Ion Selection Apparatus Asymmetric Waveform Ion Mobility
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Solution Overview
Problem
Field asymmetric ion mobility spectrometer (FAIMS) systems face difficulties in detecting substances when using ionization sources that produce predominantly ions of a single polarity, as these ions tend to collect on insulated surfaces, reducing the number of like-polarity ions entering the detection gap and affecting detection capabilities.
Innovation Solution
Incorporating an arrangement with first and second parallel electrodes that apply an asymmetric voltage creating periods of high and low fields, allowing only selected ions to pass through the drift region, and using a DC compensation field to manage ion flow, ensuring that only specific ions are detected.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If an ionization source producing predominantly single-polarity ions is used, then the ion current is enhanced, but the ions collect on insulated surfaces reducing the number of ions entering the detection gap
Solution Approach 1:
A gate electrode is introduced as an intermediary between the ionization source and the detection gap. This gate electrode collects excess ions of the dominant polarity and controls their passage, preventing charge buildup on insulated surfaces while allowing selective ion transmission to the detection region.
Solution Approach 2:
The voltage applied to the gate electrode is dynamically adjusted to change the transmission characteristics. By varying the gate voltage, the system can control which ions pass through and which are blocked, enabling selective ion transmission while managing charge accumulation effects.
2Measurement precision
If a gate is used to control ion passage, then ion selection is improved, but the system complexity increases
Solution Approach 1:
The gate electrode serves multiple functions simultaneously: it acts as an ion shutter to control ion passage timing, a charge reservoir to manage excess ion accumulation, and a selection element to filter ions based on their mobility characteristics. This multi-functionality reduces the need for additional separate components.
3Measurement precision
If the asymmetric field is increased to improve ion selection, then the separation of ions is enhanced, but more ions are deflected and lost
Solution Approach 1:
The asymmetric field is applied in periodic pulses rather than continuously. During the high-field phase, ions are separated by mobility; during the low-field or zero-field phase, ions drift forward. This periodic application allows repeated separation opportunities while maintaining overall ion transmission through the drift region.
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 solution enhances the ability to detect specific ions by selectively passing ions through the drift region, improving the system's sensitivity and specificity in identifying substances, even with ionization sources that produce predominantly single-polarity ions.
Implementation Method 1
an arrangement for applying an asymmetric voltage across the two electrodes to create in the gap between the electrodes periods of a high field followed by longer periods of a low field
Implementation Method 2
Molecules in the sample of air are ionized, such as by means of a radioactive source, UV source or by corona discharge, and are admitted into the drift region of the cell by an electrostatic gate at one end. The ionized molecules drift to the opposite end of the cell at a speed dependent on the size of the ion.
Implementation Method 3
The field preferably includes a dc compensation field selected such that only selected ones of the ions are passed through the field.
Implementation Method 4
Molecules in the sample of air are ionized, such as by means of a radioactive source, UV source or by corona discharge
Data Source
Figure 1
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Figure 4~5
AI summary
An ion mobility spectrometer or other ion apparatus has two or three grid electrodes 51 and 52; 151 to 153; 106 and 107; 106' and 107' extending laterally of the ion flowpath. An asymmetric waveform with a dc compensating voltage is applied between the electrodes to produce a field parallel to the ion flow path that affects ions differently according to their field-dependent mobility. This filters or delays different ions selectively in their passage to an ion detector 11, 111, 111 'to facilitate discrimination between ions that would otherwise produce a similar output.