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

doi:10.1016/j.aca.2023.340863

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

Anal Chim Acta. 2023 Mar 15:1246:340863. doi: 10.1016/j.aca.2023.340863. Epub 2023 Jan 20.

Authors

Yuka Fujito¹, Yoshihiro Izumi², Kohta Nakatani³, Masatomo Takahashi³, Yoshihiro Hayakawa⁴, Mitsuo Takayama⁵, Takeshi Bamba⁶

Affiliations

¹ Division of Analytical and Measuring Instruments, Shimadzu Corporation, 1 Kuwabaracho Nishinokyo Nakagyo-ku, Kyoto, 604-8511, Japan; Department of Systems Life Sciences, Graduate School of Systems Life Sciences, Kyushu University, 3-1-1 Maidashi Higashi-ku, Fukuoka, 812-8582, Japan.
² Department of Systems Life Sciences, Graduate School of Systems Life Sciences, Kyushu University, 3-1-1 Maidashi Higashi-ku, Fukuoka, 812-8582, Japan; Division of Metabolomics/Mass Spectrometry Center, Medical Research Center for High Depth Omics, Medical Institute of Bioregulation, Kyushu University, 3-1-1 Maidashi Higashi-ku, Fukuoka, 812-8582, Japan.
³ Division of Metabolomics/Mass Spectrometry Center, Medical Research Center for High Depth Omics, Medical Institute of Bioregulation, Kyushu University, 3-1-1 Maidashi Higashi-ku, Fukuoka, 812-8582, Japan.
⁴ Division of Analytical and Measuring Instruments, Shimadzu Corporation, 1 Kuwabaracho Nishinokyo Nakagyo-ku, Kyoto, 604-8511, Japan.
⁵ Mass Spectrometry Laboratory, Graduate School of Nanobioscience, Yokohama City University, 22-2 Seto, Kanazawa-ku, Yokohama, Kanagawa, 236-0027, Japan.
⁶ Department of Systems Life Sciences, Graduate School of Systems Life Sciences, Kyushu University, 3-1-1 Maidashi Higashi-ku, Fukuoka, 812-8582, Japan; Division of Metabolomics/Mass Spectrometry Center, Medical Research Center for High Depth Omics, Medical Institute of Bioregulation, Kyushu University, 3-1-1 Maidashi Higashi-ku, Fukuoka, 812-8582, Japan. Electronic address: bamba@bioreg.kyushu-u.ac.jp.

PMID: 36764769
DOI: 10.1016/j.aca.2023.340863

Abstract

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.

Keywords: Atmospheric pressure ionization; Carbon dioxide; Mass spectrometry; Supercritical fluid chromatography.