How can mass spectrometry determine molecular weight of organic compounds?

Mass spectrometry determines the molecular weight of organic compounds by measuring the mass-to-charge ratio of their ionised fragments.

Mass spectrometry is a powerful analytical technique used to quantify known materials, identify unknown compounds within a sample, and elucidate the structure and chemical properties of different molecules. The complete process involves the conversion of the sample into gaseous ions, with or without fragmentation, which are then characterised by their mass to charge ratios (m/z) and relative abundances.

The first step in mass spectrometry is to turn the molecules of the organic compound into ions. This is achieved by bombarding the sample with a beam of electrons. The high energy electrons remove an electron from the sample molecules, creating positive ions. These ions are often unstable and can break up into smaller fragments. This process is known as ionisation.

The ions are then accelerated by an electric field, which gives them a kinetic energy that is proportional to their mass-to-charge ratio. The ions then pass through a magnetic field, which deflects them. The amount of deflection depends on the mass-to-charge ratio of the ions. Lighter ions and ions with a higher charge are deflected more than heavier ions or ions with a lower charge.

The ions then hit a detector, which measures the number of ions at each mass-to-charge ratio. The result is a mass spectrum, a graph of intensity (number of ions) against mass-to-charge ratio. The molecular ion peak, the peak with the highest m/z ratio, corresponds to the molecular weight of the organic compound.

In summary, mass spectrometry determines the molecular weight of organic compounds by ionising the molecules, accelerating the ions, deflecting them with a magnetic field, and then detecting the ions. The mass-to-charge ratio of the ions, as determined by their deflection and detection, gives the molecular weight of the compound.