Proton-transfer-reaction (PTR) mass
spectrometry (MS) is capable
of detecting trace-level volatile organic compounds (VOCs) in gaseous
samples in real time. Therefore, PTR-MS has become a popular method
in many different study areas. Most of the currently reported PTR-MS
applications are designed to determine volatile compounds. However,
the method might be applicable for nonvolatile organic compound detection.
Supercritical fluid chromatography (SFC) has been studied in the last
5 decades. This approach has high separation efficiency and predictable
retention behavior, making separation optimization easy. Atmospheric
ionization techniques, such as atmospheric chemical ionization (APCI)
and electrospray ionization (ESI), are the most studied SFC-MS interfaces.
These processes require the addition of makeup solvents to prevent
precipitation or crystallization of the solute while depressurizing
the mobile phase. In contrast, the PTR process is carried out in a
vacuum; supercritical carbon dioxide may release solute into the PTR
flow tube without a phase transition as long as it is maintained above
a critical temperature. Therefore, this might constitute yet another
use for the SFC-MS interface. Caffeine and a few other nonpolar compounds
in supercritical carbon dioxide were successfully detected with time-of-flight
MS without adding solvent by using preliminarily assembled supercritical
flow injection and supercritical fluid extraction (SFE)-PTR interfaces.
We have developed a rapid and sensitive analytical method for α-tocopherol and its oxidative products by combining online hyphenation of supercritical fluid extraction-supercritical fluid chromatography (SFC) with proton transfer reaction (PTR) ionization mass spectrometry (MS). α-Tocopherol is a well-known antioxidant that plays a vital role in the antioxidant defense system in plant cells. However, studies on the cellular mechanisms of α-tocopherol have been limited owing to the lack of a rapid analytical method, which limits the comparison of plant cells incubated in various conditions. Additionally, complex sample preparation and long chromatography separation times are required. Moreover, the majority of the involved molecules are a combination of isomers, which must be separated before applying tandem MS. α-Tocopherol produces the α-tocopheroxyl radical in the first step of its antioxidant function; another ion with the same mass may also be generated from the source. SFC separation effectively distinguished the observed ions from their oxidative products in the sample and those produced during the ionization reaction process. This method enabled the measurement of α-tocopherol and its oxidative products such as α-tocopheroxyl radical and αtocopheryl quinone in approximately 3 min per sample, including the time required for sample preparation.
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