Pulsed capillary introduction applied to a miniature mass spectrometer for efficient liquid analysis.

Abstract

Capillary sampling of liquids for direct mass spectrometry (MS) analysis is introduced. The low transfer rate of the solution in the capillary will affect the analytical sensitivity and the response time; hence a pulsed capillary introduction (PCI) method was proposed and characterized. The experiments were carried out using a miniature quadrupole mass spectrometer, and liquid can be spontaneously drawn into the vacuum chamber for subsequent ionization and detection. A simple up-and-down motor platform was used to control the brief contact of the capillary inlet with the liquid sample and implement pulsed injection. The pulsed sampling parameters were optimized based on the characterization and dynamic study of liquid transfer in capillaries. Compared with continuous capillary introduction (CCI), PCI can reduce the response time of MS analysis from more than half a minute to a few seconds. In addition, it provides better detection sensitivity as the ion signals of all solution components are enhanced and the acquired limit of detection (LOD) of toluene is about eight times lower than CCI analysis. For each analysis, the consumed sample volume is only a few nanoliters and the absolute consumption of the analyte can reach the femtogram level. The proposed PCI method is proved to be successful in improving the sampling efficiency when performing direct liquid analysis without increasing the vacuum load. A miniature MS instrument with a proper capillary inlet can possess flexible operation modes to meet different application demands.