Electrospray ionization and atmospheric pressure ionization mass spectrometry of stable organic radicals

The ionization behavior of stable radicals (1-8) was studied under electros pray ionization (ESI) and atmospheric pressure ionization (APCI) conditions . With APCI, using methanol + water as the solvent, most radicals were prot onated to give the radical cation [M + 1](+.), whereas TEMPO (5) was detect ed 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 a cid, stable radicals (5-7) gave [M + 1](+.),(2-4) M+, whereas (1) and (8) s howed the [M + 2](+) ion as the base peak. Using the ESI source as a contro lled-current electrolytic cell, radicals (2-8) were efficiently oxidized to give + ions, Radical (1) was detected as the [M + 2](+) ion. Detection lim its for the radicals (1-8) were between 3 x 10(-9) mol L-1 and 5 x 10(-7) m ol L-1 using APCI or ESI in the full-scan mode. In most cases detection lim its were lower using the tandem mass spectrometry (MS/MS) full-scan mode.