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.