General Information

Molecules that fall into this section are generally volatile under high vacuum conditions. This makes them amenable to ELECTRON IMPACT and CHEMICAL IONIZATION.

In both of these ionization methods, the sample is introduced into the instrument via a direct insertion probe. The source is under high vacuum (10^-6 Torr) conditions , allowing the sample to vaporize into the ionization region.  The vaporized sample is then ionized according to the ionization method being used.

1. Criteria for ELECTRON IMPACT.

2. Criteria for CHEMICAL IONIZATION.

3. Does this sample need a HIGH RESOLUTION measurement as well ?
 
 
 
 
 
 
 
 
 
 
 

Electron Impact

An EI mass spectrum is obtained by impact with a beam of electrons.  The desired product is M^+., the molecular ion.  For organic molecules, the molecular ion is a radical species, and therefore frequently prone to fragmentation.  So, for many species, EI produces a spectrum with a lot of fragmentation information.  This is useful for structural elucidation purposes, however, the molecular peak is sometimes relatively weak, or even on occasion, so unstable that it does not survive to appear in the EI mass spectrum.
 
 

Chemical Ionization

In CI mass spectrometry, a reagent gas is employed to generate a source of reagent ions.  In this laboratory, the reagent gas we employ is ammonia, which produces ammonium ions, NH₄⁺, as reagent ions under CI conditions.  This reagent ion can then interact with the vaporized molecule.  The desired product will be either [M+H]⁺ (protonated molecule) or [M+NH₄ ]⁺ (ammoniated molecule), depending upon the molecule's structure.  In some cases, both of these species are observed.

The cationized species are even-electron species and so relatively stable compared to their molecular ion counterparts.  So, CI mass spectra typically show little fragmentation and large molecular signals.
 
 

Exact Mass Measurement (High-Resolution) Requests  - Yes or No ?

At a technical level, performing exact mass measurements is much easier in EI than in CI.  Therefore, samples submitted with high resolution requested will always have the low-resolution analysis run in EI first.  If this produces a satisfactory molecular ion, then the high-resolution analysis will be run in EI.  If low-resolution EI fails to produce a molecular ion, then low-resolution CI will be run and high resolution performed on the cationized species.

If you do NOT need a high-resolution analysis, then most samples will be run using low-resolution CI, since this tends to produce more intense molecular peaks.

If you know you need a high-resolution analysis, then it is helpful if you tell us this AT THE TIME OF ORIGINAL SUBMISSION, since this will enable us to perform the appropriate low-resolution experiment(s).