Chromatography is a practical technique used to separate and identify the components in a mixture.
Chromatography involves a mixture being dissolved in a mobile phase (which could be a liquid or a gas) that is then passed through an immobile stationary phase (which is usually a solid).
The phases are chosen so that components in the mixture have differing interactions in each phase; the balance of these two factors determines the rate of movement of a component which is recorded as either an Rf value or a retention time and used in a component’s identification.
Examples of types of chromatography include: thin-layer chromatography, column chromatography and gas chromatography.
First of all let’s look at gas-liquid chromatography. Gas-liquid chromatography is a microscale chromatographic technique that can be used for both qualitative and quantitative analysis.
It consists of a mobile phase of a gas such as helium, argon, nitrogen or hydrogen, (depending on the detection technique used) which flows over a liquid stationary phase absorbed onto a solid support.
Gas-liquid chromatography is particularly useful in the analysis and determination of volatile compounds, for instance those that vaporise easily without decomposition.
Alternatively, liquid chromatography nuclear magnetic resonance relies on the fact that every nucleus produces a magnetic field due to their spin, and will thus align with another magnetic field. This alignment can be used to identify molecules.
Radio waves are used in NMR spectroscopy. The NMR transition is very low energy. We thus use low frequency, low energy, radio waves to induce the transition. We must match the energy input to the transition.
Microwave radiation is a fast and efficient way to only heat up the specific target areas. It does not heat up the container but just the molecules, so is known as selective. It allows for much higher localised temperatures, speeding up reactions.
In liquid chromatography we separate molecules. We push molecules through a liquid chromatography column in order to separate them. Thus we can use liquid chromatography to separate a mixture, then use mass spectrometry to analyse each separate component.
That was a lot of information to take in! But you’re half way through – and this is the last section to revise in A2!
For gas chromatography, it is essential that our sample is volatile. We need a volatile sample that we can easily form a gas from to pass into the gas stream and the column.
We vaporise our analytes to produce a gas before forcing it through a column. This means we do not need to dissolve our analyte.
At this stage if you’re not sure about chromatography columns – go and look them up! Chances are you might have to navigate one of them for an exam question.
Finally let’s look at the involvement of infrared and ultraviolet radiation.
Infrared can give us some idea of functional groups. Furthermore if we already have a known molecule that matches our spectrum, we can use the fingerprint region to identify the unknown molecule.
In reality though, we must always use a number of different spectroscopic techniques, and where these agree, we have a result we can confirm.
Ultraviolet is a commonly used spectroscopic technique because different molecules have characteristic absorption bands.
In much the same way as infrared spectroscopy, different molecules absorb light in the ultraviolet spectrum in different patterns, helping us to identify certain molecules.
And finally, ultraviolet can cause molecules to undergo homolytic fission. Ultraviolet is often used to start reactions that require free radicals due to its propensity to induce homolytic fission and produce said free radicals.
And that’s it, you made it through!