Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

PMY2106 Medicinal Substances > Gas Chromatography > Flashcards

Gas Chromatography Flashcards

1
Q

Describe gas chromatography.

A

The gas is the mobile phase and the liquid is the stationary phase.
A gaseous mobile phase flows, under pressure, through a heated column either coated with a liquid stationary phase or packed with a liquid stationary phase coated onto a solid support.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What can GC be used to separate?

A

Used to separate volatile organic compounds.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Why is a column oven used?

A

The column is contained within an oven which is used to maintain a high temperature.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is the role of the soap-bubble meter?

A

Soap-bubble meter is connected to the detector to monitor and establish the flow-rate of the mobile phase.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What are some commonly used carrier gases?

A

Hydrogen, Helium, Nitrogen, Argon, Carbon dioxide.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Why is an injection port important?

A

Important because column efficiency depends on this system.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

The column efficiency requires the sample to be…. (2 points)

A

Sample has to be a suitable size and sample must be introduced as a ‘plug’ of vapour (so sample must be volatile)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What happens if oversized samples are slowly injected?

A

Causes band spreading and poor resolution.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Why is it more advantageous to use rotary sample valve injection port rather than the commonly used microflash vapouriser direct injector?

A

Reproducible results can be produced if using a rotary sample valve.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

How are solid samples injected?

A

Solid samples are sealed into thin-walled vials that can be inserted at the head of the column and punctured/crushed from the outside.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Microflash vapouriser direct injection - injection port method -

A

The sample should be prepared as a solution into the syringe. The sample is then injected directly into the vapourisation chamber so that the sample is vapourised into the chamber.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What are the advantages of the sample being injected using a syringe into a flowing stream of hot mobile phase?

A
  • The high temperature (50 degrees above bp) used causes the vapourisation of the sample.
  • It introduces a narrow plug of sample vapour into the column which reduces band-spreading and increases resolution.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

How much of the sample can be injected into packed columns?

A

Inject 1-5µL of sample.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What additional instrument must be used if capillary columns are used?

A

A split valve is used to introduce a small fraction of sample into the column.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What are preheaters in the oven used for?

A

Preheaters are used to convert the sample into its vapour form and mix them with the mobile phase.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Column temperatures must be controlled precisely.

A

For precise work, column temperatures must be controlled to within tenths of a degree.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What is the optimum column temperature to obtain an elution time of 2-30mins?

A

The optimum column temperature is dependent on the boiling point of the sample but typically is 50 degrees above the boiling point of the sample.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Minimal temperatures give good resolution but increase elution times.

A

Maximum temperatures give poor resolution but decrease elution time.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

The partition coefficient of the solute/analyte is dependent on the temperature.

A

Therefore, the temperature maintenance in the column is highly essential for efficient separation.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

The temperature can be constant or a range of temperatures can be applied. What difference does this cause?

Isothermal = temperature is constant
Programmed = temperature increased in steps
A

Depending on whether it is low or high constant (isothermal) temperatures, the elution will be slow/fast with good/poor resolution, respectively.
Whereas if a range of temperatures are used (temperatures programmed to increase over time), then resolution will be high with a suitable elution rate and more compounds can be separated.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

Are LC detectors more sensitive than GC detectors?

A

GC detectors are 4-5 orders of magnitude more sensitive than LC detectors.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Different carrier gases have different thermal conductivities.

A

Changes in the temperature of the electrically-heated wires in the detector are affected by the thermal conductivity of the gas which flows around this. The changes in this thermal conductivity are sensed as a change in electrical resistance and are measured.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

What is a disadvantage of GC in terms of type of compounds it can separate?

A

It can only separate volatile compounds. It cannot be used to separate non-volatile compounds.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

Describe the shape of the column used.

A

Column is colloid shaped not straight.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

How is the sample injected?

A

Sample is prepared in a syringe and injected directly into the injector.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

What does the two-stage pressure regulator and flow controller do?

A

Controls pressure and flow-rate.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

Describe the use of hydrogen as carrier gas.

A
  • better thermal conductivity
  • low density
  • used in thermal conductivity detector and flame ionisation detector
  • reacts with unsaturated compounds and it is inflammable
  • some samples cannot use hydrogen as carrier gas due to safety risk
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

Describe the use of helium as carrier gas.

