Salad dressing

How to measure 10^-27 kg mass? 
F.W. Aston was the first who showed that neon, that of red lamp, is a mixture of different atoms. The photo to the left is his first mass spectrometer with the magnetic deflection.
The photo to the right is his article from 1920 on existence of different isotopes in neon.

Quadrupole mass spectrometers are 
the most versatile tools. They use a variable (a few MHz) frequency electrical field to guide only the ions with a determined ratio charge-to mass. 

They work like potato-selectors: shaking ions left and right and leaving only those with a right mass.

Time-of-flight mass spectro-meter is simply a long tube.

Lighter ions travel faster than heavy, so they arrive first (assuming the same initial acceleration potential applied).