Ionisation methods
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This presentation discusses various ionization techniques used in mass spectrometry. It begins with an introduction to the history and basic principles of mass spectrometry. It then describes several ionization methods including electron ionization, chemical ionization, desorption chemical ionization, field desorption, fast atom bombardment, and matrix-assisted laser desorption ionization. For each technique, it discusses the ionization process, sample introduction methods, advantages, limitations, and applicable mass ranges. The presentation provides an overview of the key ionization techniques used in mass spectrometry and their characteristics.
![FIELD DESORPTION
• The sample is deposited onto the emitter and the emitter is based to
a high potential (several kilovolts) and a current is passed through
the emitter to heat up the filament.
• Mass spectra are acquired as the emitter current is gradually
increased and the sample is evaporated from the emitter into the gas
phase.
• The analyte molecules are ionized by electron tunneling at the tip of
the emitter 'whiskers'.
• Characteristic positive ions produced are radical molecular ions and
cation attached species such as [M+Na]+ and [M-Na].
16](https://image.slidesharecdn.com/ionisationmethods-190110135909/85/Ionisation-methods-16-320.jpg)
Ionisation methods
- 1. PRESENTATION ON IONISATION TECHNIQUES IN MASS SPECTROSCOPY PRESENTED BY: RABIYA AHSAN M.PHARM 1ST YEAR INTEGRAL UNIVERSITY 2018-2019 1
- 2. INTRODUCTION • Modern techniques of mass spectrometry were devised by Arthur Jeffrey Dempster . • And F.W. Aston in 1918 and 1919 respectively. • Sector mass spectrometers known as calutrons were developed by Ernest O. Lawrence. • And used for separating the isotopes of uranium during the Manhattan Project. 2
- 3. •Mass spectrometry. • An instrumental method for identifying the chemical constitution of a substance by means of the separation of gaseous ions according to their differing mass and charge — called also mass spectroscopy. •Principle of mass spectroscopy • The basic principle on which mass spectrometry operates is that a stream of charged particles is deflected by a magnetic field. • The amount of the deflection depends on the mass and the charge on the particles in the stream. 3
- 4. component of mass spectroscopy Inlet system Ion sources Electrostatstics accelerating system Magneti c field Ion separator Ion collector Vacuum system 4
- 5. • . Ionisation techniques • Gas-Phase Ionization -Electron Ionization (EI) . • Chemical Ionization (CI) . • Desorption Chemical Ionization (DCI) . • Negative-ion chemical ionization . • Field Desorption Ionization . • Field Ionization (FI) . • Particle Bombardment . • Fast Atom Bombardment (FAB) . • Secondary Ion Mass Spectrometry (SIMS) • Atmospheric Pressure Ionization . • Electrospray Ionization (ESI) • Atmospheric Pressure Chemical Ionization (APCI) • Laser Desorption . • Matrix-Assisted Laser Desorption Ionization (MALDI) 5
- 6. ELECTRON IONIZATION ( ELECTRON IMPACT IONIZATION) • electron bombardment ionization • Is an ionization method in which energetic electron interact with solid or gas phase atoms or molecules to produce ions. • EI was one of the first ionization techniques developed for mass spectrometry However, this method is still a popular ionization technique. • This technique is considered a hard (high fragmentation) ionization method, since it uses highly energetic electrons to produce ions. 6
- 7. • This leads to extensive fragmentation, which can be helpful for structure determination of unknown compounds. • EI is the most useful for organic compounds which have a molecular weight below 600. Also, several other thermally stable and volatile compounds in solid, liquid and gas states can be detected with the use of this technique when coupled with various separation methods. 7
- 8. BENEFITS 1. Well understood. 2. Can be applied to virtually or volatile compound. 3. Reproducible Mass Spectra. LIMITATIONS 1. SAMPLE MUST BE THERMALY VOLATILE AND STABLE. 2. The molecular ion may be weak or absence for many compound. Mass range 1. . Low Typically less than 1,000 Da. 8
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- 14. DESORPTION CHEMICAL IONISATION • This is a variation on chemical ionization in which the analyte is placed on a filament that is rapidly heated in the CI plasma. • The direct exposure to the CI reagent ions, combined with the rapid heating acts to reduce fragmentation. • Some samples that cannot be thermally desorbed without decomposition can be characterized by the fragments produced by pyrolysis DCI. • Mass range • Low Typically less than 1,500 Da. 14
- 15. • Sample introduction • sample deposited onto a filament wire . • filament rapidly heated inside the CI source. • Benefits • reduced thermal decomposition . • rapid analysis . • relatively simple equipment • Limitations • not particularly reproducible . • rapid heating requires fast scan speeds 15
- 16. FIELD DESORPTION • The sample is deposited onto the emitter and the emitter is based to a high potential (several kilovolts) and a current is passed through the emitter to heat up the filament. • Mass spectra are acquired as the emitter current is gradually increased and the sample is evaporated from the emitter into the gas phase. • The analyte molecules are ionized by electron tunneling at the tip of the emitter 'whiskers'. • Characteristic positive ions produced are radical molecular ions and cation attached species such as [M+Na]+ and [M-Na]. 16
- 17. • Benefits • simple mass spectra, typically one molecular or molecular-like ionic species per compound. • little or no chemical background . • Limitations • sensitive to alkali metal contamination and sample overloading . • emitter is relatively fragile . • the sample must be thermally volatile to some extent to be desorbed. • Mass range • Typically less than about 2,000 to 3,000 Da. 17
- 18. FIELD IONIZATION (FI) • The sample is evaporated from a direct insertion probe, gas chromatograph, or gas inlet. As the gas molecules pass near the emitter, they are ionized by electron tunneling. • Sample introduction • heated direct insertion probe . • gas inlet . • gas chromatograph • Benefits • simple mass spectra, typically one molecular or molecular-like ionic species per compound. . • little or no chemical background . • works well for small organic molecules and some petrochemical fractions • Limitations • The sample must be thermally volatile. Samples are introduced in the same way as for electron ionization (EI). • Mass range 18
- 19. FAST ATOM BOMBARDMENT • The analyte is dissolved in a liquid matrix such as glycerol, thioglycerol, m- nitrobenzyl alcohol, or diethanolamine and a small amount (about 1 microliter) is placed on a target. • The target is bombarded with a fast atom beam (for example, 6 keV xenon atoms) that desorb molecular-like ions and fragments from the analyte. • Cluster ions from the liquid matrix are also desorbed and produce a chemical background that varies with the matrix used. • Sample introduction • direct insertion probe . • Benefits • rapid . • simple . 19
- 20. • Limitations • high chemical background defines detection limits . • may be difficult to distinguish low-molecular-weight compounds from chemical background . • analyte must be soluble in the liquid matrix . • no good for multiply charged compounds with more than 2 charges. • Mass range • Moderate Typically ~300 Da to about 6000 Da. 20
- 21. MATRIX-ASSISTED LASER DESORPTION IONIZATION (MALDI) • The analyte is dissolved in a solution containing an excess of a matrix such as sinapinic acid or dihydroxybenzoic acid that has a chromophore that absorbs at the laser wavelength. • A small amount of this solution is placed on the laser target. • The matrix absorbs the energy from the laser pulse and produces a plasma that results in vaporization and ionization of the analyte. 21
- 22. • Sample introduction • direct insertion probe . • continuous-flow introduction. • Benefits • rapid and, • convenient molecular weight determination. • Limitations • requires a mass analyzer that is compatible with pulsed ionization techniques. • not easily compatible with LC/MS. • Mass range • Very high Typically less than 500,000 Da. 22
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