mass spectrometry hh

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  • 1. By K . Rakesh Gupta

2. CONTENTS Introduction Principle Working of the mass spectrometer Instrumentation Theory of mass spectrometry Applications 3. INTRODUCTION J. J. Thomson (1913) separated the isotopes 20Ne and 22Ne Atlantic Refining Company (1942), first commercial use This technique resolves ionic species by their m/e ratio Francis William Aston won the 1922 Nobel Prize in Chemistry for his work in mass spectrometry Replica of an early mass spectrometer 4. Some of the modern techniques of mass spectrometry were devised by Arthur Jeffrey Dempster and F.W. Aston in 1918 and 1919 respectively. In 1989, half of the Nobel Prize in Physics was awarded to Hans Dehmelt and Wolfgang Paul for the development of the ion trap technique in the 1950s and 1960s. In 2002, the Nobel Prize in Chemistry was awarded to John Bennett Fenn for the development of electrospray ionization (ESI) and Koichi Tanaka for the development of soft laser desorption (SLD) and their application to the ionization of biological macromolecules, especially proteins. The earlier development of matrix-assisted laser desorption/ionization (MALDI) by Franz Hillenkamp and Michael Karas has not been so recognized despite the comparable (arguably greater) practical impact of this technique, particularly in the field of protein analysis. This is due to the fact that although MALDI was first reported in 1985, it was not applied to the ionization of proteins until 1988,after Tanaka's report. 5. WHAT IS MASS SPECTROMETRY It is an analytical technique for the determination of the elemental composition of a sample or molecule 6. MS is a powerful analytical technique Identify Unknown Compounds quantify known materials Elucidation of structural and chemical properties Requires minute Quantities (6000 C) Only a small amount of analyte is utilized (< 1%) 19. Electron ionization Electron ionization (EI, formerly known as electron impact) is an ionization method in which energetic electrons interact with gas phase atoms or molecules to produce ions. This technique is widely used in mass spectrometry, particularly for gases and volatile organic molecules 20. The following gas phase reaction describes the electron ionization process : where M is the analyte molecule being ionized, e- is the electron and M+ is the resulting ion. Diagram representing an electron ionization ion source 21. Chemical Ionization (CI) Ion Source A modified form of EI Higher gas pressure in ioniation cavity (1 torr) Reagent gas (1000 to 10000-fold excess) added; usual choice is methane, CH4 Reagent gas is directly ionized instead of analyte Gentle; little fragmentation; even-electron ions produced more stable than odd-electron ions produced in EI Excess energy of excited ions removed by many ion-reagent gas collisions 22. Chemical Ionization Reactions Reagent gas ionization: CH4 CH4 + +e (also CH3 +, CH2 +) Secondary reactions: CH4 + + CH4 CH5 + + CH3 CH3 + + CH4 H2 + C2H5 + (M+29) Tertiary reactions CH5 + + MH CH4 + MH2 + (M+1) proton exchange CH3 + + MH CH4 + M+ (M1) hydride exchange CH4 + + MH CH4 + MH+ (M) charge exchange 23. Fast Atom Bombardment Ion source for biological molecules Ar ions passed through low pressure Ar gas to produce beam of neutral ions Atom-sample collisions produce ions as large as 25,000 Daltons 24. glow discharge Sputtering of the cathode material (the sample) by an argon plasma. Ionisation of the elements of the sample in the plasma. Extraction and acceleration of ions. Ions separation with a magnetic sector (Mattauch Herzog configuration). Ions detection by a Faraday cup or an electron multiplier 25. Matrix-Assisted Laser Desorption/Ionization (MALDI) Analyte mixed with radiation-absorbing material and dried Sample ablated with pulsed laser Often coupled to time-of-flight (TOF) detector Excellent for larger molecules, e.g. peptides, polymers 26. MASS ANALYZERS Quadrupole Analyzer Ions forced to wiggle between four rods whose polarity is rapidly switched Small masses pass through at high frequency or low voltage; large masses at low frequency or high voltage Very compact, rapid (10 ms) R = 700-800 27. TOF Time of Flight Mass Analyzer Separates ions based on flight time in drift tube Positive ions are produced in pulses and accelerated in an electric field (at the same frequency) All particles have the same kinetic energy Lighter ions reach the detector first Typical flight times are 1-30 sec 28. Time of Flight Mass Analyzer Separation Principle All particles have the same kinetic energy In terms of mass separation principles: Vparticle = Her/m Hold H,e, and r constant Vparticle = 1/m (constant) Vparticle is inversely proportional to mass 29. Quadrupole Ion Trap Ions follow complex trajectories between two pairs of electrodes that switch polarity rapidly Ions can be ejected from trap by m/z value by varying the frequency of end cap electrodes 30. Detectors electron multiplier Faraday cups Microchannel plate detectors 31. Electron multiplier Continuous dynode electron multiplier An electron multiplier (continuous dynode electron multiplier) is a vacuum-tube structure that multiplies incident charges. In a process called secondary emission, a single electron can, when bombarded on secondary emissive material, induce emission of roughly 1 to 3 electrons. If an electric potential is applied between this metal plate and yet another, the emitted electrons will accelerate to the next metal plate and induce secondary emission of still more electrons. This can be repeated a number of times, resulting in a large shower of electrons all collected by a metal anode, all having been triggered by just one. 32. Faraday cup A Faraday cup is a metal (conductive) cup designed to catch charged particles in vacuum. The resulting current can be measured and used to determine the number of ions or electrons hitting the cup. The Faraday cup is named after Michael Faraday who first theorized ions around 1830. Schematic of a Faraday cup 33. Faraday cup cont.. When a beam or packet of Ions hits the metal it gains a small net charge while the ions are neutralized. The metal can then be discharged to measure a small current equivalent to the number of impinging ions. Essentially the faraday cup is part of a circuit where ions are the charge carriers in vacuum and the faraday cup is the interface to the solid metal where electrons act as the charge carriers (as in most circuits). Faraday cup with an electron- suppressor plate in front By measuring the electrical current (the number of electrons flowing through the circuit per second) in the metal part of the circuit the number of charges being carried by the ions in the vacuum part of the circuit can be determined. 34. Micro-channel plate (MCP) It is a planar component used for detection of particles (electrons or ions) and impinging radiation (ultraviolet radiation and X-rays). It is closely related to an electron multiplier, as both intensify single particles or photons by the multiplication of electrons via secondary emission. However, because a micro channel plate detector has many separate channels, it can additionally provide spatial resolution. 35. A 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). 36. References Skoog, Instrumental analysis, cengage learning , Indian edition.


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