ppt of mass spectrometry

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mass spectrometry

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MASS SPECTROMETY PRESENTED BY D.SHWETHA 08FDIR0011 1

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CONTENTS INTRODUCTION PRINCIPLE INSTRUMENTATION RECORDING MASS SPECTROGRAM APPLICATIONS OF MASS SPECTROMETRY 2

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INTRODUCTION Mass spectrum is an analytical which can provide information concerning molecular structure of organic and inorganic compounds. It is used to determine the molecular weight as high as 4000. It based on sample principle, it is very complex and expensive instrument. The analytical chemist is attracted to the mass spectrometry mainly by its speed and reliability. 3

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PRINCIPLE Mass spectra is also called as positive ion spectra or line spectra. We use electron bombardment to convert neutral molecules to a positive charged one. Also there is no ground or excited state like other types of spectroscopy. Obtaining mass spectra consists of 2 types Conversion of neutral molecule into a charged molecule, preferably to a positively charged molecule. Separation of positively charged fragments formed, based on their masses, by using electrical or magnetic field or both. The Sample is bombarded with high energy electron beam (70eV), where an electron is knocked off from every molecule. Hence the molecules become positively charged. 4

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When a positive potential (accelerating potential) is applied, as molecules are positively charged, they get repelled and travel with great speed, in straight path. Potential energy = Kinetic energy of molecule eV =1/2 mv 2 Where e=charge of ion V=acceleration voltage m=mass v=velocity after acceleration When a magnetic field or electrical field applied, the positive charged fragments which were travelling in straight path, now travels in curved path. When they travel in a curved path under the influence of magnetic field, the fragments are separated into different masses because the radius of curvature depends upon their respective masses. Under the magnetic field, Hev = mv 2 /r v = reH/m 5

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Where r = radius of ion path H = strength of magnetic field e = charge of ion v = velocity after acceleration m = mass substituting the values of the first equation (i.e. eV = 1/2mv2) eV=1/2 *m*(reH/m) 2 m/e = H 2 r 2 /2v m/e is directly proportional to r 2 (H , v maintain constant). Therefore mass is directly proportional to (radius of ion path) 2 since, e = 1. 6

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INSTRUMENTATION The instrumentation of Mass Spectrometer consists the following components. The inlet system (or sample handling system) The ion source ( or ionization chamber) The electrostatic accelerating system The magnetic field The ion separator (analyzer) Ion detectors 7

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INLET SYSTEM HEATED INLET SYSTEM: Gases and less volatile liquids, the liquids vaporized externally an then slowly introduced into the ionization source. DIRECT INLET SYSTEM: Solids , nonvolatile liquids, unstable compounds directly introduced into the ion source. NON VOLATILE LIQUIDS: steroids, carbohydrates polymeric substances etc.. 8

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ION SOURCE From the inlet system the sample is introduced into ionization chamber where a beam of electrons put across the molecules of the sample. The ion sources are of different type Electron impact ion source Chemical ionization MALDI(Matrix-assisted laser desorption ionization) 9

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ELECTRON IMPACT IONISATION In this an electrically heated filament A produces thermal electrons which are then accelerated by anode A'. In this way A beam of electrons which intersects of flow of simple molecules is produced, resulting in the formation of positively charged ions. Then these ions are with -drawn by the electric field which exist between the repeller plate C and first accelerated plate B. The intermediate plate B' helps to focus the ion beam and second accelerated plate C gives a final acceleration to the ions. The energy of electron beam is controlled by potential on anode A’. 10

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The intermediate plate B' helps to focus the ion beam and second accelerated plate C gives a final acceleration to the ions. The energy of electron beam is controlled by potential on anode A'. If the energy of the electron beam is low there occurs only the production of singly charged molecular ions, resulting in a mass spectrum having almost a single peak corresponding to the mass of the origin molecule. M + e -  M + + 2e - If the energy of the electron beam is increased, this yields highly excited ion which may produce fragments if it is complex or may knock out the second electron. M + e -  M 2+ + 2e - M 2+ + e -  M 3+ + 2e - 12

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CHEMICAL IONISATION In this technique a reaction gas like methane is introduced along the sample to be analyzed by mass spectrometer in the ionization chamber. When a beam of electrons passed through the ionization chamber, the reaction gas (methane) undergoes ionization to produce ions which react further with neutral molecules to form products. The products so formed are reactive species and can interact with the sample molecules to form positive ions. 13

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MATRIX ASSISTED LASER DESORPTION In this technique low concentration of the analyte is uniformly dispersed in a solid or liquid matrix deposited on the metal plate. The metal plate put in vaccum chamber and laser beam focused on the sample. Then matrix and the sample strongly absorb the laser radiation. Then the sample gets ionized. The most common type of mass analyzer used with the is the time of flight analyzer. Various types of matrix Nicotinic acid matrix - to analyte the proteins glycoproteins Ferulic acid matrix to analyte the proteins and Caffieic acid matrix oligonucleotides Succinic acid – to analyte the proteins. 15

