Chapter 29 Chromatography And Spectroscopy

Question 1

What is TLC Chromatography?

Answer 1

Separating components within a mixture by their difference in attraction to the stationary or mobile phase.

Question 2

What is the stationary phase of TLC?

Answer 2

Solid silica (SiO2) or alumina AlO3 on an inert support material like glass or plastic

Question 3

What is the mobile phase of TLC?

Answer 3

Liquid solvent

Question 4

How fast does the component of the mixture move if it has a stronger affinity to the mobile phase?

Answer 4

The component will move faster and further up the TLC plate

Question 5

How fast does the component of the mixture move if it has a stronger affinity to the stationary phase?

Answer 5

The component will move slower and stop closer to the start of the TLC plate

Question 6

How does separation occur during TLC?

Answer 6

Relative adsorption
The components in the mixture adsorb differently to the surface of the stationary phase

Question 7

State an example of polar solvents that may be used in TLC?

Answer 7

Alcohol or water

Question 8

State an example of a non polar solvents that may be used during TLC?

Answer 8

Alkanes, hexane

Question 9

What quantitative measurement can be gained from TLC plates?

Answer 9

Retention factors (RF) of each component can be calculated and compared to known database values of pure samples

Question 10

How is Rf calculated?

Answer 10

Distance travelled by component
————————————————
Solvent front distance

Question 11

What are the limitations of TLC?

Answer 11

-Difficult to measure the exact centre of component spots
-Similar compounds can have similar Rf values so difficult to distinguish which one it may be
-Reference chromatogram may not exist on the database
-One solvent may not dissolve all components in sample or cause a component to be too soluble meaning it does not show up

Question 12

What is Gas liquid Chromatography?

Answer 12

Used to separate volatile components in a mixture, so is useful to analyse organic compounds with low boiling points

Question 13

What is the mobile phase of Gas liquid chromatography?

Answer 13

Inert carrier gas

Question 14

What is the stationary phase of Gas liquids chromatography?

Answer 14

High boiling point liquid adsorbed onto the surface of a solid support

Question 15

How is Gas liquid chromatography separated?

Answer 15

Relative solubility

Question 16

What quantitive measurement can be gained from Gas liquid chromatography?

Answer 16

Retention time which can be compared to a database of known pure samples

Question 17

What is Retention time?

Answer 17

The time taken for a compound to travel through the column to detector

Question 18

What does a longer retention time suggest?

Answer 18

The greater interaction between the component and stationary phase, the slower the component moves through the column
The component is more soluble in stationary phase than mobile

Question 19

What does a shorter retention time suggest?

Answer 19

The weaker the interaction between the component and stationary phase the, the faster the component moves through the column
The component is more soluble in mobile phase

Question 20

Suggest approximations for the retention times of a non polar or polar compound being separated using Gas liquid chromatography using a non polar liquid stationary phase

Answer 20

The non polar compound will be more soluble in the non polar stationary phase, so will move slower and have a longer retention time.
While polar compounds will be more soluble in the mobile phase, interacting less with the stationary phase allowing it to move faster and have a shorter retention time.

Question 21

What graph does the Gas liquid chromatogram produce?

Answer 21

Gas Chromatogram

Question 22

What does each peak in the Gas chromatogram represent?

Answer 22

Each compound in the mixture will produce a peak and the area under each peak is proportional to the relative amount of the compound present in the sample, however it does not give an exact concentration.

Question 23

How can % component of mixture be calculated from the Gas chromatogram?

Answer 23

Area under the peak
——————————- X 100
Total area of all peaks

Question 24

What is the calibration curve used to determine?

Answer 24

Determines the concentration of a substance analysed by Gas chromatography

Question 25

Describe how the calibration curve is calculated

Answer 25

-Prepare standard solutions of the compound under investigation -Obtain a gas chromatogram for each standard solution and measure the area under each curve -Plot a calibration curve of peak area against concentration -Compare the area under the curves for the compound under investigation with calibration curve

Question 26

What are the limitations of Gas Chromatography?

Answer 26

-Compounds can have the same Retention factor -A small amount of one component may hide behind a component of higher concentration -Unknown compounds may have no retention time data to compare against

Question 27

State another form of analysis that is often used alongside Gas chromatography

Answer 27

Mass spectrometry

Question 28

Why is mass spectrometry used with gas chromatography?

Answer 28

Gas chromatography is used to separate the components within a mixture While mass spectrometry is used to confirm the identity of each component within the mixture from accurate measurements of Mr and unique fragmentation.

Question 29

How is a sample prepared before NMR is carried out?

Answer 29

The sample to be studied is dissolved in a solvent (which contains no H1 atoms as this could hide the signal of the actual compound by appearing in the spectra)

Question 30

State the different solvents that can be used to dissolve the sample

Answer 30

CCl4 CDCl3 C6D6

Question 31

Why is deuterium used as a solvent?

Answer 31

Because it has no spin

Question 32

What molecule is used as reference to compare other compounds with in NMR?

Answer 32

TMS or Si(CH3)4 is used to provide the internal standard conditions of zero that other peaks can therefore be compared against to work out the spectra.

Question 33

State the different factors on a H-NMR spectrum that are used to identify the overall compound

Answer 33

-The number of different Hydrogen environments can be found from the number of peaks -The type of Proton environments can be found from chemical shift -The relative numbers of each proton can be found from the area under peak or integration value. -The number of protons adjacent to a given proton (within 3 bonds) can be found from the splitting pattern

Question 34

How can the number of Hydrogen environments in the NMR spectrum be determined?

Answer 34

Number of peaks

Question 35

How can the type of proton environment be determined from the NMR spectra?

Answer 35

Chemical shift table on data sheet

Question 36

How can the relative numbers of each proton be determined from NMR spectra?

Answer 36

Peak area or integration value

Question 37

How can the number of protons adjacent to a given proton (within 3 bonds) be determined from the NMR spectra?

Answer 37

Splitting pattern

Question 38

How can the integration values be calculated to determine a ratio for the number of protons involved in the compound?

Answer 38

Each intensity is divided by the smallest intensity before multiplying out to give a whole number= number of H atoms NOTE: ratio may be in simplest form so check the number of Hydrogens against a larger ratio.

Question 39

What is the rule to calculate adjacent protons from splitting patterns?

Answer 39

Rule = n + 1 Where N = number of lines

Question 40

How many protons would be adjacent to a singlet signal?

Answer 40

The H causing the signal will have no adjacent H within 3 bonds

Question 41

How many protons would be adjacent to a doublet signal?

Answer 41

The H causing signal will be adjacent to CH group

Question 42

How many protons would be adjacent to a triplet signal?

Answer 42

The H causing the signal will be adjacent to a CH2 group

Question 43

How many protons would be adjacent to a quartet signal?

Answer 43

The H will be adjacent to a CH3 group

Question 44

How does a OH or NH group always appear as on the NMR spectra?

Answer 44

Always will appear as a singlet

Question 45

How can OH and NH be determined within the sample?

Answer 45

By adding D2O to the sample and rerunning the spectra causes the groups to disappear.