Characterization Techniques CHEM241-06A
Gas Chromatography
Thin Layer Chromatography
How does it work?
What does it do?
Pros and Cons:
Video Demonstration:
Separates a mixture of analyses based on their varying affinity for a stationary phase vs. a mobile phase. This allows us to be able to identify compounds in a mixture, determine purity, and follow the progress of a reaction.
This technique takes place on a plate covered with a thin layer of polar silica gel or alumina which makes up the stationary phase. First we draw a line in pencil about 1.5cm from the bottom. Spots of our solvent mixture are placed on this plate. These spots are about 1-2mm in diameter and are evenly spaced on the line to avoid the samples from combining. This plate is then placed in a beaker with a small amount of our mobile phase. This mobile phase is a non-polar solvent or mixture such as heptane. Make sure that the solvent level is below the spots to avoid the sample dissolving into the solvent. After placing the plate in the beaker containing the mobile phase, we cover the beaker to avoid evaporation. The mobile phase will begin to travel up the plate via capillary action and will bring the sample spots with it. The less polar a substance is the farther the mobile phase is able to move it. This is due to the more polar analytes sticking to the polar silica and not reacting to the non-polar mobile phase. We let this reaction continue until the solvent front is close to the top. Do not let the front reach the top or evaporation may occur and affect your results. Then we remove the plate from the beaker and quickly use our pencil to draw a line along the solvent front. At this point we may be able to see where our spots moved to but a UV light might be needed. We use our pencil to outline this spots and mark their centers. We then use the distance from where the spot was placed to the solvent front as our distance the solvent moved. Then we use the distance from where the spots were placed to the center of each spot to determine the distance that the spot moved. Then we find the Rf value for each spot be dividing the distance the spot moved by the distance the solvent moved. The lower the Rf value the more polar the compound.
Professor Dave Explains Thin Layer Chromatography:
Pros:
-Requires smaller quantities and is faster than column chromatography
-Easily separates components
-Little equipment is needed
-Cheap
Cons:
-Difficult to reproduce results
-Only works for soluble components
-Qualitative not quantitative analysis
-Not an automatic process
-Separation process is limited to the length of the plate
What does it do?
When is appropriate to use this technique?
This technique is used on a mixture of soluble non-volatile components.
Like other forms of chromatography, it separates a mixture of compounds based on their differences in polarities and affinities for stationary and mobile phases.
How does it work?
A small sample of the mixture is injected into the gas chromatograph to be vaporized. The gaseous mixture moves along the column with the help of the carrier gas, which is the mobile phase. This gas is typically helium. The stationary phase is a hollow metal tube with resin where the separation occurs. As the sample travels through the column, the compounds interact with the stationary phase at different rates, so they travel through the column at different rates which gives the separation. A GC chart will show Rt (retention time) which is how long each compound to exit the column
Melting Points
How does it work?
Pros and Cons
What does it do?
When is it apporpriate to use this technique?
Video Demonstration
Video Demonstration:
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Pros and Cons:
Pros: -Very efficient
- Reproducible
- Can be used alongside a mass spectrometer
- Only a very small sample is needed
Cons:
- Compounds must be thermally stable and volatile
- Usually a destructive process
- Requires more advanced equipment than thin layer and column chromatography
NMR Spectroscopy
What does it do?
How does it work?
When is it appropriate to use this technique?
Pros and Cons
Video Demo
both solid and liquid samples
Pros
Cons
spectroscopic technique that observes local magnetic fields around atomic nuclei to determine the content and purity of a sample as well as its molecular structure
low sensitivity, expensive, requires a large sample
When molecules are placed in a strong magnetic field, the nuclei of some atoms will begin to behave like small magnets. If a broad spectrum of radio frequency waves are applied to the sample, the nuclei will being to resonate at their own specific frequencies. The resonant frequencies of the nuclei are then measured and converted into an NMR spectrum that displays all of the frequencies as peaks on a graph. The height of each peak represents the number of nuclei that resonates at each specific frequency. The more resonating nuclei, the higher the intensity. The value of each frequency gives information about the surroundings of the atom. When atoms are in close proximity to one another, they can even cause each other to resonate. Increasingly stronger magnets can be used to study more complex molecules.
quantitative, does not require extra steps for sample preparation, observed in natural environment while still intact, non-invasive
Melting point is a temperature where the compound starts to melt and completely becomes a liquid.
This technique can be used when trying to determine the purity of an organic compound, as well as identifying and verifying the identity of an organic compound
As a compound melts, its particles vibrate more rapidly as the solid absorbs kinetic energy. To obtain melting point, you must heat your sample by controlling the heat and gradually lowering the heat control when 10 degrees Celsius of the approximate melting point range. The smaller the melting point range, the higher the purity of the sample.
pros
cons
If sample is not solid it will not work.
melting point is quick and relatively easy.
large samples will require more heat, resulting in wider melting point range.
requires small amounts of sample.
When it is appropriate?
Gas Chromatography can be used on volatile liquid or gas mixtures