Mass Spectrometry Laboratory

School Affiliated departments and LAS

College of Liberal Arts & Sciences

Chemical Ionization

CI can produce molecular ions for some volatile compounds that do not give molecular ions in EI. CI uses a reagent gas to gently transfer protons to the sample, usually producing (M+H)+ quasimolecular ions. Such ions have very little tendency to fragment because they are even electron species and little excess energy is imparted to them. Thus CI produces less fragmentation than EI. The reagent gas (methane, isobutane, or ammonia) is present in the ion source at a pressure of 1 torr. It is ionized by an electron beam and the resulting ions undergo a complex series of ion-molecule reactions to produce species such as CH5+ in methane. These reagent gas ions, in a ratio of 10^6:1 to the sample, then collide with sample molecules producing ions by proton transfers to or from the sample molecules. Methane produces some fragmentation and is preferred for instrumental reasons. CI can be performed by direct probe, GC/MS, and by desorption CI (DCI).

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

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

The 70-VSE can perform chemical ionization using methane, ammonia, or isobutane as reagent gases. Most CI service is done by ACE since almost all users desire both EI and CI data.

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

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

This is done by scanning a small mass range that includes the unknown peak between two reference peaks. The resolution is 5,000. The data system determines the precise mass of the unknown by interpolating between the reference peaks.

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 LRCI. When you submit your sample and forms you must also include all LRCI output and your LRCI 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

tel: 217/333-2545
frs [at] illinois [dot] edu
Lab Hours: 24/7 access by key
Staff Available 8 am to 4:45 pm
Monday through Friday

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