Analysis- Chromatography and spectroscopy

Question 1

What is chromatography?

Answer 1

An analytical technique used to separate and identify the components of a mixture

Question 2

What are the 2 main types of chromatography?

Answer 2

-Thin Layer
-Gas

Question 3

What are the 2 phases in chromatography?

Answer 3

The stationary phase and the mobile phase

Question 4

Describe the method for chromatography

Answer 4

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.

Question 5

What is the equation to find Rf values?

Answer 5

Distance travelled by solute/ Distance travelled by solvent front

Question 6

Why do different amino acids have different Rf values?

Answer 6

They have different polarities/solubilities so have different affinities for stationary phase vs mobile phase

Question 7

What is retention time in gas chromatography?

Answer 7

The time a compound is held under given conditions.

Question 8

What factors affect retention time?

Answer 8

-Length and packing of the column
-Temperature
-Nature and flow rate of the carrier gas

Question 9

What is gas chromatography used for?

Answer 9

By forensic scientists to detect minute amounts of drug, alcohol, explosives, etc.

Question 10

Why should you allow the developing solvent to rise up to the top in chromatography?

Answer 10

To increase percentage error.

Question 11

What is the mobile phase in gas chromatography?

Answer 11

An inert gas carrier

Question 12

How does gas chromatography work?

Answer 12

-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.

Question 13

What is the stationary phase in gas chromatography?

Answer 13

A liquid with a high boiling point

Question 14

How would a scientist find the area under peaks in a gas chromatogram?

Answer 14

They would use integration

Question 15

What are the limitations of gas chromatography?

Answer 15

Similar compounds have similar retention times, so may not be distinguishable from each other

Question 16

How can gas chromatography be combined with mass spectrum analysis?

Answer 16

-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.

Question 17

What are the uses of gas chromatography combined with mass spectrometry?

Answer 17

-Drug identification
-Fire investigations (identification if accelerants)
-Identification of oil fingerprints

Question 18

How do you test for alkenes?

Answer 18

Add bromine water, if present it will be decolourised from orange to colour

Question 19

How do you test for a haloalkane?

Answer 19

Add silver nitrate and ethanol and warm to 50 degrees in a water bath, if present the coloured precipitates will form

Question 20

How do you test for a carbonyl?

Answer 20

Add 2,4-DNP, if present an orange precipitate will form

Question 21

How do you test for an alcohol and aldehyde?

Answer 21

Add acidified potassium dichromate and warm in a water bath, if present the colour will change from orange to green

Question 22

How do you test for a carboxylic acid?

Answer 22

Add aqueous sodium carbonate, if present you will see effervescence

Question 23

How would you identify a phenol?

Answer 23

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.

Question 24

What does nuclear magnetic resonance spectroscopy (NMR) do?

Answer 24

Differentiates between carbon environments

Question 25

What is the chemical shift?

Answer 25

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

Question 26

What does the number of different carbon environments determine?

Answer 26

Number of peaks on the spectrum

Question 27

What is used as a solvent in NMR and why?

Answer 27

CDCl3 because it doesn’t interfere with the sample’s spectrum

Question 28

Why do nuclei of the same type only produce one peak in the spectrum?

Answer 28

They are in the same environment so will resonate at the same frequency