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THE ELECTROSTATIC ACCELERATING SYSTEM The positive ions formed in the ionization chamber are withdrawn by the electric field which exist between the first accelerating plate B and the second repeller plate C. A strong electrostatic field between B and C of 400 to 4000V accelerates the ions of masses m1 m2 m3…….. to their final velocities. The ions which escape through the slit D consist of a collimated ribbon of ions having velocities and kinetic energies given by eV = ½ m v = ½ m v = ½ m v ……….. Whenever the mass spectrometer is to record the spectrum, the second accelerator is charged to an initial potential of 4000volts.Then this charge is permitted to leak off to ground at a controlled rate over a period of 25 minutes . 17

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MAGNETIC FIELD As the accelerated particles from the electrical field enter the magnetic field enter the magnetic field, the force of magnetic field requires them to move in a curved path. The radius of this curvature, r, is dependent upon the mass, m, the accelerating voltage, V, the electron charge, e, and the strength of the magnetic field, H. It is the two properties m/e and r upon which mass spectrometry is based. The mass to charge ratio and the radius to the curvature are interdependent, where as a change in either the accelerating potential or the magnetic field will change m/e and r.. All particles greater or less m/e ratio will strike the side of the separation tube and will be neutralized. 18

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THE ION SEPARATOR It is the part of mass spectrometer which separates the ions according to their masses. An analyzer must possess the following characteristics. It should have a high resolution. It must have high rate of transmission of ions. There are different types of analyzers are there they are Single focusing magnetic analyzer Double focusing analyzer Quadrupole mass spectrometer Time of flight systems 19

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Single Focusing Magnetic Analyzer It has a horse shoe shaped which is evacuated. It has a sample inlet, electron bombarding source and accelerating plates on one end. At the other end, collector slit is present. At the curvature of the tube, there is a provision to apply electrical or magnetic field. 20

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2.Double focusing mass analyzer Double beam instrument where two ion beam from independent sources pass side by through a common mass analyzer and are detected by separate collectors are available . 21

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3.Quadrupole Mass Spectrometry A quadrupole mass filter consists of four parallel metal rods arranged as in the figure below. Two opposite rods have an applied potential of (U+Vcos (wt)) and the other two rods have a potential of -(U+Vcos(wt)), where U is a dc voltage and Vcos(wt) is an ac voltage. The applied voltages affect the trajectory of ions traveling down the flight path centered between the four rods. For given dc and ac voltages, only ions of a certain mass-to-charge ratio pass through the quadrupole filter and all ions are thrown out of their original path. 22

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Quadrupole mass spectrometers consist of an ion source, ion optics to accelerate and focus the ions through an aperture into the quadrupole filter, the quadrupole filter itself with control voltage supplies, an exit aperture, an ion detector, detection electronics, and a high-vacuum system. 23

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4.Time-of-Flight Mass Spectrometry (TOF-MS) A time-of-flight mass spectrometer uses the differences in transit time through a drift region to separate ions of different masses. It operates in a pulsed mode so ions must be produced or extracted in pulses. This schematic shows ablation of ions from a solid sample with a pulsed laser. The reflection is a series of rings or grids that act as an ion mirror. This mirror compensates for the spread in kinetic energies of the ions as they enter the drift region and improves the resolution of the instrument. The output of an ion detector is displayed on an oscilloscope as a function of time to produce the mass. 24

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ION DETECTORS 1.The Faraday Cup or Cylinder The Faraday cup or cylinder electrode detector is very simple. The basic principle is that the incident ion strikes the dynode surface which emits electrons and induces a current which is amplified and recorded. The Faraday cup is a relatively insensitive detector but is very robust. 26

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2.The Electron Multiplier Electron multipliers are probably the most common means of detecting ions, especially when positive and negative ions need to be detected on the same instrument. Their are two types of electron multiplier. 1.A Faraday cup uses one dynode and as a result produces one level of signal amplification. One type of electron multiplier has series of dynodes maintained at increasing potentials resulting in a series of amplifications. 2.The other type has a curved continuous dynode where amplifications occur through repeated collisions with the dynode surface. 27

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RECORDING OF MASS SPECTROGRAM Mass spectrogram is generally represented by a bar graph in which intensity on the ordinates is plotted against m/e ratio on the abscissa. But this method is cumbersome. Therefore, the data are often represented by a graph which plots relative abundance on the ordinate and m/e ratio on the abscissa. The relative abundance of the fragment is given as the percent intensity of a given peak relative to the most intense peak in the spectrum. Time required to obtain a complete mass spectrum depends on the particular instrument. The time varies from 20 min to 1 second. 29

