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OnCampusRates2002.pdf
OffCampusRates2002.pdf
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Routine Services

• Low Resolution electron ionization

Electron impact ionization (EI) is the classical ionization method used in mass spectrometry. Samples are thermally desorbed (standard solids probe, high temperature probe) or introduced through a gas chromatograph and thus must be thermally stable. EI is a highly informative method as it provides fragmentation information in addition to molecular weight. In may cases a library search can be used to assist in structural identification.

• High Resolution electron ionization
• Low Resolution chemical ionization

Chemical ionization (CI) is a complementary technique to EI. Sample introduction is that same of EI so this method is limited to thermally stable compounds. The ion source is maintained at high pressure with a CI reagent gas such as methane or isobutane and the spectrum shows a quasimolecular ion with little or no fragmentation. CI is a soft ionization method used to get the molecular weight of a thermally stable, volatile compounds.

• Low Resolution fast atom bombardment

Fast atom bombardment (FAB) is a soft ionization method, and like CI provides quasimolecular information with little or no fragmentation. FAB is used for many thermally unstable, non-volatile compounds. The sample is dissolved in a solvent and matrix a placed on a FAB probe target. The target is introduced into the ion source and the sample, solvent, matrix combination is bombarded with energetic Cs ions often with the result of protonation of the sample. After desolation and removal of the matrix in the vacuum system the analyte is detected and analyzed. FAB is used to get the molecular weight of polar, often large molecular weight, non-volatile compounds. Caution must be taken to clean up the sample including removal of salts.

• High Resolution fast atom bombardment
• Atmospheric Pressure Ionization

Atmospheric pressure ionization (API) is a soft ionization technique and like FAB and CI provides quasimolecular information. Two API methods available at this facility are Electrospray Ionization (ESI) and Atmospheric Pressure Chemical Ionization.  ESI is used for the analysis of large polar compounds including but not limited to proteins, peptides and oligonucleotides.  Analytes are dissolved in solvent mixtures such as Methanol/H2O or Acetonitrile/H2O and introduced to the API chamber by infusion or high performance liquid chromatography.

For ESI the analyte must be ionized in solution.  The pH is adjusted to achieve either protonation or deprotonation of the sample (positive or negative overall charge).  A good starting point can be seen in the table below.

                - Acids                   pH < pKa-2

                - Bases                   pH > pKa+2

In APCI the ionization, usually protonation occurs for the vaporized neutral analyte.  Thus the pH is adjusted to suppress ionization is solution.  The table below can be used as a guide in the pH adjustment process.

                - Bases                   pH > pKa+2

                - Acids                   pH < pKa-2

Further adjustments can be made from this point to optimize instrument response.

All samples MUST be purified before submission for analysis.  It is essential that non-volatile buffers not be used.  The used of volatile buffer systems is preferred.  Samples MUST also be desalted.  Either precipitation or dialyzing can be used for this.  The use of trifluoroacetic (TFA) acid is discouraged.  If TFA is necessary for the chromatographic profile the concentration must be less than 0.1% v/v.

Other Services

• High Resolution GC/MS (EI only - definition above)
• Low resolution GC/MS (EI or CI - definition above)

For information on the Mass Spectrometry Facility, please contact: