A Level Chemistry – The Mass Spectrometer



The time of mass spectrometer is an instrument used for measuring the masses of atoms and molecules. It can also be used to measure the relative abundance of different isotopes and to predict the structure of more complex molecules.

How does the mass spectrometer works

The workings of the mass spectrometer can be summarized in four stages:


  • Electrospray ionisation

The sample is dissolved in a polar solvent and pushed through a small nozzle at high pressure. A high voltage is applied, causing the particles to lose an electron. The ionised particles are then separated from the solvent, leaving a gaseous sample of ions


The positive ions are accelerated by an electric field; smaller ions have a higher speed than larger ions


Ions leave the electric field at different speeds depending on their mass/charge ratio. The heavier the particle, the lower the speed and the greater the time taken to reach the detector.


The ions create a current when they reach the detector. The detector records the time taken to reach it and the current. The greater the abundance of the isotope, the larger the current.


What are the factors that affect the time of flight?

The time of flight depends on the mass and the charge; the greater the mass, the longer the time, and the greater the charge, the shorter the time. It can be shown that the time of flight is inversely proportional to the m/e ratio.


In most cases, however, the charge is +1, so the time of flight depends essentially on the relative mass of the species in the mass spectrometer. If the spectrometer is calibrated, the masses of all the species can be directly measured.


The greater the number of particles landing at a single point on the detector, the greater the electric current and the larger the peak. Thus the relative abundance of different isotopes can be measured.


Thus the relative atomic mass of the element can be calculated from its mass spectrum.