Vacuum Technology Know How - Triumf

Formula 4-7
Shot orifice
Formula 4-8
Shot angle
Formula 4-9
Maximum acceleration
voltage U zmax
Formula 4-10
HF power
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The filter thus acts as a high pass. As HF amplitude (V ) increases, ever-heavier types of ions
become unstable, starting with the light masses, and are thus sorted out. This operating
mode produces an integral spectrum.
The shot conditions are crucial for transmission of ions through the filter.
Ions parallel to the rod system must be shot in within the following diameter:
1 M
D = . r0 .
2 �M
The maximum shot angle must satisfy the condition:
tgψ < 11.85 .
2 r0 L 2
and the energy must be as uniform as possible. The advantages of the Pfeiffer Vacuum ion
sources described in 4.1.4.1 translate into high transparency and thus high sensitivity.
In order for the amplitudes of the unstable ions to become large enough to strike the rods,
where they are neutralized, these ions must perform a minimum number of oscillations in the
separating field. The following equation applies for the maximum acceleration voltage in the
Z direction:
kg �M
U zmax � 4.2 . 10 - 6 . L 2 . f 2 . M .
A . s M
In practical operation, the ratio U / V is activated as a function of the mass number in such a
manner that the actual resolution �M / M, does not remain constant, but that instead the line
width �M remains constant. This means that resolution increases proportionally to the mass
number. Due to Formula 4-5 (V is proportional to M ), the quadrupole (as opposed to
the sector field mass spectrometer) produces a linear mass scale.
One point of major significance for a QMS is the required HF power. If C is used to designate
the entire capacity of the system and Q to designate the factor quality of the power circuit,
the required HF power
C
N � HF . M 2 . f 5 . r0 Q
will increase with high powers of f and r 0 . An enlargement of field radius r 0 will lessen the
occurring relative mechanical tolerances, thus resulting in improved behavior. Essentially, it is
advantageous to select f and r as large as possible. However there are limits in this regard
0 0
due to the associated increase in HF power (Formula 4-10). While extending the rod system
permits a lower operating frequency, the size of a production unit should not exceed certain
dimensions.
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