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Analytical > Chromatography > Flashcards

Chromatography Flashcards

1
Q

What is the key fact about chromatography?

This is about spendint time in phases

A

Analytes spend the same amount of time in the mobile phase. Only the time spent in the stationary phase changes

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2
Q

What are the 5 ways of separating analytes?

A
  1. Adsorption
  2. Partition
  3. Ion-exchange
  4. Size exclusion
  5. Affinity
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3
Q

Define dead time

A

Time for unretauned solute to go through the column

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4
Q

Define adjusted retention time

A

Retention time - dead time

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5
Q

Define retention factor

A

Adjusted retension time / dead time

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6
Q

What is the column separation factor?

A
  • A fraction >= 1 showing how well the sample separate
  • Retention factor (A) / Retention factor (B)
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7
Q

Define resolution

A

Ratio between two peak maxima and their average width at base

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8
Q

Define partition coefficient

A

Equilibrium between mobile and stationary phases

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9
Q

What does peak fronting signify?

A

Column overloading

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10
Q

What does peak tailing signify?

A

Uneven stationary phase

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11
Q

What causes peak tailing?

A

Some sites retaining the solute mole strongly than others

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12
Q

Define Plate

A

Represents the theoretical distance required for one adsorption-desorption step of a solute between the stationary and mobile phases

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13
Q

Define peak capacity

A

Maximum theoretical number of solutes that can be baseline resolved on a given column

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14
Q

Purnell equation

Which three factors does the equation combine?

A
  1. Efficiency
  2. Retention factor
  3. Selectivity
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15
Q

Purnell equation

How can efficiency be increased?

2 ways

A
  • Increasing column length
  • Using more efficient column
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16
Q

Purnell equation

Which factor affects the resolution most?

A

Selectivity

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17
Q

What is the principle underlying optimising resolution by adjusting retention factor?

A

Make solute spend less time in the mobile phase and more in the stationary

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18
Q

How to increase retention factor in GC?

A

Lower the temperature

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19
Q

What is the general elution problem?

A

Adjusting the retention factor to improve the resolution between one pair of solutes increases the retention times of later solutes

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20
Q

3 methods to improve resolution

A
  1. Adjust retension factor (k’)
  2. Adjust column selectivity (alpha)
  3. Adjust column efficiency (N)
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21
Q

How to adjust column selectivity (alpha) in GC?

A

Change stationary phase

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22
Q

How to adjust column selectivity (alpha) in LC?

A

Change mobile phase

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23
Q

3 factors affecting peak broadening

A
  1. Multiple paths (eddie diffusion)
  2. Longitudinal diffusion
  3. Resistance to mass transfer
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24
Q

What is a key contributor towards Eddie diffusion?

A

Particle size. For larger particles the solute needs to travel further to go around it

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25
Q

How can logitudinal diffusion be reduced?

A

Make the solute go through the column faster

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26
Q

GB

How does gas mass affect the diffusivity

A

The lighter the gas the greater the diffusivity

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27
Q

GC

What are the requirements for mobile phase?

A
  1. Chemically inert
  2. Must not significantly influence analyte partition coefficient between phases
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28
Q

GC

Name the two types of columns

A
  1. Packed
  2. Capillary (tubular)
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29
Q

GC

Which column is longer?

A

Capillary

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30
Q

GC

Which column is broader

A

Packed

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31
Q

GC

Which column needs less material?

A

Capillary

32
Q

GC

What is the disadvantage of needed less material?

A

Difficult to inject the same amount of sample reproducubly

33
Q

GC

Which column has no Eddie diffusion?

A

Capillary

34
Q

GC

Which column has taller plates?

A

Packed

35
Q

GC

Which phase has a greater contol over partitioning?

A

Stationary

36
Q

GC

What 2 factors does the elution order depend on?

A
  1. Boiling point of the analyte (aka vapour pressure)
  2. Interaction between the analyte and the stationary phase
37
Q

GC

What are the requirements for the stationary phase?

5 points

A
  • Partitions the analyte without reacting
  • Does not react to mobile phase
  • Thermally stable
  • Low vapour pressure
  • Polarity appropriate to sample’s components
38
Q

GC

What happens to gas viscosity as temperature increases?

A

Increases

39
Q

GC

What can happen to column as it is heated?

A

It starts to bleed

40
Q

GC

How is bleeding observed on a spectrum?

A

Constantly raising base line

41
Q

GC

Why does column bleeding happen?

