4.3- Analysis

Question 1

What is the mobile phase

Answer 1

The phase that moves in chromatography

Question 2

What is the stationary phase

Answer 2

The phase that doesn’t move in chromatography

Question 3

What are the two types of chromatography

Answer 3

Thin-layer chromatography (TLC) and gas chromatography (GC)

Question 4

What is the principle/basis that chromatography works on

Answer 4

Different components have different affinities for a stationary phase and for a mobile phase.

(The stationary phase interacts with the components in the mixture, slowing them down. The greater the interaction the more the components are slowed down. This allows different components to flow over the stationary phase at different speeds separating the components)

Question 5

What are the phases of the mobile and stationary phase in TLC

Answer 5

Stationary phase- solid

Mobile phase- liquid

Question 6

What are the phases of the mobile and stationary phase in GC

Answer 6

Stationary phase- liquid or solid on a solid support.

Mobile phase- Gas

Question 7

How does TLC separate components

Answer 7

By adsorption- the components that adsorb most strongly to the stationary phase will not travel as far up the plate with the solvent, thus have a smaller Rf value

Question 8

How does GC separate components

Answer 8

Solid Stationary phase- By adsorption- the components that adsorb most strongly to the stationary phase are slowed down and have longer retention times.
Liquid Stationary phase- By relative solubility- components that are more soluble in the liquid stationary phase are slowed down and have longer retention times. Volatile components have shorter retention times.

Question 9

How do you identify a component in TLC

Answer 9

By Rf values- compare to known values under same conditions

Rf = distance moved by component/distance moved by solvent

(Note: number of spots= number of components)

Question 10

How do you identify a component in GC

Answer 10

By retention times- compare to known values

Note: number of peaks= number of components

Question 11

What is the retention time

Answer 11

The time from the injection of the sample for the component to leave the column

Question 12

What does the relative peak area on a GC absorption-time graph tell you

Answer 12

The approximate proportions of the components in the mixture.

Question 13

What is chromatography

Answer 13

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

Question 14

What are 4 limitations of TLC

Answer 14

1. Similar compounds have similar Rf values
2. Unknown compounds have no Rf values for comparison
3. Difficulty finding an ideal solvent- so components have the right amount of solubility in the solvent so they are able to travel a convenient distance up the plate.
4. Doesn’t give info on how much of each component there is.

Question 15

What are 2 limitations of GC

Answer 15

1. Similar compounds have similar retention times

2. Unknown compounds have no reference retention times for comparison.

Question 16

What other analytical technique can you use alongside GC

Answer 16

Mass spectrometry (making GC-MS) 
-> GC separates components in mixture and then Mass spectrometer produces distinctive fragmentation pattern for each component, which can be analysed further to provide a more powerful analytical tool.

Question 17

What are 4 uses of GC-MS

Answer 17

1. Forensics- use as evidence in court
2. Environmental analysis- pollutants
3. Airport security- explosives
4. Space probes- evidence of different materials on foreign planets

Question 18

What is nuclear magnetic resonance spectroscopy

Answer 18

An analytical technique that involves the interaction of materials with the low energy radiowave region of the electromagnetic spectrum

Question 19

What compound is used as the standard reference for measuring NMR chemical shifts

Answer 19

Tetramethylsilane (TMS) provides a reference signal from which all other NMR signals are compared. This is as TMS has 12 equivalent protons so a single peak is produced and called 0ppm

Question 20

What does the size of the peak in a carbon-13 NMR spectra tell you

Answer 20

Nothing

Question 21

Why must Deuterated solvents used when running NMR spectrum

Answer 21

As normal organic solvents would give rise to peaks due to the hydrogen atoms they contain. Deuterium is an isotope of Hydrogen with one proton and one neutron in it’s nucleus and it produces no signal on NMR spectra. An example is CDCl3

Question 22

What is the purpose of D2O in NMR spectra

Answer 22

Deuterium oxide or ‘Heavy Water’ is used to identify peaks containing -OH or -NH groups which can be tricky to identify. You run the NMR spectrum as normal, then run it again with D2O present, so the deuterium atoms replace the hydrogen atoms in -OH or -NH, removing any peak caused by these groups. Therefore compare the two spectra to see if any -OH or -NH peaks are present.

Question 23

What is a medical use of NMR spectroscopy

Answer 23

MRI scans (magnetic resonance imaging) are the same technology as an NMR spectroscopy and used to obtain diagnostic information about internal structures in body scanners.

Question 24

What is the molecular ion peak

Answer 24

The peak furthest to the right at the highest m/z value on a mass spectrometry graph