19. Analytical Techniques II

Question 1

What is a property of an atom required to be detected by NMR

Answer 1

nuclei must have an odd number of nucleons so that nuclear spin is generated

Question 2

What type of solvents are used in NMR

Answer 2

deuterated solvents

Question 3

What are deuterated solvents and why are they used in NMR

Answer 3

hydrogen atoms are replaced with deuterium to prevent interfering with signals in proton NMR

Question 4

Give an example of a solvent used in NMR

Answer 4

CDCl3
- good solvent as lots of substances will readily dissolve in it

Question 5

Why is tetramethylsilane (TMS) used as a standard molecule in NMR

Answer 5

1. it produces a single intense peak
2. inert - doesn’t react with unknown substance
3. volatile - has low boiling point so is easily separated from unknown substance

Question 6

Why does multiplicity occur in proton NMR

Answer 6

occurs because the spin of one nucleus affects that of a chemically different nucleus on an adjacent atom

Question 7

Give the equation for the number of splitting peaks in proton NMR

Answer 7

no of peaks = number of hydrogens on adjacent chemically different atoms + 1

Question 8

What does the area underneath a peak in proton NMR represent

Answer 8

area is proportional to the number of hydrogen atoms present in that proton environment

Question 9

What is the use of a D2O shake

Answer 9

used to determine which signal is due to an OH proton

after a spectrum is run, the sample is shaken with a small amount of D2O
any signal due to an OH proton disappears

Question 10

What is the stationary phase in TLC

Answer 10

thin metal sheet coated in alumina or silica

Question 11

What is the mobile phase in TLC

Answer 11

polar or non-polar liquid
polar - water or alcohol
non-polar - alkanes

Question 12

Explain why different solutes travel different distances along the stationary phase in TLC

Answer 12

solute molecules adsorb onto the surface of the stationary phase
the distance travelled depends on the strength of the interactions between the mobile and stationary phase

Question 13

Give three locating agents that are used to identify spots on a chromatogram in TLC

Answer 13

1. UV light
2. ninhydrin (carcinogenic)
3. iodine vapour

Question 14

Explain how mixtures of amino acids are separated

Answer 14

1. proteins are hydrolysed using concentrated hydrochloric acid
2. resulting mix is separated using TLC
3. amino acids are coloured using either ninhydrin or UV light
4. plate is heated at 100*C

Question 15

Why must the baseline on a TLC plate be drawn in pencil

Answer 15

any other medium would interact with the sample component and solvents

Question 16

What is the stationary phase in column chromatography

Answer 16

adsorbent powdered solids e.g. silica and alumina
- INERT

Question 17

What is the mobile phase in column chromatography

Answer 17

liquid solvent

Question 18

Describe the process of column chromatography

Answer 18

1. inert solid is placed in the column
2. liquid solvent is added until it is saturated
3. sample mixture is dissolved in the solvent and introduced at the top of the column using a pipette
4. more solvent (eluent)
5. sample flows through the column via gravity

Question 19

How can the process of column chromatography be sped up in school laboratories

Answer 19

attaching a gas syringe to the top of the chromatography column

Question 20

How can the process of column chromatography be sped up in industry

Answer 20

attaching an air line to the top of the chromatography column

Question 21

Which components take the longest time to flow through the column in column chromatography

Answer 21

components with the greatest attraction/affinity to the stationary phase

Question 22

How can coloured components in column chromatography be identified

Answer 22

using the Rf value

Question 23

How can colourless components in column chromatography be identified

Answer 23

using fluorescence under UV light to show their position in the column

Question 24

What is the use of HPLC

Answer 24

used to separate components in a mixture that are very similar to each other

Question 25

Why do silica particles have a diameter of 4mm in HPLC

Answer 25

the use of fine particles increases the surface area of the stationary phase so separation is more efficient

Question 26

Give the 5 main differences in HPLC compared to column chromatography

Answer 26

1. HPLC sample is pumped through by the solvent rather than using gravity 2. stationary phase particles are much smaller - greater separation of compounds 3. in HPLC there is a detector at the end of the column measuring retention time 4. HPLC is automated and uses a computer

Question 27

Define retention time

Answer 27

the time taken from sample being injected to the sample being detected

Question 28

Why is the use of a computer in HPLC a benefit compared to column chromatography

Answer 28

allows for quicker analysis and comparison of results against known compounds in a database

Question 29

What is the use of GLC

Answer 29

used for analysing gases, volatile liquids and solids in their vapour form

Question 30

What is the stationary phase in GLC

Answer 30

inner surface of column of non-volatile liquid

Question 31

What is the mobile phase in GLC

Answer 31

an inert carrier gas e.g. helium or nitrogen

Question 32

Describe how GLC works

Answer 32

the sample is injected into the column through a self-sealing disc the vapour formed is carrier through the stationary phase using the carrier gas mobile phase

Question 33

What is the equipment that measures GLC

Answer 33

measured on a chromatogram each peak represents a volatile compound in the sample

Question 34

What does the area of the peak on a GLC chromatogram represent

Answer 34

proportional to the abundance present in the mixture

Question 35

How are retention times used to identify molecules

Answer 35

retention times are compared to data book values to identify unknown molecules

Question 36

What effect does size of molecule have on retention time in GLC

Answer 36

the larger the molecule, the more strongly it is adsorbed onto the stationary phase the longer the retention time

Question 37

What effect does volatility have on retention time in GLC

Answer 37

the more volatile the molecule, the quicker it will move though the column the shorter the retention time

Question 38

What effect does branching have on retention time in GLC

Answer 38

branched molecules will be less strongly adsorbed onto the stationary phase branched molecules have a shorter retention time

Question 39

What effect do London forces have on retention time in GLC

Answer 39

molecules with more London forces will be less volatile will move slower through the column will have a longer retention time

Question 40

Give 5 uses of HPLC and GLC

Answer 40

1. providing forensic evidence 2. drug testing 3. analysis of environmental pollution 4. detecting explosives in baggage 5. presence of chemicals in industrial processes

Question 41

Give a limitation of HPLC and GLC

Answer 41

it is not very useful at identifying specific compounds

Question 42

Give three reasons why is HPLC and GLC not very useful at identifying specific compounds

Answer 42

1. different compounds may have the same retention time 2. it can be difficult to manage conditions e.g. temperature and pressure 3. some unknown compounds may not have a reference for comparison in the databases