WM - modern analytical techniques Flashcards

1
Q

Mass spectra - what does it tell us?

A
helps identify compounds 
relative atomic mass
relative molecular mass
relative isotopic abundance
molecular structure
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2
Q

Mass spectra - how does it work?

A
  1. vapourisation - the sample is turned into a gas by an electrical heater
  2. ionisation - the gas particles are bombarded with high energy electrons to ionise them. Electrons are knocked off the particles leaving +ve ions
  3. acceleration - the +ve ions are accelerated by an electric field
  4. detection - the time taken for the +ve ions to reach the detector is measured. This depends on an ion’s mass and charge. Lighter, highly charged ions first.
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3
Q

Mass spectrum - what does it look like?

A

y-axis gives the abundance of ions - often as a percentage.
For an element the height of each peak gives the relative isotopic abundance
x-axis units are given as mass/charge ratio - since the charge on the ions is mostly +1 can often assume the x-axis is simply the relative mass

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4
Q

mass spectra - what is the M+ peak?

A

gives the relative molecular mass of a compound
the mass/charge value is the molecular mass
its the far right peak

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5
Q

mass spectra - what is the M+1 peak?

A

a smaller peak to the right of the molecular ion peak

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6
Q

mass spectra - what causes the M+1 peak?

A

cause by the presence of the carbon isotope 13C

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7
Q

mass spectra - fragmentation

A

the bombarding electrons make some of the molecular ions break up into fragments
- the fragments the are ions show up on the mass spectrum making a fragmentation pattern

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8
Q

mass spectra - what is fragmentation used for?

A

used to identify molecules and their structure

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9
Q

mass spectra - how do you work out structural formula?

A
  1. work out what ion could have made each peak from its m/z value
  2. identify the fragments
  3. piece them together to form a molecule with the correct Mr
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10
Q

infrared spectra - how does it work?

A
  1. a beam of infrared radiation is passes through a sample of a chemical
  2. the IR radiation is absorbed by the covalent bonds in the molecules
    - this increases their vibrational energy (they vibrate more)
  3. bonds between different atoms absorb different frequencies of IR radiation and bonds in different places in a molecule also absorb different frequencies
    eg. O-H bond in alcohol and O-H bond in carboxylic acids absorb different frequencies
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11
Q

infrared spectra - what does it tell us?

A

An infrared spectrometer produces a spectrum that shows you what frequencies of radiation the molecules are absorbing

can use this to identify the functional groups in a molecule

also means that you can tell if a functional group has changed during a reaction.

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