Time of flight mass spectroscopy is a powerful analytical tool, however it is quite straightforward in principle. Initially a population of singly charged ions are created with a fixed energy permitting them to fly a fixed distance and then their arrival is measured at a detector. Their mass can be inferred from the time taken to travel the distance. A mass spectrum is obtained consisting of several peaks corresponding to the mass of the ions detected, and the peak height shows how much mass is being analyzed.
In an ideal scenario, all the ions of the same mass reach the detector at the same time, but practically speaking, the peaks in a mass spectrum have a finite width, which determines the ability of the mass spectrometer to separate ions having similar masses.
This quantity, the mass resolution of the instrument, is obtained by comparing this width to the total time of flight. In Figure 1, we have a peak which is 2.2 ns wide and a flight time of 90.7 µs, resulting in a mass resolution of 20,000.
There is a lot of research being done on how these machines need to be improved. Whatever the method and scale of improvement, however, the objective is the same: to bring down the in-system peak width. Anything that increases the measured ion peak width brings down the instrument’s mass resolution, as shown in Figure 1.
Figure 1. Any travel-time difference for ions of the same mass in aTOF system reduces the mass resolution.
To attain the required sensitivity and detect individual ions, all time of flight mass spectrometers make use of an electron multiplier as a detector. In these devices, the incoming ion strikes the energetic surface, where emission of at least one secondary electron occurs.
An electric field accelerates the secondary electron, and the multiplier is struck again, causing emission of multiple electrons. In a microchannel plate, this process happens many times an a large number of parrallel electron multipliers, producing an output pulse of a million electrons from a single ion.
Figure 2. Microchannel plates are parallel arrays of individual electron multipliers with each pore or channel having a diameter of the order of microns or tens of microns.
Figure 3. The channels are typically not normal to the input surface of the microchannel plate but instead pitched at a slight angle called the bias angle.
Detector Time Jitter
Detector time jitter occurs in the microchannel plate due to the following:
- The detector has a finite pulse width. The multiplier’s physical dimensions and the electric fields in it control the pulse width so that MCPs with dimensions in microns are normally fast, also MCPs with smaller pore size are smaller than those with larger pores.
- The detector contributes to the width by varying the ion arrival time. Individual ion peaks come within the envelope of the time of flight peak and from this the term jitter originates since the pulse jitters back-and-forth. A part of this jitter is because of the flight instrument.
Reducing Detector Time Jitter
The detector time jitter can be reduced by:
- Ensuring the detector pulse is fast and electrons reach the anode at the same time.
- Avoid the presence of electric or magnetic fields so that the flight of the incoming ions are not affected
- Ensuring the ions strike a parallel and planar surface to the incoming ion packet.
It is important to make sure that the microchannel plate surface is planar. In order to determine the flatness of the flatness of the incoming surface of the microchannel plate, the total focus plane deviation is measured using a machine vision system similar to those used in the semiconductor industry, that can measure flatness down to about 0.2 µm.
The global flatness is a single number with units of distance.
Figure 4. The machine vision system helped measures the absolute flatness as well determine the shape of the input surface.
MountingPad Microchannel Plates
Normally the field of channels is surrounded by a solid glass border, which enables a secure mechanical connection to the MCP while mounting it to the detector hardware. This can cause considerable bowing or warping - enough to be visible with the naked eye.
The Mounting Pad architecture was created, using four individual mounting pads evenly spaced on the outer edge of the MCP instead of a solid glass border. This eliminates the warping, resulting in better flatness values.
Hardware Effect on Global Flatness
Several experiments were done to indicate how the detector impacted the global flatness.
Figure 5. Ion detector assembly with two MCPs to measure the effect of global flatness
Two MCPs were considered with known measured flatness. The detector assembly was repeated using the same housing but with eight different hardware for the front flange changing the assembler and date of assembly.
The flatness of the complete MCP surface mounted in the detector housing is measured. It was found that good hardware improved the MCP flatness and bad hardware can degrade it. The circled red dot shows that assembly errors will lead to worse flatness values than the capabilities of the hardware or the MCPs.
Figure 6. Assemby error has a greater impact on results than the hardware quality
Time Spread due to Ion Arrivals
The flight time of the ions can be modelled using the ion optic modelling program Simion to include the ion motion in the MCP channel electric field, and determine a total time spread that will depend on the ion mass. The time delay in picoseconds as a function of mass is shown in Figure 7.
Figure 7. Model of ion motion in the MCP, with the time delay given as a function of mass.
It is possible to improve time of flight mass resolution by:
- Controlling MCP global flatness
- Increasing MCP bias angle
- Reducing MCP pore size
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This information has been sourced, reviewed and adapted from materials provided by PHOTONIS Technologies S.A.S.
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