RF Multipole Ion Guides With Segmented Electrodes
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Solution Overview
Problem
Quadrupole rod systems in ion guides have an upper mass limit, leading to significant losses and discrimination against heavy ions during injection and ejection, particularly in peptide analysis, where the upper mass limit is unacceptable, and existing alternatives like hexapole or octopole systems suffer from poor collision focusing and ion distribution.
Innovation Solution
Introducing pole electrode systems with structured surfaces, such as grids or reshaped solid rods with edges or tips, to generate a more inhomogeneous near field that enhances the reflection of heavy ions while maintaining a low lower mass limit, allowing for a significantly increased upper mass limit by creating a quadrupole field with bipolar or multipole grids and varying RF voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If solid pole rods with smooth surfaces are used in quadrupole rod systems, then the system maintains simple structure and ease of manufacture, but the upper mass limit is restricted leading to significant losses and discrimination against heavy ions
Solution Approach 1:
The solid pole rod surface is segmented into multiple discrete wire elements arranged in a grid pattern. This segmentation creates a structured surface that generates more inhomogeneous RF fields, enhancing the reflection of heavy ions while maintaining transmission of lighter ions, thereby resolving the mass limit restriction without compromising manufacturing feasibility
Solution Approach 2:
The pole rod surface is transformed into a porous-like wire grid structure with gaps between wires. This structure allows the RF field to penetrate and create complex field distributions that improve heavy ion reflection, effectively increasing the upper mass limit while maintaining a structure that can be manufactured using conventional techniques
2Reliability
If hexapole or octopole rod systems are used as alternatives, then the upper mass limit is increased, but collision focusing and ion distribution deteriorate
Solution Approach 1:
The wire grid structure introduces controlled asymmetry in the RF field distribution by creating more inhomogeneous fields at the pole surfaces. This asymmetric field enhancement improves collision focusing by creating stronger field gradients that better confine ions, while the overall quadrupole symmetry is maintained to preserve proper ion distribution
Solution Approach 2:
The wire grid structure modifies the local field quality at the pole surfaces without changing the global quadrupole field configuration. This local modification enhances field inhomogeneity precisely where needed for heavy ion reflection and collision focusing, while maintaining the overall field quality required for proper ion transmission and distribution
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 enables the effective transmission and detection of heavy ions up to 30 to 40 times the lower mass threshold, improving ion focusing and reducing losses, thus enabling more comprehensive mass spectrometry analysis without compromising the lower mass limit.
Implementation Method 1
The multipole systems are based on the effect of so-called 'pseudopotentials', which are produced in inhomogeneous alternating fields
Implementation Method 2
If a rod system such as this is filled with a collision gas at a pressure between 0.01 and 1 Pascal, injected ions give up most of their kinetic energy as a result of collisions with this gas and collect with only thermal energy in this potential channel along the axis. This effect is also observed when the ions are in slow flight. This process, which has been known since the early 1980s, is now termed 'collisional focusing'.
Implementation Method 3
The field produced by long, parallel wires also forms an ion reflector if every other wire is fed one phase of the RF voltage and the remaining wires the other phase
Implementation Method 4
neighboring structural elements can each be fed with different RF voltages so that a near field is created in front of each pole electrode system, said near field being formed from the strongly inhomogeneous electric RF dipole fields between the structural elements
Data Source
AI summary
In a multipole rod ion guide system operated with RF voltages to collect or transmit ions, the inhomogeneity of the electric RF fields is increased in front of the ion guide rods by forming the rod surfaces from a plurality of spaced electrodes. The inhomogeneous fields produced by the plurality of electrodes increases the mass range over which the ions are guided effectively while still maintaining a pseudopotential minimum which is as well defined as possible close to the axis. Particularly favorable ion guides of this type make it possible to apply an axial DC field to the guide system for the active transport of the ions.