instrumentation of mass spectrometry

all things about instrumentation of mass spectrometry, with brief description about Mass Analyzers

1. Instrumentation of mass spectrometry Reported by Manali parab m.Pharmacy first year Pharmaceutics (sem ii) 1

2. Mass spectrometers Instrument that produces ions and separate them according to their mass to charge ratios, m/z. Following are components of mass spectrometer 1) inlet system 2) ion sources 3) mass analysers 4) detectors 5) vacuum system 2

3. Block diagram of Components of Mass Spectrometer 3

4. Description of instrument components 4

5. Ion sources Basic type Name of ion source Ionizing agents Gas phase Electron Impact (EI) Energetic electrons Chemical Ionization (CI) Reagent gaseous ions Field Ionization (FI) High potential electrode Desorption Field Desorption (FD) High potential electrode Matrix Assisted Laser Desorption Ionization (MALDI) Laser beam Fast Atom Bombardment (FAB) Energetic atomic beam 5

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7. Theory behind mass spectrometry Sample to introduce into the ion source of the mass spectrometer in the vapour form what is bombarded by electrons of about 70 eV The positive ions formed in the Ion source are accelerated by using the plates which are kept at the potential difference of about 3000 to 6000 volts the accelerated ions enter the magnetic field which is applied perpendicular to the direction of motion of ions this produces a force which is perpendicular both to the motion of the electrons and also to the direction of the applied magnetic field this force causes the deflection of Ions based on there mass to charge ratio Then the ions passed through and exits slit or collectors list and impinge upon a collector the signal received is then amplified in the form of peaks in the mass spectrum The ions of a different mass to charge values follow the path of different curvature  In the analyser in electric field all the ions acquired the same kinetic energy. The kinetic energy of the ions is expressed as m is the mass of ion and v is velocity. 7

8. The velocity of the heavier ions is lesser as compared to the light ions. Because of the higher mass heavier ion resists that changed in the direction of the motion also they are accelerated do a lesser extent as a compared to the light ions. Therefore they follow the path of the greater curvature because of them smaller mass. The light ions get deflected easily also they are accelerated more compared to the heavy ions Therefore they follow the path of a smaller curvature and reach the detector faster than the heavier ions By changing either the accelerating voltage(V) or the magnetic field(H), the radius (r) of the ion path can be changed which allows the ions of different mass to charge ratios to impinge upon the collector at the same point. The variation in the magnetic field provides a wide range of ions to be covered in a single sweep but the voltage sweep makes the scanning very rapid The relation between the magnetic field and accelerating voltage the radius of the ions path and the m/z value of an ion can be obtained as follows in an electric field the potential energy of i on is a converted to its kinetic energy in magnetic field as follows 8

9. In the electrical field, the potential energy(P.E.) of an ions converted to kinetic energy(K.E.) P.E. = K.E. Zv=1/2 mv 2 In the magnetic field the ions follws the semi-circular path and hence centripetal force is a balanced by the centrifugal force CPF=CFF Or Hzv=mv 2 /r Or v= Hzr/m by putting the value of v from equation the m/z of an ion can be obtained Zv=1/2 m(Hzr/m) 2 ½ H 2 z 2 r 2 /m V=1/2 H 2 zr 2 /m Hence m/z= H 2 r 2 /2V Above equation shows the radius of the ion paths various for the ions having the different mass to charge ratio if the values of H and V are kept constant. If all the ions of a different m/z values to be collected at the same point on a detector the value of a radius should be kept constant 9

10. This makes the fabrication of the mass spectrometer much simpler and allows the collection of a different ions at one point without the need for a change in a position of a collector's slit. This can be achieved by changing the other magnetic of a electric field at the magnetic field or both In some mass spectrometer the magnetic field is kept constant and the accelerating potential is gradually increased If H and r kept constant on the right hand side of this equation, then the value of the inversely proportional to the V that is heavier ions can be collected by applying a small and accelerating potential and vice versa If V and r are kept constant then the value of m/z is directly proportional to H. In this case at a lower have values of H, ions with the smaller values would be collected, and as we increase the value of H ions of higher m/z values would get recorded 10

11. Double focusing spectrometry The term double focusing is applied to mass spectrometers in which the directional aberrations and the energy aberrations of a population of ions are simultaneously minimized. Careful selection of combinations of electrostatic and magnetic fields. Ion beam is first passed through an electrostatic analyser (ESA) consisting of two smooth curved metallic plates across which a dc voltage is applied. Effect of limiting the kinetic energy of the ions reaching the magnetic sector to a closely defined range. Ions with energies greater than average strike the upper side of the ESA slit and are lost to ground. Ions with energies less than average strike the lower side of the ESA slit and are thus removed. The most sophisticated of these are capable of resolution in the 10 5 range. 11

