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DESCRIPTIONJ.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 - PowerPoint PPT Presentation
Discoverer of the Electron
Background InformationCathode RaysForm 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 workTravel in straight lines in the absence of an external field
A Cathode Ray TubeZumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58
Cathode Ray Experiment1897 ExperimentationUsing 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 RaysDorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117cathodesource ofhigh voltageyellow-greenfluorescenceshadow
The Effect of an Electric Field on Cathode RaysDorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117
Cathode Ray ExperimentDeflectionregionDrift regionDisplacement+-Anodes / collimatorsCathodeVolts
Thomsons CalculationsCathode Ray ExperimentThomson used magnetic and electric fields to measure and calculate the ratio of the cathode rays mass to its charge.
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 Daltons 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.
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
Mass spectrometers that break up molecules into fragments that can be characterized by electrical methods. [image link] DetectorplateLeast massive ionsIon-acceleratingelectric fieldMagnetic fieldHeating device to vaporize samplePositive ionsSampleElectron beamacceleratedIon beamMost massive ionsSlits
Mass Spectrophotometerelectron beammagnetic fieldgas stream of ions of differentmasseslightest ionsheaviest ionsDorin, Demmin, Gabel, Chemistry The Study of Matter 3rd Edition, page 138
Inlet - ensures that the sample enters the machine with minimal lossSource - sample components are ionized (the method by which this is done depends on the specific mass spectrometer being used.)Analyzer - accelerates ion and separates themDetector - 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.Components of a Mass SpectrometerInletSignalprocessorSourceAnalyzerDetectorVacuum
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 BeamIon AcceleratingArrayMolecularSourceMagnetic FieldBends Path of ChargedParticlesCollectorExit SlitHo
Mass Spectrometry-+Photographic plateHill, Petrucci, General Chemistry An Integrated Approach 1999, page 320Stream of positive ions
Mass Spectrum for Mercury196 197 198 199 200 201 202 203 204Mass numberRelative number of atoms30
196 199 201 204198 200 202Mass spectrum of mercury vaporThe percent natural abundances for mercury isotopes are:
Hg-196 0.146% Hg-198 10.02% Hg-199 16.84% Hg-200 23.13% Hg-201 13.22% Hg-202 29.80% Hg-204 6.85%(The photographic record has been converted to a scale of relative number of atoms)
The percent natural abundances for mercury isotopes are:
Hg-196 0.146% Hg-198 10.02% Hg-199 16.84% Hg-200 23.13% Hg-201 13.22% Hg-202 29.80% Hg-204 6.85%(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 = xx = 200.63956 amu(% "A")(mass "A") + (% "B")(mass "B") + (% "C")(mass "C") + (% "D")(mass "D") + (% "E")(mass "E") + (% F)(mass F) + (% G)(mass G) = AAMABCDEFG
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
Using our estimated % abundance data 55% Cl-35 and 45% Cl-36calculate 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
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 ions mass.
Whats mass got to do with it?
Magnetic field B
m/z = B2r2e/2V
Where m = mass,e = electron charge,z = # of charges/ionr = radius of curvature
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 44.121628C+O+CO+CO2+M+
Mass spectrums reflect the abundance of naturally occurring isotopes.HydrogenCarbonNitrogenOxygenSulfurChlorineBromine1H = 99.985%2H = 0.015%12C = 98.90%13C = 1.10%14N = 99.63%15N = 0.37%16O = 99.762%17O = 0.038%18O = 0.200%32S = 95.02%33S = 0.75%34S = 4.21%36S = 0.02%35Cl = 75.77%37Cl = 24.23%79Br = 50.69%81Br = 49.31%Natural Abundance of Common Elements
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 molecules1 carbon is carbon-13
100 Base peak
Why is the Mass Spectrometer an Important Analytical Instrument?
Mass Spectrometers have been used in:
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.
J. J. Thomson - English physicist. 1897Made a piece of equipment called a cathode ray tube.It is a vacuum tube - all the air has been pumped out.
Mass spectrometers that break up molecules into fragments that can be characterized by electrical methods. [image link] A gaseous sample is ionized by bombarding it with electrons in the lower part of the apparatus (not shown), producing positive ions. The ions pass through an electric field in which they are brought to a particular velocity. The ions then pass through a narrow slitinto a curved chamber. A magnetic field is applied perpendicular to the beam of ions. All the ions with the same mass-to-charge ratioare deflected into the same circular path. (In most cases