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PMY2106 > Gas-Liquid Chromatography > Flashcards

Gas-Liquid Chromatography Flashcards

1
Q

What is Gas-Liquid Chromatography

A

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

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2
Q

GC is a technique used to seperate what?

A

Volatile organic compounds

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3
Q

What does GC use?

A

GC uses the presence of a inert carrier gas through a capillary or packed column. The flow of analytes through the column is measured by detectors

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4
Q

What carrier gases are commonly used? (5)

A
  1. Nitrogen
  2. Helium
  3. Hydrogen
  4. Argon
  5. Carbon dioxide
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5
Q

Why is hydrogen used as a carrier gas?

A

Better thermal conductivity & low density. Used in thermal conductivity detector & flame ionization detector. It reacts with unsaturated compounds and it is inflammable.

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6
Q

Why is helium used as a carrier gas?

A

It also has excellent thermal conductivity, but it is expensive

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7
Q

Why is nitrogen used as a carrier gas?

A

It is inexpensive but has reduced sensitivity.

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8
Q

What are the requirements for a carrier gas? (5)

A
  1. Inertness
  2. Suitable to detector used
  3. Easily available
  4. Cheap less risk of explosion/fire hazard
  5. Should give best column performance (incl. speed of analysis)
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9
Q

What is the choice of carrier gas often dependent on?

A

Choice of carrier gas is often dependent upon the type of detector which is used

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10
Q

Injection port

A

Column efficiency requires sample of suitable size and sample has to be introduced as a “plug” of vapor.

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11
Q

What does slow injection of oversized samples cause?

A

Slow injection of oversized samples causes band spreading and poor resolution.

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12
Q

What do preheaters do ?

A

Preheaters are used to convert the sample into its vapor form and mix them with the mobile phase (carrier gas). They are present along with injecting devices.

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13
Q

For precise work, how should column temperature be controlled?

A

column temperature must be controlled to within tenths of a degree.

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14
Q

What different types of detectors can be used in GC? (5)

A
  1. Hot Wire Detector/ Thermal Conductivity Detector
  2. Flame Ionisation Detector
  3. Electron Capture Detector
  4. Nitrogen Phosphorous Detector
  5. Argon Ionisation Detector
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15
Q

How much more sensitive are GC detectors compared to LC detectors?

A

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

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16
Q

What are the advantages of Hot Wire Detector or Thermal Conductivity Detector (TDC) (4)

A
  1. Applicable to most compounds
  2. Linearity is good
  3. Sample is not destroyed
  4. Simple, easy to maintain & inexpensive
17
Q

What are the disadvantages of Hot Wire Detector or Thermal Conductivity Detector (TDC) (3)

A
  1. Low sensitivity
  2. Affected by fluctuations in temp & flow rate
  3. Biological samples cannot be analyzed
18
Q

What are the advantages of Flame Ionisation Detectors (FID)? (4)

A
  1. The detector is extremely sensitive & background noise is low (in μg).
  2. Stable and insensitive to small changes in the flow rate of carrier gas and water vapor
  3. Respond to most of the organic compounds
  4. Linearity is excellent
19
Q

What are the disadvantages of Flame Ionisation Detectors (FID)?

A
  1. Destructive
  2. Less sensitive to non hydrocarbon groups
  3. Insensitive to H2O,CO2,SO2
20
Q

What are Flame Ionisation Detectors (FID) useful for?

A

Useful for the detection of pollutants in natural water samples, because it is insensitive to water

21
Q

What are the advantages of Electron Capture Detector (ECD)?

A

The detector is highly sensitive (in ng can be detected).
* Halogenated compounds, several pesticides etc., can be detected using this type of detector.

22
Q

What are the disadvantages of Electron Capture Detector (ECD)?

A

Can only be used for compounds with electron affinity

23
Q

Why is Electron Capture Detector (ECD) choice of detectors for environmental samples?

A

It is choice of detectors for environmental samples because this detector selectively detects halogen containing compounds, such as pesticides and polychlorinated biphenyls

24
Q

What are the two types of columns used in GC?

A
  1. Packed columns: metal column packed with solid material
  2. Open tubular columns: a non-packed capillary tube
25
Q

What is the stationary phase in packed columns?

A

Stationary phase is attached to the solid material

26
Q

What is the mobile phase in packed columns

A

Mobile phase is the inert carrier gas

27
Q

What is the mobile phase in open tubular columns?

A

Either in thin layer coat on the inside the capillary tube or just the silica, polystyrene support, etc., as a means to support the stationary phase

28
Q

What is the mobile phase in open tubular columns?

A

Mobile phase is the inert carrier gas

29
Q

What are the three different types of capillary (open tubular) columns?

A
  1. Wall-coated Open Tubular - Inner wall is liquid phase
  2. Support-coated Open Tubular - Inner wall is solid support coated with liquid phase
  3. Porous Layer Open Tubular - Inner wall is porous adsorbant
30
Q

Choice of Liquid Stationary Phases in GC Columns

A
  1. Based on “like dissolves like” rule
    - Nonpolar columns for nonpolar solutes
    - Strongly polar columns for strongly polar compounds
  2. To reduce “bleeding” of stationary phase:
    - Bond (covalently attached) to silica
    - Or covalently cross-link to itself
31
Q

How is order of elution and retention time determined?

A

Order of elution is mainly determined by volatility:
- Least volatile = most retained.
- Polar compounds (e.g. amino alcohols) are the least volatile and will be the most retained on the GC system.
Second factor is similarity in polarity between compound and stationary phase

32
Q

Only about 25% of all known organic compounds can be chromatographed directly by GLC. All other compounds require derivatisation. Why is this?

A

One reason for this is that many compounds are thermolabile at the temperatures required for their chromatography

33
Q

in terms of derivatisation, what does it mean if a compund is highly polar?

A

If a compound is highly polar, generally derivatisation is required prior to GC in order to obtain a good chromatographic peak shape. Compounds may be strongly absorbed by the column thus leading to non-ideal behaviour and poor peak shape.

34
Q

What could also cause poor peak shape to occur?

A

This poor peak shape may also occur due to low volatility of the sample

35
Q

Examples of such materials that need to be derivatized (4)

A

organic acids, amides, poly hydroxy compounds, amino acids etc.

36
Q

What needs to happen to materials that need to be derivatives to render them more volatile?

A

In order to render such materials more volatile, they are either esterified, silanated or acetylated using one of a number of different methods of derivatization.

37
Q

Advantages of GC

A
  • Fast analysis, typically in minutes
  • Reliable and relatively simple (automated injection)
  • Efficient and high resolution (very good separation)
  • Small samples are required, typically (μL or μg)
  • Highly accurate quantification (1-5% RSD)
  • Good detection system (sensitive detectors for ppm level)
  • Non-destructive (usable for on-line coupling to mass Spectrometer)
  • Inexpensive (~£9000-£15000)
38
Q

Disadvantages of GC

A

Limited to only volatile samples
* Analytes should have boiling points below 400 !C
* Temperature of column is limited to ca. 400 !C
* Not suitable for thermally labile samples
* Samples must be soluble and not react with the GC column * Some samples may require intensive preparation * Some GC chromatograms can be complicated and require the mass spectrometer to confirm peak identity

PMY2106 (15 decks)

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