Dynode Multiplier Gain Control via Stage Switching
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Solution Overview
Problem
Current secondary electron multipliers in mass spectrometers face limitations in dynamic measuring range and lifetime due to saturation issues and aging of dynode surfaces, particularly in high-speed analytical applications like gas or liquid chromatography.
Innovation Solution
The implementation of pulse switching electronics to selectively activate and short-circuit dynode stages in a discrete dynode multiplier, using a feedback control circuit to adjust the number of active dynode stages based on the ion signal, allowing for ultra-fast gain control and extended dynamic range without requiring fast changes in high voltage supply.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the trans-impedance amplifier gain is changed to enhance dynamic measuring range, then the dynamic range is improved, but the solution is limited by saturation of the SEM output current and requires slow adjustment of high voltage power supply
Solution Approach 1:
The patent divides the single series of discrete dynode stages into multiple parallel series, each with its own trans-impedance amplifier. This segmentation allows independent measurement of secondary electron currents from each series, enabling fast switching between different gain ranges without saturating a single amplifier. The dynamic range is extended by selecting appropriate series based on signal intensity, achieving both wide dynamic range and fast response speed.
2Measurement precision
If the SEM gain is increased to detect weak ion signals, then the sensitivity is improved, but the SEM output current becomes saturated when detecting strong ion signals
Solution Approach 1:
The patent implements dynamic gain control by selectively activating different series of dynode stages based on the ion signal strength. For weak signals, more series are activated with higher gain to improve sensitivity. For strong signals, fewer series are activated or lower gain is used to prevent saturation. This dynamic adaptation allows the detector to maintain optimal performance across a wide dynamic range of 10^15.
3Measurement precision
If the dynode surfaces are critically conditioned to low work function to yield high gain, then the secondary electron gain is improved, but the multiplier ages faster due to electron bombardment changing surface conditions
Solution Approach 1:
The patent uses multiple parallel series of dynode stages, where each series can operate independently at lower gain levels. This distributes the electron bombardment load across multiple series, reducing the aging effect on any single series. By activating only the necessary number of series based on signal requirements, the overall lifetime of the multiplier is extended while maintaining the required gain performance.
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 significantly increases the dynamic range to greater than 10^15 and extends the detector's lifetime by reducing dynode aging, enabling real-time operation with robust and cost-effective electronics, and allows for simultaneous detection of positive and negative ions without high voltage switching.
Implementation Method 1
ions convert to electrons on the first dynode. Using the subsequent series of dynodes, each biased by a positive voltage, the electrons are accelerated into the next dynode, creating multiple secondary electrons
Implementation Method 2
The secondary electron current is increased from dynode to dynode forming a kind of electron avalanche
Data Source
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AI summary
The invention relates to mass spectrometers having secondary electron multipliers with series of discrete dynode stages. The invention particularly relates to an operation with extended dynamic measuring range and extended lifetime. The invention is based on not adapting the dynamic measuring range by control of the gain of the trans-impedance amplifier, nor controlling the multiplier operating voltage, which both are usually too slow, but alternating a number of active and passive dynode stages of a discrete dynode multiplier. Each dynode stage is connected to a discrete voltage supply circuit, being able to be de-energized and short-cut; the multiplier gain is feedback-controlled by energizing or short-cutting dynode stages, serially from the end of the multiplier, as a function of a last measured ion signal; and the multiplier has a single trans-impedance amplifier and a single analog-to-digital converter, measuring and digitizing the output current of the last active dynode stage.