4.1 Testing of the vacuum system
The vacuum system is redesigned to offer an ability to couple with
atmospheric ionizations while preserving the ultra-high vacuum inside
the MRTOF chamber. We recorded the pressure change in the analyzer using
a cold cathode gauge (PKR251, Pfeiffer) with respect to the pumping time
at a heating temperature of 100 ℃, which corresponds to about 60 ℃ on
the chamber.
The pumping process depicted in Fig. 5 is described by an exponential
reduction in pressure, with a slower decreasing trend after the pumping
period exceeds 9 hours. For metal vacuum chambers, the desorption of
water vapor and adsorbed gases and the outgassing from seals lengthen
the pump-down time 53.
The process can be shortened by baking out the chambers. After long-time
baking, the best pressure in the analyzer reached up to 6.8 ×
10–6 Pa for our API-MRTOF-MS, corresponding to a mean
free path of 2 km. The result indicates that a baking time of several
days is necessary to achieve high vacuum.
Fig. 5. Experimental results of pressure in analyzer as a function of
pumping time.
As we reduced the length of the DT between the FLT and the MRTOF
analyzer in the design, the analyzer pressure changes must be tested
with different cooling gas pressures. We regulated the pressure in the
FLT by manually adjusting the intake gas valve. The pressure variation
was recorded and compared with the results calculated by the method
mentioned in Section 3.
As illustrated in Fig. 6, the FLT outer chamber pressure (Fig. 6A) and
the DT chamber pressure (Fig. 6B) increase with the buffer gas pressure,
showing a similar tendency with the calculation results. While the
experimental results do not change much in comparison with the
calculation results, according to the different scales of y-axes, the
calculated pressures are about five to six times higher than those in
the experiment. In contrast in
Fig. 6C, the experimental data differs from the calculated trend. The
buffer gas injection process appears to have no effect on the analyzer
pressure which is mainly due to the elastomer sealing components
(“O-ring”) we used. The diffusion through elastomer seals and their
desorption rates caused limitations of the vacuum. These considerations
will be used in future vacuum chamber designs to realize an improved
vacuum. The differential vacuum system was carefully arranged with a
pressure of 6.8 × 10–6 Pa.
Fig. 6. Measured pressures for different buffer-gas pressures in the
FLT. Comparison of calculation and experimental results of the pressure
outside the FLT (A), the pressure in the drift tube chamber (B), and the
pressure in the MRTOF analyzer (C).