Exploring 20 eV electron impact ionization in gas chromatography-tandem mass spectrometry for the determination of estrogenic compounds

In electron ionization mass spectrometry (MS), the generation of characteristic fragmentation patterns allows reliable and sensitive identification of compounds. However, loss or a less intense signal of the molecular ion (or more diagnostic ions) can often be observed, which can be detrimental for identification and/or sensitivity, even when MS/MS approaches are applied for quantification. The benefits of applying lower ionization energy (i.e., 20 eV compared to 70 eV) using a gas chromatography (GC) - tandem MS (MS/MS) instrument were investigated in the detection of three estrogenic compounds, namely estrone (E1), 17β-estradiol (E2), and 17α-ethynylestradiol (EE2), emerging aquatic pollutants included in the European Commission Watch List. As expected, the relative intensity of molecular ions (M+.) or high-mass fragments closely related (M+.-CH3) increased significantly at 20 eV compared to 70 eV (from 4.6 % to 35.0 % for EE2, from 22.5 % to 87.3 % for E2, and from 76 % to 100 % for E1). This change in the spectrum profile led to an overall increase in the sensitivity of the compounds when detected using the multiple reaction monitoring mode. These results were compared with the instrumental limit of quantification obtained in liquid chromatography - MS/MS showing a limit of quantification of about 100-folds lower for GC-MS/MS and a completely neglectable matrix effect, thus posing the base for the development of a miniaturized sample preparation method (with an overall lower concentration factor) to achieve the challenging low limits of detection required by the EU regulation for estrogenic compounds.