One of the recurring practical challenges we face in PTC process development is performing analysis of our products at the end of the PTC reaction to assure that there is no phase-transfer catalyst residue in our products. This is especially true for analyzing samples for the presence of tetraalkyl quaternary ammonium salts since they have no chromophores for typical detector in HPLC and they are not volatile to go through a GC injection port, though they do break down in injector ports tp form trialkylamines that can sometimes be analyzed.
A patent was issued last week that appears to address the issue of analyzing for tetraalkyl quaternary ammonium cations. I must admit that I am not an expert in mass spectrometry. I know just enough about MS to understand that cations are hurled through a magnetic field and are separated by their mass that determines their trajectory on a curve with a radius depending on the mass of the cation.
Tetraalkyl ammonium quats are already cations and don’t need to be bombarded by anything to form cations. Accordingly, the invention described in Cooks, R.; Baird, Z.; Wei, P.; (Purdue Research Foundation) US Patent 10,720,316, 21-Jul-2020, utilizes the concept used in mass spectrometry to analyze for tetraalkyl quaternary ammonium cations. I have read the patent a couple of times and, frankly, I need someone to explain it to me in layman’s terms.
A key concept in the patent is that “the invention provides sample analysis systems that are configured to analyze ions at or above atmospheric pressure and without the use of laminar gas flow.”
The preferred analyzer appears to consist of three curved electrodes separated from each other by a non-conductive spacer and at least one of the three curved electrodes includes an opening through which a probe may be inserted.
A solution of 10 microM each of tetrapropyl-, tetrabutyl-, tetrahexyl-, and tetradodecylammonium bromide in acetonitrile was analyzed by performing separation of the ions in air without the use of a vacuum or a flowing gas. Instead, pulsed voltages were employed with the multi-electrode system as a means to inject ions into the curved ion path and effect a separation of the tetraalkylammonium cations.
The patent contains many diagrams of the analyzer configuration as well as graphs and pictures of the analysis of the tetraalkyl ammonium cations.
If you have faced the challenge of analyzing residual quat in your product, you may want to forward this patent to the creative problem solvers in your analytical department to see if they can utilize this information, or buy a piece of equipment that can perform such analysis.
