Axis BTA 2100D Manuel de l'opérateur télécharger pdf (Page 209)

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2000 MS Method MS/MS Option:

Theory of Operation

Overview

This section describes the four basic operations in Saturn Ion Trap Tandem

Mass Spectrometry (MS/MS) for electron ionization (EI):

 Ion formation and matrix ion ejection

 Parent ion isolation

 Product ion formation

 Product ion mass scanning.

Chemical ionization (CI) MS/MS uses a subset of these operations.

Note that the utility of the MS/MS technique derives from the following:

 optimally filling an ion trap with the selected parent ion;

 obtaining qualitative structural information about the sample by forming the

product ion spectrum; and

 increasing the signal-to-noise ratio by eliminating interfering matrix ions in

the product ion spectrum during isolation.

Ion Formation and Matrix Ion Ejection

Ionization of the sample and any co-eluting matrix occurs by electron ionization.

During ionization, a broadband multifrequency waveform is applied to the end

caps of the ion trap to resonantly eject ions below the specified parent ion mass.

This removes unwanted low-mass ions whose space charge would otherwise

affect the storage efficiency of the parent ion. Removal of most of the space

charge from the trap improves mass resolution in subsequent MS/MS steps.

Following ionization, a second broadband waveform is applied to the trap to eject

those ions whose masses lie above the specified parent ion mass. This removes

most of the remaining unwanted ions from the trap. There now remains in the

trap the optimum number of parent ions having the desired m/z value, along with

a few ions with values of m/z slightly above and below it.

The advantage of this two-step approach is that it only requires one calculation of

the waveforms. If subsequent adjustments are needed to accommodate changes

in mass calibration, they can be made automatically by software that

independently modulates the rf fields in each ion-ejection step.

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Axis BTA 2100D Manuel de l'opérateur Page 209

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