Mass Spectrometry 1

Mass Spectrometry and Related Techniques 1 Lecture Date: February 25th, 2008 Ion and Particle Spectrometry 1 - Outline  Atomic and Molecular Mass Spectrometry – Skoog et al. Chapter 11 (atomic) and Chapter 20 (Molecular). – Cazes Chapter 14 1 Mass Spectrometry  Mass Spectrometry (a.k.a. MS or mass spec) – a method of separating and analyzing ions by their mass-to-charge ratio  MS does not involve a specific region of the electromagnetic spectrum (because it is not directly interested in the energies of emitted photons, electronic or vibrationaltransitions,nuclear spin transitions,etc…) Ion abundance Ion Ion Ion m/z Up to m/z= 100000! General Notes on Atomic and Molecular Mass  Helpful units and conversions: – 1 amu = 1 Da = 1/12 the mass of a neutral 12C atom. – 1 kDa = 1000 amu  Atomic weights of other elements are defined by comparison.  Mass-to-charge ratio (m/z): the ratio of the mass of an ion (m) to its charge (z)  Molecular ion: an ion consisting of essentially the whole molecule 2 Mass Spectrometers  Ablock diagram of a “generic” mass spectrometer: Ionization Source Mass Analyzer Detector Ionization Sources  Electron Ionization (EI)  Chemical Ionization (CI/APCI)  Photo-ionization(APPI)  Electrospray (ESI)  Matrix-assisted Laser Desorption (MALDI)  FieldDesorption (FD)  Plasma Desorption (PD)  Fast atom bombardment (FAB)  High-temperature Plasma (ICP) Gas Phase Desorption Ionization Source Mass Analyzer Detector Seealso Table 20-1 in Skoog, et al. 3 EI: Electron Ionization/Electron Impact  The electron ionization (EI) source is designed to produce gaseous ions for analysis. Heated Incandescent Tungsten/Rhenium Filament E-  EI, which was one of the earliestsources in wide use for MS, usually operates on vapors (such as those eluting from a GC) Accel! 70 eV Vaporized Molecules Ions To Mass Analyzer EI: Electron Ionization/Electron Impact  How EI works: – Electronsare emittedfrom a filamentmade of tungsten, rhenium, etc… – They are accelerated by a potential of 70 V – The electrons and moleculescross (usually at a right angle) and collide – The ions are primarly singly-charged, positive ions, that are extractedby a small potential (5V) through a slit  See also Fig. 20-3, pg. 502 in Skoog, et al. Diagram from F. W. McLafferty, “Interpretation of Mass Spectra”, 3rd Ed., University Science Books, Mill Valley, CA(1980). 4 EI: Electron Ionization/Electron Impact  Whenelectrons hit – the molecules undergo rovibrationalexcitation (the mass of electrons is too small to really “move” the molecules)  About one in a million molecules undergo the reaction: M+ e- M+ + 2e- EI: Electron Ionization/Electron Impact  Advantages: – Results in complexmass spectra with fragmentions, useful for structural identification  Disadvantages: – Can produce too much fragmentation,leading to no molecular ions! (makes structural identificationdifficult!) 5 ... - tailieumienphi.vn