2.3 Vacuum system
As stated above, the ion flight distance shall be extended, which in
turn requires a long mean free path length of the ions.
\(\lambda\ =\ \frac{kT}{\sqrt{2}\sigma p}\),
with the gas pressure p , the collision cross-section σ ,
the temperature T , and the Boltzmann’s constant k . Here,
the mean free path is inversely proportional to pressure
(\(\lambda\propto 1/p\)); thus, the analyzer chamber must be maintained
in an ultra-high vacuum to reduce the collision rate and prolong the ion
flight length.
Compared with the ZD-MRTOF-MS at SLOWRI, the vacuum system of the
API-MRTOF-MS became more complex. A large acceptance is required to
achieve high transmission efficiency with the continuous sampling from
atmospheric ionizations. Considering the high vacuum analyzer chamber,
the differential vacuum system that covers a massive pressure difference
is precisely designed using gas-flow calculations.
The Knudsen number (Kn ) and the Reynolds number
(Re ) are calculated to characterize the type of
gas flow (see Ref. 53page 14). Then, the conductance of a orifice and a long pipe in laminar
flow and molecular flow are well described by simple formulas in Ref.53.
In a balanced system, the gas flow \(Q\) is then given by
\(\operatorname{Q}=(\operatorname{}-\operatorname{})\times C=\operatorname{}\times
S\),
which is proportional to the pressure difference of neighboring domains\((\operatorname{}-\operatorname{})\), with the flow conductanceC , and is equal to the product of the pressure of next stage
vacuum P1 and the pumping speed of a vacuum pumpS .
We translate the theoretical concepts, calculations, and requirements
into a dedicated design.
As
a prototype, we prefer to use the previous components and chambers. The
vacuum system for the API-MRTOF-MS includes four metal chambers, as
shown in Fig. 4. These chambers are custom made of cast aluminum alloy
and sealed by “O-ring” seals (Fluoroelastomer). This design is solely
for the prototype and not a true ultra-high vacuum system, but it still
achieves the required vacuum level. Our miniature MRTOF-MS will make use
of the improved vacuum design.