Chromatography Forum

Is it theoretically possible that CI is more sensitive than EI? I am measuring compounds that break down in small ions when using EI.

Absolutley , but it also depends on your coumpound
as the energy for formation of the pseduo molecular ion doesn't cause as much fragmentation.
the traditional CI gas is methane, but depending on the stability of your compound, that too, could cause fragmentation.
We use ultra pure Ammonia Gas to see the pseudo-molecular ion and adducts, as the heat of formation is less than methane.
Please note that Ammonia, beside being a hazardous chemical, is also hazardous to your equipment, you need to speak to your instrument rep to determine if your system can handle it, eg have ammonia resistant seals in your roughing pump, ( I don't know anything about diffusion pumps, or even if they can do CI?), if you need two gas lines, as our instruments tune in methane and saves the parameters into a ammonia tune file.

Alex

It really depends on what you're looking for. Typically Positive CI mode is less sensitive then EI mode but Negative CI is significantly more sensitive.

So if you're looking at halogen containing compounds, especially those containing Chlorine and Florine (Chlorinated pesticides, PCBs) you can get a significant boost in sensitivity.