Analysis

Question 1

What is chromatography?

Answer 1

An analytical technique that separates components in a mixture between a mobile phase and a stationary phase

Question 2

Define mobile phase

Answer 2

The phase that moves in chromatography. May be liquid or gas

Question 3

Define stationary phase

Answer 3

The phase that doesn’t move in chromatography. May be a solid (as in thin-layer chromatography) or either a liquid or a solid on a solid support (as in gas chromatography)

Question 4

How does as solid stationary phase separate?

Answer 4

By adsorption to the solid phase

Question 5

How does a liquid stationary phase separate?

Answer 5

By relative solubility in the liquid stationary phase

Question 6

What is the R(f) value?

Answer 6

A number between 0 and 1 that shows how far a component has moved compared with the solvent front.
R(f) = distance moved by component/distance moved by solvent front

Question 7

What is the retention time?

Answer 7

The retention time in gas chromatography is the time for a component to pass from the column inlet (injection) to the detector

Question 8

How can the relative proportions of compounds in a mixture be deduced from gas chromatograms?

Answer 8

The area under the peak is proportional to the amount of a compound in the sample

Question 9

What are the limitations of gas chromatography?

Answer 9

• thousands of chemicals may have the same retention time, peak shape and detector response, so it may not positively identify all components
• not all substances in a sample will necessarily be separated and detected. A small amount of a substance can hide beneath another that has a higher concentration and the same retention time
• unknown compounds have no reference retention times for comparison. To make results meaningful, analysts need to know which components are to be expected.

Question 10

Why might you combine gas chromatography with mass spectrometry?

Answer 10

Gas chromatography-mass spectrometry is a far more powerful analytical tool than the techniques used alone: first the components are separated (GC), and retention times may provide a preliminary identification, and then they are analysed (MS) for a positive identification of the compound, as the mass spectrum is unique and definitive for a single compound.

Question 11

What are the uses of gas chromatography-mass spectrometry?

Answer 11

• Forensics: analysing minute particles found at the scene of the crime; detecting minute quantities of drugs in the hair of a suspect, weeks after they have been taken
• Environmental analysis: monitoring and analysing organic pollutants in the environment, including waste and drinking water. Detecting pesticides in food.
• Airport security: detecting explosives in luggage and on human beings
• Space probes: analysing materials from planets (surface and atmosphere).

Question 12

What is NMR spectroscopy?

Answer 12

It involves interaction of materials with the low-energy radiowave region of the electromagnetic spectrum

Question 13

In carbon-13 NMR, what must be included in the answer given?

Answer 13

• different types of carbon present with reference to chemical shift
• possible structures of the molecules

Question 14

In proton NMR, what must be included in the answer given?

Answer 14

• different types of proton present with reference to chemical shift
• relative numbers of each type of proton present with reference to relative peak areas from integration traces or ration numbers
• the number of non-equivalent protons adjacent to a given proton with reference to spin-spin splitting pattern, using the n + 1 rule
• possible structures for the molecule

Question 15

What is the standard for chemical shift measurements in NMR and why is it used?

Answer 15

Tetramethylsilane (TMS): (CH(3))(4)Si. Defined as 0 on the delta scale.

• has 12 equivalent protons, so gives a single, sharp peak that can be identified easily
• is chemically unreactive and volatile so can be removed from sample easily after running an NMR spectrum.

Question 16

What kind of solvents are required for MNR spectroscopy and why?

Answer 16

Organic solvents contain Cs and Hs which will produce a signal, so deuterated solvents are used: solvents with the isotope of hydrogen called Deuterium - as it has an even number of nucleons, it doesn’t give a signal in an NMR spectrum. Often use CDCl(3).

Question 17

What are the issues with identifying -OH and -NH groups in proton NMR?

Answer 17

The peaks appear over a wide range of chemical shift values; signals are often broad; usually no splitting pattern

Question 18

How can we identify -OH and -NH peaks in proton NMR?

Answer 18

Use deuterium oxide, D(2)O. Any peak due to an -OH or an -NH group disappears, as there is an equilibrium:
-OH + D(2)O -OD + HDO (also works with -NH). Run a before adding D(2)O and after adding D(2) spectra, thus eliminating any peaks due to -OH or -NH

Question 19

Where is the technology of NMR spectroscopy used?

Answer 19

In magnetic resonance imaging (MRI) to obtain diagnostic information about internal structures in body scanners. Tissues containing few hydrogen atoms (like bone) come up dark, whilst those containing many hydrogens (like fat) come up lighter.