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2000 MS Method Ion Preparation

The Ion Preparation Method

The two fundamental processes in mass spectrometry are ion creation and ion

analysis. For some applications, the ability to manipulate the ion population

following ion creation but prior to ion analysis through such techniques as

selective storage or dissociation may be advantageous. Although this may

require additional instrumentation when using other mass spectrometers, the ion

trap can accomplish it by means of an ion preparation method (IPM), which

modifies the scan function. Advantages associated with IPMs are similar to those

of other sample preparation methods, e.g., reduction of noise, increased

selectivity, etc.

Intermediate Scan Segments (ISS)

The use of the Ion Preparation technique introduces intermediate scan segments

(ISSs) into the basic rf scan function. These allow you to customize the prescan

and analytical scan(s) for both electron- and chemical-ionization modes of

analysis.

In the case of electron ionization (EI), ISS insertion takes place between creation

of the ions (ionization) and ion analysis. Electron ionization uses the automatic

gain control (AGC) scan function, which consists of a prescan and up to four

analytical scans. As seen in the figure, the prescan and analytical scans include

ionization periods, ISSs, and six rf voltage ramps (only two ramps shown) over a

user-defined range.

Automatic gain control scan function showing location of intermediate scan segments

With chemical ionization (CI), there are two steps in ion creation (ionization and

reaction). The ion preparation method is capable of inserting ISSs following

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