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J.J. Thomson Discoverer of the Electron Background Information • • • • Cathode Rays Form when high voltage is applied across electrodes in a partially evacuated tube. Originate at the cathode (negative electrode) and move to the anode (positive electrode) Carry energy and can do work Travel in straight lines in the absence of an external field A Cathode Ray Tube Source of Electrical Potential Stream of negative particles (electrons) Metal Plate Gas-filled glass tube Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58 Metal plate Cathode Ray Experiment 1897 Experimentation • Using a cathode ray tube, Thomson was able to deflect cathode rays with an electrical field. • The rays bent towards the positive pole, indicating that they are negatively charged. The Effect of an Obstruction on Cathode Rays High voltage source of high voltage shadow cathode yellow-green fluorescence Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117 The Effect of an Electric Field on Cathode Rays source of high voltage High voltage cathode negative plate _ + anode positive plate Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117 Cathode Ray Experiment Displacement Volts Anodes / collimators Cathode + Deflection region Drift region Thomson’s Calculations Cathode Ray Experiment • Thomson used magnetic and electric fields to measure and calculate the ratio of the cathode ray’s mass to its charge. Electric = deflection Magnetic = deflection charge of ray particle x charge of ray particle x length of electric x deflection region x field mass of ray velocity of 2 x particle ray particle length of drift region length of magnetic x deflection region x field mass of ray velocity of x particle ray particle length of drift region magnetic deflection electric deflection magnetic field = electric field x velocity Conclusions • He compared the value with the mass/ charge ratio for the lightest charged particle. • By comparison, Thomson estimated that the cathode ray particle weighed 1/1000 as much as hydrogen, the lightest atom. • He concluded that atoms do contain subatomic particles - atoms are divisible into smaller particles. • This conclusion contradicted Dalton’s postulate and was not widely accepted by fellow physicists and chemists of his day. • Since any electrode material produces an identical ray, cathode ray particles are present in all types of matter - a universal negatively charged subatomic particle later named the electron So what does J.J. Thomson have to do with mass spec? • Just as J.J. Thomson used a magnetic field to affect charged particles, so does a mass spectrometer. source of High voltage _ negative plate high voltage cathode + positive anode plate • The machine sorts ions according to their mass to charge ratio, something Thomson was able to calculate for the electron using the results of his cathode ray experiments. What is mass spectrometry? Mass spectrometry is a technique used to separate a substance into ions based on their mass. Molecules are bombarded by high energy particles that cause them to lose one electron and carry a +1 charge. These ions undergo further fragmentation producing smaller positive ions. The spectrum produced plots intensity (abundance of ions) against the ions’ mass-to-charge ratio. Substances can be identified by their characteristic fragment ions represented on a mass spectrum Detector plate Ion-accelerating electric field Least massive ions accelerated Ion beam Positive ions Sample Most massive ions Electron beam Slits Magnetic field Heating device to vaporize sample Mass spectrometers that break up molecules into fragments that can be characterized by electrical methods. [image link] Mass Spectrophotometer magnetic field heaviest ions stream of ions of different masses electron beam gas Dorin, Demmin, Gabel, Chemistry The Study of Matter 3rd Edition, page 138 lightest ions Components of a Mass Spectrometer Signal processor Inlet Source Analyzer Detector Vacuum Inlet - ensures that the sample enters the machine with minimal loss Source - sample components are ionized (the method by which this is done depends on the specific mass spectrometer being used.) Analyzer - accelerates ion and separates them Detector - records the charge induced when an ion passes by or hits a surface. Signal Processor - produces a mass spectrum, a record of the m/z's at which ions are present. *A vacuum must be used to maintain a low pressure. A low pressure reduces the collisions among the ions. The general operation of a mass spectrometer is: 1. create