A Dedicated Mass Spectrometer Tuning Method for SFC–MS and Evaluation of Two Different Linear Restrictor Types Used in the Operation of a Split Flow Pre-Back Pressure Regulator SFC–MS Interface
Abstract The operation of a SFC–MS system has been improved. The SFC–MS interface uses a post-column make-up flow of liquid whose supply line to the SFC–MS interface mixing tee is equipped with an injection valve. Prior to analysis, this arrangement permits a stable flow of a dilute tune solution of analyte(s) for the exact amount of time required to optimize mass spectrometry tuning. Advantageously, with this arrangement tuning is performed in the presence of the exact split flow and same composition of SFC mobile phase and make-up liquid to be used for subsequent SFC–MS operation. The SFC–MS interface was constructed to provide a pre-back pressure regulator split flow to the mass spectrometer using a linear restrictor. Two different linear restrictors constructed from either 50 µm i.d. PEEK tubing or 25 µm i.d. fused silica tubing were evaluated. For best electrospray performance, each restrictor required a different post-column make-up flow rate. The use of either restrictor retained a high degree of chromatographic fidelity for the SFC separation of six pharmaceutical compounds using a cyanopropyl column with a mobile phase of supercritical fluid carbon dioxide modified with methanol and aqueous ammonia solution. Each restrictor provided a different level of SFC–MS performance for each of the six pharmaceutical compounds tested. Based upon overall performance, a linear fused silica restrictor was selected to develop a chromatographic separation using on-line SFC–MS for three structurally very similar monensin sodium salt analogues that are not directly amenable to off-line SFC using UV/Vis detection. Graphical abstract
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