Electron Impact
Fast Atom Bombardment
Electrospray Ionization

If you are unsure of which ionization technique is required, ask one of the spectroscopists to discuss your class of compounds with you. IN GENERAL, if your sample decomposes while obtaining a melting point, you will not be able to request electron impact (EI). Fast atom bombardment (FAB) or electrospray (ESI) will be required. All salts require either FAB or ESI.

Electron Impact    top

The electron impact technique is straightforward. The sample must be delivered as a gas, which is usually accomplished by heating the sample to vaporize it off of the probe. Once in the gas phase, the compound passes into an electron ionization region where it interacts with a beam of electrons of nearly homogeneous energy (70 electron volts), typically causing electron ejection and some degree of fragmentation.
Electron impact is most useful for compounds below a molecular weight of 400 Da since larger molecules tend to thermally degrade during vaporization. Electron impact is principally used as a detector for gas chromatography (GC/MS) in a wide variety of areas including synthetic organic chemistry, hydrocarbons analysis, pharmaceutical compounds and drugs of abuse (for example it is widely used in the Olympic drug testing program), and environmental studies such as water testing.
Non-air sensitive samples for EI analysis can be submitted in an eppendorf tube (supplied by the lab and can be picked up along with the submission forms) or any other small vial. The staff will transfer the appropriate amount to special capillaries. Sample requirements for EI analysis are on the order of 50 micrograms (µg) to 1 mg max. One or two good-sized crystals should be enough. If you are uncertain, ask.

Fast Atom Bombardment    top

The FAB ionization technique is a soft ionization method that typically requires the use of a direct insertion probe for sample introduction and a high energy beam of xenon atoms or cesium ions to sputter the sample and matrix from the probe's surface. The matrix, such as m-nitrobenzyl alcohol, is used to dissolve the sample and facilitate desorption as well as ionization. The FAB matrix is a nonvolatile liquid material that serves to constantly replenish the surface with new sample as the incident ion beam bombards this surface. The matrix also serves to minimize sample damage from the high-energy particle beam by absorbing most of the incident energy and the matrix is believed to facilitate the ionization process.
Non-air sensitive samples for FAB analysis can be submitted in an eppendorf tube or any other suitable small vial. Although we prefer samples submitted as crystals/dry material, you can submit samples in solution (NO DEUTERATED SOLVENTS!) or as an oil. Sample requirements are on the order of 50 micrograms (µg) to 1 mg (max).

Electrospray Ionization    top

Electrospray ionization generates ions directly from solution (usually an aqueous or aqueous/organic solvent system) by creating a fine spray of highly charged droplets in the presence of a strong electric field (typically 3.5 kV). As the droplet decreases in size, the electric charge density on its surface increases. The mutual repulsion between like charges on this surface becomes so great that it exceeds the forces of surface tension, and ions begin to leave the droplet through what is known as a "Taylor cone". The ions are then electrostatically directed into the mass analyzer. Vaporization of these charged droplets results in the production of singly or multiply-charged gaseous ions. The number of charges retained by an analyte can depend on such factors as the composition and pH of the electrosprayed solvent as well as the chemical nature of the sample. For small molecules (< 2000 Da) ESI typically generates singly or doubly charged ions, while for large molecules (> 2000 Da) the ESI process typically gives rise to a series of multiply charged species. Because mass spectrometers measure the mass-to-charge (m/z) ratio, the resultant ESI mass spectrum contains multiple peaks corresponding to the different charged states.
ESI is very suitable for analysis of polar biological samples. Peptides and proteins are usually easily analyzed by ESI. Bio-inorganic compounds can also be analyzed by ESI. ESI is extremely sensitive towards impurities so make sure your (biological) samples do not contain salt or buffers. For bio-inorganic compounds we usually get the best results for samples that have been re-crystallized. The best solvents are methanol (MeOH), acetonitrile (AcN), MeOH/H₂O and AcN/H₂O. Other solvents such as DMF, CHCl₃ and THF can be used if mixed with either MeOH or AcN. See a spectroscopist before submitting your sample if it is not soluble in MeOH, AcN or H₂O.