Why is it important that the pressure inside a mass spectrometer gets low as possible?
1 Answer
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
We can accomplish this by lowering the pressure in the spectrometer to something like
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.