Pulse Leading Edge Characteristics and Mass Resolution of High Voltage Extraction Power Supply for Time of Flight Mass Spectrometer
Time of flight mass spectrometry measures the mass to charge ratio of ions by timing their flight over a known distance. Ions are accelerated to a constant kinetic energy by an extraction field, then drift through a field free region where their arrival time at a detector is recorded. The flight time depends on the ion mass, with lighter ions arriving earlier than heavier ions. The mass resolution, the ability to distinguish ions of similar mass, depends critically on the timing precision, which is affected by the characteristics of the high voltage extraction pulse.
The time of flight mass spectrometer operates in a pulsed mode where ions are created or extracted in discrete packets. In the simplest configuration, ions are formed in a source region and then extracted by applying a high voltage pulse to create an extraction field. The ions are accelerated through the extraction region, gaining kinetic energy proportional to the charge and the extraction voltage. They then enter a drift region where they separate according to their velocities, with the flight time proportional to the square root of the mass to charge ratio.
The mass resolution of a time of flight instrument is typically defined as the mass divided by the peak width, where the width is measured at half maximum or another defined level. The resolution is limited by several factors including the initial spatial and velocity distributions of the ions, the timing jitter of the extraction and detection, and the stability of the extraction voltage. The extraction pulse characteristics affect several of these factors, making the high voltage power supply design critical for achieving high resolution.
The leading edge of the extraction pulse, the transition from zero to the full extraction voltage, is particularly important for the mass resolution. During the leading edge, ions at different positions in the extraction region experience different acceleration histories. Ions near the extraction grid begin accelerating earlier than ions deeper in the source region, creating a correlation between initial position and final velocity that can degrade the resolution. Faster rise times reduce this effect by making the acceleration more simultaneous for all ions.
The rise time requirement depends on the extraction region dimensions and the ion masses of interest. For typical extraction regions of a few millimeters and ion masses in the hundreds to thousands of atomic mass units, rise times of tens of nanoseconds or faster are needed for high resolution. The required rise time becomes faster for larger extraction regions or lighter ions. The high voltage power supply and switching circuitry must achieve these rise times while delivering the required voltage amplitude.
Voltage stability during the flat top of the extraction pulse affects the kinetic energy spread of the extracted ions. Variations in the extraction voltage during the ion acceleration cause variations in the final ion energy, which translate to variations in flight time and peak broadening. The voltage must remain stable within a fraction of a percent throughout the ion extraction period, which is typically microseconds. Droop or ripple on the pulse flat top degrades the mass resolution.
The fall time of the extraction pulse, the transition back to zero voltage, is less critical for mass resolution but affects the duty cycle and the sensitivity of the instrument. A slow fall time extends the time required between extraction pulses, reducing the maximum repetition rate and the number of ions that can be analyzed per unit time. Fast fall times enable higher repetition rates and greater sensitivity, but may require additional circuit complexity.
Jitter in the extraction pulse timing causes uncertainty in the start time for the flight time measurement. Timing jitter directly translates to uncertainty in the flight time, broadening the mass peaks and reducing resolution. The jitter sources include the trigger circuit, the switching elements, and the voltage rise time variations. Sub nanosecond jitter is required for high resolution time of flight instruments. The trigger and switching circuits must be designed for low jitter operation.
Reflectron designs can compensate for some of the resolution degradation from imperfect extraction pulse characteristics. A reflectron is an ion mirror that reverses the ion trajectory, with an electric field gradient that compensates for the correlation between ion energy and penetration depth. Ions with higher energy penetrate deeper into the reflectron and travel a longer total path, partially compensating for their higher velocity. The reflectron can improve resolution even with nonideal extraction conditions, but the improvement is limited and optimal performance still requires good extraction pulse characteristics.
Extraction pulse generation circuits for time of flight mass spectrometry include direct switching of high voltage, pulse transformer coupling, and cascade multiplier circuits. Direct switching using fast high voltage switches provides the fastest rise times but requires switches with high voltage and current ratings. Pulse transformers can step up the voltage from a lower voltage fast switch, but the transformer response affects the pulse shape. The circuit design must balance rise time, voltage amplitude, and reliability for the specific application requirements.
