TOF timing electronics
When ions have travelled around the time-of-flight mass spectrometer, they strike an electron multiplier detector, which outputs a pulse of electrons to some sort of pre-amplifier. It is important to realise that the electron multiplier is so sensitive that it is perfectly capable of converting the arrival of a single ion into a measurable pulse. The arrival time of the pulse gives us the measured mass, so after pre-amplification some sort of timing circuit is used.
The timing strategies can be broadly divided into two groups:
A very fast digitiser, or analogue-to-digital converter (ADC), records the shape of the pulse waveform into memory. All the pulses resulting from a single spectrometer start event are recorded in a single trace; so in principle a complete mass spectrum can be recorded from one spectrometer cycle. However to achieve acceptable precision, it is often necessary to add a number of traces together to give an average.
The arrival time of each ion pulse is recorded as a simple number, or time-stamp, by a time-to-digital converter (TDC). The implicit assumption is that each pulse represents a single ion; all information about the shape of individual pulses is discarded. To create a mass spectrum, many spectrometer cycles are performed, typically a few thousand, and the time-stamps built into an arrival-time histogram.
The best strategy depends on the experiment, in particular on the number of ions expected in a single cycle and on any constraints to the cycle repetition rate e.g. a laser that won't pulse faster than 10Hz. A more detailed comparison can be found here
Last updated: 16:53 26/02/2014
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