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