Basic Principles of Mass Spectrometry.ppt

This Presentation gives a brief overview and description of the principles of mass spectroscopy, its various components and the various uses of mass spectroscopy

1. MASS SPECTROMETRY

2. INTRODUCTION Mass spectrometry is a microanalytical technique requiring only a few picomoles of the sample to obtain characteristic information pertaining to the structure and molecular weight of analyte. It involves the production and separation of ionised molecules and their ionic decomposition product and finally the measurement of the relative abundance of different ions produced. It is, thus a destructive technique in that the sample is consumed during analysis .

3. The first mass spectrometer was developed in 1912 by J.J. Thompson. The essential features of all mass spectrometers are therefore: production of ions in the gas phase; acceleration of the ions to a specific velocity in an electric field, separation of the ions in a mass analyser; and detection of each species of a particular m/z ratio.

4. P R IN C IPLE It is based on the principle of ion generation, which is often unstable and with the increase in energy (50-70eV) according to bond strength, they break into fragments. Ions formed are separated in analyser on the basis of their m/z( mass/charge) ratio under the influence of electric and magnetic field and are recorded by the detector to give rise a mass spectrum.

5. COMPONENTS OF MASS SPECTROMETER It mainly consists of following components: Inlet System Ion generation chamber Mass Analyser Detector Data collection system A high vacuum system ( 10 -6 torr) is required. All mass analysers operate under vacuum in order to minimise the collision between ions and air molecules.

6. IONIS A TION Ions may be produced from a neutral molecule by removing an electron to produce a positively charged cation, or by adding an electron to form an anion. Electron impact ionisation Chemical Ionisation

7. Fast atom bombardment(FAB) Electrospray Ionisation(EI) MALDI-TOF( Matrix assisted laser desorption ionisation- time of flight

8. MASS ANALYSER Separates the ions on the basis of m/z ratio. In majority of instruments, a particular type of ionisation is coupled to a particular mass analyser i.e., EI,CI and FAB with magnetic sector analyser; ESI with quadrupole and MALDI with TOF detection. Magnetic sector mass analyser Quadrupole mass analyser

9. MAGNETIC SECTOR ANALYSER :-

10. TIME-OF-FLIGHT (TOF) : The TOF is a mass analyzer that allows ions to flow down a field free region; which allows the ions with a greater velocity, lighter ions, to hit the detectors first. The kinetic energy of an ion leaving a source is given by: T = eV = mv 2 /2 ……(i) Where velocity v is defined by the length of the path divided by time: v = L/t ; t = L/v …….(ii) Substituting the value of v from eq. (i), From the above eq. it is clear that mass is directly proportional to time, hence, an ion of greater mass will strike the detector at a lower rate than ion of comparatively smaller mass.

11. DETECTOR The ions from the mass analyser impinge on a surface of a detector where the charge is neutralised, either by collection or donation of electrons. That leaves a space amongst the electrons in the metal, and the electrons in the wire shuffle along to fill it. A flow of electrons in the wire is detected as an electric current that is amplified and ultimately converted into a signal that is processed by a computer. Electron multiplier are used as detectors for many types of MS. These are frequently combined with a conversion dynode

12. CONVERSION DYNODE ELECTRON MULTIPLIER

13. TYPES OF PEAKS IN MS Molecular ion peak : When a molecule is bombarded with an electron of 9eV-15eV energy, the molecular ion is produced by loss of a single electron. Fragment ion peak : When an energy is given further more upto 70eV, fragment ion is produced. Metastable ion peak : Ions resulting from the decomposition between the source region and the analyser, are the metastable ion. These appear as broad peak as metastable ion peaks. Multicharged ion: Peaks produced by the ions with 2 or 3 charges instead of single charge is called as multicharged ion peak.

14. Data from MS showing different peaks

15. APPLICATIONS OF MS Elucidation of the structure of the organic and biological molecules. Determination of molecular mass of peptides, proteins and oligonucleotides. Analyses of aerosol particles. Determination of pesticide residues in food. Identification of drugs abuse and metabolites of drugs of abuse in blood, urine and saliva.

16. COMPARISON OF MASS SPECTROMETRY AND EDMAN DEGRADATION S.No. Mass Spectrometry S.No. Edman Degradation 1. Protein characterization involves confirmation of the N- and C-termini of the protein of interest. 1. Characterization of the protein or peptide from the amino terminus. 2. Proteins can be identified even after post- translational modification. 2. It does not work if the N-terminus has been chemically modified. 3. More sensitive as it requires small amount of sample less than a picomole 3. Requires sample amount of at least 1 picomole or more than that. 4. Long peptides can be sequenced 4. Peptides sequenced cannot have more than 50-60 residues (better works with 30 amino acid residues)

17. LIMI T A TIONS OF MS The sample subjected to MS cannot be recovered further. Cannot distinguish between isomers of a compound having the same m/z ratio.