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Mass Spectrometry.  Used to identify organic compounds mainly through molar mass and identification of major fragments.  A mass spectrometer is often.

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Presentation on theme: "Mass Spectrometry.  Used to identify organic compounds mainly through molar mass and identification of major fragments.  A mass spectrometer is often."— Presentation transcript:

1 Mass Spectrometry

2  Used to identify organic compounds mainly through molar mass and identification of major fragments.  A mass spectrometer is often used as the “detector” for a gas chromatograph (GC-MS) A mixture is separated and a total ion chromatogram (TIC) obtained. A TIC is the GC-MS equivalent of a gas chromatogram. The MS then allows identification of the components without having to rely on a comparison of retention times.

3 Mass Spectrometers

4 Mass Spectrometry  Analysis time once the GC has been set up (column chosen, gas flow set, oven program finalized) is about 20 minutes.  Not too expensive Our benchtop model was $30K.  Compounds that can be analyzed are those that can be analyzed on a GC: Low to moderate boiling liquids (and, of course, gases)

5 Mass Spectrometers  analyze particles…cations!, not photons. Impact from a high-energy electron knocks an electron from the organic compound and creates the Molecular ion (aka parent ion) a cation with an unpaired electron M + It has the m/z of the molecule, so the value of m/z in most cases is the value of the molar mass (provided z, the charge of the cation, is +1)

6 Mass Spectrometers  analyze cations, not photons Impact from a high-energy electron also causes fragmentation.  Base peak is the most intense m/z peak is the most stable fragment  Other fragments form as well, some from complex rearrangements

7 Mass Spectrometers  The MS detects the m/z of each fragment by directing the ionized particles through a magnetic quadrupole The trajectory of the ions in the magnetic field bends as a function of the mass of the particle and its charge:

8 Mass Spectrometers  The magnetic quadrupole thus acts as a mass filter. For a given magnetic field strength, light particles are bent more and heavier particles less. By changing the magnetic field strength, m/z can be focused on the detector (an electron multiplier) in succession and yield a mass spectrum  A plot of signal vs m/z

9 OCCC’s GC-MS  Shimadzu GC  GC-17A version 3  Restek Rtx-XLB 30 meter fused silica capillary column with 0.25 mm inner diameter and a 0.25μm coating (the liquid phase is low polarity…and proprietary) for a column volume of 1.5 mL  Column operates from 30° - 340°C

10 OCCC’s GC-MS  Shimadzu MS parameters  GCMS-QP5000 requires an operating pressure of approximately 2 Pa  quadruple mass filter  electron multiplier detector  turns on after 2 min to clear solvent  set up for 2 scans of m/z per second

11 Fragmentation Patterns - Benzene Ring Peaks at 51 and 77 are very typical of the benzene ring. Odd m/z suggests N may be present.

12 Fragmentation Patterns - the benzylic cation, the allylic cation Extraneous peaks happen.

13 Fragmentation Patterns - Amines

14 Fragmentation Patterns - alcohols Alcohols can fragment so easily that the M+ peak may be missing altogether.

15 Fragmentation Patterns - aldehydes and ketones

16 Fragmentation Patterns - halogens  Chlorine has 3:1 ratio of 35 Cl : 37 Cl Look for peaks at 35 and 37 Look for a gap of 35. Look for M:M+2 ratio of 3:1.  Bromine has 1:1 ratio of 79 Br : 81 Br Look for peaks at 79 and 81 Look for a gap of 79. Look for M:M+2 ratio of 1:1.  Iodine Look for peak at 127. Look for a gap of 127.

17 Rule of Thirteen  Once you have the molecular weight (MW)… Divide MW by 13 and express the result as an integer (n) and a remainder (r). If your compound is a hydrocarbon, its formula is approximated as C n H n+r. You may then find elements of unsaturation from the molecular formula.

18 Rule of Thirteen  If you have hetero atoms in your compound, adjust the formula to accommodate: For O, add O and subtract CH 4. For N, add N and subtract CH 2. For Cl, add Cl and subtract either C 2 H 11 or C 3 H -1. Again, calculate elements of unsaturation from the formula.

19 How to Analyze a Mass Spectrum  Identify the molecular ion peak, if present. Note if the MW is odd.  Mark the base peak and show the fragment which gives rise to it.  Mark significant fragment peaks and identify the fragments giving rise to them.  Mark peaks or gaps that indicate the halogens, Cl (M : M+2 peaks = 3:1), Br (M : M+2 peaks = 1:1), I (M or gap = 127).  The mass of the compound with Br will be M+1.


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