Evidence of Reactivity in the Membrane for the Unstable Monochloramine during MIMS Analysis

Abstract

Membrane Inlet Mass Spectrometry (MIMS) was used to analyze monochloramine solutions (NH2Cl) and ammonia solutions in a compact FTICR. Chemical ionization enables identification and quantification of the products present in the permeate. The responses of protonated monochloramine and ammonium increase linearly with the solution concentration. The enrichments were respectively 1.2 and 5.5. Pervaporation is dependent on pH and only the basic form of ammonia NH3 pervaporates through the membrane. Unexpectedly, the small ammonia molecule permeated very slowly. It could be due to interactions with water molecules inside the membrane that create clusters. Moreover, NH2Cl solutions, in addition to the NH3Cl+ signal, presented a strong NH4+ signal at m/z 18.034. Ammonia presence in the low-pressure zone before ionization is probable as NH4+ was detected with all the precursors used, particularly CF3+ and trimethylbenzene that presents a proton affinity higher than monochloramine. Ammonia may be formed inside the membrane due to the fact that NH2Cl is unstable and may react with the water present in the membrane. Those results highlight the need for caution when dealing with chloramines in MIMS and more generally with unstable molecules.

Tracking Monochloramine Decomposition in MIMS Analysis

“…Figure 3 shows the mass spectrum of the NH 2 Cl aqueous solution at a concentration of 10 mg L −1 (as Cl 2 ). This spectrum agreed well with that reported by Louarn et al [11]. The dominant peak at m/z 19.018 represents the H 3 O + reagent ion signal, and the secondary peak at m/z 121.102 represents the C 6 H 3 (CH 3 ) 3 H + protonated trimethylbenzene.…”

“…This suggests that the detected NH 4 + resulted from decomposition of monochloramine into ammonia inside the MIMS instrument. A similar conclusion was reported recently by Louarn et al, who suggested that this decomposition takes place across the membrane, since the low pressure inside the mass spectrometer source prevents adsorption and degradation reactions downstream of the membrane [11]. Monochloramine decomposition on membrane has never been reported in other studies.…”

“…contrary to our expectations, the acquired mass spectrum also had an intense ion at m/z 18.034, which is attributed to the NH 4 + . The recent study by Louarn et al suggests that this ion results from monochloramine decomposition inside the membrane [11]. This decomposition has never been reported in other studies.…”

“…In the present research, the membrane inlet and ICR cell were spaced at more than 70 cm distance. Louarn et al estimated that this phenomenon should not be observed, due to the low pressure inside the ICR cell [11]. The total pressure measured by Hansen et al inside their ion source was 0.7 mTorr [28].…”

“…The effect of sample flow rate was investigated at 1, 10, and 100 mL min −1 (at 22 • C membrane temperature). MIMS signals were examined at six temperatures: 20, 30, 40, 50, 60, 70, and 80 • C. Using a PDMS membrane similar to that used in this research, Louarn et al reported that steady state was not reached after 15 min of analysis; the NH 2 Cl solutions were therefore circulated through the membrane for 45 min, to equilibrium response [11].…”