Axis BTA 2100D Manual do Operador descarregar pdf (Página 155)

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2000 MS Method Basic Scan

Functions

Introduction

The Saturn GC/MS analyzes the gas-phase ions formed from a sample in terms

of their mass-to-charge (m/z) ratios and their relative abundances in the resulting

spectra. The mass spectrum is a graphical representation of the ion intensities

versus the mass-to charge ratio.

The Saturn GC/MS creates the mass spectrum with an ion-trap analyzer. This

type of analyzer differs from beam transport analyzers such as magnetic sectors,

quadrupoles, and time-of-flight instruments where the ions to be separated pass

through a sequence of fixed electromagnetic fields. Instead, the ion trap confines

the ions within a single region where they experience time-dependent

electromagnetic fields.

Sample analysis with an ion trap analyzer may be divided into several steps:

Sample Introduction: Compounds are introduced from the transfer line to the ion

trap analyzer through the direct coupled capillary column.

Sample Ionization: The compound is ionized in the mass spectrometer by either:

 destabilizing its molecular structure, causing an electron to be removed from

somewhere on the molecule (Electron Impact/Ionization) (EI).

 pressuring the MS with a selected reagent gas; e.g., methane, performing EI

on the gas to form reagent ions, and allowing ion-molecule reactions to occur

between the compound and the reagent ion (Chemical Ionization) (CI).

Sample Fragmentation: Depending on the compound’s original structure and the

excess destabilizing (internal) energy present in the structure, the ionized

compound (molecular ion) breaks apart (fragmentation). This process forms

fragment ions and neutral fragments.

Ion Storage: The ions (molecular and fragment ions) which were just produced

are stored and stabilized in the ion trap cavity, where they travel in defined orbits.

Helium buffer gas is also present in the cavity. It helps to focus the ions into more

compact orbits which produce sharper mass peaks as they are scanned out.

Helium is used because it does not ionize as readily as the analyte molecules.

While helium ions are the most dominant species in the trap, they are not stored,

but are pumped away as soon as they are formed.

Ion Analysis: The ions are analyzed by applying a radio frequency (rf) (1.1 MHz)

voltage to the ring electrode encircling the trap cavity. As the voltage increases

on the ring electrode, ions are sequentially ejected from the trap according to

their mass-to-charge ratio. A small AC voltage (axial modulation voltage) of fixed

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