Electron Ionization for GC–MS

Electron ionization (EI), (formerly known as electron impact ionization [1]) is used in gas chromatography–mass spectrometry (GC–MS) to ionize and fragment analyte molecules before mass spectrometric analysis and detection. A GC column is connected via a transfer device to a mass spectrometer, the simplest and most popular of which will contain an electron ionization source with a single-quadrupole mass analyzer and a detector.

A typical electron ionization source exposes the analyte, under vacuum, to a stream of thermionic electrons produced from a resistively heated filament. Close passage of highly energetic electrons to the neutral analyte molecule causes large fluctuations in the local electric field, which induces ionization via the removal of a single electron from the highest occupied molecular orbital (HOMO) of an atom within the molecule, forming a radical cation according to the general scheme (2):

In general, the order of ease with which electrons are lost under electron ionization conditions is lone pair > π-bonded pair > σ-bonded pair.

The cations formed within the ion source can then be electrostatically ejected from the ionization source using a repelling voltage applied to the rear element of the source. The analyte ions are collimated, focused, and accelerated through a series of electrostatic lenses and subsequently into the mass analyzing device (3).

To induce analyte ionization, "standard" electron ionization uses 70 electron volt (eV) electrons, a level that is higher than the energy required for ionization of most common organic compounds (typically between 6 and 15 eV). Most of the excess energy imparted by the ionizing electron will be internalized and dissipated via vibrational and electronic excitation modes.

The radical cation formed typically undergoes a single or multiple fragmentations or a rearrangement reaction, resulting in the elimination of a neutral species to achieve a more stable state. The degree of fragmentation or intermolecular rearrangement will depend on the analyte molecule ionization cross-sectional area and the ability of the molecule to stabilize charge (4).

Fragmentation and rearrangement pathways are well characterized and predictable according to some fairly rudimentary principles. A typical mass spectrum produced from electron ionization is shown in Figure 1. The mass-to-charge ratio and intensity of the parent (pseudomolecular) and fragment ions act as a "fingerprint" that can be used to deduce analyte chemistry or entered into a spectral library for identification against thousands of library entries acquired under similar operating conditions.

Figure 1: A typical electron ionization spectrum of the cocaine molecule. Note the large number of intense fragments created.

Electron ionization is known as a "harsh" technique because of the degree of fragmentation induced. It is possible to reduce the energy of ionizing electrons to reduce the degree of fragmentation, however the sensitivity of the technique decreases markedly. Chemical ionization (CI) is a complementary technique that uses a reagent gas to create a plasma of charged gas molecules that then transfer charge to the analyte via a range of gas-phase chemical and charge transfer reactions.

Electron ionization typically is the first method of choice when screening samples and is applicable to the vast majority of analytes that can be analyzed by gas chromatography.

References

(1) IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). (Online corrected version: 2006–).

(2) R.J. Anderson, D.J. Bendell, and P.W. Groundwater, Organic Spectroscopic Analysis (The Royal Society of Chemistry, Cambridge, UK, 2004), pp 120–148.

(3) G. Zschornack, V.P. Ovsyannikov, F. Grossmann, and O.K. Koulthachev, "Electron Ionization Ion Source," United States Patent No 6717155.

(4) J. McMurry, Organic Chemsitry, 3rd ed. (Brooks/Cole Publishing Company, Pacific Grove, California, 1992).