure to provide the large number of collisions necessary for effective KED.

• An ion guide that maintains effective ion focusing across the mass range, even at high cell gas pressure.

• Ions that enter the cell with a very narrow spread of ion energy.