Axis BTA 2100D Manuel de l'opérateur télécharger pdf (Page 158)

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The figure below shows the scan function for chemical ionization.

A Reagent gas ionization, time determined by the previous scan

B Ejection of unwanted ions (i.e., non-reagent ions) using SECI waveform

C Reaction of reagent gas ions with sample molecules, time determined from

previous scan

D Ejection of ions with masses lower than that of the background mass

E Acquisition of CI mass spectrum

CI Scan Function

Chemical ionization occurs by forming reagent ions by electron impact, which

react with the sample to form product ions by ion molecule reactions. The

ionization and reaction storage rf are maintained at the same level. Unwanted

sample ions formed by electron ionization during the formation of the reagent

ions are ejected using a low frequency waveform prior to the chemical ionization

step.

The ion trap operates in a pulsed mode. The supply of reagent ions is created

during the ionization pulse and consumed during the reaction period to form

analyte ions. The number of analyte ions depends on the concentration of the

analyte, the initial reagent ion intensity, and the reaction time.

Because the spectral intensity is proportional to sample concentration and

reaction time, linear calibration curves can be obtained.

Then, during the analytical scan, the following steps occur:

1. The reagent gas is ionized for the length of time determined by the previous

scan.(A)

2. All ions with a mass greater than the reagent ions are ejected, i.e., all EI ions

from the sample. ( B)

3. Reagent gas ions react with sample molecules to form sample ions. (The

reaction time was determined by the previous scan.) (C)

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