Ionization Source
The ESI operating voltage was 3800 V, the gas pressure was 0.4 MPa (N2 standard gas with purity ≥ 99. 999%, Guangzhou Yuejia Gas), and the inlet temperature was 180 °C. With the strong electric field between the liquid and the inlet capillary of the interface, a so-called Taylor cone was formed. The instable charged droplets evaporated from the tip of the cone and drift to the inlet. Coulomb fission of the droplet happened as its radius approaches the Rayleigh limit 54, resulting in gas-phase ions. The heated inlet capillary efficiently improved the ionization process and kept neutral gas away.
3.2 Sample Preparation and Data Acquisition
Cesium ions with a nominal average molecular weight of 133 Da were used to test the characteristics of the API-MRTOF-MS. Solutions were prepared in acetonitrile at different concentrations (CsI, Sigma-Aldrich) and infused at a flow rate of 5 μL min–1 using a syringe pump (Harvard Pump 11 Elite, Harvard).
Ion signals were acquired with an oscilloscope (500 MHz, Keysight DSOX3052A, Keysight) with 2 ns resolution and subsequently transferred to a computer for display analysis.
3.3 MS measurement pattern
As described in Section 2, at this stage in the experiment, the API subsystem works in a continuous transport pattern; all three quadrupoles are RFQs that work as ion coolers, cooling ions by collision with residual gases without selection or bunch function, bunch function is not enable. The ions are cooled by collision with lighter buffer gases for extra 5 ms at a pressure of ~7 Pa. The transfer section following the FT is similar to that in Ref.42 but without the pulsed drift tube. The measurement cycle is triggered by a free-running timing system.
Results and discussion