simple question on GC MS

[just me]

hello

I have been reading about how a trace of ammonia or formic acid is added to the mobile phase during electrospray ionisation to enable compounds to be ionised and detected using the mass spectrometer. I was wond if gas chromatography is used how does the compound become ionised is there something added to the carrier mobile phase gas ?

[indium]

Nothing is added to the carrier gas. In GC-MS the sample ionization is accomplished by bombarding the analyte molecules with electrons (70eV). This is a high energy process resulting in complete fragmentation and formation of radical cations. Electrospray ionization is a soft ionization technique that relies on the mobile phase as the source of the positive or negative charge and results in little, if any, fragmentation.

[james little]

GC-MS in CI mode uses additives for ionization such as ammonia to form ammonium ions and methane to form CH5+ ions. These ions then protonate compounds or form ion clusters. Much like APCI. However, usually no sodium adducts formed like electrospray.

Also can do negative ion GC-MS using gases to either moderate the energy of electrons to molecular anions, M-., or chloride attachment, proton abstraction with CH4/N2O to give OH- which abstracts protons.

Very powerful technique, great book, see

http://www.campusi.com/ta_Chemical_Ioni ... rrison.htm

or my website.

http://users.chartertn.net/slittle/