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Premium member Presentation Transcript Slide 1: Mass Spectroscopy: Separation of Ions on the Basis of Mass to Charge RatioGeneral Setup: General Setup Common to all mass spectrometers are Sample Inlet (2) Ionization Source (3) Mass Analyzer (4) Ion Detector (5) Vacuum SystemMASS ANALYZER: MASS ANALYZER Mass analyzer separates the ion according to their mass to charge ratio Characteristics: It should have high reslution Should be capable of differentiating very minute difference Should allow passage of a sufficient no of ions to yield readily measurable currents Should have low pressurein the range of 10 -5 to 10 -7 so that no collision in mass analyzer occur It should have high rate of transmission.RESOLUTION: RESOLUTION Capacity of Mass analyzer is denoted by resolution It is the capability of analyzer to separate ion based on mass to charge ratio. R=m / m where m =higher mass peak or average mass peak m=mass difference between two adjacent peaks that are just resolved Two peaks are considered to be resolved when height between valley is less than 10 %ANALYZER: ANALYZER 1.Magnetic sector analyzer 2.Double focussing analyzer 3.Time of flight analyzer 4.Quadrupolar analyzer 5.Ion cyclotron analyzer 6.Ion trap analyzer 7.Radiofrequency analyzer 8.Cycloidal focussingSlide 6: Magnetic sector analyzer, Double focussing & Cycloidal focussing analyzer depends on Voltage or magnetic field Time of Flight, which depends on time Electric Quadrupole, which depends on AC/DC Currents Common to these Mass Analyzers is the separation of particles on the basis of mass to charge ratiosSlide 7: MAGNETIC SECTOR ANALYZERMagnetic sector analyzer: Magnetic sector analyzerSlide 9: Any ion possesses either Kinetic energy & Potential energy. If an ion which is formed in an ionic source initially has no Kinetic energy, the energy that is imposed by the potential drop V between the accelerating electron is zV E= z V where z=charge on ion V= potential applied an accelerating slit Once ion enters the slit potential energy is equivalent to kinetic energy E= z V =1/2mv 2 v=velocity of ion with which it is posseses V 2 =2zV/m------------------(1) When this type of ions enters in the magnetic field two types of forces exists.Slide 10: 1. Centripetal forces(exerted by magnetic field.) F M =Bzv-------------(1) v=velocity B=magnetic field strength. 2. Centrifugal force Fc=mv 2 /r r = radius When ion gets in equliberium then both forces are equal Fm=Fc Bzv = mv 2 /r therefore v= Bzr/m------(2) Putting 2 in 1 we get B 2 z 2 r 2 /m 2 = 2zV/m therefore 2V = B 2 r 2 z/m Therefore m/z= B 2 r 2 /2V For a fixed radius of curvature the m/z of ions which strike the detector can be controlled etiher by VarryingSlide 11: 1. Magnetic field strenght B or 2. Accelerating potential V. The angle of deflection of ionic beam in a mass analyzer is normally fixed at 60 0 , 90 0 or 180 0 Advantage: 1.Simple in construction 2. Cost is high 3.Produce low resolutionMagnetic-Sector Mass Spectrometry: Magnetic-Sector Mass Spectrometry In summary, by varying the voltage or magnetic field of the magnetic-sector analyzer, the individual ion beams are separated spatially and each has a unique radius of curvature according to its mass/charge ratio.DOUBLE FOCUSSING ANALYZER: DOUBLE FOCUSSING ANALYZER In this instruments ion s from the source pass the accelerated slit and enters the electrostatic field. Lighter ions are deflected the most and heavier the least.The dispersed ions then fall on the photographic plate and are thus recorded. It consists of two devices 1.Electrostatic analyzer(electrostatic sector) 2. Magnetic sector analyzer 1 Electrostatic analyzer(electrostatic sector ) It consists of two curved metallic plates across which dc potential is applied.This potential has the effect of limiting the kinetic energy reaching the magnetic sector to a closely defined rangeElectrostatic sector: Electrostatic sector If ions has energy greater than average it strikes the upper side of ESA slit It ion has energy less than average it strikes the lower side of ESA slitand are thus removed. All those ions that have nearly identical kinetic energy are focussed on slit 2 between electric and magnetic field. After passing through slit 2 ion enters the magnetic sector There are two double focussing design in common use: A) Neir John’s design: It make use of 90 o sector B)Mattauch herzog geometry: It make use of 31 o 50Time of FlightAnalyzer: INTRODUCTION: Separates ions based on flight time Operates in pulsed mode Ions accelerated by an electric field Lighter ions reach the detector first Time of FlightAnalyzer Time of FlightAnalyzer: Time of FlightAnalyzer All the ions from the ionic source have same kinetic energy butthey possess different massto charge ratio.Hence different velocity Hence lighter ions will moved faster and will strike the detector Thus time required by an ion to reach the detector is measured. From this time and known experimental parameter one can find mass to charge ratio of the particle.This method is a powerful tool for finding the m/z ratio of charged particle atoms & moleculesTime of FlightAnalyzer: Time of FlightAnalyzer Ion from ionic source enters the evacuated flight tube through accelerating grid.which is kept more positive to applying potential.The potential between acceleratin g grid and slit is kept zero during measurement.A repelling potential is placed on the slit after an ion of