Mass Spectrometry Laboratory

School Affiliated departments and LAS

Chemistry
College of Liberal Arts & Sciences

Electron Ionization

EI is the most commonly used method of ionization, and a great number of organic compounds are amenable to EI. To give an EI spectrum, the compound must be volatile. Specifically, it must have a vapor pressure of at least 10^-6 torr. The sample may be solid, liquid, or gas and it can be heated to 400 C to achieve the necessary pressure. Ions are formed when a 70 eV beam of electrons hits the sample molecules in the gas phase. This gives the sample molecules a great deal of excess energy and many fragment ions are formed. These ions can be useful in determining the structure of the molecule. Since samples must usually be heated, thermally labile samples may not work, although it is possible to cool the ion source from the usual 200 C to about 50 C. Unfortunately, some compounds will fragment completely and not give molecular ions. Ionic samples generally do not work by EI. EI can be performed by direct probe and GC/MS. For compounds that do not work by EI, alternate methods of ionization have been developed.


Low resolution EI Instrument:
70-VSE Sample size: 1-2 mg (ideally)

(The 70-VSE can handle amounts as small as 1-10 microgram.)

70 eV fragmentation pattern: The sample is admitted into the mass spectrometer and several scans are taken, usually with increasing direct probe temperature. The output will be one or more computer-generated bar plots and mass lists.

Special instructions: If you submit a sample for low-res and intend to have high-res done on the sample, then please check the "Save crucible for high resolution" box on the form. We will save the crucible and use the sample for high resolution.

The 70-VSE operator should be consulted in advance for special handling of air sensitive, especially difficult compounds, or gas samples. Special care should be made to insure that samples are solvent-free and as pure as possible. Air sensitive samples can be loaded into a crucible in your dry box and then stored there until the sample is called for by the Mass Spectrometry Laboratory personnel.


High resolution EI Instrument:
70-VSE Sample size: 1-2 mg (ideally)

(The 70-VSE can handle amounts as small as 20 microgram.)

  1. Voltage scanning: This method is used when the elemental composition of only one or a few peaks is desired. The sample and the reference compound (usually a perfluoroalkane mixture) are simultaneously bled into the ion source and the voltage is scanned over a range including two reference peaks and the sample peak between them. The data system acquires the data at a resolution of 5,000 and determines the precise mass of the unknown peaks by interpolation between the reference peaks. Mass accuracy is +/-2.0 mDa. Output is in the form of a high resolution elemental composition mass list.
  2. Magnetic scanning: This method is used when the elemental compositions of many or all the ions in a mass spectrum are desired. It is otherwise similar to voltage scanning.

Special instructions: The user should give some idea of an expected elemental composition for the compound under study if possible. You must have previously run your sample by LREI. When you submit your sample and forms you must also include all LREI output and your LREI form.



University of Illinois at Urbana-Champaign
Mass Spectrometry Laboratory
35 Noyes Laboratory
MC-712 Box 51-1
505 South Mathews Ave.
Urbana, IL 61801
Furong Sun
Director

tel: 217/333-2545
frs [at] illinois [dot] edu
Lab Hours: 24/7 access by key
Staff Available 8 a.m. to 5 p.m.
Monday through Friday

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