gas-phase ions 2. separate the ions based on their mass-to-charge ratio 3. measure the quantity of ions of each mass-to-charge ratio Electron Beam Magnetic Field Bends Path of Charged Particles Molecular Source Ion Accelerating Array Collector Exit Slit Ho Mass Spectrometry 198 200 202 Photographic plate 196 - + Stream of positive ions Hill, Petrucci, General Chemistry An Integrated Approach 1999, page 320 199 201 204 Mass spectrum of mercury vapor Mass Spectrum for Mercury (The photographic record has been converted to a scale of relative number of atoms) The percent natural abundances for mercury isotopes are: Hg-196 Hg-198 Hg-199 Hg-200 Hg-201 Hg-202 Hg-204 Relative number of atoms 30 25 198 0.146% 10.02% 16.84% 23.13% 13.22% 29.80% 6.85% 196 200 199 15 10 5 197 198 201 204 Mass spectrum of mercury vapor 20 196 202 199 200 Mass number 201 202 203 204 80 Hg 200.59 The percent natural abundances for mercury isotopes are: A B C D E F G Hg-196 Hg-198 Hg-199 Hg-200 Hg-201 Hg-202 Hg-204 0.146% 10.02% 16.84% 23.13% 13.22% 29.80% 6.85% (% "A")(mass "A") + (% "B")(mass "B") + (% "C")(mass "C") + (% "D")(mass "D") + (% "E")(mass "E") + (% F)(mass F) + (% G)(mass G) = AAM (0.00146)(196) + (0.1002)(198) + (0.1684)(199) + (0.2313)(200) + (0.1322)(201) + (0.2980)(202) + (0.0685)(204) = x 0.28616 + 19.8396 + 33.5116 + 46.2600 + 26.5722 + 60.1960 + 13.974 = x x = 200.63956 amu 17 Cl 35.453 • Assume you have only two atoms of chlorine. • One atom has a mass of 35 amu (Cl-35) • The other atom has a mass of 36 amu (Cl-36) • What is the average mass of these two isotopes? 35.5 amu • Looking at the average atomic mass printed on the periodic table...approximately what percentage is Cl-35 and Cl-36? 55% Cl-35 and 45% Cl-36 is a good approximation 17 Cl 35.453 Using our estimated % abundance data 55% Cl-35 and 45% Cl-36 calculate an average atomic mass for chlorine. Average Atomic Mass = (% abundance of isotope "A")(mass "A") + (% "B")(mass "B") + ... AAM = (% abundance of isotope Cl-35)(mass Cl-35) + (% abundance of Cl-36)(mass Cl-36) AAM = (0.55)(35 amu) + (0.45)(36 amu) AAM = (19.25 amu) + (16.2 amu) AAM = 35.45 amu What’s mass got to do with it? An electric or magnetic field can deflect charged particles. The particles have kinetic energy as they move through a magnetic field (KE=1/2mv2). The particles’ inertia depends on their mass. A mass analyzer can steer certain masses to the detector based on their mass-to-charge ratios (m/z). by varying the electrical or magnetic field. Typically ions in a mass spectrometer carry a +1 charge so the m/z ratio is equivalent to the ion’s mass. What does a mass spectrum look like? Intensity or ion abundance is plotted on the y-axis. The m/z ratio is plotted on the x-axis. The base beak is from the ion that is the most abundant and is assigned an intensity of 100%. The molecular ion peak, M+, is the peak due to the parent ion (the original molecule minus one electron). Mass spectrum of carbon dioxide, CO2 molecular ion is seen at m/z 44. CO2+ M+ % RELATIVE INTENSITY 100 90 80 70 60 50 40 30 C+ 12 20 10 O+ CO+ 28 16 0 m/z 5 10 15 20 25 30 35 40 45 50 Mass spectrums reflect the abundance of naturally occurring isotopes. Natural Abundance of Common Elements Hydrogen 1H = 99.985% 2H = 0.015% Carbon 12C = 98.90% 13C = 1.10% Nitrogen 14N = 99.63% 15N = 0.37% Oxygen 16O = 99.762% 17O = 0.038% Sulfur 32S = 95.02% 33S = 0.75% 34S = 4.21% 36S = 0.02% Chlorine 35Cl = 75.77% 37Cl = 24.23% Bromine 79Br = 50.69% 81Br = 49.31% 18O = 0.200% For example….Methane For carbon 1 in approximately 90 atoms are carbon-13 The rest are carbon-12 the isotope that is 98.9% abundant. So, for approximately 90 methane molecules…1 carbon is carbon-13 C-13 Where’s Waldo? Where’s Waldo? The Mass Spectrum of Methane 100 Base peak 86 M+ = 15 C12H3+ M+ = 16 Molecular ion [C12H4]+. [C12H2]+. C12H+ [C12]+. 3 8 16 1.11 12 13 14 15 16 17 M +1 = 17 [C13H4]+. m/z Ethyl Bromide M = 29 C2H5+ C2H5Br81 C2H5Br79 Why is the Mass Spectrometer an Important Analytical Instrument? Mass Spectrometers have been used in: 1) Forensics 2) Organic synthesis laboratories 3) The analysis of large biomolecules: proteins and nucleic acids 4) Drug Test 5) Determination of isotopic abundance 6) Identification of impurities in pharmaceutical products 7) Diagnosis of certain diseases. References • http://www.aist.go.jp/RIODB/SDBS/ • http://www.infochembio.ethz.ch/links/en/spectrosc_m ass_lehr.html • http://dbhs.wvusd.k12.ca.us/AtomicStructure/Disc-ofElectron-Intro.html • http://wps.prenhall.com/wps/media/objects/340/3482 72/Instructor_Resources/Chapter_12/47