17.1 Mass Spectrometry
Learning Objectives:
1. Describe how mass spectrometry can be
used to identify unknown compounds.
Used to Find Molecular Mass of
Organic Compounds
1. Compound enters MS as gas.
2. Electron gun ionises sample.
3. Positive ions accelerated through MS as beam of
ionised molecules.
4. Ions deflected by magnetic field into a detector.
5. Strength of the magnetic field related to m/z
(m/z = mass as charge = +1).
6. Detector measures abundance.
Mass Spectrum
Why are there so many peaks?
• molecular ion = ion formed from whole
molecule
• The main peak furthest to the right
corresponds to the mass of the molecular ion.
• Fragmentation = molecule breaks up during
the MS process to form smaller ions
• The pattern can then be used as a fingerprint
to identify the molecule.
Mass Spectrum of Pentane
High Resolution MS
• MS usually to nearest whole number.
• Can be difficult to differentiate between
different molecules with same similar Mr.
• High res MS can measure masses to three or
four decimal places.
• More accurate, can be used to differentiate
different molecules of similar masses.
17.2 Intra-Red Spectroscopy
Learning Objectives:
1. Describe IR spectroscopy.
2. Be able to interpret IR spectra.
All bonds are vibrating
• Each bond has a certain frequency of
vibration.
• This frequency corresponds with certain
frequencies (wavelengths) of IR radiation.
• Bonds can absorb IR radiation to vibrate more.
• The frequencies which are absorbed can be
detected and used to identify what bonds are
in a molecule.
Infra-Red Spectrometer
1. Beam of IR radiation with range of
frequencies passed through a sample.
2. The sample absorbs certain frequencies.
3. Instrument plots the transmittance (intensity
of frequencies which make it through the
sample).
4. The absorbed frequencies will be “missing”
from the chart.
IR Spectrum
Peaks Correspond to Specific Bonds
FINGERPRINT REGION
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•
•
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organic molecules have a lot of C-C and C-H bonds within their structure
spectra obtained will have peaks in the 1400 cm-1 to 800 cm-1 range
this is referred to as the “fingerprint” region
the pattern obtained is characteristic of a particular compound the frequency
of any absorption is also affected by adjoining atoms or groups.
IR SPECTRUM OF A CARBONYL COMPOUND
• carbonyl compounds show a sharp, strong absorption between 1700 and 1760 cm-1
• this is due to the presence of the C=O bond
IR SPECTRUM OF AN ALCOHOL
• alcohols show a broad absorption between 3200 and 3600 cm-1
• this is due to the presence of the O-H bond
IR SPECTRUM OF A CARBOXYLIC ACID
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•
•
•
carboxylic acids show a broad absorption between 3200 and 3600 cm-1
this is due to the presence of the O-H bond
they also show a strong absorption around 1700 cm-1
this is due to the presence of the C=O bond
IR SPECTRUM OF AN ESTER
• esters show a strong absorption between 1750 cm-1 and 1730 cm-1
• this is due to the presence of the C=O bond
WHAT IS IT!
One can tell the difference between alcohols, aldehydes
and carboxylic acids by comparison of their spectra.
O-H STRETCH
ALCOHOL
C=O STRETCH
ALDEHYDE
O-H STRETCH
CARBOXYLIC ACID
AND
C=O STRETCH