Sensitivity and selectivity of switchable reagent ion soft chemical ionization mass spectrometry for the detection of picric acid

We have investigated the reactions of NO⁺, H₃O⁺, O₂⁺, and Kr⁺ with picric acid (2,4,6 trinitrophenol, C₆H₃N₃O₇, PiA) using a time-of-flight mass spectrometer with a switchable reagent ion source. NO⁺ forms a simple adduct ion PiA·NO⁺, while H₃O⁺ reacts with PiA via nondissociative proton transfer to form PiAH⁺. In contrast, both O₂⁺ and Kr⁺ react with PiA by nondissociative charge transfer to produce PiA⁺. For Kr⁺, we also observe dissociation of PiA, producing NO₂⁺ with a branching percentage of approximately 40%. For the reagent ions H₃O⁺ and O₂⁺ (and operating the drift tube with normal laboratory air), we find that the intensities of the PiAH⁺ and PiA⁺ ions both exhibit a peak at a given drift-tube voltage (which is humidity dependent). This unusual behavior implies a peak in the detection sensitivity of PiA as a function of the drift-tube voltage (and hence E/N). Aided by electronic-structure calculations and our previous studies of trinitrotoluene and trinitrobenzene, we provide a possible explanation for the observed peak in the detection sensitivity of PiA.