Axis BTA 2100D Manual do Operador descarregar pdf (Página 168)

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Reagent Ion Spectrum for Ammonia

Tuning for CI Operation

In CI, as in EI, the Saturn GC/MS is tuned to achieve the best combination of

maximum peak height (sensitivity), optimum resolution of peaks, and smooth

peak shape. In addition, as in EI, the key to proper CI tuning is to optimize the

number of ions in the ion trap: too few ions, sensitivity suffers; too many ions,

resolution and peak shape suffer.

In CI, the two main factors that affect the number of ions in the ion trap are the

ionization time and the reaction time. (The filament emission current is an

important third factor.)

To a first approximation, the number of reagent ions formed is directly

proportional to the ionization time, and the number of sample ions formed is

directly proportional to the reaction time.

To tune the Saturn GC/MS for CI operation, you simply tune and calibrate the

instrument from System Control Auto Tune section. You do not need to run a

different automatic setup, tuning, and calibration program for CI. The setup

parameters and mass calibration established for EI are also satisfactory for CI.

Low-Pressure CI vs. High Pressure CI

The conventional high pressure CI source in quadrupole or magnetic mass

spectrometers requires reagent gas pressures on the order of 1 Torr. Such high

pressures are necessary because the residence time of ions is very short, about

-5

seconds. Higher pressures result in the generation of the large number of

reagent ions that are necessary for successful chemical ionization in this short

time frame.

The Saturn GC/MS ion trap accumulates reagent ions and holds them for

reaction periods on the order of 1 to 128 milliseconds. In this longer time frame, a

lower reagent gas pressure (ca. 10

-5

Torr) produces a large population of sample

ions.

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