Axis BTA 2100D Instrukcja Operatora Pobierz pdf (Strona 191)

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2000 MS Method Electron

Ionization

The Electron Ionization Scan Function with Automatic

Gain Control (AGC)

The ion trap has a maximum storage capacity, beyond which mass resolution

and spectral quality deteriorate. The number of ions created is proportional to the

ionization time, with more ions being produced the longer the ionization time.

Automatic Gain Control (AGC) controls the ionization time in order to maintain

the number of ions in the trap at the optimum level.

The AGC scan function consists of a prescan and up to six scan segments. The

prescan is done using resonance ion ejection and the number of ions detected in

the prescan is used to calculate the ionization time for the six scan segments.

The figure below illustrates four scan segments correspond to the following four

portions of the mass range (default values): 10 to 99 m/z, 100 to 249 m/z, 250 to

399 m/z, and 400 to 650 m/z. One scan involves one complete ramping of the rf

voltage over the mass range selected by the user. The rf voltage is ramped at a

constant rate of approximately 5600 m/z per sec. One complete scan at this fixed

rate produces a microscan.

The AGC Scan Function

The AGC software automatically selects an ionization time from 10 µsec to 65

msec for each of the six scan segments of the microscan, depending on the

amount of analyte in the ion trap and the tuning parameters. An estimate of the

number of ions formed in the ion trap is provided by a short prescan that consists

of a 0.1 millisecond ionization period. For low levels (e.g., for baseline or small

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