Ionazation in Mass spectroscopy
- 1. 1 Mass spectrometry: A technique for measuring and analyzing molecules,that involves introducing enough energy into a target molecule to cause its ionizationand disintegration.The resulting primary ions and their fragments are then analyzed, based on their mass/ charge ratios, to produce a “molecular fingerprint.”
- 2. 2 Electron impact Ionization (EI) : In the EI process,the sample is vaporizedinto the mass spectrometer Ion source,where it is impacted by the a beam of electrons with Sufficientenergy to ionize the molecule. Used for volatile and thermally stable compounds. e.g. M (g) + e- M+(g) + 2 e-
- 3. 3 This method is suitable for volatile organic compounds e.g. = hydrocarbon, oils, flavors’, Probability of breaking Bond is maximum at 70eV but less than 70eV energy electionpass through the molecule without breaking bond . Disadvantage ; produce abdunt fragmention.
- 4. 4 Chemical ionization (CI) : Chemical ionizationinvolves ion-moleculereactions to produce ions fromthe analyte. A high reagent gas pressure (60 Pa), results in ion- molecule reactions between the reagent gas ions and reagent gas neutrals The products of these ion-molecule reactions can react with the analyte molecules to produce analyte ions
- 5. 5 Field ionization (FI) : The sample is evaporated froma direct insertion probe, gas , or gas inlet. As the gas molecules pass near the Emitter, they are ionizedby electron tunnelling.( Quantum tunnelling) The sample must be thermally volatile. If compound is non-volatile or thermolabile soln Prepared in Volatile solvent applied to fieldion emitter the sample then Subjected to high electric field this modificationis called field Desorption. Field desorption(FD) is a method of ion formation used in mass spectrometry (MS) in which a high- potential electricfield is applied to an emitter with a sharp surface, such as a razor blade, or more commonly,a filamentfromwhich tiny "whiskers" have formed. This results in a high electricfieldwhich can result in ionizationof gaseous molecules of the analyte.
- 6. 6 Fast atom bombardment ionization(FAB): The sample is deposited on a target that is bombarded with atoms, Neutrals, or ions. Dissolve the analyte in a liquid matrix with low volatility and to use a Relatively high current of bombarding particles (4000 to 10,000 eV) m-nitrobenzylicalcohol (MNBA) is a good liquid matrix for non-polar compounds It is useful for non-volatile,polar and thermally unstable compounds And also high molecular weight compounds like insulin, vitamins.
- 7. 7 Matrix assisted laser desorption ionization (MALDI) : The analyte is dissolvedin a solution containing an excess of a matrix such as sinapinic acid or dihydroxybenzoicacid that has a chromophore that absorbs at the laser wavelengthA small amount of this solutionis placed on the laser target The matrix absorbs the energy fromthe laser pulse and produces a plasma that results in vaporization and ionizationof the analyte. It is used as non-volatile,polar and thermally unstable compounds. Good for large molecules (proteins, polymers,carbohydrates)
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- 9. 9 Atmospheric Pressure Chemical Ionization (APCI): • gas phase chemical ionization(CI) process where the vapourizedLC Mobile phase acts as the CI reagent gas to ionize the sample • Mobile phase and analyte are first nebulized (N2) and vapourisedby Heating to 350-550⁰C • The resulting vapour is ionizedusing a corona discharge (source of Electrons) • Subsequent ion/molecule reactions (CI) then cause ionizationof the Analyte • Unlike ESI, analyte ions do not need to exist in solution • Unlike ESI, best sensitivity is achieved at high liquid flowrates i.e. 200μL–1mL/min therefore easily interfaced to conventional HPLC Samples containing heteroatoms:urea’s, benzodiazepines,Carbamates
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- 12. 12 Electrospray ionization (ESI): The sample solutionis sprayed across a high potential difference (3_4 KV) froma needle into an orifice in the interface.Heat and gas flows are used to desolvate the ions existing in the sample solution. Production of charged droplets at the electrospray capillary tip; an evolutionof the charged Droplets by repeated droplet disintegrationand formationof very small highly charged droplets Capable of producing gas- phase ions.
- 13. 13 Atmospheric pressure photoionization(APPI): • Experimentally,you can viewAPPI as an APCI source where the coronadischarge has been replaced with a uv lamp • The 1st step is complete vapourizationof the mobile phase used in the LC separation employing nebulization (N2) and heating to 350-550⁰C • gas phase photoionizationprocess • where the vapourizedmobile phase may be photoionizedto form a CI plasma • or a modifier (dopant = e.g. toluene and acetone. ) is added to aid the photoionization process and formationof the CI plasma • or the analyte can be directly photoionizedby photons from the uv lamp
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