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Chem. 133 –2/13 Lab Lecture

Chem. 133 –2/13 Lab Lecture. Announcements. Experiments Covered Today GC-MS HPLC Fluorescence Graphite Furnace Atomic Absorption Other Labs (NMR, FTIR, and GC will be covered next time) Additional time to complete electronics lab Set 2 lab periods will be shifted by 1 lab period

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Chem. 133 –2/13 Lab Lecture

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  1. Chem. 133 –2/13Lab Lecture

  2. Announcements • Experiments Covered Today • GC-MS • HPLC • Fluorescence • Graphite Furnace Atomic Absorption • Other Labs (NMR, FTIR, and GC will be covered next time) • Additional time to complete electronics lab • Set 2 lab periods will be shifted by 1 lab period • Electronics Lab Report – due 2/24

  3. Equipment

  4. GC-MS Lab • Purposes: • Learn basic operation of GC-MS • Identify unknown compounds (6 compounds: 3 halogenated, 1 ketone, and 2 aromatic compounds) • Explain peaks identified in mass spectra • Learn how “extract ions” works • Theory • Can be used for both quantitative and qualitative analysis

  5. GC-MS Lab Total Ion Current • Theory – cont. • Provides 2 dimensions of qualitative information (time + m/z) • Typical viewing of data is in 2 2D plots: • TIC vs. time • ion intensity vs. m/z (at single time) Ion current Time Time (min) m/z = 77 Ion m/z

  6. GC-MS Lab • Theory – cont. • Identification uses: • retention time • compound MW • fragmentation pattern • isotope effect (pattern seen from secondary isotopes)

  7. GC-MS Lab • Procedure • Select an Unknown • Learn how to run GC-MS • Select an EI method • If 6 peaks show up, that method works, but may want to optimize • Less than 6 peaks means missing or overlapping compounds: • peaks before MS starts (in first ~4 min) • after chromatogram ends (not yet eluted) • overlapping

  8. GC-MS Lab • Procedure – cont. • Adjust Chromatographic program: • change temperature program (but RENAME FIRST) • try to get 6 separate peaks eluted efficiently • overlapping peaks are o.k. (it justifies using the extract ion program) • If identification is unclear (e.g. dimethylphenol vs. ethylphenol), you may need to prepare and run standards • Mass Spectral Identification

  9. GC-MS Lab • Mass Spectral Identification • see example at end of section in lab manual • should identify and/or discuss: • parent ion (if it exists) and compound molecular weight (most common isotopes) • major fragment ions (including an indication of how they formed for common fragmentation/rearrangement) • ions with significant M+2/M+2n isotope peaks (Cl or Br containing compounds). You will need to compare the measured M+2n to M intensities to that expected.

  10. GC-MS Lab • Mass Spectral Identification • Fragments formed • many fragments from simple bond cleavage • McLafferty rearrangements (g H extraction and a-b bond cleavage) • Loss of phenols (- CO is common) • Disubstituted aromatics (- X, then – HX) O MW = 86 O MW = 86 – 15 = 71 + OH+ MW = 58

  11. GC-MS Lab • Mass Spectral Identification • Isotope peaks - Example: CH2Br2 MW = 173.8 (not useful) • ion with most common isotopes = 12C1H279Br2 MW = 171.9 • M+2 peak occurs due to 1 81Br isotope and M+4 due to 2 81Br isotopes • abundance can be calculated from abundance ratios and from probability: • 1Br is 97.3/100 81 isotope:79 isotope • 2 Br distributions 79Br 79Br or 79Br 81Br or 81Br 79Br or 81Br 81Br (so 1 to 2 to 1 for M: M+2: M+4 • Net ratio M+2/M = 2(97.3)(100)/(100)(100) = 195/100 • Net ratio M+4/M = (97.3)(97.3)/(100)(100) = 94.7/100 • Somewhat trickier for CHClBr2

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