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.