Understanding the mechanism of CO2-Assisted electrospray ionization for parameter optimization in supercritical fluid chromatography mass spectrometry

Yuka Fujito, Yoshihiro Izumi, Kohta Nakatani, Masatomo Takahashi, Yoshihiro Hayakawa, Mitsuo Takayama, Takeshi Bamba

Research output: Contribution to journalArticlepeer-review


Supercritical fluid chromatography (SFC) is often coupled with electrospray ionization mass spectrometry (ESI-MS) for analyte detection because of its detection capability to a wide range of chemical properties. However, MS sensitivity is highly dependent on the chromatographic conditions, so that it is important to understand the ionization mechanism to determine the optimal chromatographic conditions. The ionization mechanism in SFC/ESI-MS is different to that of liquid chromatography because of the use of CO2 as a mobile phase. Some studies have suggested that alkoxycarbonic acids are formed in the mixture of CO2 and the alcohol modifier, and these species contribute to ionization in CO2-assisted SFC/ESI-MS. Therefore, in this study, we investigated CO2-assisted ESI to test this hypothesis, and we confirmed that methoxylcarbonic acid is generated in CO2/methanol mixtures and contributed to ion generation and detection because it acts as a proton donor in positive-ion mode. However, methoxylcarbonic acid interfered with ionization in negative-ion mode. Addition of ammonium acetate, which is often added to the modifier for negative ion detection in SFC/MS analysis, did not contribute to the recovery of MS sensitivity, although it tended to suppress the formation of metoxylcarbonic acid. This is likely due to ion suppression and neutralization of the negative sites of the analytes by anions or cations derived from ammonium acetate in the negative ion mode. Thus, additive-free methanol/CO2 was the most suitable mobile phase for obtaining high sensitivity in SFC/MS. To demonstrate the practicality of these findings, we tested our optimal mobile phase selection for pesticide analysis. In addition, we tested the addition of 0, 1, and 5 mM ammonium formate to the modifier and make-up solvent, and found that the addition of 1 mM ammonium formate gave the best results in pesticides analysis. In SFC/MS, salt is often added to improve separation or prevent desorption, but our findings suggest that the concentration of salt must be kept as low as possible to achieve highly sensitive MS detection. The results of this study reveal the best selection of the optimal conditions for the modifier and make-up solvent for CO2-assisted SFC/MS analysis and will be useful for the method development in SFC/MS.

Original languageEnglish
Article number340863
JournalAnalytica Chimica Acta
Publication statusPublished - Mar 15 2023

All Science Journal Classification (ASJC) codes

  • Analytical Chemistry
  • Biochemistry
  • Environmental Chemistry
  • Spectroscopy


Dive into the research topics of 'Understanding the mechanism of CO2-Assisted electrospray ionization for parameter optimization in supercritical fluid chromatography mass spectrometry'. Together they form a unique fingerprint.

Cite this