Direct detection of diphenylamino radical formed by oxidation of diphenylamine using APCI-MS.

RATIONALE Diphenylamine (DPAH) and its derivatives have been widely used as antioxidants. Its antioxidation behaviors are started to form diphenylamino radical (DPA radical, DPA• ) by loss of hydrogen atom through reaction with oxygen or ozone. DPAH was oxidized by ozonation to produce DPA• and the DPA• was directly detected using APCI-MS. Influence of the oxidation time on generation of the DPA• and consumption of DPAH was also investigated. METHODS Experiments were performed using the home-made ozone bubbling apparatus for oxidation of DPAH. The 1,000 ppm DPAH solution in acetone was stirred during the ozonation. The oxidized sample was diluted 100 fold and analyzed using APCI-MS. Structures and energies of DPAH, DPA radical, and detected ions were obtained by electronic structure calculations using Spartan'10 and Gaussian09. RESULTS The DPA• was detected in the positive APCI-MS as a form of DPAH•+ by proton transfer reaction with protonated acetone. Formation of the DPAH•+ from DPA• was more favorable than that of the [DPAH + H]+ from DPAH and it was also much more favorable than that of the DPA+ from DPA• . By increasing the oxidation time, relative abundance of the DPAH•+ was increased whereas that of the [DPAH + H]+ was notably decreased. CONCLUSIONS Existence of the DPA• was directly analyzed using APCI-MS. The DPAH and DPA• were differently detected as [DPAH + H]+ and DPAH•+ , respectively. Direct APCI-MS analysis was a suitable technique for identification and quantitation of the DPA radical.