Chapter 29 - Chromatography and Spectroscopy

Question 1

What is chromatography used for?

Answer 1

To separate individual components from a mixture of substances

Question 2

What is the stationary phase in chromatography?

Answer 2

A phase that doesn’t move, usually solid or liquid supported on a solid

Question 3

What is the mobile phase in chromatography?

Answer 3

A phase that moves, normally a liquid or gas

Question 4

What can chromatography used to anaylyze?

Answer 4

Drugs, plastics, flavourings, and air samples

Question 5

What are the benefits of TLC?

Answer 5

It’s quick, inexpensive, and shows how many components are in a mixture

Question 6

What is a TLC plate made of?

Answer 6

Plastic or glass coated with a thin layer of solid adsorbent (e.g. silica) which acts as the stationary phase

Question 7

How is separation achieved in TLC?

Answer 7

Different components have different affinities for the adsorbent and bind with varying strength

Question 8

What is adsorption in the context of TLC?

Answer 8

The process by which silica holds different substances in the mixture to its surfacr

Question 9

How do you carry out a TLC experiment?

Answer 9

Draw a pencil baseline
Spot the solution with a capillary tube
Prepare tank (beaker with solvent + watch glass)
Ensure solvent level is below baseline
Place TLC plate in tank
Let solvent rise close to top
Remove and mark solvent front
Dry the plate
Visualise spots (UV lamp or spray with locating agent like iodine or ninhydrin)

Question 10

How is TLC analysed?

Answer 10

By calculating the retention factor
Rf = distance moved by component/distance moved by solvent

Question 11

How can you identify compounds in TLC?

Answer 11

Run the sample alongside a known pure compound and compare visually

Question 12

What is gas chromatography used for?

Answer 12

To separate and identify volatile organic compounds

Question 13

What are stationary and mobile phases in gas chromatography?

Answer 13

Stationary phase: High boiling point liquid adsorbed on inert solid
Mobile phase: Inert carrier gas (e.g. helium or neon)

Question 14

How does separation occur in gas chromatography?

Answer 14

Components slow down depending on their solubility in the liquid stationary phase. More soluble = slower = larger retention time

Question 15

What does each peak on a gas chromatogram represent?

Answer 15

A different component in the mixture

Question 16

How are compounds identified in gas chromatography?

Answer 16

By comparing retention times with known values

Question 17

How is concentration determined in gas chromatography?

Answer 17

Prepare standard solution
Obtain chromatogram and plot calibration curve
Run unknown sample under same conditions
Use curve to determine unknown concentration

Question 18

How do you test for a carboxylic acid?

Answer 18

Add aq sodium carbonate - effervescence indicates a positive result

Question 19

How do you test for a phenol?

Answer 19

Add bromine water at room temperature - decolourisation and formation of a white precipitate indicate a phenol

Question 20

What is nuclear spin, and when is it significant?

Answer 20

Nuclear spin is a property of the nucleus and is significant when there’s an odd number of nucleons

Question 21

What is resonance in NMR?

Answer 21

It occurs when a nucleus flips between two spin states under a strong magnetic field and radio frequency radiation

Question 22

What determines the frequency required for resonance?

Answer 22

It is proportional to the strength of the magnetic field

Question 23

What equipment is used to generate the magnetic field in NMR?

Answer 23

A large cylinder storing an electromagnet cooled by liquid helium

Question 24

Where else is NMR technology commonly used?

Answer 24

In hospitals as MRI scanners

Question 25

What is chemical shift and what unit is it measured in?

Answer 25

It is the frequency shift measured in parts per million (ppm)

Question 26

What is the standard reference chemical in NMR?

Answer 26

Tetramethylsilane (TMS), (CH3)4Si

Question 27

Why is TMS used as a reference in NMR?

Answer 27

Because it has a well-defined chemical shift and does not interfere with other signals

Question 28

How is an NMR experiment carried out?

Answer 28

Sample dissolved in solvent Placed in NMR tube with small amount of TMS Inserted into spectrometer Spun to even out imperfections Zeroed against TMS Given radiation pulse Absorptions detected and displayed

Question 29

Can the sample be recovered after NMR?

Answer 29

Yes by evaporating the solvent

Question 30

What is deuterated solvent and why is it used?

Answer 30

A solvent where 1H is replaced by 2H (deuterium), so it doesn't produce an NMR signal. CDCl3 is commonly used

Question 31

What information does carbon-13 NMR provide?

Answer 31

Number of carbon environments (number of peaks) Types of carbon environments (chemical shift)

Question 32

What factors can affect chemical shift range in 13C NMR?

Answer 32

Solvent, concentration, and substituents

Question 33

How are symmetrical carbon atoms represented in 13C NMR?

Answer 33

They are equivalent and contribute to the same peak

Question 34

How does proximity to oxygen affect carbon-13 chemical shift?

Answer 34

Carbon closer to an oxygen atom will have a larger chemical shift (move more to left)

Question 35

What does proton NMR provide?

Answer 35

Number of proton environments (peaks) Types of proton environments (chemical shift) Relative numbers of each proton type (integration traces) Number of nonequivalent adjacent protons (splitting pattern)

Question 36

What can affect proton NMR chemical shifts?

Answer 36

Solvent, concentration, and substituents

Question 37

How do equivalent protons appear in NMR?

Answer 37

They absorb at the same chemical shift, increasing peak size

Question 38

What does the area under a proton NMR peak represent?

Answer 38

The relative number of protons, shown by integration trace

Question 39

What is spin-spin coupling in 1H NMR?

Answer 39

Splitting of a main peak into sub-peaks due to interactions with adjacent protons

Question 40

How do you determine the number of sub-peaks from adjacent protons?

Answer 40

Use the n+1 rule: if a proton has n adjacent protons, it splits into n+1 sub-peaks

Question 41

What are examples of splitting patterns?

Answer 41

Singlet, doublet, triplet, quartet, etc.

Question 42

Are OH and NH protons involved in spin-spin coupling?

Answer 42

No

Question 43

How are OH and NH protons identified in NMR?

Answer 43

Run NMR spectrum Add deuterium oxide (D2O) Shake the mixture Run a second spectrum - OH/NH peaks disappear

Question 44

Why do OH/NH peaks disappear after adding D2O in NMR?

Answer 44

Deuterium replaces OH/NH protons and does not absorb in the 1H NMR range

Question 45

What is the sequence of analytical techniques for identifying compounds?

Answer 45

Elemental analysis Mass spectrometry Infrared spectroscopy NMR spectroscopy