Improving GC–MS with Cold EI

A paper recently published by researchers at the School of Chemistry of Tel Aviv University (Tel Aviv, Israel) inMass Spectrometry Reviews (1) discussed the benefits of gas chromatography–mass spectrometry (GC–MS) with cold electron ionization (EI) compared to the technique coupled with standard EI. It is the opinion of the authors that incorporating cold EI offers over 60 benefits and advantages over the more traditional method with no corresponding disadvantages, thus becoming a driving factor in the future implementation of GC–MS (2).

A central analytical technique that serves a broad range of applications and disciplines, a major strength of GC–MS with its standard electron ionization (EI) ion sources is its ability to provide easy sample identification, with names and structures—including at the isomer level within 70 eV EI-MS libraries. However, the authors of the study believe that the technique suffers from two major limitations. First, the range of volatile, thermally stable compounds that are amenable to analysis is small, and many analyses therefore require liquid chromatography (LC)–MS or cannot be done. Second, EI mass spectra have limitations because of the frequent absence or inadequate amount (if not out-and-out absence) of molecular ions. This limitation of molecular ions leads to a diminished confidence level regarding the identification of samples via the library, which makes it impossible to identify those compounds which are not part of the library. As a result of these two limitations, the use of LC–MS has increased.

Within the last 35 years, the performance capabilities of a new type of GC–MS interface and ion source were developed by Aviv Amirav of Tel Aviv University, the review’s lead author. The technique known as “cold EI” is based on the use of a supersonic molecular beam (SMB) for interfacing the GC and MS and as a medium for electron ionization of vibrationally cold sample compounds in the SMB, in a contact-free fly-through ion source, and developed by Amirav and his group in 1990 (3,4).

With cold EI, a supersonic molecular beam is created by the expansion of gas through a small pinhole or shaped nozzle into a vacuum chamber. In this expansion, the same final velocity of the carrier gas (typically helium) and sample molecules is obtained, accelerating the sample compounds to the velocity of the carrier gas, which ensures slow intra-beam collisions and results in internal vibrational-rotational cooling of the sample compounds. The SMBs are characterized by a few features which are highly advantageous for GC–MS; these features, and the benefits they provide, include:

• vibrational cooling of the seeded sample compounds (enhanced molecular ions);
• high column flow rate compatibility up to 100 mL/min (extended range);
• compatibility with a contact-free fly-through EI ion source (many benefits);
• unidirectional molecular motion in space with heavy species concentration along the beam axis (jet separation);
• controlled amount of sample species kinetic energy up to 16 eV because of its acceleration as neutral compounds to the helium velocity (enables vacuum background filtration) (1).

The authors of the review article state that the advantages of GC–MS with cold EI are mainly based on the ionization of vibrationally cold molecules during their axial flight path in a fly-through ion source and are the result of its capability to study samples with high column flow rates of up to 100 mL/min without affecting their sensitivity. They have explored several hundred applications using GC–MS with cold EI and describe many of them in various publications, as well as in their Advanced GC–MS Blog Journal (6), where 58 application notes describing its use in various circumstances are housed (1).

It is the opinion of Amirav and his team that GC–MS with cold EI is a highly superior replacement ion source and destined to lead the way for the future of GC–MS. Furthermore, they state it will fully replace GC–MS with standard EI in a short time once the technique is commercialized (1).

Gas chromatography instrument. © MJ Iceberg - stock.adobe.com

References

1. Amirav, A.; Neumark, B.; Elkabets, O.; Yakovchuk, A. Cold EI-The Way to Improve GC–MS and Increase Its Range of Applications. Mass. Spectrom. Rev. 2025. https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/full/10.1002/mas.21928 (accessed 2025-03-10).

2. Amirav, A.; Fialkov, A. B.; Alon, T. What Can Be Improved in GC–MS—When Multi Benefits Can Be Transformed into a GC–MS Revolution. Int. J. of Anal. Mass Spectrom. Chromatogr. 2013, 1 (1), 31–47. DOI: 10.4236/ijamsc.2013.11005

3. Amirav, A.; A. Danon, A. Electron Impact Mass Spectrometry in Supersonic Molecular Beams. Int. J. Mass Spectrom. Ion Processes 1990, 97, 107–113. DOI: 10.1016/0168-1176(90)85042-Z

4. Amirav, A. Electron Impact Mass Spectrometry of Cholesterol in Supersonic Molecular Beams. J. Phys. Chem. 1990, 94, 5200–5202. DOI: 10.1021/j100376a002

5. Advanced GC–MS Blog Journal (2012-2023). http://blog.avivanalytical.com (accessed 2025-03-10).