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Analytical Chemistry > Gas Chromatography > Flashcards

Gas Chromatography Flashcards

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1
Q

what is chromatography

A
  • Separation process that is achieved by distributing the components of a mixture between 2 phases
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2
Q

what are the two phases

A

o A stationary phase – chemical partitioning
o A mobile phase – contains the analytes

  • Mobile phase flow will drive separation
  • Analytes interact different with the stationary phase
    o Analytes favouring the mobile phase will take short time to elute
    o Analytes favouring the stationary phase will take longer time to elute
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3
Q

separation fundamentals

A
  • The stationary phase is usually fixed in a column
  • Mobile phase
    GC- inert gas such as N2, He or H2
    LC- water mixed with organic solvent
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4
Q

how is separation achieved

A

2 component mixture

  • Onto stationary phase (polar)
  • Continuous flow of mobile phase (varying polarity)
  • Called norm phase chromatography
  • More polar the component the slow it will travel through the column
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5
Q

Components have different affinities for the phases

Greater affinity for stationary phase (more polar)

A
  • Spends more time in that phase not moving
  • Less time moving in mobile phase
  • Moves through system slowly
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6
Q

Components have different affinities for the phases

Less affinity for stationary phase (less polar)

A
  • Spends less time in that phase
  • More time moving in mobile phase
  • Moves through system quickly
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7
Q

chromatographic parameters

A
  • Resolution Rs
  • Retention (capacity) factor, K
  • Selectivity factor (α)
  • Efficiency- number of theoretical plates (N)
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8
Q

resolution

A

see powerpoint for resolution equation

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9
Q

Gas chromatography

A

Sometimes referred as GLC – Gas-Liquid-Chromatography

  • Mobile phase is a gas (usually nitrogen or helium)
  • Stationary phase is a liquid (very high boiling point)
    Separates volatile organic compounds (VOC)
    Analyte in gas phase (suitable for thermostable, non polar compounds
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10
Q

Gas chromatography instrument

A

see powerpoint

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11
Q

mobile phase

A
  • Three main gases (nitrogen, hydrogen and helium, helium most frequently used)
  • Carrier gases drives the analyte forward through the column
  • Must be chemically inert
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12
Q

gas choice- van deter plots

A
  • You want a high linear flow in GC so that the separation is quick
  • The higher the linear flow, the higher the back pressure so want a gas with low viscosity
  • Also there is less time for retention of analytes (less efficient due to high theoretical plate height)
  • Carrier gas choice and linear velocity significantly af fect column separation efficiency (illustrate using van Deemter plots)

SEE POWERPOINT FOR GRAPH

The optimum linear velocity for each gas is at the lowest point on the curve, where plate height H is minimised and efficiency is maximised

  • Nitrogen provides the best efficiency
    Steepness of its van Deemter plot on each side of optimum means that small changes in linear velocity can result in large negative changes in efficiency
  • Helium has a wider range for optimal linear velocity, but offers slightly less efficiency
    Only a small decrease in efficiency when velocity changes slightly
  • Hydrogen has the shortest analysis times and the widest range of average linear velocity over which high efficiency is obtained
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13
Q

Gas inlet

A
  • Gas is fed from cylinders through supply piping to the instrument
  • It is usual to filter gases to ensure high gas purity
  • Gas supply may be regulated at the bench to ensure an appropriate supply pressure
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14
Q

sample inlet- injector

A
  • Heats injected liquid samples to gas phase
  • Temperature of the sample inlet is usually about 500C higher than the b.pt of the least volatile component of the sample
  • Many inlet types exist including
    Split/splitless
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15
Q

two sample inlet modes

A
  • Split: proportion of the analyte/solvent gas passes onto the column, most exits through the split outlet
  • Splitless: all analyte/solvent gas enters the column
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16
Q

splitless injection

A
  • In splitless mode the split vent is closed during the first part of injections
  • All sample goes into the column (much higher detection limits)
  • Splitless is used for low concentration samples
  • Must be careful with solvent peak
17
Q