A

The stationary phase degrades and elutes as the column is heated

42
Q

GC

2 ways to prevent column bleeding

A
  • a bonded stationary phase that is chemically attached to the capillary’s silica surface
  • a cross linked stationary phase that chemically links together separate polymer chains
43
Q

GC

3 ways of sample injection

A
  • Direct vaporisation
  • Split/splitless injection
  • Cold on column injection
44
Q

GC

Describe the principles of direct vaporisation

Used only for direct vaporisation

A
  • Inject through a septum into a heated chamber
  • Rapid vaporisation to minimuse width of initial sample band
45
Q

Describe the principles of split/splitless injection

A
  • Similar to direct vaporisation, but has 2 outlets to avoid overloading
  • Amount of sample injected can be monitored
46
Q

Describe the principles of cold on-column injection

A
  • Inject cold onto column before volatilising the sample at a low T
  • Inject into uncoated silica to allow different droplets to collect together
47
Q

GC

List some of the desired detector qualities

7 total, not all neeeded

A
  • Large linear range
  • Universal, sensitive to all analytes or to specific class of analytes
  • Insensitive to flow rate or temperature change
  • Fast response
  • Non destructive of the analyte
  • Able to provide some information on the identity of an analyte
48
Q

GC

Principle of work of Thermal conductivity detector (TCD)

A
  • Mobile phase passes over Re-W filament
  • Its electrical resistance depends on T which depends on thermal conductivity of gas
  • When analyte elutes the thermal conductivity of the gas decreases causing the T of the filament and its resistance to increase
49
Q

GC

Advantages of Thermal conductivity detector (TCD)

2 points

A
  • Universal
  • Non destructive
50
Q

GC

Disadvantages of Thermal conductivity detector (TCD)

1 point

A

Poor detection sensitivity

51
Q

GC

Advantages of Flame ionisation detector (FID)

2 points

A
  • Almost universal for organic compounds
  • Most inorganic and many gases are not detected, perfect for atmospheric and environmental samples
52
Q

GC

Disadvantages of Flame ionisation detector (FID)

1 point

A

Destructive

53
Q

HPLC

What does smaller packing material require?

A

The use of higher pressure to push the analyte through

54
Q

Which phase is more common for HPLC?

normal/reverse

A

Reverse

55
Q

What is the polarity of the stationary phase of normal phase chromatography?

A

More polar

56
Q

What is the polarity of the stationary phase of reverse phase chromatography?

A

Less polar

57
Q

HPLC

What is normal phase commonly used for?

A

Separation of compounds that engage in polar interactions

58
Q

HPLC

What is the point of the secondary equilibrium

A

Introduce additional partitioning equilibria with the stationary phase and impart extra selectivity to the separation

59
Q

HPLC

What is the solvent triangle used for?

A

Find the best separation for binary mixtures with three different mobile phases

60
Q

HPLC

Why is the loop injector used instead of direct injection?

A

Due to high pressure

61
Q

HPLC

Name 3 type of detectors

A
  • Spectroscopic
  • Electrochemical
  • Mass spectrometer
62
Q

SFC

Supercritical’s fluid density is closer to liquids or gas?

A

Liquid

63
Q

SFC

What has a greater diffusion?

Superctitical liquid/liquid

A

Supercritical fluid

64
Q

SFC

What has a greater viscosity?

Superctitical liquid/liquid

A

Liquid

65
Q

Ion exchange

What is the purpose of ion-suppressor

A

To get rid of solvent ions as both analyte and solvent are charged

66
Q

Size exclusion

What material to be used for stationary phase?

A

Porous

eg: porous silica 5-400 nm

67
Q

Size exclusion

Particles of what size elute first?

A

Larger

68
Q

Electrophoresis

What separates the analytes

A

How they move through a conductive medium

69
Q

Electrophoresis

What is used as a conductive medium?

A

An aqueous buffer in an applied electric field

70
Q

Electrophoresis

Electrophoretic mobility

A

Movement of analyte in direct response to electric field; cations towards cathode (with E), anions towards anode (against E)

71
Q

Electrophoresis

Electroosmotic flow

A

Movement of analyte because of buffer’s movement due to electric field, usually towards cathode (same for all analytes)

72
Q

Electrophoresis

Why does buffer viscosity change?

A

As the current is applied the viscousity is increased

73
Q

Advantages of Electrophoresis

4 points

A
  • Greater resolution and peak capacity
  • Needs only small sample volumes
  • Fast
  • Needs only few micro litres of buffer
74
Q

Disadvantages of electrophoresis

A

Detection limits 100-1000x poorer than GC and HPLC

75
Q

Chromogentic agent

A

Analytes with weak or no absorbance in UV-Vis can be reacted with other specices to produce a stong absorption

Analytical (5 decks)

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