Molecular Mass SpectroscopyIntroductionMass SpectroscopyTechnique is capable of providing information aboutThe four main components of a molecular mass spectrometerThe Sample Inlet SystemMagnetic Sector AnalyzersDouble Focusing InstrumentsA schematic of a double-focusing instrument is shownTime-of-Flight AnalyzersIsotopesGas-Phase SourcesAdvantages of Electron Impact sourcesDisadvantages of Electron Impact sourcesIdentification of Pure Compounds by Mass SpectroscopyReferencesMolecular Mass Molecular Mass SpectroscopySpectroscopy Chem. 331Chem. 331IntroductionIntroduction •In Mass Spectroscopy (MS), atomic and In Mass Spectroscopy (MS), atomic and molecular weights are generally molecular weights are generally expressed in terms of expressed in terms of atomic mass unitsatomic mass units (amu). The atomic mass unit is based on (amu). The atomic mass unit is based on upon a relative scale in which the upon a relative scale in which the reference is the carbon isotope 126C, reference is the carbon isotope 126C, which is assigned a mass of exactly 12 which is assigned a mass of exactly 12 amu. Thus the amu is defined as 1/12 amu. Thus the amu is defined as 1/12 of the mass of one neutral carbon atom. of the mass of one neutral carbon atom.Mass SpectroscopyMass Spectroscopy•Mass spectroscopy is perhaps one of Mass spectroscopy is perhaps one of the most widely applicable of all the the most widely applicable of all the analytical tools available to the analytical tools available to the analytical chemist in the sense that analytical chemist in the sense that this technique is capable of providing this technique is capable of providing information about information aboutTechnique is capable of Technique is capable of providing information aboutproviding information about •the qualitative and quantitative composition the qualitative and quantitative composition of both organic and inorganic analytes in of both organic and inorganic analytes in complex mixturescomplex mixtures•this instrument measures compounds with this instrument measures compounds with molecular masses up to 200, 000 Daltons.molecular masses up to 200, 000 Daltons.•the structures of a wide variety of complex the structures of a wide variety of complex molecular species molecular species •isotopic ratios of atoms in samples andisotopic ratios of atoms in samples and•the structure and composition of solid the structure and composition of solid surfaces.surfaces.The four main components of a The four main components of a molecular mass spectrometermolecular mass spectrometerThe Sample Inlet SystemThe Sample Inlet System •Batch Inlet SystemsBatch Inlet Systems::• These systems are the simplest These systems are the simplest and simply involve the volatilization and simply involve the volatilization of the sample externally and then of the sample externally and then the gradual leakage of the the gradual leakage of the volatilized sample into the volatilized sample into the evacuated ionization chamber. For evacuated ionization chamber. For gases, the sample is introduced into gases, the sample is introduced into the metering volume container and the metering volume container and then expanded into the reservoir then expanded into the reservoir flask where it is then leaked into the flask where it is then leaked into the ionization chamber. For liquids, a ionization chamber. For liquids, a small quantity of sample is small quantity of sample is introduced into the reservoir and introduced into the reservoir and the pressure of the system is the pressure of the system is reduced to about 10-5 torr. The inlet reduced to about 10-5 torr. The inlet system is lined with glass to avoid system is lined with glass to avoid losses of polar analytes by losses of polar analytes by adsorption.adsorption.•The Direct Probe Inlet:The Direct Probe Inlet: •These systems are used for These systems are used for solids and non-volatile liquids solids and non-volatile liquids and in these systems the and in these systems the sample is introduced into the sample is introduced into the ionization region by means of a ionization region by means of a sample holder, or probe, which sample holder, or probe, which is inserted through a vacuum is inserted through a vacuum lock. Probes are also used when lock. Probes are also used when the amount of the sample to be the amount of the sample to be analyzed is small. With a analyzed is small. With a probe, the sample is generally probe, the sample is generally held on the surface of a glass or held on the surface of a glass or aluminum capillary tube and aluminum capillary tube and positioned within a few meters positioned within a few meters of the ionization source. of the ionization source.Magnetic Sector Magnetic Sector AnalyzersAnalyzers•Magnetic sector analyzers employ a permanent magnet or electromagnet Magnetic sector analyzers employ a permanent magnet or electromagnet to cause the beam from the ion source to travel in a circular path of 180, to cause the beam from the ion source to travel in a circular path of 180, 90, or 60 degrees. Here, ions are formed by electron impact.90, or 60 degrees. Here, ions are formed by electron impact.•The translational energy of an ion of mass The translational energy of an ion of mass mm and charge and charge zz upon exciting upon exciting slit slit BB is given by is given by •K. = K. = ZevZev = ½ mv2 = ½ mv2 Equation 1Equation 1•where V is the voltage between A and B, v is the velocity of the ion after where V is the voltage between A and B, v is the velocity of the ion after acceleration, and e is the charge of the ion.acceleration, and e is the charge of the ion.•The path in the sector described by the ions of a given mass and charge The path in the sector described by the ions of a given mass and charge represents a balance between two forces acting upon them. These two represents a balance between two forces acting upon them. These two forces are the centripetal force and the magnetic force and equating forces are the centripetal force and the magnetic force and equating these two forces yields:these two forces yields:•Bzev = mv2/rwhich rearranges to v = Bzer/mBzev = mv2/rwhich rearranges to v = Bzer/m•Substituting the above equation into (1) givesSubstituting the above equation into (1) gives m/z = B2r2e/2V m/z = B2r2e/2V•The last equation shows how mass spectra can be obtained by varying The