Question

Describe two methods for ionizing liquid samples for mass spectral analysis

Answer 1

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Electrospray ionization (ESI) - ESI is the ionization technique in which electrospray is created by putting a high voltage on a flow of liquid at atmospheric pressure, sometimes this is assisted by a concurrent flow of gas. The created spray is directed to an opening  vacuum system of the mass spectrometer, where the droplets are de-solvated by a combination of heat, vacuum and acceleration into gas by voltages. Eventually the ions are ejected from the droplets and accelerated into the mass analyzer by voltages. For larger molecules, the ions may contain multiple charges, allowing the detection of very large molecules on analyzers that have limited mass to charge (m/Z)) ratio ranges. Because of the natural use of a flowing liquid, it is easily adapted to liquid chromatography (LC).

Fast Atom Bombardment (FAB) - it was the first technique that allowed ionization of non-volatile compounds that could be done simply. It was done by bombarding a sample in a vacuum with a beam of atoms, typically Ar or Xe, accelerated to Kilovolt energies. The sample was typically mixed in a matrix. The two most common matrixes were glycerol and 3 Nitro-benzoic acid. The matrix allowed the sample to refresh itself. The ions formed by FAB were adducts to the molecule, where the adducts could be protons, sodium ions, potassium ions or ammonium ions. A variation of FAB was replacement of the atom beam with a beam of ions, typically cesium ions, which was called secondary ion mass spectrometry (SIMS). SIMS spectra were typically identical to FAB spectra and the terms became interchangeable.