Varian 4000 MS User Manual

Also see for 4000 MS: Hardware operation manual

Contents

Varian 4000 MS User Manual Manual pdf 10 page image

10•++• +→+ 3544 CHCHCHCH )(25243 HHCCHCH +→+ ++Chemical Ionization Reactions in PCIIn the second step, the reagent gas ions react with sample molecules in the iontrap to form sample ions. The four principal reactions between reagent gas ionsand sample molecules are as follows:(A) Proton transfer: +)RH( + M → (MH) + + R(B) Hydride abstraction: +R + M → [M-H] + + RH(C) Association: +R + M → (MR) +(D) Charge transfer: +R + M → +M + Rwhere R + is the secondary reagent gas ion and M is the neutral sample molecule.For methane CI, proton transfer (A) is the major reaction, and hydride abstraction(B) is the next most often observed reaction. In both cases the resulting even-electron ions are often relatively stable, and the observation of strong (M+1) or(M-1) ions is possible even if the EI spectrum of the same component shows nomolecular ion. The exothermicity of the reactions determines the amount ofenergy deposited; therefore the degree of fragmentation can be controlled by thechoice of a suitable CI reagent gas. The proton affinities of some commonreagent gases of this type, known as proton transfer agents or Bronsted acids,range from 130 kcal/mol to 200 kcal/mol in the following order: methane, water,isobutane, and ammonia (with ammonia resulting in the “softest” ionization).Among the common liquid CI reagents, methanol has a proton affinity of 180.3kcal/mol, while acetonitrile is 1862 kcal/mol. By choosing a suitable reagent gas,you can obtain high specificity (i.e., less efficient detection of background ormatrix interferences compared to the analyte) as well as molecular weightinformation for the compounds of interest.Association reactions (C) typically have very low reaction rates, and the reactionproducts require rapid collisional stabilization. The products of these associationreactions are called adduct ions because the reagent ion has been added to theanalyte. They are typically seen on the 4000 GC/MS in Internal configuration atmuch lower abundance then the (M+1) ion, but when (M+29) and (M+41) adductions are observed using methane, they are useful for verifying the molecularweight.The charge transfer reaction (D) produces a radical molecular ion (i.e., an ionwith an odd number of electrons) that dissociates quickly, giving EI-like spectra.However, the energy deposited in the molecular ion and the resultingfragmentation pattern does not depend on the electron energy of the ionizingelectrons.Ion Preparation OptionsThe 4000 MS uses a combination of waveforms and RF to isolate, or remove,specific ions after they are formed and are stored in the trap. Selected IonStorage (SIS) and Tandem Mass Spec (MS/MS) can be performed on the ionsstored in the ion trap before mass analysis takes place.