Detection and Behaviors of TEMPO Derivatives in Seven Mass Spectrometry Ionization Methods

Four 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) derivatives with molecular weights of 339-1131 Da were synthesized to investigate their ionization behaviors in fast atom bombardment (FAB), electron ionization (EI), direct analysis in real time (DART), electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI), and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). These include 4-dodecanoylamino-2,2,6,6-tetramethyl-1-piperidinyloxy (1), 4-dodecylamino-2,2,6,6-tetramethyl-1-piperidinyloxy (2), N,N'-bis(2,2,6,6-tetramethyl-1-oxyl-4-piperidinyl)dodecanediamide (3), and bis-cholesterol TEMPO derivative 4. For TEMPO derivatives 1 and 2 (molecular weights: 353 Da and 339 Da), [M]⁺, [M+H]^+•, and [M+2H]⁺ peaks were detected. TEMPO derivative 3, containing two TEMPO moieties, showed [M]^+•, [M+H]⁺, [M+2H]^+•, and [M+3H]⁺ peaks. EI was optimal for smaller derivatives, as molecular ion peaks were predominant. For larger analytes, molecular ion intensities weakened, favoring hydrogen adduct peaks. EI and DART-MS failed to detect bis-cholesterol TEMPO derivative 4. Under APCI and APPI-MS, N-O bond cleavage was observed. MALDI-MS detected only hydrogen adduct peaks. FAB, ESI, APCI, and APPI-MS detections varied with scan numbers, unlike EI, DART, and MALDI-MS.