A
  • excellent thermal conductivity

- expensive

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

Describe the use of nitrogen as carrier gas.

A
  • inexpensive

- reduced sensitivity

30
Q

What are the 6 factors to be taken into account before choosing a carrier gas?

A
  1. inertness - should not react with sample.
  2. suitable to the detector used.
  3. easily available.
  4. cheap.
  5. less risk of explosion/fire hazard.
  6. should give best column performance.
31
Q

Which type of device is used to remove water and other impurities?

A

Molecular sieve, contained within carrier gas system.

32
Q

How is the flow rate controlled?

A

Controlled by the two-stage regulator on the gas cylinder and additional controls within the instrument.

33
Q

Inlet pressure range from 10-50 psi above room pressure which leads to what flow rates in packed columns and open-tubular columns?

A

Flow rates of:
25-150ml/min with packed columns
1-25ml/min with open-tubular columns

34
Q

What happens to the flow rates if inlet pressure remain constant?

A

If inlet pressures are constant, flow rates will be constant.

35
Q

List some detectors used in GC.

A 
Hot wire detector/ thermal conductivity detector
Flame ionisation detector
Electron capture detector
Nitrogen phosphorus detector
Argon ionisation detector
36
Q

How does thermal conductivity detector (TCD)/ katharometer work?

A

TCD compares the thermal conductivity of 2 gas flows - the pure carrier gas and the sample.
ie pure mp and the mp containing sample.
Changes in the temperature of the electrically heated wires in the detector are affected by the thermal conductivity of the gas which flows around this. The changes in thermal conductivity are sensed as a change in electrical resistance and measured.

37
Q

What are the thermal conductivities for different carrier gases?

A

Different carrier gases have different thermal conductivities.

38
Q

Describe the 5 advantages of TCD.

A
  1. Applicable to most compounds.
  2. Linearity is good.
  3. Sample is not destroyed.
  4. Simple and easy to maintain.
  5. Inexpensive.
39
Q

Describe the 3 disadvantages of TCD?

A
  1. Low sensitivity.
  2. Affected by fluctuations in temperature and flow rate.
  3. Biological samples cannot be analysed.
40
Q

Describe the flame ionisation detector (FID).

A

FID is based upon the electrical conductivity of carrier gases. At normal temperature and pressure, gases act as insulators but become conductive if ions are present. Ions produced by burning the gas flow.

41
Q

Describe the 4 advantages of FID.

A
  1. The detector is extremely sensitive and the background noise is low.
  2. Stable and insensitive to small changes in the flow rate of carrier gas and water vapour.
  3. Responds to most organic compounds.
  4. Linearity is excellent.
42
Q

Describe the 3 disadvantages of FID.

A
  1. Destructive process - sample will be destroyed.
  2. Less sensitive to non-hydrocarbon groups.
  3. Insensitive to H2O, CO2 and SO2.
43
Q

Why is FID useful in detecting pollutants in natural water samples?

A

FID is insensitive to water.

44
Q

Describe electron capture detector (ECD).

A

ECD uses a radioactive Beta emitter (electron) to the carrier gas and produces a current between a biased pair of electrons.
ECD is sensitive as FID but has limited dynamic range and is used in analysis of halogenated compounds as halogenated compounds are capable of capturing electrons and thus reducing the current.

45
Q

What are the 2 advantages of ECD?

A
  1. Detector is highly sensitive (can measure nanograms).

2. Halogenated compounds an be detected using ECD.

46
Q

What is the disadvantage of using ECD?

A

Can only be used for compounds with an electron affinity.

Selectively detects halogen containing compounds.

47
Q

Why are columns set up in ovens?

A

To allow it to operate at high temperatures.

48
Q

Give the ranges of length, inner diameters, film thickness of typical GC columns.

A

Length: 5-200m
Inner diameters: 50-500mm
Film thickness: 0.25, 1.0, 5,0 mm

49
Q

Describe the main difference between packed and capillary/open tubular columns.

A

Packed column is fully packed with solid materials whereas capillary columns are not packed, core region is empty.

50
Q

Describe packed column.

A

Metal column packed with solid material. Stationary phase attached to the solid material (liquid coated on the solid support). Mobile phase is the inert carrier gas.

51
Q

Describe capillary/open tubular system.

A

Stationary phase can be thin layer coat on the inside of the capillary tube or as a solid support made of silica or polystyrene (supports stationary phase). Mobile phase is the inert carrier gas.

