Question

Why is it important that the pressure inside a mass spectrometer gets low as possible?

Answer 1

The higher the pressure inside the spectrometer, the shorter a distance a molecular ion can fly before it collides with something else (like another molecular ion). This distance is called the mean-free path.

Each collision (with the inert gas in the system, like `"N"_2` or `"Ar"`) has the potential to fragment these molecular ions, but sometimes we like control over when the ions collide.

We can accomplish this by lowering the pressure in the spectrometer to something like `10^-5 "torr"` and reducing the mean-free path of each molecular ion.

That way, we don't have to worry about unexpected fragmentation.

This is important because sometimes many equivalents of the same sample are injected, and consistency in fragmentation is helpful in identifying specific molecules in the sample if they are unknown. It also minimizes erratic peaks in the final mass spectrum from irregular fragmentation.

This low pressure has been useful in the older magnetic sector mass spectrometers, and these days, in some orbitraps and time-of-flight spectrometers.