interest has passed and before ions of next m/z ration enters.that allows the detector and readout device that respond relatively slowly to measure the intensity of a single ion. Potential energy of a charged particle in an electric field is related to its charge and to strength in field Ep=zV-------(1) The kinetic energy of any mass is Ek=1/2mv 2-----(2) In effect potential energy is converted to kinetic energy Ep=Ek where z=charge of ion ,V=applied potential zv=1/2mv 2 v=velocityTime of FlightAnalyzer: Time of FlightAnalyzer A velocity of charged particle can be determined in a time of flight since the length of the path of the flight of ion is known and the time of flight of ion can be measured using sophisticated stopwatch technology m/z-2V/v 2 m/z=2Vt 2 /d 2 since d&V are fixed m/z=kt 2 Heavier ions will require more time than lighter ones Advantage: Simple Ease of accessibility of the ion source Unlimited mass range Disadvantage:Less satisfactory than magnetic focussing Quadrupole Mass Analyzers : Quadrupole Mass AnalyzersBenefits Quadrupole Mass Analyzers: Benefits Quadrupole Mass Analyzers • easy to use • simple construction • fast • low cost • can achieve unit to 0.1 m/q resolutionBasis of Quadrupole Mass Filter: Basis of Quadrupole Mass Filter consists of 4 parallel metal rods, or electrodes opposite electrodes have potentials of the same sign one set of opposite electrodes has applied potential of [U+Vcos(ωt)] other set has potential of - [U+Vcosωt] U= DC voltage, V=AC voltage, ω= angular velocity of alternating voltageOperation of Quadrupole Mass Filter: Operation of Quadrupole Mass Filter voltages applied to electrodes affect trajectory of ions with the m/q ratio of interest as they travel down the center of the four rods these ions pass through the electrode system ions with other m/z ratios are thrown out of their original path these ions are filtered out or lost to the walls of the quadrupole, and then ejected as waste by a vacuumsystem in this manner the ions of interest are separatedSummary: Summary Magnetic Mass Spectroscopy shows the relationship between voltage and mass to charge ratio: m/q = B 2 r 2 /(2V) Time of Flight Mass Spectroscopy shows the relationship between tube time and mass to charge ratio: m /q = 2Vt 2 /d 2 Electric Quadrupole shows the relationship between AC/DC currents and mass to ratios.Ion Trap Analyzer: Ion Trap AnalyzerIon Trap Analyzer: Ion Trap Analyzer The ion trap works on the same physical principles as the quadrupole mass analyzer, but the ions are trapped and sequentially ejected. Ions are trapped in a mainly, in a space defined by a ring electrode (usually connected to the main RF potential) & between two endcap electrodes (typically connected to DC or auxiliary AC potentials). The sample is ionized either internally (e.g. with an electron or laser beam), or externally, in which case the ions are often introduced through an aperture in an endcap electrode. Advantage: Compact in size Less cost than sector or quadrupole ION CYCLOTRON ANALYZER: ION CYCLOTRON ANALYZER Fourier transform mass spectrometry , is a type of mass analyzer (or mass spectrometer ) for determining the mass-to-charge ratio (m/z) of ions based on the cyclotron frequency of the ions in a fixed magnetic field.  The ions are trapped in a Penning trap (a magnetic field with electric trapping plates) where they are excited to a larger cyclotron radius by an oscillating electric field perpendicular to the magnetic field. The excitation also results in the ions moving in phase (in a packet). The signal is detected as an image current on a pair of plates which the packet of ions passes close to as they cyclotron. The resulting signal is called a free induction decay (FID), transient or interferogram that consists of a superposition of sine waves . The useful signal is extracted from this data by performing a Fourier transform to give a mass spectrum .RADIOFREQUENCY ANALYZER: RADIOFREQUENCY ANALYZER If the ions coming out of the accelerator are of uniform energy an assembly of grid is employed to select ions according to their velocities. The alternative grids are connected to radio frequency source. Remaining grids are connected together to a steady potentials. In front of radio frequency analyzer an energy sector is adjusted.RADIOFREQUENCY ANALYZER: RADIOFREQUENCY ANALYZER A positive voltage is applied to the ion repeller which will repel all those ions which have received less then a specified function of the total available energy. Mass Spectrum will be obtained by varying the frequency of the rf section of the analyzer.OMEGATRON ANALYZER : OMEGATRON ANALYZER It is based on cyclotron principle of accelerating ions in a spiral path In this analyzer resonant ions are collected at a detector electrode It is mostly used as residual gas analyzerDETECTORS: DETECTORS It records the charge induced or current produced when an ion passes by or hits a surfaceTYPES OFDETECTORS: TYPES OFDETECTORS FARADAY CUP ELECTRON MULTIPLIER PHOTOGRAPHIC PLATE SCINTILLATION COUNTER DINODIC STRIP ELECTRON CHANNEL ELECTRON MULTIPLIR MICROCHANNEL PLATE DALY DETECTOR FARADAY CUP DETECTOR : FARADAY CUP DETECTOR the incident ion strikes the dynode surface (see Fig. 1) which emits electrons and induces a current which is amplified and recorded. The dynode electrode is made of a secondary emitting material like CsSb, GaP or BeO. The Faraday cup is a relatively insensitive detector but is very robust. It is ideally