Instrumentation of Mass Spectrometry

By;
Sowmya B U (Student)
Under the Guidance;
Mrs. Nethravathi P C
Department of Studies and Research in Organic Chemistry
Tumkur University Tumakuru.

1. Department of Studies and Research in Organic Chemistry Seminar presentation on “Instrumentation of Mass Spectrometry” B y Ms. Sowmya B U Register number: 19POC131 II MSc, IV Sem Department of Studies and Research in Organic Chemistry Tumkur University, Tumakuru Under the guidance Mrs. Nethravathi P C Department of Studies and Research in Organic Chemistry Tumkur University, Tumakuru Submitted to Dr. Aruna Kumar D B The Coordinator Department of Studies and Research in Organic Chemistry Tumkur University, Tumakuru 2020-21 TUMKUR UNIVERSITY

2. 2 CONTENTS Introduction to Mass Spectrometry Basic Principles of Mass Spectrometry Mass Spectrometer Overview Major components of Mass Spectrometer Conclusion References Sample Inlet Ion Source Mass Analyzer Detector, Amplifier and Recorder Data System 1 2 3 4 5 6 A B C D E

3. 3 1. Introduction to Mass Spectrometry Mass spectrometry is unlike the other spectroscopic techniques that it does not measure the interaction of molecules with the spectrum of energies found in the electromagnetic spectrum, but the output from the instrument has all other spectroscopic characteristics, in showing an array of signals corresponding to a spectrum of energies; to highlight this distinction , the name mass spectrometry is preferred. The fundamental principles date to the late 1890s when J. J. Thomson determined the mass-to-charge ratio of the electron, and Wien studied magnetic deflection of anode rays and determined the rays were positively charged. Each man was honored with the Nobel Prize (Thomson in 1906 and Wien in 1911) for their efforts. In 1912–1913, J. J. Thomson studied the mass spectra of atmospheric gases and used a mass spectrum to demonstrate the existence of neon-22 in a sample of neon-20, thereby establishing that elements could have isotopes. The earliest mass spectrometer, as we know it today, was built by A. J. Dempster in 1918. J. J. Thomson (1856-1940)

4. 4 2. Basic Principles of Mass Spectrometry The first step in the mass spectrometric analysis of compounds is the production of gas-phase ions (molecular ion) of the compound, for example by electron ionization; M + e - → M •+ + 2e - This molecular ion normally undergoes fragmentations. Because it is a radical cation with an odd number of electrons, it can fragment to give either a radical and an ion with an even number of electrons or a molecule and a new radical cation. m 1 + + m • M •+ m 1 •+ + m 2 All these ions are separated in the mass spectrometer according to their mass-to-charge ratio and are detected in proportion to their abundance . The plot of ion abundance versus mass-to-charge ratio will gives the mass spectra. (Molecular ions having lifetime of at least 10 -5 sec will reach the detector without breaking into fragments).

5. 5 B D A C E H J K L F G I Sample inlet Sample vapourization chamber Heating coil Ionization chamber Electron source Neutral ions collector Accelerating plates( a , b and c ) Magnet Mass analyzer Detector Amplifier Recorder 3. Mass Spectrometer Overview

6. 6 Flow Chart Sample inlet (Sample insertion) Sample vapourization chamber (Vapourization) Ionization chamber (Ionization and Fragmentation) (Acceleration) Recorder Amplifier (Amplification) Detector (Detection) Mass analyzer (Mass separation) Magnetic analyzer (Deflection) Mass spectrometer

7. 7 4. Major Components of Mass Spectrometer A. Sample Inlet A sample inlet system provides stream of molecules. A sample studied by mass spectrometry may be a gas, a liquid, or a solid. The sample is introduced into a larger reservoir from which the gaseous molecules (from vaporization using heating coils) can be drawn into the ionization chamber. Handling of gas sample: It involves the transfer of sample containers of known volume coupled to mercury manometer. Introduction of liquid sample: The sample is converted into gaseous state then injected by using a micropipette to a sintered glass disk under a layer of molten gallium. Introduction of solid sample: Samples with lower vapor pressures are inserted directly into the ionization chamber on the end of a probe, and their volatilization is controlled by heating the probe tip . Sample introduction methods: Gas Chromatography Direct infusion Liquid chromatography Capillary electrophoresis Direct ionization Direct insertion probe

8. 8 B. Ion Source The several methods available for inducing the ionization of organic compounds but electron bombardment is routinely used . Organic molecules react on electron bombardment in two ways: either an electron is captured by the molecule , giving a radical anion, or an electron is removed from the molecule, giving a radical cation. Most organic molecules form molecular ions (M •+ ) when the energy of the electron beam reaches 10-15 eV (≈ 103 kJ mol-I). While this minimum ionization potential is of great theoretical importance, fragmentation of the molecular ion only reaches substantial proportions at higher bombardment energies, and 70 eV (≈ 6 X 103 kJ mol") is used for most organic work . When the molecular ions have been generated in the ionization chamber, they are expelled electrostatically by means of a low positive potential on a repeller plate a in the chamber. Once out, they are accelerated down the ion tube by the much higher potential between the accelerating plates b and c .