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Resolution of Mass Spectrometer The ability of a mass spectrometer to distinguish between the ions of nearly equal masses is termed as the ''resolution'' of the instrument. For many inorganic and organic application, the resolution is expressed as follows Resolution = m/ /\ m Where m and /\ m are the mass numbers of two neighboring peaks of equal intensity in the mass spectrum. The resolution of the instrument is generally decreased due to the following factors Distribution of kinetic energies produced in the electron beam. Variation in the accelerating voltage Variation in the magnetic field Poorly collimated ion beam Space charge of the ion beam Width of the ion beam as determined by the slits Pressure in the spectrometer 30

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TYPES OF IONS PRODUCED IN MASS SPECTROMETRY Molecular ions The observed % abundance (we usually refer to intensity when we are speaking of signals) of the suspected molecular ion must correspond to expectations based on the assumed molecular structure. Molecules containing p- or non-bonding electrons are less likely to fragment readily and will often yield intense signals for M+ ions in the spectrum. Conversely, highly branched molecules (which can generate relatively unstable tertiary radical cations) or molecules that can lose neutral stable radicals (like halides) will give less intense or completely absent signals for M+. The relative abundance of the molecular ion will usually correspond to the following sequence: Aromatic and conjugated compounds > alicyclic compounds > sulfides > unbranched hydrocarbons > mercaptans > ketones > aldehydes > amines > amides > esters > ethers > carboxylic acids > branched hydrocarbons > alcohols. 31

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Base peak If an electron beam of energy of 70eV is used in a mass spectrometer, the molecular ion is produced by the loss of a single electron which undergoes splitting to form many fragments, and the parent peak in mass spectrum is called the base peak and the heights of all other peaks are measured with respect to it .Generally the ion abundances are expressed in terms of the base peak. Multiple charged ions In mass spectrometer the ions are generally carrying a single positive charge. However sometimes doubly charged or even triply charged ions are found in the mass spectrum. The doubly and triply charged ions are recorded at a half or a third of the m/e value of the singly charged ions. The formation of these multiply charged ions are more common in heteroaromatic molecules . 32

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Negative ions In addition to positive ions negative ions may be formed from electron bombardment of the sample. The formation of negative ions is very rare but these can be produced in three ways : AB+e ----------- A + B - (dissociation resonance capture) AB+e ----------- AB - (resonance capture) AB+e -----------A + + B - +e - (ion pair production) Rearrangement ions In some cases, fragments are observed which are not a part of the original molecule. These are known as rearrangement ions which are formed from the molecular ions by redistribution of atoms or group of atoms at the moment of decomposition of the molecular ion. 33

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Metastable ions The life time of an ion may be so small that it undergoes decomposition during its passage between the source and collector units in the spectrometer .The ions resulting from the decomposition between the source region and the magnetic analyzer are called metastable ions which appear in the spectrum as broad peaks at non-integral mass numbers. The relationship between the apparent m/e of the metastable ion and its parent is given by the following formulation. m1 +  m2 + + m0 The metastable ion is observed at a mass m* which is related to m and m by the eqation. m*= m2 2 / m1 Where m is the mass of parent ion, m the mass of daughter (metastable) ion , m the mass of the neutral fragment and m* apparent mass of the metastable ion . 34

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Fragmentation The molecular ions are energetically unstable, and some of them will break up into smaller pieces. The simplest case is that a molecular ion breaks into two parts - one of which is another positive ion, and the other is an uncharged free radical. The uncharged free radical won't produce a line on the mass spectrum. Only charged particles will be accelerated, deflected and detected by the mass spectrometer. These uncharged particles will simply get lost in the machine - eventually, they get removed by the vacuum pump. The ion, X + , will travel through the mass spectrometer just like any other positive ion - and will produce a line on the stick diagram. 35

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All sorts of fragmentations of the original molecular ion are possible - and that means that you will get a whole host of lines in the mass spectrum. For example, the mass spectrum of pentane looks like this: 36

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General rules for interpretation of mass spectra The exact molecular weight 2.The Isotope effect 3.Nitrogen rule 4.Ring rule 37

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APPLICATION OF MASS SPECTROMETRY 2.Isotopic abundance 1.Molecular mass determination 3.Quantitative analysis of mixtures 4.Distinction between the cis and trans-Isomer 5.Determination of Ionization Potential 6.Bonding 7.Reaction Kinetics 8.Impurity Detection 9.Identification of the unknown compound 10.Characterization of polymers 38

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REFERENCES 1. Instrumental Methods of Chemical Analysis by GURDEEP R CHATWAL, SHAM K. ANAND. 2. Text Book of Pharmaceutical Analysis, Third Edition, Dr. S. RAVI SANKAR. 3. www.google.com 39

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