Ionisation techniques

Mass Spectrosocopy ionisation technquies

1. IONISATION TECHNIQUES OF MASS SPECTROSCOPY SEMINAR BY:- BHAVESH M CHOUDHARY SUBJECT :-AAT

2. CONTENTS:- Basic concept of ionization Types of Ionization Fast atom bombardment Matrix Assisted Laser Desorption Surface Enhanced Laser Desorption/Ionization Advantages Disadvantages References

3. Ionization Ionization is a process of charging a molecule. The sample molecule must be charged in order to measure them using a mass spectrometer. The atom is ionized by knocking one or more electrons off to positive ion. The particles in the sample (atoms or molecules) are bombarded with a stream of electrons to knock one or more electrons out of the sample particles to make positive ions.

4. Classification of ionization techniques Gas Phase Sources. • Electron Impact Ionization (EI). • Chemical Ionization (CI). • Field Ionizations (FI). Desorption Sources. Electro spray Ionization (ESI). Matrix assisted desorption/Ionization (MALDI). Fast Atom Bombardment (FAB). Surface Enhanced Laser Desorption ionization (SELDI).

5. Fast Atom Bombardment A high-energy beam of neutral atoms, typically Xe or Ar, strikes a solid sample causing desorption and ionization. It is used for large biological molecules that are difficult to get into the gas phase. FAB causes little fragmentation and usually gives a large molecular ion peak, making it useful for molecular weight determination. The atomic beam is produced by accelerating ions from an ion source though a charge-exchange cell. The ions pick up an electron in collisions with neutral atoms to form a beam of high energy atoms.

6. Advantages Used for ionization of polar high molecular weight samples. Provides rapid heating of samples and reduces sample fragmentation. Rapid ionization

7. Disadvantages Difficult to distinguish between low molecular weight compounds. Compounds must be soluble in liquid matrix. Not good for multiply charged compounds

8. MATRIX Matrix is used in MALDI to Absorb the laser energy. Prevent analyte agglomeration. Protect analyte from being destroyed by direct laser beam. Matrix consists of a crystallized molecules of which the most commonly used are :- 3,5 – dimethoxy – 4 – hydroxy cinnamic acid (sinapinic acid). α – cyano – 4 – cinnamic acid ( α – cyano or α – matrix). 2,5 – dihydroxy benzoic acid (DHB)

9. SELDI Surface-enhanced laser desorption/ionization (SELDI) is a soft ionization method in mass spectrometry (MS) used for the analysis of protein mixtures. In MALDI, the sample is mixed with matrix material and applied to a metal plate before irradiation by a laser, whereas in SELDI, proteins of interest in a sample become bound to a surface before MS analysis. SELDI is typically used with time-of-flight (TOF) mass spectrometers and is used to detect proteins in tissue samples, blood, urine, or other clinical samples, however, SELDI technology can potentially be used in any application by simply modifying the sample surface.

10. STEPS IN SELDI SELDI can be seen as a combination of solid-phase chromatography andTOF-MS. The sample is applied to a modified chip surface, which allows for the specific binding of proteins from the sample to the surface. Contaminants and unbound proteins are then washed away. After washing the sample, an energy absorbing matrix, such as sinapinic acid (SPA) or α-Cyano-4-hydroxycinnamic acid (CHCA), is applied to the surface and allowed to crystallize with the sample. Alternatively, the matrix can be attached to the sample surface by covalent modification or adsorption before the sample is applied.[4] The sample is then irradiated by a pulsed laser, causing ablation and desorption of the sample and matrix.

11. References 1. Principles of Instrumental analysis. Fifth Edition by Douglas. A. Skoog, F.James Holler and Timothy A. Niemen. Page No. 499 – 511. 2. https://en.wikipedia.org/wiki/Surfaceenhanced_laser_desorption/ionization

12. Questions Note:- MALDI FAB Write a note on ionisation techniques used in mass spectroscopy.