Linear Ion Trap Octopole Field for Space Charge Reduction
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Solution Overview
Problem
The performance of ion trap mass spectrometers is limited by space charge density, which affects mass accuracy and resolution, particularly as ion population increases, leading to reduced analytical capabilities.
Innovation Solution
A linear ion trap system with auxiliary electrodes is used to create a two-dimensional substantially quadrupole field with a significant octopole component, reducing the impact of space charge effects by introducing non-linear RF and electrostatic fields, thereby improving ion dynamics and mass accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a pure quadrupole field is used in a linear ion trap, then ion trapping stability is improved, but mass accuracy deteriorates due to space charge effects at high ion populations
Solution Approach 1:
The patent applies asymmetry by introducing a controlled octopole field component (breaking the perfect quadrupole symmetry) to counteract space charge effects. The auxiliary electrodes are positioned asymmetrically relative to the main quadrupole rods, creating a field with deliberate higher-order components that compensate for the destabilizing effects of high ion density on mass accuracy.
Solution Approach 2:
The patent changes the field composition parameter by superimposing an octopole harmonic (amplitude A4) on the quadrupole harmonic (amplitude A2), where A4 is maintained between 0.01-5% of A2. This parameter modification allows the system to maintain trapping stability while improving mass accuracy at high ion populations by altering the mathematical form of the electric potential.
2Measurement precision
If higher order field components are added to improve mass accuracy, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The electrode system is segmented into two functional groups: main quadrupole rods for trapping and auxiliary electrodes for field modification. This segmentation allows independent optimization of each component's function, reducing overall system complexity by clearly defining the role of each electrode set.
Solution Approach 2:
The auxiliary electrodes serve multiple functions: they generate the octopole field component, provide dipolar excitation capability, and enable mass spectral scanning. This multi-functionality reduces device complexity by consolidating several operations into a single electrode structure rather than requiring separate components for each function.
3Productivity
If ion population is increased to improve productivity, then productivity is improved, but mass accuracy deteriorates due to space charge effects
Solution Approach 1:
The patent converts the harmful space charge effect into a beneficial control parameter. By intentionally adding octopole field components that scale with ion population, the system transforms the previously detrimental space charge interaction into a controllable field component that can be optimized to maintain mass accuracy even at high ion densities.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach maintains a relatively pure quadrupole field with minimal higher order components, enhancing mass accuracy and resolution by reducing the deleterious effects of space charge, allowing for increased ion population without compromising analytical performance.
Implementation Method 1
establishing and maintaining a two-dimensional substantially quadrupole field, the field comprising a quadrupole harmonic of amplitude A2 and an octopole harmonic of amplitude A4
Implementation Method 2
the field comprising a quadrupole harmonic of amplitude A2 and an octopole harmonic of amplitude A4, wherein A4 is greater than 0.01 % of A2, A4 is less than 5% of A2
Implementation Method 3
a dipolar excitation AC to either the first pair of rods or a diagonally oriented pair of auxiliary electrodes at a lower frequency than the first frequency to radially excite the selected portion of the ions having the selected m/z
Implementation Method 4
a voltage supply connected to the first pair of rods, the second pair of rods and the four auxiliary electrodes. The RF voltage supply is operable to provide i) a first RF voltage to the first pair of rods at a first frequency
Data Source
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Figure 3a
AI summary
A two-dimensional substantially quadrupole field is provided. The field comprises a quadrupole harmonic of amplitude A2 and an octopole harmonic of amplitude A4, wherein A4 is greater than 0.01 % of A2, A4 is less than 5% of A2, and, for any other higher order harmonic with amplitude An present in the field, n being any integer greater than 2 except 4, A4 is greater than ten times An.