9. 9 C. Mass Analyzer Seperation of the ions in the analyzer: In a magnetic analyzer, ions are separated on the basis of m/z values. The kinetic energy, E of an ion of mass m travelling with velocity v is given by ; E =1/2mv 2 . The potential energy of an ion of charge z being repelled by an electrostatic field of voltage V is zV . When the ion is repelled, the potential energy, zV is converted into the kinetic energy, 1/2mv 2 so that; zV = 1/2mv 2 v 2 = 2zV/m 1 When ions are shot into the magnetic field of the analyzer, they are drawn into circular motion by the field, and at equilibrium the centrifugal force of the ion (mv 2 /r) is equated by the centripetal force exerted on it by the magnet ( zBv ) . mv 2 /r = zBv v = zBr /m 2 Combining equation 1 and 2 2zV/m = [ zBr /m] 2 m/z = B 2 r 2 /2V Where; B is strength of the focusing magnet V is accelerating voltage.

10. 10 Types of Mass Analyzer The Magnetic Sector Mass Analyzer Double Focusing Mass Analyzer Quadrupole Mass Analyzer Time-of-Flight Mass Analyzer Fourier Transform-Ion Cyclotron Resonance Deflection: Ions are deflected by a magnetic field due to difference in their masses. The lighter mass, more they are deflected. It also depends upon the charge on the ion the more positive charge, more it will be deflected.

11. 11 D. Detector, Amplifier and Recorder The focused ion beam passes through the collector slit to the detector , which must convert the impact of a stream of positively charged ions into an electrical current. This must be amplified and recorded , either graphically or digitally. Several different amplification systems are used by different manufacturers, but the most common is the electron multiplier, which operates in a manner similar to the photomultiplier detector. A series of up to twenty copper-beryllium dynodes transduces the initial ion current, and the electrons emitted by the first dynode are focused magnetically from one dynode to the next; the final cascade current is thus amplified more than one million times. Two essential features of the recording system in a mass spectrometer are that it must (a) have a very fast response, and be able to scan several hundred peaks per second, and (b) be able to record peak intensities varying by a factor of more than 103 .

12. 12 E. Data System The analog signal coming from the detector is first converted to digital form in an analog-to-digital convertor, or ADC, and the digitized data are stored in computer memory. Computer-controlled instruments produce the mass spectral data in several forms, either as a list of fragment ions or plotted directly as a bar diagram. Improvements in instrumentation have largely eliminated the need for sets of mirror galvanometers. Accurate mass calibration is carried out each day by recording the mass spectrum of appropriate reference compounds such as perfluorokerosene (PFK) or cesium iodide clusters for very high molecular masses. Identification of known compounds can be carried out by searching through computer-held digitized mass spectra; many collections are available commercially containing up to 100 000 compounds on file. Example: Mass Spectra of 1-pentanol

13. 13 5. Conclusion Organic chemists use mass spectrometry in three principal ways: To measure relative molecular masses (molecular weights) with very high accuracy. (2) To detect the places at which it prefers to fragment; from this can be deduced the presence of recognizable groupings within the molecule. (3) As a method for identifying analytes by comparison of their mass spectra with libraries of digitized mass spectra of known compounds.

14. 14 6. References Introduction to Spectroscopy- Pavia, Lampman, Kriz, Vyvyan- Fourth Edition (Page. No; 418-432) Organic Spectroscopy- William Kemp- Third Edition(Page. No; 285-292) A Text Book of Mass Spectrometry- Jurgen H. Gross (Page. No; 1-4) Principles and Applications of Mass Spectrometry- Edmond de Hoffmann, Vincent Stroobant- Third Edition-(Page. No; 1-5) https://en.m.wikipedia.org/wiki/Sample_preparation_in_mass_spectrometry Mass Spectrometry animation, instrumentation and working: https://www.youtube.com/watch?v=PLElbY8S8u_DKLj5f46jYuC

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