Mass spectrometry is one of the most important instrumental techniques used for the identification and structural elucidation of organic compounds. In mass spectrometry, the sample is first converted into ions, and these ions are then separated according to their mass-to-charge ratio (m/z) and detected. The process of converting neutral molecules into charged particles is called ionisation. The ionisation method is particularly important because it determines the type of molecular ions and fragment ions produced and, consequently, the nature of the mass spectrum. Different organic compounds require different ionisation techniques depending upon their molecular mass, volatility, thermal stability, polarity and structural characteristics. The major ionisation methods used in mass spectrometry include electron ionisation, chemical ionisation, fast atom bombardment, electrospray ionisation, atmospheric pressure chemical ionisation, matrix-assisted laser desorption/ionisation, field ionisation and field desorption. Among these, electron ionisation and chemical ionisation are particularly important in conventional organic mass spectrometry, whereas electrospray ionisation and MALDI are widely used for polar, non-volatile and high-molecular-mass compounds. Electron ionisation (EI) is one of the oldest and most widely used ionisation methods. In this technique, the sample is introduced into a high-vacuum ionisation chamber in the gaseous state. A beam of high-energy electrons, commonly having an energy of about 70 eV, is passed through the vapourised sample molecules. Collision between an energetic electron and a neutral molecule results in the removal of an electron from the molecule, producing a molecular radical cation. M + e⁻ → M⁺• + 2e⁻ The molecular radical cation is called the molecular ion. Because the electron beam possesses considerably more energy than is required for ionisation, the molecular ion is often formed with excess internal energy. It therefore undergoes fragmentation to produce a series of smaller positively charged ions and neutral fragments. These fragment ions give the characteristic pattern of the mass spectrum. EI is called a hard ionisation technique because extensive fragmentation generally occurs. The fragmentation pattern is highly useful for structural identification because particular functional groups and structural arrangements give characteristic fragments. The molecular ion peak also provides information about the molecular mass, provided that the molecular ion is sufficiently stable. EI is particularly suitable for volatile and thermally stable organic compounds such as hydrocarbons, alcohols, ethers, aldehydes, ketones, esters and many aromatic compounds. Its major limitation is that thermally unstable, non-volatile and very high-molecular-mass compounds cannot generally be analysed directly by conventional EI. Chemical ionisation (CI) is a softer ionisation technique developed to reduce the extensive fragmentation observed in electron ionisation. In CI, a reagent gas such as methane, isobutane or ammonia is introduced into the ionisation chamber at a relatively high pressure. The reagent gas is first ionised by electron impact. The resulting reagent-gas ions undergo ion-molecule reactions and subsequently transfer a proton or another charged species to the analyte molecule. For example, with methane as the reagent gas, protonated species can be formed: M + CH₅⁺ → [M + H]⁺ + CH₄ The ion [M + H]⁺ is called the protonated molecular ion or pseudomolecular ion. Depending on the reagent gas and the nature of the compound, adduct ions such as [M + NH₄]⁺ may also be produced. CI produces considerably less fragmentation than EI because the energy transferred to the analyte molecule is lower. It is therefore referred to as a soft ionisation method. The main advantage of CI is that it gives a strong molecular-weight-related ion, making determination of molecular mass easier. However, because fragmentation is limited, CI generally provides less structural information than EI. Positive-ion CI is commonly used, while negative-ion chemical ionisation can also be employed for compounds capable of forming stable negative ions. Field ionisation (FI) is another soft ionisation technique. In this method, the sample molecules are exposed to an extremely strong electric