Analytical potential of rf-PGD-TOFMS for depth profiling of an oxidized thin film composite

Abstract

The capabilities of radiofrequency pulsed glow discharge time of flight mass spectrometry (rf-pulsed-GD-TOFMS) for the analysis of thin film composite membranes have been investigated in this work. These semi-permeable membranes, used for water purification and desalinization, consist of a thin film of polyamide on top of a polysulfone porous layer deposited on a support sheet of polyester. The analysed samples were four oxidized swatches obtained using NaClO or ClO₂ (diluted in seawater) as oxidizing agents, and a non-oxidized virgin membrane. The performance of the rf-pulsed-GD-TOFMS prototype was investigated for the detection of both positive and negative analyte ions. Improved depth profiles could be achieved using an Ar pre-chamber that prevented the entry of air in the discharge. The detection of polyatomic ion signals was needed to monitor properly the transition between the subsequent sputtered membrane layers. In particular, NH₃⁺ and ¹²C¹⁴N⁻ polyatomic ions were mainly found at the polyamide layer (in the positive and negative detection modes, respectively), while ³⁴S⁺ and ³³S¹H⁻ ion signals appeared in the polysulfone layer. Qualitative depth profiles of the oxidized samples showed an intense Br ion signal in the polyamide layer. Moreover, the Br ion signal was significantly enhanced when NaClO was used as the oxidizing agent, in agreement with previous studies. It is considered that the incorporation of Br into the membranes is a consequence of its presence in seawater and of the oxidation process.