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Mass spectrometry, • Mass spectrometry, also called mass spectroscopy, is an, analytic technique by which chemical substances are identified by the, sorting of gaseous ions in electric and magnetic fields according to, their mass-to-charge ratios., • The instruments used in such studies are called mass, spectrometers and mass spectrographs, and they operate on the principle, that moving ions may be deflected by electric and magnetic fields., • The two instruments differ only in the way in which the sorted charged, particles are detected., • In the mass spectrometer they are detected electrically, in the mass, spectrograph by photographic or other nonelectrical means; the, term mass spectroscope is used to include both kinds of devices., • Since electrical detectors are now most commonly used, the field is, typically referred to as mass spectrometry., • Mass spectrometry technique used to:, – quantify known materials,, – to identify unknown compounds within a sample, and, – to elucidate the structure and chemical properties of different molecules.
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History, • The foundation of mass spectroscopy was laid in, 1898, when Wilhelm Wien, a German physicist,, discovered that beams of charged particles could, be deflected by a magnetic field., • In more refined experiments carried out between, 1907 and 1913, the British physicist J.J. Thomson,, who had already discovered the electron and, observed its deflection by an electric field, passed a, beam of positively charged ions through a, combined electrostatic and magnetic field., • The two fields in Thomson’s tube were situated so, that the ions were deflected through small angles, in two perpendicular directions., • The net result was that the ions produced a series, of parabolic curves on a photographic plate placed, in their paths., , Electric field, , Beam of charged, particle, , Magnetic field
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Principle of Mass Spectrometry (MS), • In this technique, molecules are bombarded with a beam of energetic, electrons., • The molecules are ionized and broken up into many fragments, some of, which are positive ions., • Each kind of ion has a particular ratio of mass to charge, i.e. m/e ratio, (value)., • For most ions, the charge is one, and thus, the m/e ratio is simply the, molecular mass of the ion., • The ions pass through magnetic and electric fields to reach the detector, where they are detected and signals are recorded to give mass spectra., • A mass spectrum is a plot of relative abundance against the ratio of, mass/charge (m/e)., • These spectra are used to determine the elemental or isotopic, signature of a sample, the masses of particles and of molecules, and to, elucidate the chemical structures of molecules and other chemical, compounds.
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v/B
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H
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Working of Mass Spectrometry (MS), • In a typical procedure, a sample, which may be solid, liquid, or gas,, is ionized, for example by bombarding it with electrons., • This may cause some of the sample’s molecules to break into, charged fragments. These ions are then separated according to, their mass-to-charge ratio, typically by accelerating them and, subjecting them to an electric or magnetic field:, • Ions of the same mass-to-charge ratio will undergo the same, amount of deflection., • The ions are detected by a mechanism capable of detecting charged, particles, such as an electron multiplier. Results are displayed as, spectra of the relative abundance of detected ions as a function of, the mass-to-charge ratio., • The atoms or molecules in the sample can be identified by, correlating known masses (e.g. an entire molecule) to the identified, masses or through a characteristic fragmentation pattern.
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Instrumentation, A. Sample Inlet, • A sample stored in the large reservoir from which molecules reach, the ionization chamber at low pressure in a steady stream by a, pinhole called “Molecular leak”., B. Ionization, • Atoms are ionized by knocking one or more electrons off to give, positive ions by bombardment with a stream of electrons., • These electrons move between cathode and anode., • When the sample passes through the electron stream between the, cathode and anode, electrons with high energy knock electrons out, of the sample and form ions., • Most of the positive ions formed will carry a charge of +1., • Ionization can be achieved by :, – Electron Ionization (EI-MS), – Chemical Ionization (CI-MS), – Desorption Technique (FAB)
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... Instrumentation, C. Acceleration, • The ions placed between a set of charged parallel plates get, attracted to one plate and repel from the other plate., • The acceleration speed can be controlled by adjusting the, charge on the plates., D. Deflection, • Ions are deflected by a magnetic field due to differences in, their masses., • The lighter the mass, the more they are deflected., • It also depends upon the no. of +ve charge an ion is, carrying; the more +ve charge, the more it will be, deflected.
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... Instrumentation, E. Detection, • The ions with correct charge and mass move to the detector., • The ratio of mass to charge is analyzed through the ion that hits the, detector., • When an ion hits the metal box, the charge is neutralized by an electron, jumping from the metal onto the ion., • Types of analyzers:, – Time of Flight analyzers (TOF): does not use magnetic fields depends, on the differing speeds of ions with the same energy but different, masses., – Magnetic sector mass analyzers, – Double focussing analyzers, – Quadrupole mass analysers, – Ion trap analyzer, – Ion cyclotron analyser
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Applications of Mass Spectrometry, • Environmental monitoring and analysis (soil, water, and air pollutants,, water quality, etc.), • Geochemistry – age determination, soil, and rock composition, oil and, gas surveying, • Chemical and Petrochemical industry – Quality control, • Identify structures of biomolecules, such as carbohydrates, nucleic acids, • Sequence biopolymers such as proteins and oligosaccharides, • Determination of the molecular mass of peptides, proteins, and, oligonucleotides., • Monitoring gases in patients’ breath during surgery., • Identification of drug abuse and metabolites of drugs of abuse in blood,, urine, and saliva., • Analyses of aerosol particles., • Determination of pesticides residues in food.
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Applications of MS
