Steady-state asymmetric nanospray dual ion source for accurate mass determination within a chromatographic separation

Here we report the design, implementation, and initial use of an asymmetric steady-state continuous dual-nanospray ion source. This new source design consists of two independently controlled and continuously operating nanospray interfaces with funnel shaped counter electrodes. A steady-state ion mixing region combines the ions from the two sources into a single ion beam in the intermediate region after ion extraction from the nanospray sources but before the bulk of the pressure gradient of the vacuum interface. With this design we have achieved robust mixing of ions with no loss of duty cycle and remarkable ionization characteristics that appear entirely noncompetitive and potentially beneficial. This allows continuous introduction of internal mass calibration ions during a liquid chromatography-mass spectrometric analysis. This in turn allows for recalibration of individual spectra yielding sub part per million mass accuracy throughout the run. The steady-state approach presented here has several advantages over previous approaches. Since neither the voltage nor positions of the sprayers are changed, the nanospray has greater spray stability. The ions produced by the analyte sprayer are continuously sampled, as opposed to time-sharing which necessitates that the analyte ion stream be interrupted for some part of the duty cycle. There are no moving parts, no rapid changes to high voltages requiring additional control electronics, and no need for completely separate vacuum interfaces and the associated complexity. The sprayers are independently controlled and do not exhibit competition or mutual ionization suppression. This novel source has been implemented with a Bruker Apex II 9.4 T FTICR with a modified Apollo electrospray ion source as part of a nanoflow liquid chromatography-Fourier transform ion cyclotron resonance mass spectrometry analysis platform. Because of the low cost of implementation, the new source could potentially be applied to other forms of mass spectrometry, such as electrospray ionization-time-of-flight (ESI-TOF), which can benefit from internal mass calibration.