N-Nitrosopiperazines form at high pH in post-combustion capture solutions containing piperazine: a low-energy collisional behaviour study.

During the process of exploring aqueous piperazine chemistry under simulated flue-gas scrubbing conditions, positive-ion electrospray ionisation mass spectrometric (ESI-MS) analyses of the resulting reaction mixtures in a triple quadrupole system revealed the presence of peaks at m/z 116 and 145, the putative N-nitroso derivatives of piperazine. Confirmation of the presence of these species in the reaction mixtures was achieved using collision-induced dissociation experiments. A purchased standard, together with in-house synthesised N-nitrosopiperazine standards (including N-nitroso derivatives derived from deuterium-labelled precursor materials), were used for this purpose. Across a small range of collision energies, large fluctuations in the abundance of the two major product ions of protonated N-nitrosopiperazine, m/z 86 and 85, were observed. Using B3LYP/6-311 + +G(d,p) computations, the potential energy surface was determined for loss of NO and [H,N,O]. At an activation energy slightly in excess of 1 eV, intramolecular isomerisation precedes loss of NO (m/z 86) via a 4,1 H-shift, and at activation energies between 2.1-2.3 eV, consecutive loss of NO and atomic hydrogen competes with the direct loss of nitrosyl hydride (m/z 85). It is recommended that any multiple reaction monitoring method for quantifying N-nitrosopiperazines at low collision energies use the sum of both transitions (m/z 116 ← 85, m/z 116 ← 86) to avoid errors that could be introduced by subtle changes in ES source conditions or collision voltages. This approach is adopted in an HPLC/MS/MS method used to monitor the degradation of N-nitrosopiperazine exposed to (i) broad-band UV light and (ii) heat typical of an amine regeneration (stripper) tower. The results reveal that aqueous N-nitrosopiperazine is thermally stable at 150°C but will degrade slowly upon exposure to UV light.