Low Energy Electron Ionization and Chemical Ionization Mass Spectrometry

This experiment was developed by S.E. Van Bramer for Chemistry 465 at Widener University.

Introduction:

The fragmentation observed in mass spectrometry depends upon the ionization method and the ionization energy. In this experiment we will study the fragmentation of 1-propanol (C3H7OH), methyl ethyl ether (CH3OC2H5), 1-decanol (n-C10H21OH), tributyl amine ((n-C4H9)3N), hexadecane (n-C16H34) or some other similar compounds. In the first part of this experiment the ionization energy is controlled by varying the potential of the electron beam. This changes the fragmentation patterns observed in the mass spectrometer. This technique is used to enhance low energy fragments, to help identify unknown compounds, and to study fragmentation energetics.

In the second part of this experiment methane chemical ionization (CI) is used to produce ions. Because CI is much less energetic than electron ionization, CI does not induce as much fragmentation. This is very useful for identifying the molecular ion and determining the molecular weight of a compound. The first part of this experiment is based upon; Maccoll, A. Org. Mass Spectrom., 1986, 21, 601-611. Low Energy, Low Temperature Mass Spectra 6-A Synoptic View. This second part of this experiment is based upon Munson, M.S.B.; Field, F.H. J. Am. Chem. Soc., 1966, 88, 2621-2630. Chemical Ionization Mass Spectrometry. I. General Introduction. (See Reading Notes.)

With low ionization energy, the ionization cross section is much lower and as a result the signal is much smaller at low energies. Chemical Ionization produces a number of very intense background peaks in the spectrum. To maintain adequate S/N and dynamic range, a variety of paramenters will be adjusted during the experiment. This includes changing the injection technique, the electron multiplier voltage, the amplification gain, mass range and the source region configuration.

Experimental:

  1. Overview of the instrument. Introduction to the ionization energy and chemical ionization controls.

  2. Calibrate the mass spectrometer.

  3. Setting up the experiment.

  4. Collect Data.
    1. Collect mass spectra for the selected compound at 10 different ionization energies between 11 and 70 eV.
    2. CI spectra. Collect CH4 CI spectra at sevral different source pressures between 0.05 torr and 0.25 torr.

    Laboratory Write-up:

    This Laboratory write-up will be a formal lab report. I expect that you will have questions about this report, after looking at your data and coming up with some ideas make an appointment to discuss your results. Your final report should include the following information.
    1. (10 pts) An abstract of your results.

    2. (20 pts) A 1 page introduction (describes the significance of this experiment).

    3. (50 pts) A results section. This is the bulk of the report. The goal is to organize and present your experimental results in a clear and concise discussion. This section should include mass spectra, graphs, tables, and reaction mechanisms. This section should not include all of your data, the idea is for you to use examples from the data to demonstrate your results.

      To help you get started, think about what this experiment demonstrates and what the data show. Spend some time looking at the data to find the major trends. Then find an efficient and effective way to show these trends. You will need to spend some time thinking about how to normalize (scale) your data.

      You may find the following software useful:

      • Word. For word processing, you should copy and paste from other software.
      • ChemSketch. For chemical structure drawing to show reactions and fragmentation.
      • Excel. For graphs to show ion intensity vs EI energy.

    4. (10 pts) Appendix. Clearly labeled spectra to show the mass spectra for each compound at different ionization energies.

    5. (10 pts) The Carbon pages from your laboratory manual.

This page is maintained by
Scott Van Bramer
Department of Chemistry
Widener University
Chester, PA 19013

Please send any comments, corrections, or suggestions to svanbram@science.widener.edu.

This page has been accessed times since 1/5 /96 .
Last Updated Thursday, September 02, 1999 12:52:00 PM