F324 - Analysis

Question 0

What is meant by the term mobile phase?

Answer 0

The solvent which moves through the chromatography column or over paper. It is either a liquid or a gas.

Question 1

What is chromatography?

Answer 1

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

Question 2

What is meant by the term stationary phase?

Answer 2

The separating material in chromatography, such as solid particles packed into a column, water held in the fibres of paper, or a viscous liquid coated onto a solid surface.

Question 3

How do solid stationary phases separate the mixtures in chromatography?

Answer 3

Adsorption

Question 4

How do the liquid stationary phases separate the mixtures in chromatography?

Answer 4

Relative solubility

Question 5

What is meant by the term Rf value?

Answer 5

Retardation factor

Rf = distance moved by solute/distance moved by front of mobile phase

Question 6

What is meant by the term retention time?

Answer 6

The time taken for a component of a mixture to pass through a gas chromatography column.

Question 7

What are the limitations to analysing compounds by gas chromatography?

Answer 7

• similar compounds will often have similar retention times

- unknown compounds have no reference retention times for comparison

Question 8

How can the approximate proportions of the components of a mixture be determined by gas chromatography?

Answer 8

Take the area of the peak from the chromatogram, or if very narrow use the peak height and divide by the total areas (or heights) x100%.

Question 9

Why are gas chromatography and mass spectrometry combined?

Answer 9

Provides a more powerful analytical tool than chromatography alone as the compounds with similar retention times can be ‘fingerprinted’ by their unique mass spectra.

Question 10

What are GC-MS used for?

Answer 10

Airport security checks, petroleum and food industries, and in forensic and medical testing.

Question 11

How does NMR spectroscopy work?

Answer 11

Interaction of materials with the low-energy radiowave region of the electromagnetic spectrum. As odd mass number nuclei have a spin property which causes them to behave like very small bar magnets. The nuclei with this property absorb the energy and the absorption are analysed.

Question 12

Why are different chemical shifts observed in proton NMR spectra?

Answer 12

Protons are in different chemical environments.

Question 13

What do the area ratios of the signals correspond to in a proton NMR?

Answer 13

The number of protons in the different chemical environments.

Question 14

What do the protons on neighbouring carbon atoms do to the signals produced?

Answer 14

Split them in an n+1 pattern.

Question 15

What does a carbon-13 NMR spectra represent?

Answer 15

The number of different types of (non-equivalent) carbon atoms in a compound and their molecular environments.

Question 16

How can -OH and -NH proton signals be identified?

Answer 16

Arise as single peaks in the original proton NMR spectrum due to rapid proton exchange with water molecules. Removed from spectrum using D2O (deuterium oxide) which does not absorb in the same region so the signal disappears and the two spectra can be compared.

Question 17

Why are deuterated solvents used in NMR?

Answer 17

The nuclei in the molecules of a solvent will interact with the magnetic field and complicate the spectrum. Deuterated solvents do not contain hydrogen atoms or CCl4.

Question 18

What is tetramethylsilane (TMS) used for?

Answer 18

Standard for chemical shift measurements using NMR.