Varian 4000 MS User Manual

Also see for 4000 MS: Hardware operation manual

Contents

Varian 4000 MS User Manual Manual pdf 9 page image

9scan. There are actually two types of mass scanning in internal EI. First there is aprescan to count the number of ions formed in a short fixed ionization time. Aftera calculation based on the prescan ion count, ions are formed for the ionizationtime recommended by the AGC prescan algorithm and the analytical scan iscarried out. The analytical scan can be broken up into up to six segments and therelative ionization times for the segments can be adjusted to meet tuningrequirements for methods such as those of the US EPA for the compoundsDFTPP and BFB.Library Searching and Data HandlingThe mass spectra collected from the MS are analyzed through Varian MS DataReview. The identity of most compounds is determined by the comparison of thecollected spectrum with a reference library. The mass and intensity listing iscompared to results collected on other instruments. Such listings include theNIST library, the Wiley MS library, and the PMW library. Each library has adifferent focus, from pharmaceutical to environmental analysis. Custom librariescan also be generated from results collected on the 4000 MS system.About Internal Chemical IonizationChemical ionization (CI) provides mass spectral data that complement electronionization (EI) data for the analysis of complex compounds. The generation ofions in chemical ionization is a two-step process. A CI reagent gas is introducedinto the ion trap analyzer. The reagent gas is ionized by EI. Then, the samplemolecules 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 thesample molecules than does EI. Thus, the ionized sample molecule undergoesless fragmentation, and an ion indicative of the molecular weight is more likely tobe observed. In addition to molecular weight confirmation, CI mass spectra oftenprovide other significant structural information that may not be available from EImass spectra.Forming Reagent IonsIn the first step, reagent gas ions are formed as the reagent gas is ionized byinteraction with electrons emitted by the filament. The ion trap operates in apulsed mode. The supply of reagent ions is created during the ionization pulseand consumed during the reaction period to form analyte ions.In internal mode, the CI reagent can be either liquid or gas. The most commonreagents are methane, methanol, acetonitrile, and isobutane.Reagent ion formation can be a complex process. For example, when methane isused as the reagent gas, reagent gas ions are formed as follows:First, methane is ionized to form two primary ions:−+•− +→+ e2CHeCH 44 )(−−+− ++→+ HeCHeCH 34These primary ions then react very rapidly to form predominantly the secondaryions, :++525 HCandCH