What is the difference between 'Electrospray Ionisation' and 'Electron Impact' during the ionisation stage in a mass spectrometer?

Mass Spectrometry is often used by scientists to determine all the isotopes present in a sample (using their isotopic masses) and thus identify the element. The first stage is called the 'ionisation stage' and interestingly there are 2 methods undertaken to ionise a sample. Electrospray ionisation is where a sample is dissolved in a volatile, polar solvent. It is then injected through a fine hypodermic needle creating a fine mist or aerosol. A high voltage is now applied to the tip of the needle, causing the sample molecule, M, to gain a proton and form MH+. The solvent then evaporates away, leaving the MH+ ions, which are then attracted and accelerated towards the negative plate. In contrast, 'Electron Impact' is where a vaporised sample is injected at low pressures, and high energy electrons are fired towards the sample. This removes/knocks off one outer electron forming positive ions. Electron impact is used for elements and substances with low formula masses as it would otherwise cause larger organic molecules to fragment.

Answered by Rohil C. Chemistry tutor

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