split injection

A
  • In split mode the split vent is opened during the first part of the injection
  • A fraction of the sample enters the column (usually ratios such as 1:10, 1:20, 1:50, 1:100 and leads to average detection limits
  • Primarily used for non-trace analysis of volatile samples
18
Q

two types of GC columns

A

packed

capillary

19
Q

packed GC columns

A
  • Finley divided, solid support materials coated with liquid stationary phase
  • Made of glass or stainless steel
  • 1.5 – 10m in length
  • Internal diameter of 2-4mm
  • Large sample capacity – used for preparative work
20
Q

capillary/ open tubular column

A
  • Internal diameter of a few tenths of a mm
  • 10-80m in length
  • Film thickness of 0.1- 5 micrometers
  • High efficiency
  • Small sample size
  • Used for analytical application
  • WCOT stationary phase include unreactive silicones, saturated hydrocarbons, esters and amines
21
Q

WCOT

A

wall coated open tubular

22
Q

PLOT

A

porous layer open tubular

23
Q

SCOT

A

support coated open tubular

24
Q

GC oven settings

A
  • The column is contained in a thermostatic oven controllable to 0.1 degrees C (separation of the analytes depends on vapour pressure which depends on temperature)
  • Separation can be improved by adjusting column temperature
25
Q

temperature oven settings

A
  • When a single temperature is used – isothermal (isocratic)
  • A changing temp profile is called a temperature gradient
  • Temp changes can be finely controlled
  • Can be increased (or decreased) either quickly or slowly
    (Useful when compounds have a wide range of boiling points)
  • Adjustments are made to
    Increase separation
    Increase resolution
    Decrease run time
26
Q

low temp in oven settings

A

For a range of compounds of the same type
but with a wide range of boiling points;
at a low temp all are separated but higher
boiling ones are broad and take longer to
elute.

27
Q

higher temp in oven

A

At a higher temp, above boiling point, all will
be vapour but lower boiling ones will not be
separated.

28
Q

temp gradient in oven

A

A temp gradient allows all to be separated
and resolved – as T rises higher boiling
components will vaporise and be separated.
All within a reasonably short time.

29
Q

types of GC detector

A
  • The following are common types of GC detectors
    • Flame ionisation detector (FID)
    • Thermal conductivity detector (TCD)
    • Electron capture detector (ECD)
    • Nitrogen-phosphorus detected (NPD)
    • Mass spectrometers (MS) – covered in another lecture
  • The choice of detector will depend on the analyte and what the purpose of the analysis is
30
Q

Flame ionisation detector

A
  • Ignition of effluent from column
  • Organic compounds burning the flame produce ions and electrons
  • Conduct electricity through the flame
  • A large electrical potential is applied at the burner tip and a collector electrode is located above the flame
  • Quantities of the analyte down to µg-level can be detected
  • Good linearity
  • Signal is proportionate to concentration
31
Q

analysis of BAC using GC-FID

A

• Blood alcohol concentration (BAC) corresponds directly to the level of impairment of an intoxicated driver
• Breathalyser and field sobriety test provide subjective indication of impairment
• Any court requires quantitation of ethanol content
– The most widely run test in toxicology labs
• Due to the large number of samples and their relative short hold times there is a need for rapid and accurate tests
• Headspace GC is widely used
• This combined with a FID is the most common set-up
• Also one of the practicals for this module

32
Q

GC detector comparison

A

see powerpoint

33
Q

advantages of GC

A

fast analysis

high efficiency- leading to high resolution

sensitive detectors

high quantitative accuracy

requires small samples

rugged and reliable techniques

34
Q

disadvantages of GC

A

limited to volatile samples

not suitable for samples that degrade at elevated temperatures

not suited to preparative chromatography

requires MS detector for analyse structural elucidation

most non-MS detectors are destructive

Analytical Chemistry (20 decks)

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