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Analogue vs digital timing for time-of-flight mass spectrometry

Relative merits and discussion

Here is a brief description of the two strategies.

Analogue advantages

Analogue disadvantages

Digital advantages

Digital disadvantages

In general, if you can use a counting solution you should! If there is a choice between having fewer cycles with lots of ions, or many cycles with a few ions, there is no contest, the counting solution is much better. Although the "one ion per peak per cycle" saturation limit sounds a little radical, typical spectrometer repetition rates range from 50kHz for small spectrometers to 10kHz for larger models. A few 10 - 20 milliseconds of data collection in counting mode will gave the same dynamic range as an 8 bit digitiser, at much higher resolution and much lower cost.

However, there are occasions where an analogue system is the best choice, for example, a situation where the spectrometer repetition rate is limited by a laser that will only fire at 1Hz, each "shot" gives thousands of ions at the detector, and unit mass resolution is sufficient. In this case a good digital oscilloscope with at least 300MHz bandwidth, a sampling rate around 1GHz and at least 50,000 point memory would be a reasonable choice.

As luck would have it, many lasers will fire at about 10Hz, but at this speed a regular digital oscilloscope won't be able to keep up with the averaging; some more specialist hardware will be required. This is the regime where the choice between analogue and digital can be finely balanced. The optimum choice will depend on the detailed experimental priorities and the available budget.


Last updated: 16:53 26/02/2014

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