Investigating Mycobacteria FAME Profiles Using Alternative Ionization Energies in GC–MS

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Highly repeatable fragmentation of the compounds into the ion source is one of the major advantages of using gas chromatography coupled with electron ionization mass spectrometry (GC–EI-MS) for the analysis of volatile and semivolatile compounds. The generation of intense and diagnostic fragmentation when the ionization is performed at 70 eV has been used for the creation of many established databases-enabling the analyte identification process.

However, such an intense fragmentation can cause the loss of the molecular ion or more diagnostic ions, which can make the identification of unknown analytes, homologues, or isomers difficult. This is particularly true for the analysis of fatty acids. These issues have led to a demand for lower-energy or “soft” ionization techniques in MS as well as calls for alternative detectors.

The use of lower electron kinetic energy and lower ion source temperature was attempted in the 1980s (1,2) as possible solutions, but the technique was never routinely applied because of the lower fragmentation repeatability and a general loss of sensitivity (2,3). However, with the advent of modern instrumentation the technique may be viable under different ionization conditions and may offer researchers a new technique to analyze fatty acids.

To test this researchers explored different ionization voltages using a conventional quadrupole MS, coupled to a prior gas chromatographic separation (4). The investigation used two different mixtures of fatty acids methyl esters (FAME) standards and also investigated the  advantages of a “softer” ionization energy when classifying and identifying FAME profiles of different mycobacteria species.

Despite reports in the literature to the contrary (4), the fragmentation pattern in terms of relative intensity was found to be very repeatable, with an average CV% of less than 5% at all the ionization energies tested. The softer fragmentation obtained at 20 eV simplified the interpretation of the mass spectra by providing more information on diagnostic ions as they were found to be significantly higher in intensity when compared to analyses carried out at 70 eV.

Despite the current lack of MS databases, researchers believe the technique is complementary to the current methods and could offer extra information. It was also noted that more studies are required on different chemical classes to fully flesh out the technique and invited researchers to investigate and question some basic assumptions that may no longer hold true.

References

1. A. Maccoll, Org. Mass Spectrom.17, 1–9 (1982).
2. A. Maccoll, Org. Mass. Spectrom.21, 601–611 (1986).
3. J.H. Gross, Mass Spectrometry (Structure. Springer, 3rd ed. Switzerland, 2017).
4. M. Beccaria et al., Anal. Bioanal. Chem.410, 7987–7996 (2018).