Edexcel Chemistry - Topic 19: Modern Analytical Techniques II

• Mass Spectrometry is used to identify compounds based off of a mass/charge ratio
  (m/z value) 
• The parent ion will have an m/z ratio that is equal to the relative molecular mass of a compound
• High Resolution Mass Spectrometry can measure the relative atomic masses and relative molecular masses of atoms and elements to a precision of four decimal places
• This allows a compound to be predicted from its mass spectrum more accurately as the precision of measurements is higher
• Hydrogen has a relative atomic mass of 1.0079
  Carbon has a relative atomic mass of 12.0107
  Oxygen has a relative atomic mass of 15.9994
• Therefore the relative molecular masses of butane (C4H10) and propanal (C3H6O) which both have a relative molecular mass of 58 to the nearest whole number will have a slight difference based of their High Resolution Mass Spectrums:
  C4H10 = (4 x 12.0107) + (10 x 1.0079) = 58.1218
  C3H6O = (3 x 12.0107) + (6 x 1.0079) + 15.9994 = 58.0789

• NMR spectroscopy involves interaction of materials with the low-energy radiowave region of the electromagnetic spectrum
• The radio waves used in Proton NMR cause the hydrogen nucleus to change its spin state
• NMR spectroscopy is the same technology that is used in Magnetic Resonance Imaging (MRI)

There are two forms of NMR:

• Proton NMR (H NMR):

*Gives information about the number of hydrogen atoms that are in a molecule and the environments they are in

• Carbon-13 NMR (C-13 NMR):

*Gives information about the number of carbon atoms that are in a molecule and the environments they are in

• In an H NMR spectrum, there is one signal for each set of equivalent H atoms
• In addition the intensity (integration value) of each signal is proportional to the number of equivalent H atoms it represents
• e.g. Ethanol has 3 different hydrogen environments so there are 3 peaks
• The samples used for creating the NMR spectrum needs to be dissolved in a solvent without any H-1 atoms e.g. CCl4 so the solvent doesn't give any peaks in the H NMR spectrum
• Tetramethylsilane (TMS) is added to the sample to calibrate the spectrum
• TMS is used because:
  Its signal is away from all the others
  It only gives one signal
  It is non-toxic and is inert
  It has a low boiling point and so can be removed from the sample easily
• The spectra are recorded on a scale known as the chemical shift δ which is how much the field has shifted away from the field for TMS
• The δ is a measure in parts per million (ppm) and is a relative scale of how far the frequency of the proton signal has shifted away from that for TMS
• The δ depends on what other atoms/groups are near the H - more electronegative groups give a greater shift

• In high resolution H NMR each signal in the spectrum can be split into further peaks due to inequivalent H's on neighbouring C atoms
• Nuclei in identical chemical environments do not show coupling amongst themselves
• Splitting of peak = number of inequivalent Hs on neighbouring C atoms + 1
• The number of peaks can be used to determine the hydrogen environments in a molecule

• Carbon-13 NMR Spectra give information on the carbon environments present in a molecule
• The number of peaks present in the spectrum = the number of different carbon environments in the molecule
• The data sheet provided in the exam can be used to determine what carbon environment causes each chemical shift
• These environments are then matched to the number of peaks and the molecular formula to determine the structure of a molecule
  

• Chromatography is an analytical technique that seperates components in a mixture between a mobile phase and a stationary phase
• Seperation by column chromatography depends on the balance between solubility in the mobile phase and retention in the stationary phase
• The mobile phase may be a liquid or a gas
• The stationary phase may be a solid (TLC) or either a liquid or solid on a solid support (GC)
• A solid stationary phase seperates by adsorption. A liquid staionary phase separates by relative solubility
• If the stationary phase was polar and the moving phase was non-polar e.g. Hexane then non-polar compounds would pass through the column more quickly than polar compounds as they would have a greater solubility in the non-polar mobile phase

• Gas-liquid chromatography (GC or GLC) can be used to seperate mixtures of volatile liquids
• In Gas-liquid chromatography, the mobile phase is a gas such as helium and the stationary phase is a high boiling point liquid adsorbed onto a solid
• The time taken for a particular compound to travel from the injection of the sample to where it leaves the column to the detector is known as the retention time and can be used to identify a substance
• Some compounds have similar retention times so won't be distinguished
• Basic Gas-liquid chromatography will tell us how many components there are in the mixture by the number of peaks
• The area under each peak will be proportional to the abundance of that compound
• It is also possible for a gas-liquid chromatography machine to be connected to a mass spectrometer, IR spectrometer or NMR machine enabling all the components to be identified
• GC-MS is used in analysis in forensics, environmental analysis, airport security and space probes

• A mixture can be seperated by chromatography and identified from the amount they have moved

Method

• Wearing gloves, draw a pencil line 1cm above the bottom of a TLC plate and mark spots for each sample to be added
• Use a capillary tube to add a tiny drop of each solution to a different spot and allow the plate to air dry
• Add solvent to a charger or large beaker with a lid so that it is no more than 1 cm in depth
• Place the TLC plate into the chamber making sure that the level of solvent is below the pencil line. Replace the lid to get a tight seal
• When the level of solvent reaches about 1cm from the top of the plate, remove the plate and mark the solvent level with a pencil
• Allow the plate to dry in the fume cupboard
• Place the plate under a UV lamp in order to see the spots and draw around them lightly in pencil
• Calculate the Rf values of the observed spots:
  (Distance moved by substance)/(Distance moved by solvent)