A. Fourier-Transform MSFourier-transform mass spectrometry takes advantage of ion-cyclotron resonance to select and detect ions. (see below)B. Ion-trap MSC. Magnetic-sector MSD. Quadrupole MSChemistry 331Chapter 11 Atomic Mass SpectrometryIntroduction:Mass spectrometers use the difference in mass-to-charge ratio (m/e) of ionized atoms or molecules to separate them from each other. Mass spectrometry is therefore useful for quantitation of atoms or molecules and also for determining chemical and structural information about molecules. Molecules have distinctive fragmentation patterns that provide structural information to identify structural components. The general operation of a mass spectrometer is: 1.create gas-phase ions 2.separate the ions in space or time based on their mass-to-charge ratio 3.measure the quantity of ions of each mass-to-charge ratio The ion separation power of a mass spectrometer is described by the resolution, which is defined as: R = m / m, where m is the ion mass and m is the difference in mass between two resolvable peaks in a mass spectrum. E.g., a mass spectrometer with a resolution of 1000 can resolve an ion with a m/e of 100.0 from an ion with an m/e of 100.1. Instrumentation:In general a mass spectrometer consists of an ion source, a mass-selective analyzer, and an ion detector. Since mass spectrometers create and manipulate gas-phase ions, they operate in a high-vacuum system. The magnetic-sector, quadrupole, and time-of-flight designs also require extraction and acceleration ion optics to transfer ions from the sourceregion into the mass analyzer. The details of mass analyzer designs are discussed in the individual documents listed belowMass analyzer designs:A. Fourier-Transform MS Fourier-transform mass spectrometry takes advantage of ion-cyclotron resonance to selectand detect ions. (see below)B. Ion-trap MS The ion-trap mass spectrometer uses three electrodes to trap ions in a small volume. The mass analyzer consists of a ring electrode separating two hemispherical electrodes. A mass spectrum is obtained by changing the electrode voltages to eject the ions from the trap. The advantages of the ion-trap mass spectrometer include compact size, and the ability to trap and accumulate ions to increase the signal-to-noise ratio of a measurement. C. Magnetic-sector MS The ion optics in the ion-source chamber of a mass spectrometer extract and accelerate ions to a kinetic energy given by: K.E. = 0.5 mv2 = eV where m is the mass of the ion, v is it's velocity, e is the charge of the ion and V is the applied voltage of the ion optics. Theions enter the flight tube between the poles of a magnet and are deflected by the magneticfield, H. Only ions of mass-to-charge ratio that have equal centrifugal and centripetal forces pass through the flight tube: (see below)D. Quadrupole MS A quadrupole mass filter consists of four parallel metal rods arranged as in the figure below. Two opposite rods have an applied potential of (U+Vcos(wt)) and the other two rods have a potential of -(U+Vcos(wt)), where U is a dc voltage and Vcos(wt) is an ac voltage. The applied voltages affect the trajectory of ions traveling down the flight path centered between the four rods. For given dc and ac voltages, only ions of a certain mass-to-charge ratio pass through the quadrupole filter and all other ions are thrown out of theiroriginal path. A mass spectrum is obtained by monitoring the ions passing throughthe quadrupole filter as the voltages on the rods are varied. There are two methods: varying w and holding U and V constant, or varying U and V (U/V) fixed for a const. w.E. Time-of-flight MS A time-of-flight mass spectrometer uses the differences in transit time through a drift region to separate ions of different masses. It operates in a pulsed mode so ions must be produced or extracted in pulses. An electric field accelerates all ions into a field-free driftregion with a kinetic energy of qV, where q is the ion charge and V is the applied voltage.Since the ion kinetic energy is 0.5mv2, lighter ions have a higher velocity than heavier ions and reach the detector at the end of the drift region sooner. An example of a time-of-flight mass spectrum is shown below.Useful Websites Dealing With Mass SpectrometryThe Analytical Chemistry Springboard at Umea U. http://www.anachem.umu.se/jumpstation.htmMurray's Mass Spectrometry