Modern Analytical Techniques II Flashcards
What is high resolution mass spectrometry?
High resolution mass spec is the same as normal mass spec, but measures to more decimal places so can be used to distinguish compounds with very similar molar masses.
How does NMR work?
Nuclei which have uneven numbers of protons and neutrons (odd mass number) (e.g. H-1, C-13, P-31) have an overall spin. When a strong magnetic field (radio frequency) is applied, this causes a spin flip, resulting in two energy levels (low energy (spins align with magnetic field) and high energy (spins are against magnetic field)). The energy required to induce this spin flip is measured. In a molecule, atoms contain electrons, which have their own magnetic field that is experienced by the nucleus. The change in electronic environment changes the magnetic field experienced by a nucleus and therefore the frequency required to spin flip the nuclei. This allows inequivalent environments to be identified and plotted on a spectrum.
What is tetramethylsilane (TMS) and why is it used in NMR?
TMS is a silicon atom bonded to 4 methyl groups. It is used as a reference in C-13 and H-1 NMR because it only produces one signal in both types of NMR and absorption peak is lower than almost everything else, so provides a useful reference point. The TMS peak is given a value of 0 and everything else is measured in chemical shifts against it (in ppm (parts per million)). TMS is non-toxic and inert, so won’t react with the chemicals being analysed. It has a low boiling point, so can be distilled off and used again. There are 12 hydrogens and 4 carbons, so produces strong signals (you don’t need to use much).
How do you analyse C-13 NMR?
The number of peaks in the carbon NMR corresponds to the number of carbon environments in the molecule. The relative areas of the peaks correspond to the ratio of the number of carbon atoms in each environment. The chemical shifts can be compared to the values on the data book to work out which functional groups are present. Put this information together to work out the possible structures.
What is chemical shift?
The difference in radio frequency absorbed by the nuclei in the molecule being analysed and that absorbed by the same nuclei (carbon or hydrogen) in TMS.
What is low resolution H-1 NMR?
Low resolution hydrogen NMR is not used anymore but it shows the number of hydrogen environments and the ratios of the number of hydrogens in each one (similar to carbon NMR) but it doesn’t show any splitting patterns.
What is high resolution proton NMR?
High resolution proton NMR shows the number of hydrogen environments, the relative areas of each one (ratio of number of hydrogen atoms) and splitting patterns, which helps determine the number of hydrogens on adjacent carbon atoms.
How does splitting occur in high resolution proton NMR?
Signals from hydrogen atoms are affected by hydrogens on adjacent carbon atoms (spin-spin coupling). The splitting pattern of a peak is the number of hydrogens on adjacent carbons +1. Only hydrogens bonded directly to carbon are considered, (e.g. carbons in -OH or -NH groups will not contribute to splitting of other peaks, or have any splitting in their own peaks).
What are integration traces in NMR?
Integration traces are a way of representing the ratio of numbers of hydrogen atoms in each environment. The ratio is given by the ratio of the heights of the vertical sections of the integration trace.
How can proton NMR be used to work out structures?
The number of peaks in the H-1 NMR spectrum gives the number of hydrogen environments in the molecule. The ratios of the numbers of hydrogen atoms in each environment are shown using integration traces or numbers above the peaks. The chemical shift of the peaks can be compared to the H1 NMR shifts data in the data booklet to identify possible functional groups. This information can be combined to work out possible structures.
What is chromatography used for?
Chromatography separates substances in a mixture and can often be used to identify those substances.
How does chromatography work?
A mobile phase moves through or over the stationary phase. The distance each substance moves/time spent in a chromatography column depends on the substances’ affinity (attraction- solubility in a solvent or adsorption to a solid) for the mobile and stationary phases. The stronger the affinity for the mobile phase, the further/quicker the substance will move, the stronger the affinity for the stationary phase, the less far/slower the substance will move.
What are mobile and stationary phases in chromatography?
The mobile phase is where the molecules can move (always a liquid or a gas (e.g. water, alcohols, alkanes)). The stationary phase is where molecules can’t move (this will be a solid, liquid, or a liquid on solid support (e.g. SiO2 (silica) on plastic plate).
How do you set up paper/TLC chromatography?
Draw a start line on the paper/plate with pencil. Spot the samples on the start line, spaced out (wait for them to dry). Place in a shallow layer of solvent than does not go over the start line. Put a lid on the tank (e.g. watch glass on a beaker). Wait until the solvent gets close to the you then remove from the solvent and draw a line in pencil to mark the solvent front. Then allow to dry and measure Rf values. If the spots are colourless, a developing agent (e.g. iodine (for many compounds), ninhydrin (for amino acids) or UV light can be used if the substances are fluorescent. The spots are drawn around in pencil.
How do you calculate Rf values?
Rf = distance travelled by substance / distance travelled by solvent
How are Rf values helpful in identifying compounds?
Rf values are the same for the same mobile and stationary phases, so the Rf values obtained for your substance in particular conditions can be compared to data book Rf values for those conditions to identify the substances.
What can you do if the substance does not fully separate in chromatography?
Use a different mobile or stationary phase (it is likely that the substances which did not separate before will have different affinities for the new phases, so will separate). 2-D chromatography can also be used (turn the paper the other way around (90 degrees) and run the solvent up the other side of the paper). This gives the substances another chance to separate.
What is column chromatography?
A sample is dissolved in ‘eluent’ (solvent) and put at the top of the chromatography column (looks like a big burette). The tap is opened and more solvent (mobile phase) is put on top. As the solvent passes through the column, the components of the mixture separate due to different levels of attraction to the stationary phase (usually silica gel bonded to various hydrocarbons).
What is high-performance liquid chromatography (HPLC)?
HPLC is column chromatography but the sample and solvent are forced through the column faster at high pressure by a pump. This produces higher resolution readouts as there is less diffusion of the samples. HPLC is usually connected to mass spec or UV vis spectroscopy, to further analyse the sample and produce a chromatogram. This shows retention times, which is the time taken for the substance to pass through the column and reach the detector. Experimental retention times can be compared to data book values to identify substances.
How does gas chromatography work?
The sample to be analysed is injected into a stream of gas (inert gas such as argon or nitrogen), which carries it through a coiled column coated with a viscous liquid or solid. The components of the mixture will dissolve in the liquid/adsorb onto the solid then evaporate back into the gas (it is done at very high temperatures). The different components in the mixture can be identified by their retention times. A mass spectrometer can be hooked up to the machine to further analyse the sample.
