18.104.22.168 Quadrupole mass filter
The filter system of a quadrupole mass spectrometer consists of four parallel rods arranged in the form of a square. Each pair of opposite rods in Figure 4.4, designated (+) or (−), is connected with the other. Voltage
Quadrupole deflection voltage
is applied between the two pairs of rods.
Ideal quadrupole fields require rods that have a hyperbolic profile. In actual practice, however, round rods are used, with the rod radius being equal to 1.144 times the field radius r0. An electrical quadrupole field is formed between the rods. Ions of varying mass are shot axially into the rod system at approximately equal energy and move through the rod system at uniform velocity. The applied quadrupole field deflects the ions in the X and Y directions, causing them to describe helical trajectories through the mass filter. To solve the movement equations, the dimensionless variables
Stability parameter a
Stability parameter q
are introduced to obtain Mathieu's differential equations. Their solutions yield the stable area with oscillation amplitudes of less than r0 beneath the triangle formed by the two solubility curves in Figure 4.5. The values ap = 0.23699 and qp = 0.706 apply for the apex of the triangle. All solutions outside result in increasing oscillation amplitudes and thus in neutralization of the ions on the rods of the quadrupole filter.
Dividing the two equations by one another yields: .
This is the pitch of the so-called load line of the mass filter.
From Figure 4.5, it can be seen that:
- All ions whose parameters a and q are located in the triangle above the load lines will reach the detector
- The ions will only reach the detector under voltage conditions < 0,1678.
Introducing the ratio between the atomic mass unit 1 amu = 1.66055 · 10-27 kg and the elementary charge and multiplying it by the dimensionless mass number M of the corresponding ion yields the following conditions for U and V for the apex of the stability triangle:
Stability condition U
Stability condition V
With the DC voltage de-energized, U = 0, all trajectories of the ions where q < 0.905 will be stable; according to Formula 4-5, these will all be masses where
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
The maximum shot angle must satisfy the condition:
and the energy must be as uniform as possible. The advantages of the Pfeiffer Vacuum ion sources described in 22.214.171.124 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:
Maximum acceleration voltage Uzmax
In practical operation, the ratio U/V is activated as a function of the mass number in such a manner that the actual resolution ΔM/M, does not remain constant, but that instead the line width ΔM 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
will increase with high powers of f and r0. An enlargement of field radius r0 will lessen the occurring relative mechanical tolerances, thus resulting in improved behavior. Essentially, it is advantageous to select f0 and r0 as large as possible. However there are limits in this regard 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.
The required mass range and desired resolution are governed by the dimensions of the filter and the selection of the operating frequency. Devices with 6, 8 and 16 mm rod diameters and appropriately matched electronics are available to satisfy the respective requirements.
What follows is a brief digression on the relationship between resolution and mechanical precision. Let us consider a quadrupole mass filter that works at the apex of the stability diagram; i.e. that works at high resolution.
applies for the DC amplitude and
for the AC amplitude.
Here, M designates the mass of the ion, r0 the field radius and f the frequency at which the filter is operated. We are making the idealized assumption that both voltages U and V, as well as frequency f, can be set and maintained “as precisely as desired”.
What follows from this is: (ck is a constant), and following differentiation and division by M, the filter scatter caused by r0 is:
Let us assume that the field radius r0 changes by Δr0 =
0.03 mm over the length of the mass filter. Now let us consider the effect of this change on scatter
ΔM/M for two mass filters of different sizes. For optimal transmission, the resolution
set on the spectrometer (we select: ΔM/M = 1/100) must be greater than the scatter generated
by the fluctuation of r0:
For a filter (a) having a field radius of 3 mm, this results in ΔM/M = 0.02, i.e. a contradiction; and for a filter (b) having a field radius of 12 mm, this results in ΔM/M = 0.005, i.e. coincidence with the above condition. In other words: If a resolution of ΔM/M = 0.01 has been set for both filters, most of the ions will not be able to pass through the filter in case (a). In the case of the larger filter (b), all ions will be able to pass through the filter, since the set resolution is greater than the scatter.
Although this simplified error calculation does not nearly take into account all of the effects that can contribute to transmission, it does teach several fundamental relationships:
- The field radius must be maintained significantly better than 1% over the entire length of the filter, depending on the selected mass range. Fluctuations in the field radius will lead to transmission losses
- The larger the dimensions of the rod system are selected, the lower the influence of the absolute mechanical tolerances will be
- The higher the mass range, and if differentiation between adjacent masses is still to be made, the stricter will be the requirements relating to the relative accuracy of the mass filter. In this simple picture, the required relative accuracy is scaled linearly with the mass range
A quadrupole mass filter is a dynamic mass filter for positive and negative ions. The mass scale is linear to the applied amplitude of the HF voltage. Mass resolution can be conveniently and electrically set by means of the ratio between the DC voltage U and the high-frequency voltage amplitude V. Due to their small dimensions and light weight, quadrupole mass spectrometers are suitable both as pure residual gas analyzers and, in higher-quality design, as sensors for gas analysis.
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