12. Analysis of ions The curvature in the ion path is obtained by applying the magnetic field in the direction perpendicular to the direction of motion of ions The ions with higher m/z values follow the path of larger radius and the ions with lower m/z values follows the path of smaller radius Hence time taken by the particles of smaller m/z value to reach the collector is shorter as compared to the required by the higher m/z value ions Thus in magnetic field, the ions are separated based on their m/z ratio. 12

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14. Quadrapole mass ANALYZER They are generally considerably more compact than magnetic sector instruments and are commonly found in commercial bench top mass spectrometers. The heart of a quadrupole instrument is the four parallel cylindrical (originally hyperbolic) rods that serve as electrodes. Opposite rods are connected electrically, one pair being attached to the positive side of a variable dc source and the other pair to the negative terminal. Variable radio-frequency ac voltages, which are 180 out of phase, are applied to each pair of rods. To obtain a mass spectrum with this device ions are accelerated into the space between the rod; by a potential difference of 5 to 10 V Meanwhile, the ac and dc voltages on the rods are increased simultaneously while maintaining their ratio constant. At any given moment, all of the ions except those having a certain mlz value strike the rods and are converted to neutral molecules. Thus, only ions having a limited range of mlz values reach the transducer. 14

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16. Time of flight mass analyser The ions produced are then accelerated by an electric field pulse of 10 3 to 10 4 V. The accelerated particles pass into a field-free drift tube about a meter long. All ions entering the tube ideally have the same kinetic energy, their velocities in the tube must vary inversely with their masses, with the lighter particles arriving at the detector earlier than the heavier ones. Typical flight times are in the microsecond range for a 1m flight tube The transducer in a TOF mass spectrometer is usually an electron multiplier whose output is displayed on the vertical deflection plates of an oscilloscope and the horizontal sweep is synchronized with the accelerator pulses; an essentially instantaneous display of the mass spectrum appears on the oscilloscope screen. Because typical flight times are in the microsecond range, digital data acquisition requires extremely fast electronics. 16

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18. Disadvantages of TOF In resolution and reproducibility, instruments equipped with TOF mass analysers are not as satisfactory as those with magnetic or quadrupole analysers. 18

19. Ion trap analysers An ion trap is a device in which gaseous anions or cations can be formed and confined for extended periods by electric and magnetic fields. Components of ion trap analysers: 1) It consists of a central doughnut-shape ring electrode and a pair of end cap electrodes. 2) A variable radio-frequency voltage is applied to the ring electrode while the two end-cap electrodes are grounded. 3) Ions with an appropriate mlz value circulate in a stable orbit within the cavity surrounded by the ring. 4) As the radio-frequency voltage is increased, the orbits of heavier ions become stabilized and those for lighter ions become destabilized, causing them to collide with the wall of the ring electrode. 19

20. Operation of ion trap analysers 20

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23. Vacuum system Diffusion and turbomolecular pumps often used to achieve the high vacuum necessary for operating many mass spectrometers. These pumps are used with a rough pump (or forepump) to move gas molecules from inside a vacuum chamber (a mass spectrometer) to outside the system. Although the pressures achieved by these two pumping systems are similar, the cost of equipment, the operating costs, and the procedures and speed of pump down and vent cycles are quite different. 23

24. Necessity of vacuum pump At any pressure, gas molecules move in random directions, changing directions of motion only through collision. The mean free path — the average distance between collisions — of gas molecules can be calculated, and this parameter serves to define the conditions of viscous and molecular flow When the mean free path of the gas molecules exceeds the dimensions of the vacuum container, the system is under molecular flow conditions. Under such conditions, the residual gas molecules move without colliding with other gas molecules, instead ultimately colliding with the surfaces and devices within the vacuum chamber. Pumping is accomplished when a net (non random) direction to the movement of residual gas molecules in the vacuum chamber is attained. 24

25. Types of pumps In both cases, the net direction imparted to the residual gas molecules is into a region of higher pressure and toward the exhaust of the high vacuum pump. The exhaust of the high vacuum pump is connected through the foreline to a rough pump that accomplishes the transport of the residual gas molecules to the final exhaust at atmospheric pressure 25

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27. Reference Principles of Instrumental Analysis, by Douglas A. Skoog ( Stanford University), F James Holler( University of KentuckJ‘), Stanley R. Crouch ( Michigan State University), 8 th edition, published by Thomson Brooks/Cole Instrumental methods of analysis, by Dr. Supriya Mahajan, Published by CBS Publishers & Distributors Pvt. Ltd, 2010 High vacuum pumps in Mass spectrometry, by Kenneth L Busch, Published by Mass Spectrometry Forum, May 2001, www.spectroscopyonline.com 27

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