Challenges in Detecting Hydroxyl Radicals Generated in Water Droplets with Mass Spectrometry

Water is sprayed into the air using three distinct methods: (1) nanoelectrospray ionization, (2) a vibrating membrane nebulizer, and (3) a pneumatic nebulizer. The resulting droplets are analyzed by three different mass analyzers: (a) a linear ion trap mass analyzer on an <i>LTQ-XL</i> mass spectrometer, (b) the <i>Velos Pro</i> dual-pressure linear ion trap mass analyzer on an <i>Orbitrap Elite</i> mass spectrometer, and (c) the Orbitrap mass analyzer on the same <i>Orbitrap Elite</i> system. We searched for hydroxyl radical adducts with hydronium ions (OH•-H₃O⁺) or reaction products with caffeine dissolved in water and with melatonin dissolved in water. These experiments were repeated in several different laboratories, and all results were the same. The oxidation products of caffeine and melatonin were not detected when using the Orbitrap mass analyzer having a much longer holding time in the ion trap (500 ms) but could be observed with reduced intensity at much shorter holding times (<10 ms). The signal of OH•-H₃O⁺ was also significantly reduced when using the <i>Velos Pro</i> dual-pressure linear ion trap mass analyzer. These results suggest that ion signals from fragile radicals may be diminished or lost depending on the mass detection system and operating conditions employed, and there may be a risk of obtaining spurious results when using an Orbitrap mass spectrometer.