suited to isotope analysis and IRMS.ELECTRONMULTIPLIER: ELECTRONMULTIPLIER They are of two types : One type of electron multiplier (Fig. 2a) has series of dynodes maintained at increasing potentials resulting in a series of amplifications. The other type (the channel multiplier, Fig. 2b) has a curved ('horn' shaped) continuous dynode where amplifications occur through repeated collisions with the dynode surface Their are two types (Fig. 2) of electron multiplier, but they both work essentially by extending the principles of the Faraday cup. A Faraday cup uses one dynode and as a result produces one level of signal amplification. One type of electron multiplier (Fig. 2a) has series of dynodes maintained at increasing potentials resulting in a series of amplifications. The other type (the channel multiplier, Fig. 2b) has a curved ('horn' shaped) continuous dynode where amplifications occur through repeated collisions with the dynode surface. In both cases, ions pass the conversion dynode (depending on their charge) and strike the initial amplification dynode surface producing an emission of secondary electrons which are then attracted either to the second dynode, or into the continuous dynode where more secondary electrons are generated in a repeating process ultimately resulting in a cascade of electrons. Typical amplification is of the order of one million to one.Electron Multiplier: Electron Multiplier In both cases, ions pass the conversion dynode (depending on their charge) and strike the initial amplification dynode surface producing an emission of secondary electrons which are then attracted either to the second dynode, or into the continuous dynode where more secondary electrons are generated in a repeating process ultimately resulting in a cascade of electrons. Typical amplification is of the order of one million to one.Channel Electron Multiplier: Channel Electron MultiplierDaly detector: Daly detector It consists of a metal "doorknob", a scintillator ( phosphor screen) and a photomultiplier . [ WORKING: Ions that hit the doorknob release secondary electrons. A high voltage (ca. -20,000 V) between the doorknob and the scintillator accelerates the electrons onto the phosphor screen where they are converted to photons. These photons are detected by the photomultiplier .Scintillation counter: Scintillation counter Here the ions initially strike a dynode which results in electron emission. These electrons then strike a phosphorous screen which in turn releases a burst of photons. The photons then pass into the multiplier where amplification occurs in a cascade fashion - much like with the electron multiplier. The main advantage of using photons is that the multiplier can be kept sealed in a vacuum preventing contamination and greatly extending the lifetime of the detector. Most commmonly usedMicro Channel Plate Detector: Micro Channel Plate Detector micro-channel plate is a slab made from highly resistive material of typically 2 mm thickness with a regular array of tiny tubes or slots (microchannels) leading from one face to the opposite, densely distributed over the whole surface. The microchannels are typically approximately 10 micrometers in diameter (6 micrometer in high resolution MCPs) and spaced apart by approximately 15 micrometers; they are parallel to each other and often enter the plate at a small angle to the surface (~8° from normal).Working:: Working: Each microchannel is a continuous-dynode electron multiplier , in which the multiplication takes place under the presence of a strong electric field. A particle or photon that enters one of the channels through a small orifice is guaranteed to hit the wall of the channel due to the channel being at an angle to the plate and thus the angle of impact. The impact starts a cascade of electrons that propagates through the channel, which amplifies the original signal by several orders of magnitude depending on the electric field strength and the geometry of the micro-channel plate. After the cascade, the microchannel takes time to recover (or recharge) before it can detect another signal. The electrons exit the channels on the opposite side where they are themselves detected by additional means, often simply a single metal anode measuring total current. In some applications each channel is monitored independently to produce an image. Phosphors in combination with photomultiplier tubes have also been used. Dual microchannel plate detector: Dual microchannel plate detector Most modern MCP detectors consist of two microchannel plates with angled channels rotated 90° from each other producing a chevron (v-like) shape. The angle between the channels reduces ion feedback in the device. In a chevron MCP the electrons that exit the first plate start the cascade in the next plate. The advantage of the chevron MCP over the straight channel MCP is significantly more gain at a given voltage. The two MCPs can either be pressed together or have a small gap between them to spread the charge across multiple channels. Z stack MCP: This is an assembly of three microchannel plates with channels aligned in a Z shape. Single MCPs can have gain up to 10000 but this system can provide gain more than 10 million.REFERENCE: REFERENCE Instrumental method of analysis –G.R Chatwal Instrumental method of analysis-Willard www.google .comTHANK YOU: THANK YOU You do not have the permission to view this presentation. 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