Selected ion flow tube mass spectrometry analyses of isobaric compounds methanol and hydrazine in humid air

Abstract

Rationale: The volatile compounds generated by the electrochemical reduction of atmospheric carbon dioxide and nitrogen include isobaric methanol (CH₃ OH) and, potentially, hydrazine (N₂ H₄ ). To achieve quantification of hydrazine molecules by selected ion flow tube mass spectrometry (SIFT-MS), its reactions with H₃ O⁺ , NO⁺ and O₂ ⁺ reagent ions must be understood.

Methods: A SIFT study (using a SIFT-MS instrument) was carried out to obtain rate coefficients and product ions for the reactions of H₃ O⁺ , NO⁺ and O₂ ⁺ reagent ions with N₂ H₄ and CH₃ OH molecules present in the humid headspace of their aqueous solutions. Using the kinetics data obtained, solution headspace concentrations were determined for both compounds as a function of their liquid-phase concentrations at 10, 20 and 35°C.

Results: Both compounds react with H₃ O⁺ ions via rapid proton transfer to produce CH₃ OH₂ ⁺ and H₅ N₂ ⁺ ions with the common m/z value of 33. It is revealed that NO⁺ rapidly transfers charge to N₂ H₄ (rate coefficient k = 2.3 × 10^-9 cm³ s^-1 ) but only slowly associates with CH₃ OH (k_2eff = 7.1 × 10^-11 cm³ s^-1 ). Thus, selective analysis can be achieved using both H₃ O⁺ and NO⁺ reagent ions. The headspace methanol vapour concentration was found to increase with increasing solution temperature, but that of hydrazine decreased with an associated increase of ammonia (NH₃ ) as measured with O₂ ⁺ reagent ions.

Conclusions: The isobaric compounds methanol and hydrazine can be separately analysed in real time by SIFT-MS using H₃ O⁺ and NO⁺ reagent ions, even when they co-occur in humid air. The evolution of hydrazine from aqueous solutions can be quantitatively monitored together with its decomposition at elevated temperatures.