Electron Impact
Fast Atom Bombardment
Electrospray Ionization
Please feel free to ask one of the facility staff if you are not sure which ionization technique would be best for your sample. IN GENERAL, if your sample decomposes while obtaining a melting point, then you will not be able to request electron impact (EI). Instead, fast atom bombardment ( FAB) or electrospray ionization ( ESI) will be required. All salts require either FAB or ESI.
Electron Impact top
The electron impact (EI) technique is fairly straightforward. The sample must be delivered as a gas, which is usually accomplished by heating the sample to vaporize it from 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. EI is most useful for compounds below a molecular weight of 400 Da because larger molecules tend to thermally decompose during vaporization. EI 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 (available in the Facility along with the submission forms) or any other small vial. The staff will transfer the appropriate amount to special capillaries prior to analysis. Sample requirements for EI analysis are on the order of 50 micrograms (µg) up to a maximum of 1 mg.
Fast Atom Bombardment top
Fast atom bombardment ( FAB) is a soft ionization method that typically requires the use of a direct insertion probe for sample introduction. A high energy beam of xenon atoms or cesium ions is used to sputter the sample and matrix from the probe's surface. The FAB matrix, such as m-nitrobenzyl alcohol, is used to dissolve the sample and facilitate desorption and ionization. The matrix is a nonvolatile liquid that constantly replenishes the surface with new sample as the incident ion beam bombards this surface. The matrix serves to minimize sample damage resulting from the high-energy particle beam by absorbing most of the incident energy. The matrix is also 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 it is preferable that samples be submitted as dry crystals, samples can be submitted in solution (NO DEUTERATED SOLVENTS!) or as an oil. Sample requirements are on the order of 50 micrograms (µg) up to a maximum of 1 mg.
Electrospray Ionization top
Electrospray ionization ( ESI) generates ions from solution (typically an aqueous or aqueous/organic solvent system) by forming a fine spray of highly charged droplets in the presence of an electric potential (typically 1.0 to 3.5 kV). As a charged droplet decreases in size due to solvent evaporation, the electric charge density on its surface increases. The mutual repulsion between like charges on the surface eventually becomes so large that it exceeds the surface tension, causing the ejection of highly charged nanodroplets and/or solvated ions through what is known as a " Taylor cone". Evaporation of the remaining solvent molecules results in unsolvated, gas-phase ions that are guided through ion optics into the mass analyzer. ESI can produce singly or multiply charged ions. The number of charges retained by a particular analyte depends on several factors such as the size, chemical composition, and higher order structure of the analyte molecule, the solvent composition, the presence of co-solutes, and the instrument parameters. For small molecules (< 2000 Da) ESI typically generates singly, doubly, or triply charged ions, while for large molecules (> 2000 Da) ESI can produce a series of multiply charged ions. Because mass spectrometers measure the mass-to-charge (m/z) ratio, the ESI mass spectrum for a large molecule typically contains multiple peaks corresponding to the different charge states. For example, the ESI mass spectrum of intact carbonic anhydase II (29,024 Da), shown below, exhibits charge states (M+19H)19+ through (M+41H)41+.
ESI is very suitable for a wide range of biochemical compounds including peptides and proteins, lipids, oligosaccharides, and oligonucleotides. Bio-inorganic compounds, synthetic polymers, and intact non-covalent complexes can also be analyzed by ESI. ESI is extremely sensitive to impurities so please be sure your samples do not contain salt or buffers. More specific guidelines for preparing samples for ESI mass spectrometry can be obtained here. For bio-inorganic compounds the best results are usually obtained for samples that have been re-crystallized. Typical ESI solvents are methanol (MeOH), acetonitrile (AcN), water, and MeOH/water and AcN/water mixtures, although other solvents such as dimethylformamide (DMF), trichloromethane (CHCl3) and tetrahydrofuran (THF) can also be used if mixed with either MeOH or AcN. Please consult the Facility staff if you wish to submit a sample in a non-typical solvent.
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