Electrospray Ionization and Atmospheric Pressure Ionization Mass Spectrometry of Stable Organic Radicals

The ionization behavior of stable radicals (1–8) was studied under electrospray ionization (ESI) and atmospheric pressure ionization (APCI) conditions. With APCI, using methanol + water as the solvent, most radicals were protonated to give the radical cation [M + 1]^+•, whereas TEMPO (5) was detected as the M⁺ ion and galvinoxyl (1) gave the [M + 2]⁺ ion. Using benzene as the solvent, all radicals (1–8) were efficiently oxidized to give M⁺ as the base peak. Under ESI conditions using methanol + water with 1% acetic acid, stable radicals (5–7) gave [M + 1]^+•, (2–4)M⁺, whereas (1) and (8) showed the [M + 2]⁺ ion as the base peak. Using the ESI source as a controlled-current electrolytic cell, radicals (2–8) were efficiently oxidized to give M⁺ ions. Radical (1) was detected as the [M + 2]⁺ ion. Detection limits for the radicals (1–8) were between 3 × 10⁻⁹ mol L⁻¹ and 5 × 10⁻⁷ mol L⁻¹ using APCI or ESI in the full-scan mode. In most cases detection limits were lower using the tandem mass spectrometry (MS/MS) full-scan mode.