Chemical Ionization using the 1200 GC/MS

The Varian 1200 GC/MS offers both EI and Chemical Ionization (CI) as standard with the instrument, using both positive and negative CI techniques. Townsend Discharge ionization is offered as an option for the petroleum and petrochemical industries.

CI on the Varian 1200 MS can be done using methane, isobutane, and ammonia as the reagent gas without any valve modification.

The vacuum interlock feature allows for fast ion volume changes without breaking vacuum and taking down the system. This makes the task of switching to CI mode, for molecular ion confirmation, quick and easy. The Varian 1200 MS can switch between positive and negative CI in less than 2 seconds during an acquisition.

Negative Ion CI (NCI)
Negative CI (NCI) provided as part of the Varian 1200 CI technique. NCI is used to selectively detect halogenated compounds, providing a very sensitive method with lower noise level than positive CI.

NCI on the Varian 1200 MS gives an analytical advantage due to improved selectivity, and a greatly reduced noise level. This provides better sensitivity for halogenated target compounds. Derivitization can be used to analyze non-halogenated compounds.

The Varian 1200 GC/MS gives better than 20:1 signal-to-noise for 10 fg OFN on column in NCI mode.

Townsend Discharge Ionization
Townsend Discharge is similar to CI except that, instead of a filament providing the excitation energy, a high voltage is applied to a discharge cup. The high voltage then discharges to the surrounding oxidizing CI gas to form highly reactive reagent ions. Townsend Discharge Ionization is softer than normal CI (eg ammonia CI).
It is a very useful technique for the characterization of hydrocarbons in the petroleum industry as it can be used to determine the concentrations of n-paraffins, isoparaffins, cycloparaffins, and aromatics, and their carbon number distributions.