2.1 Apparatus structure
Our API-MRTOF-MS for molecular analysis consists of two subsystems. The API subsystem, modified from our API-TOF44, 45, is designed for high-efficiency transport of ions from atmospheric ion sources. The API is composed of a heated stainless-steel capillary to separate atmospheric pressure and reduce depositions, a small radio-frequency-only quadrupole (RFQ) labeled MIR46, 47 in Fig. 1, and two larger quadrupoles serve as RFQs 48 (RFQ1 and RFQ2) to cool and transport the ions, providing efficient ion transport with the capability of mass selection to improve the sensitivity for low-abundance materials. RFQ1 is designed with the ability to bunch ions, as introduced in Ref.49. Through cooperating with the following ion trap, bunch mode could theoretically improve the duty cycle to 100% and supply an alternative ion selection scheme. RFQ2 serves as a quadrupole mass filter and can assist MS/MS experiments. Moreover, the API offered great flexibility in the choice of the ion source.
The following MRTOF subsystem consists of a flat ion trap (FLT)50, an accelerating einzel lens (AC), a double steerer unit (ST), a set of drift tubes (DT), and the MRTOF analyzer. In the symmetric coaxial MRTOF analyzer, the mirror electrodes are aligned coaxially by eight independent gridless ring-mounted electrodes. Low-pass RC filters are applied to improve the stabilization of the voltages at the mirror endcap electrodes (1 MΩ, 4–8 μF). The axisymmetric electrostatic field made by MRTOF analyzer is presented in Fig. 1. MRTOF-MS uses the concept of recycling the path length in a time-of-flight analyzer for multiple reflections51. The components of the MRTOF analyzer are described in great detail in Refs.42, 52. Unlike the design in these references, we designed a shorter DT and moved the MRTOF analyzer closer to the FLT. This helps to expand the mass range of ions injected into the analyzer, but it will increase the gas load in the analyzer. The symmetric coaxial mirror electrode arrangement gives the analyzer more potential for higher resolution.