Analysis- Chromatography and spectroscopy Flashcards
What is chromatography?
An analytical technique used to separate and identify the components of a mixture
What are the 2 main types of chromatography?
-Thin Layer
-Gas
What are the 2 phases in chromatography?
The stationary phase and the mobile phase
Describe the method for chromatography
1) Draw the base line with pencil 1cm from bottom of chromatography paper.
2) Using a capillary tube, spot small amounts of the sample mixture and pure samples of each component on the pencil line .
3) Add 0.5cm of solvent to a beaker and place paper in it, making sure solvent does not go over the pencil line. Place lid on.
4) Allow the solvent to rise up the paper until about 1cm from the top.
5) Remove from the beaker and immediately label how far the solvent has travelled (solvent front).
6) Allow to dry.
7) Circle any visible spots and if necessary use a UV light or chemical agents to show invisible ones.
8) Calculate the Rf value of each spot and compare to data values.
What is the equation to find Rf values?
Distance travelled by solute/ Distance travelled by solvent front
Why do different amino acids have different Rf values?
They have different polarities/solubilities so have different affinities for stationary phase vs mobile phase
What is retention time in gas chromatography?
The time a compound is held under given conditions.
What factors affect retention time?
-Length and packing of the column
-Temperature
-Nature and flow rate of the carrier gas
What is gas chromatography used for?
By forensic scientists to detect minute amounts of drug, alcohol, explosives, etc.
Why should you allow the developing solvent to rise up to the top in chromatography?
To increase percentage error.
What is the mobile phase in gas chromatography?
An inert gas carrier
How does gas chromatography work?
-The components have different affinities for the mobile phase compared to the stationary phase, and they emerge from the column at different times.
-The more soluble a component is in the stationary phase, the slower it travels through the column.
-Components with a bigger affinity for the mobile phase are most volatile so emerge first.
What is the stationary phase in gas chromatography?
A liquid with a high boiling point
How would a scientist find the area under peaks in a gas chromatogram?
They would use integration
What are the limitations of gas chromatography?
Similar compounds have similar retention times, so may not be distinguishable from each other
How can gas chromatography be combined with mass spectrum analysis?
-The gas chromatogram is linked to a mass spectrometer which can analyse the components separated by the gas chromatography.
-Using the mass spectrum results (the fragmentation pattern and molecular ion peak), the component separated is then identified.
What are the uses of gas chromatography combined with mass spectrometry?
-Drug identification
-Fire investigations (identification if accelerants)
-Identification of oil fingerprints
How do you test for alkenes?
Add bromine water, if present it will be decolourised from orange to colour
How do you test for a haloalkane?
Add silver nitrate and ethanol and warm to 50 degrees in a water bath, if present the coloured precipitates will form
How do you test for a carbonyl?
Add 2,4-DNP, if present an orange precipitate will form
How do you test for an alcohol and aldehyde?
Add acidified potassium dichromate and warm in a water bath, if present the colour will change from orange to green
How do you test for a carboxylic acid?
Add aqueous sodium carbonate, if present you will see effervescence
How would you identify a phenol?
Add bromine water at room temperature, as it will be decolourised and form a white precipitate.
Can also use pH indicator paper to show acidity.
What does nuclear magnetic resonance spectroscopy (NMR) do?
Differentiates between carbon environments
What is the chemical shift?
Found on the horizontal axis, it allows information about the environment of protons to be gained by comparing with data tables to determine the structure of a compound
What does the number of different carbon environments determine?
Number of peaks on the spectrum
What is used as a solvent in NMR and why?
CDCl3 because it doesn’t interfere with the sample’s spectrum
Why do nuclei of the same type only produce one peak in the spectrum?
They are in the same environment so will resonate at the same frequency