52
Q

What are the disadvantages and advantages of the packed columns?

A

Adv: can holder high sample capacity compared to capillary column ie large scale.
Disadv: high amount of sample means broader peaks, longer retention times, less resolution.

53
Q

What is most commonly used support in packed columns?

A

Diatomaceous earth - mainly calcium silicate. Material is silanised to remove polar Si-OH groups on the surface of the support.

54
Q

What are capillary columns made of?

A

Metal, glass, quartz (mainly used in pharm tech)

55
Q

During manufacture of capillary columns, the metal/glass/quartz is coated with polyimide resin. What is the use of this?

A

This coating provides mechanical strength and prevent atmospheric erosion.

56
Q

What are the 3 types of capillary columns/ open tubular columns?

A

WCOT - wall coated open tubular
SCOT - support coated open tubular
PLOT - porous layer open tubular

57
Q

Simply describe the design of WCOT, SCOT, PLOT.

A

WCOT - very thin liquid phase lining the core.
SCOT - solid support attached to inner surface and thin layer of liquid lines support and acts as stationary phase.
PLOT - porous solid support attached to inner core, acts as stationary phase for separation and purification of the compounds.

58
Q

What are the advantages and disadvantages of capillary columns?

A

Adv: cap columns provide higher separation efficiency than packed columns.
small sample capacity so allows for higher resolution, shorter analysis time and greater sensitivity.
Disadv: easily overloaded by too much sample, low sample capacity.

59
Q

In what 3 ways can you increase the resolution of capillary volumns?

A

Narrow columns (reduce the diameter)
Longer columns
Thicker stationary phase (increase the film of liquid)

60
Q

Which column is more efficient and why?

A

Capillary

- has more plates per metre.

61
Q

How do you choose liquid stationary phases of GC columns?

A

Choice based on like dissolves like rule.

Nonpolar columns for nonpolar compounds and polar columns for polar compounds.

62
Q

What is ‘bleeding’ of the stationary phase and how can it be prevented?

A

Bleeding is when the liquid stationary phase leaks/bleeds out of the column because it is not permanently attached to the solid support.
Can be reduced by bonding the liquid covalently to the silica support or covalently crosslink to itself.

63
Q

What is the order of elution determined by?

A

Mainly determined by volatility.
Least volatile = retained for longer = less elution.
Polar compounds = least volatile.

Also similarity in polarity between the compound and stationary phase determines elution.

64
Q

Derivatization may need to be done to compounds prior to GC process. Why should this be done?

A
  • some compounds are thermolabile at temperatures required for their chromatography.
  • for compounds that are highly polar, derivatisation is required to produce good peak shape.
  • for compounds which have poor peak shape due to low volatility.
65
Q

Why does poor peak shape occur?

A

Due to low volatility of the sample resulting in high retention time.

66
Q

What compounds are derivatised?

A

Organic acids, amides, poly hydroxy compounds, amino acids.

67
Q

What are some methods of derivatisation which render compounds more volatile?

A

Esterification, Silanation, Acetylation.

68
Q

Describe the derivatisation of acids.

A

Free COOH acids are strongly hydrogen bonded - difficult to vapourise and is bonded strongly to columns.
- can be overcome by esterification of acid.
Diazomethane (reagent) used to convert acid to methyl esters - have reduced H bonding, increased volatility and increased thermostability.

69
Q

Describe the derivatisation of hydroxy and amino compounds.

A

Strong binding between compounds and stationary phase/column for compounds which have high molecular weight.
Reagent = TMS - forms trimethylsilyl ether - thermally stable, volatile, show good chromatographic characteristics.

70
Q

Describe the derivatisation of alcohols, phenols, amines.

A

Reagent = reagents containing perfluoroalkyl groups.

- produce derivatives which are thermally stable and volatile

71
Q

What are the applications of GC?

A
  • Separation and analysis of volatile organic compounds (gases and liquids)
  • Testing purity of compounds
  • Compound identification
  • Isolation of pure compounds (microscale work) from complex mixtures
  • Determine relative amounts of components in mixture
  • Determination of partition coefficients and absorption isotherms
72
Q

What does it mean when additional peaks are present on the chromatogram?

A

Impurities present so compound is not pure.

PMY2106 Medicinal Substances (9 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions