Varian 4000 MS, User Manual

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6
Scan Functions Control MS Processes
Scan functions are programmable time sequences for applying voltages to the
external source, lens, and ion trap electrodes. The scan function controls the
variations in time of the RF potential applied to the ring electrode as well as any
supplementary waveforms applied to the endcap electrodes. The 4000 MS
module has two techniques, Electron Ionization (EI) and Chemical Ionization (CI),
for generating sample ions in the external source. Consequently, there are two
basic scan functions to perform electron ionization and chemical ionization (either
positive or negative). The diagram below illustrates the RF voltage profile of a
typical EI scan function. These scan functions are discussed in more detail in the
4000 GC/MS Software Operation Manual.
R
F V
oltag
e
Steps to Create a Mass Spectrum
Sample analysis with an ion trap analyzer may be divided into several steps:
Sample Introduction
Sample components are introduced from the transfer line to the ion trap analyzer
through the direct-coupled capillary column.
Ionization
The sample is ionized in the mass spectrometer by one of the following method:
•
Electron Ionization (EI): The sample molecules are struck with energetic
electrons removing an electron from a molecular orbital, to create
molecular ion.
•
Positive Chemical Ionization (PCI): A reagent gas is introduced in the ion
trap and EI is performed on that gas to form reagent ions. The reagent
ions then undergo ion-molecule reactions with the sample molecules to
create ions of the sample molecules and their fragments.