Axis BTA 2100D Manual do Operador descarregar pdf (Página 157)

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2000 MS Method Chemical

Ionization

Introduction

Chemical ionization (CI) provides mass spectral data that complement electron

ionization (EI) data for the analysis of complex compounds. In the CI mode of

operation, a CI reagent gas is introduced into the ion trap analyzer. The reagent

gas is ionized by EI. Then, the sample molecules are ionized by ion molecule

reactions with the reagent gas ions.

CI is a softer ionization technique than EI. That is, CI imparts less energy to the

sample molecules than does EI. Thus, the ionized sample molecule undergoes

less fragmentation, and an ion indicative of the molecular weight is more likely to

be observed. In addition to molecular weight confirmation, CI mass spectra often

provide other significant structural information that may not be available from EI

mass spectra.

The Chemical Ionization Scan Function

EI space charge control uses the integrated ion current from a fixed ion time

prescan to calculate the analytical scan ionization time. In contrast, CI space

charge control does not use a fixed ionization time prescan to measure the

charge formed. CI uses a portion of the ions detected during the previous

analytical scan to calculate the ionization and reaction times for the next

analytical scan.

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