WM - modern analytical techniques Flashcards
Mass spectra - what does it tell us?
helps identify compounds relative atomic mass relative molecular mass relative isotopic abundance molecular structure
Mass spectra - how does it work?
- vapourisation - the sample is turned into a gas by an electrical heater
- ionisation - the gas particles are bombarded with high energy electrons to ionise them. Electrons are knocked off the particles leaving +ve ions
- acceleration - the +ve ions are accelerated by an electric field
- 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.
Mass spectrum - what does it look like?
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
mass spectra - what is the M+ peak?
gives the relative molecular mass of a compound
the mass/charge value is the molecular mass
its the far right peak
mass spectra - what is the M+1 peak?
a smaller peak to the right of the molecular ion peak
mass spectra - what causes the M+1 peak?
cause by the presence of the carbon isotope 13C
mass spectra - fragmentation
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
mass spectra - what is fragmentation used for?
used to identify molecules and their structure
mass spectra - how do you work out structural formula?
- work out what ion could have made each peak from its m/z value
- identify the fragments
- piece them together to form a molecule with the correct Mr
infrared spectra - how does it work?
- a beam of infrared radiation is passes through a sample of a chemical
- the IR radiation is absorbed by the covalent bonds in the molecules
- this increases their vibrational energy (they vibrate more) - 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
infrared spectra - what does it tell us?
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.