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Chemistry > Chapter 37 Analytical Techniques (Chromatography) > Flashcards

Chapter 37 Analytical Techniques (Chromatography) Flashcards

Thin-layer Chromatography & Gas/liquid Chromatography

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1
Q

What is chromatography used for?

A
  • to separate & analyse small amounts of mixtures
  • uses the principle of a mobile phase (liquid or gas) moving past a stationary phase
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2
Q

Define mobile phase.

A
  • allows molecules to move through the stationary phase
  • can be liquid or gas
  • more soluble products move further with the mobile phase
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3
Q

Define stationary phase.

A
  • a substance that has affinity to molecules in the mixture being analysed
  • the greater the affinity of a molecule to the stationary phase, the shorter the distance it moves with the mobile phase
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4
Q

Two mechanisms of chromatography.

A
  • separation by partition
  • separation by adsorption
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5
Q

Chromatographic separation by partition. State:
- stationary phase
- mobile phase
- how components separate

A
  • stationary phase - liquid film on an inert surface
  • mobile phase - liquid or gas
  • components separate according to solubility in each phase
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6
Q

Chromatographic separation by adsorption. State:
- stationary phase
- mobile phase
- how components separate

A
  • stationary phase - solid
  • mobile phase - liquid or gas
  • components separate according to strength of bond with the stationary phase
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7
Q

The 4 types of chromatography.

A
  • Paper chromatography
  • High Pressure Liquid Chromatography
  • Thin-layer chromatography (in syllabus)
  • Gas Liquid Chromatography (in syllabus)
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8
Q

Rf value formula.

A

= distance moved by solute / distance moved by solvent
- Calculated Rf values are compared with reference values obtained under identical conditions (e.g. same temperature & same solvent used).

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9
Q

Thin-layer chromatography. State:
- stationary phase
- mobile phase
- mechanism
- method of separation

A
  • stationary phase - thin layer of silica gel or alumina coated on to a piece of glass, metal or plastic to make it rigid
  • mobile phase - organic solvent that moves up the TLC plate by capillary action
  • mechanism - adsorption
  • method of separation - Components in a mixture are separated based on their different abilities to bind or adsorb to the stationary phase, & on their different abilities to desorb or dissolve in the mobile solvent phase.
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10
Q

Advantages of TLC.

A
  • faster
  • inexpensive
  • achieves better separations (due to greater choice in stationary & mobile phases)
  • works with very small samples
  • separates a wide range of mixtures
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11
Q

How does silica gel cause the separation of solutes in TLC?

A
  • the surface of silica gel is very polar
  • because of -OH groups, can form Hydrogen bonds, London Dispersion Forces & Dipole-dipole attractions with certain compounds
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12
Q

What happens to polar molecules with a polar stationary phase in TLC?

A
  • Polar molecules have a greater attraction for a polar solid & are adsorbed more strongly onto its surface.
  • They travel more slowly up the thin layer of alumina or silica & separation occurs.
  • Solutes are located on chromatogram & identified by comparing Rf values.
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13
Q

Uses of TLC.

A
  • Identification of biological sugars
  • Identification of components that might damage paper like -COOH(s)
  • Separation of insecticides, steroids
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14
Q

Gas liquid chromatography. State:
- stationary phase
- mobile phase
- mechanism

A
  • stationary phase - high boiling point non-polar liquid on an inert solid support
  • mobile phase - inert gas (helium, nitrogen)
  • mechanism - partition
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15
Q

Method of separation of GLC.

A
  • Small amount of sample injected into the machine.
  • Injector is contained in an oven.
  • Sample boils & is carried along a thin column by an inert carrier gas.
  • Column contains a liquid stationary phase, adsorbed onto an inert solid.
  • Time taken to travel through the tube will depend on how much time is spent moving with the gas rather than being attached to the liquid.
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16
Q

Define retention time.

A
  • Time taken for a compound to travel through the column to the detector.
  • Measured from the time the sample is injected to the time its peak shows maximum height.
  • Detector is connected to a mass spectrometer.
17
Q

What 2 factors is retention time dependent on?

A
  • Boiling point: High b. p. = long retention time
  • Solubility in the liquid phase: greater solubility = long retention time
18
Q

How to interpret GLC?

A
  • Each compound in the mixture will produce a peak.
  • Areas under the peaks are proportional to the amount of a compound.
  • Retention times are used to identify compounds - they are found out by putting known compounds through the system under similar conditions. Each component has a different retention time.
19
Q

Advantages of GLC.

A
  • Fast
  • Used for organic compounds, Mr < 300
  • Rt measured
  • % composition can be calculated - area under peak measures amount of component present
20
Q

Disadvantages of GLC.

A
  • expensive
21
Q

Uses of GLC.

A
  • analysis of crude oil / oil spill
  • analysis of volatile compound in perfume
  • analysis of air borne pollutants
22
Q

Define baseline.

A

​The pencil line where the samples are placed at the start in paper chromatography.

23
Q

Define solvent front.

A

The position on the TLC plate indicating the furthest distance travelled by the developing solvent.

24
Q

Paper chromatography. State:
- stationary phase
- mobile phase
- mechanism

A
  • stationary phase: water bound to paper fibres
  • mobile phase: liquid solvent
  • mechanism: partition
25
Q

High Pressure Liquid Chromatography. State:
- stationary phase
- mobile phase
- mechanism
- apparatus needed

A
  • stationary phase: high boiling point involatile liquid coat on inert solid
  • mobile phase: liquid solvent
  • mechanism: partition / adsorption
26
Q

What causes different components to be separated in the column?

A
  • strength of bonds formed with the stationary phase
  • Mr of the component
27
Q

What locating agent is used to make amino acids visible on the chromatogram?

A

ninhydrin

28
Q

How are components separated by partition?

A

Substance is distributed between the stationary & mobile phase.
OR
Has different solubility in each phase.

29
Q

How are components separated by adsorption?

A

Substances form bonds of varying strength with stationary phase.

Chemistry (13 decks)

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