Chemical Ionization (CI) is a soft ionization technique that produces molecular ions (M+ or M"), adduct ions (M+CI reagent), and fragment ions. Instrumentation is more expensive but CI permits isomer differentiation. CI reagents are usually methane, isobutane, or ammonia. The degree of fragmentation is less than in EI.
Linear quadrupole instruments are widely spread. Recent developments in the technology now make it possible to work simultaneously with fullscan and selected ion monitoring (SIM) modes in a single run. In GC tandem MS, a first quadrupole acts as a mass selective filter and the second quadrupole is used as the collision cell with addition of a collision gas such as helium or argon. In a third quadrupole the full mode is performed to obtain the full mass spectrum of the product ions.
The mass range is 2 to 1000 Da. Triple quadrupole can solely perform MS-MS and the ratio performance/cost is low.
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