Axis BTA 2100D Manual do Operador descarregar pdf (Página 159)

157

4. Reagent gas ions are ejected. (D)

5. The CI mass spectrum is acquired for the sample ions. (E)

SECI ejects all ions above a cutoff mass by applying a waveform between the

ionization and reaction periods. This is the preferred mode of operation on the

Saturn GC/MS, since all EI artifact ions are eliminated. It should be noted that

fragmentation can still be observed in CI spectra due to energy imparted when

the proton is transferred from the reagent ion to the sample molecule.

Ion Formation By Chemical Ionization

In CI, ionization of sample molecules is a two-step process. In the first step,

reagent gas ions are formed as the reagent gas is ionized by interaction with

electrons emitted by the filament.

In the second step, the reagent gas ions react with sample molecules in the ion

trap to form sample ions. There are four principal reactions between reagent gas

ions and sample molecules. They are:

(A) Proton transfer: (RH)

+ M  (MH)

+ R

(B) Hydride abstraction: R

+ M  [M-H]

+ RH

+ M  (MR)

(D) Charge transfer: R

+ M  M

+ R

where 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 no

molecular ion. The exothermicity of the reactions determines the amount of

energy deposited; therefore, the degree of fragmentation can be controlled by

the choice of a suitable CI reagent gas. The proton affinities of some common

reagent 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). By

choosing a suitable reagent gas, you can obtain high specificity (i.e., less

efficient detection of background or matrix interferences compared to the

analyte) as well as molecular weight information for the compounds of interest.

The association or clustering reactions (C) typically have very low reaction rates,

and the reaction products require rapid collisional stabilization. They are typically

seen on the Saturn GC/MS at much lower abundance then the (M+1) ion, but

when (M+28) and (M+41) ions are observed using methane, they are useful for

verifying the molecular weight.

The charge transfer reaction (D) produces a radical molecular ion (i.e., an ion

with an odd number of electrons) that dissociates quickly, giving EI-like spectra.

However, the energy deposited in the molecular ion and the resulting

fragmentation pattern does not depend on the electron energy of the ionizing

electrons. Common charge-transfer reagent gases are nitrogen and argon.

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