Application Note and Selection Chart for Time-Of-Flight Mass Spectrometry Instrumentation

 

Basics by Stefan Kaesdorf

Time-of-flight (TOF) spectrometers combine a relatively simple mechanical setup with extremely fast electronic data acquisition. They can offer mass resolutions > 10000, a mass range of up to 500000 Dalton, an ion optical transmission > 10% and large acceptance volumes. TOF mass spectrometry is based on the fact that for a fixed kinetic energy E the mass m and the velocity v of the ions are correlated:
 
m = 2*E/v2 [1]

By measuring the time T the ions need for travelling a fixed distance s the velocity v and from [1] the mass of the ions can be calculated. Typically the length of the flight path in commercially available TOF spectrometer systems is about 1000...2000 mm, the kinetic energies used are ca. 2000 eV, the flight times are 5...100 µs, the width of individual TOF peaks a few ns. In the following some technical details of time-of-flight spectrometers are discussed:
 
Ion creation:

To meet the requirement that all the ions start the travel through the spectrometer the same time two different ionization schemes are used:

  • all the ions are created in a static electric extraction field within a very short time interval (several ns). This scheme is used in laser ionization, TOF-SIMS (time-of-flight secondary ion mass spectrometry), laser desorption and laser ablation.
  • if a continuous ionization process like electron impact ionization is employed the ions are collected for a certain time interval (normally some µs), afterwards the ionization process is stopped and the ions are extracted into the TOF spectrometer by a high voltage pulse with a rise time < 10ns.

Ion detection:

Most TOF spectrometers employ multichannel plate (mcp) detectors which have a time response < 1 ns and a high sensitivity (single ion signal > 50 mV). The large and plane detection area of mcp's results in a large acceptance volume of the spectrometer system. Only few mcp channels out of thousands are affected by the detection of a single ion i.e. it is possible to detect many ions at the same time which is important for laser ionization where hundreds of ions can be created within a few nanoseconds.





Space focussing and energy focussing

When designing a TOF mass spectrometer two problems are encountered:

  • the length s of the flight path in the spectrometer is not identical for all the ions because the creation of the ions takes place in a finite volume. By using an appropriate potential distribution of the ion extraction field ("Wiley-McLaren-criterium") it can be achieved that the ions which have to travel a longer distance have larger kinetic energies i.e. all the ions arrive at the same time at the detector independent of their starting positions ("spatial focussing")
  • the time resolution can be further reduced by different starting energies of the ions (caused by space charge effects and processes like fragmentation, desorption, ablation,...). This effect can be compensated by passing the ions through an ion reflector ("reflectron TOF spectrometer"). Faster ions penetrate deeper into the repelling electric field of the ion reflector i.e. they have to travel a larger distance. Potential distributions can be found where all the ions arrive at the same time at the detector independent of their starting energy ("energy focussing")

Both focussing methods have in common that the increase in resolution doesn't lead to a smaller signal amplitude (if the resolution of an optical monochromator is enhanced by narrowing the slit widths, the signal height will be reduced !)

Time-of-flight data acquisition


The performance of a TOF spectrometer depends strongly on the electronic data acquisition used. TOF spectrometers allow in principle to measure the complete mass distribution with one single shot i.e. huge amounts of data are created within a very short time interval. The challenge in TOF data acquisition is to design a system which combines extreme velocity (time resolution < 1 ns), zero inter-bin deadtime and high data throughput rates. Depending on the ionization scheme TOF spectra are either measured using very fast AD converters (digital storage scopes, transient recorders) or fast ion counting techniques (TOF/multichannel analyzer, time-to-digital converters).

In the case of laser ionization one laser pulse can create hundreds of ions which arrive in a pulse a few nanoseconds wide at the detector. As counting was quite difficult analog registration was normally employed. To reduce statistical uncertainties hundreds to thousands of TOF spectra have to be averaged. Typically a spectrum consists of 32k time channels with a vertical resolution of 8 bit i.e. after every laser pulse 32 kbyte of data have to be transferred and summed. The data transfer rate limits the spectrometer repetition frequency to less than 100 Hz which is compatible with conventional pulsed high power lasers.

A new class of transient recorders designed as PC plug-in cards for example from Signatec have a much higher data throughput which can be further increased by using fast signal processors such as the new Signatec Model PMP8.




These data acquisition cards will allow to make use of the high repetition frequency of up-coming high power diode-pumped solid state lasers.
If the number of individual ionic species detected per sweep is smaller than one single particle counting techniques can be used. Time-to-digital converters (TDC) allow extremely high sweep frequencies (> 1 MHz) whereas TOF/multichannel analyzers (TOF/MCA) offer the possibility to register a large number of events per sweep.
Both techniques allow time resolutions below 1 ns. Because of their multistop capabilities TOF/MCA's have to transfer larger amounts of data per sweep than TDC's.
However new instruments designed as PC plug-in cards like the FAST Model 7886 allow repetition frequencies of more than 20 kHz as demanded by TOF spectroscopy.
The screen shots show time-of-flight spectra measured by a STEFAN KAESDORF Model RFT10 reflectron TOF spectrometer (electron impact ionization, 70 eV electron energy, data acquired with FAST 7886E TOF/multichannel analyzer)