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gas chromatography Flashcards

(40 cards)

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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 is the mobile phase

A

contains the analytes

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3
Q

what is the stationary phase

A

enables chemical partitioning

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4
Q

which phase drives separation

A

mobile

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5
Q

analytes will interact differently with the stationary phase

A

Analytes favouring the mobile phase will take short time to elute

Analytes favouring the stationary phase will take longer time to elut

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6
Q

separation fundamentals - stationary phase

A

The stationary phase is usually fixed in a column

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7
Q

how is separation achieved

A

2 component mixture

  • onto stationary phase (polar)
  • continuous flow of mobile phase (varying polarity)
  • called ‘normal phase’ chromatography
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8
Q

components have difference affinities for the phases

A

•greater affinity for the stationary phase = more polar

  • spends more time in that phase not moving
  • less time moving in mobile phase
  • moves slowly through system

•less affinity for stationary phase = less polar

  • spend less time in that phase not moving
  • more time moving in mobile phase
  • moves through system quickly
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9
Q

Normal phase

A

polar stationary phase

less polar mobile phase

non-polar components travel further (faster) than polar

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10
Q

chromatographic parameters

A

resolution Rs

retention (capacity) factor, K

selectivity factor (a)

efficiency – number of theoretical plates (N)

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11
Q

SEE POWERPOINT FOR..

A

RESOLUTION CALCULATIONS

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12
Q

what is GC sometimes also referred to as

A

GLC

gas-liquid chromatography

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13
Q

what state is the mobile phase in GC

A

gas

usually nitrogen or helium (sometimes hydrogen)

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14
Q

what state is the stationary phase in GC

A

liquid

very high boiling point

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15
Q

what does GC do

A

separate volatile organic compounds

analyte in gas phase
-suitable for thermostable, non-polar compounds

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16
Q

SEE POWERPOINT FOR….

A

GC instrument

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17
Q

mobile phase in GC

Study These Flashcards
A

nitrogen, hydrogen, helium

carrier gases drive the analyte forward through the column

must be chemically inert

18
Q

what do you use to decide on a gas for GC

Study These Flashcards
A

van Deemter plots

19
Q

van Deemter plots

Study These Flashcards
A

want a high linear flow in GC so that the separation is quick

the higher linear flow, the higher the back pressure so want a gas with low viscosity

there is less time for retention of analytes
-less efficient due to higher theoretical plate height

carrier gas choice and linear velocity significantly affect column separation efficiency
-illustrated using van Deemter plots

20
Q

gas inlet

Study These Flashcards
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

21
Q

sample inlet - injector

Study These Flashcards
A

heats injected liquid samples to gas phase

temperature of the sample inlet is usually about 50 degrees Celsius higher than the b.pt of the least volatile component of the sample

many inlet types exist including
-split/split less

22
Q

split less injection

Study These Flashcards
A

a proportion of the analyte/solvent gas passes onto the column, most exists through the split outlet

in split less mode, the split vent is closed during the first part of the injection

all sample goes into the column
-leads to much higher detection limits

split less is used for low concentration samples

must be careful with solvent peak

23
Q

split injection

Study These Flashcards
A

all analyte/solvent gas enters the column

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
  • leads to average detection limits

primarily used for non-trace analysis of volatile samples

24
Q

what are the two types of sample inlet

Study These Flashcards
A

split

splitless

25
what are the two types of GC column
packed capillary
26
packed GC columns
finely divided, solid support materials coated with liquid stationary phase made of glass or stainless steel 1.5 – 10m length Internal diameter of 2-4mm Large sample capacity – used for preparative work
27
capillary/open tubular column
Internal diameter of a few tenths of a mm 10-80m in length Film thickness of 0.1-5um High efficiency Sample size sample Used for analytical application WCOT stationary phases include unreactive silicones, saturated hydrocarbons, esters and amines
28
what does WCOT stand for
wall coated open tubular
29
what does PLOT stand for
porous layer open tubular
30
what does SCOT stand for
support coated open tubular
31
GC oven settings
The column is contained in a thermostatic oven controllable to 0.1 degrees Celsius -Separation of the analytes depends on vapour pressure which depends on temperature Separation can be improved by adjusting column temperature
32
temperature control
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
33
GC temperatures
for a range of compounds of the same type but with a wide range of boiling points: at a low temp are all separated but higher boiling ones are broad and take longer to elute at a higher temp, above boiling point, all will be vapour but lower boiling ones will not be separated a temp gradient allows all to be separated and resolved, as T rises higher boiling components will vaporise and be separated.
34
name 5 types of GC detector
flame ionisation detector thermal conductivity detector electron capture detector nitrogen-phosphorus detector mass spectrometers choice of detector will depend on the analyte and what the purpose of the analysis is
35
flamed ionisation detector
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 is located above the flame Quantities of the analyte down to ug-level can be detected Good linearity Signal is proportionate to concentration
36
analysis of BAC using GC-FID
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 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
37
SEE POWERPOINT FOR...
Gc detector comparison
38
GC positives
fast analysis high efficiency - leading to high resolution sensitive detectors high quantitative accuracy requires small samples rugged and reliable techniques
39
GC negatives
limited to volatile samples not suitable for samples that degrade at elevated temp not suited to preparative chromatography requires MS detector for analyte structural elucidation most non-MS detectors are destructive
40
separation fundamentals -mobile phase
GC – inert gas such as N2, He or H2 | LC – water mixed with organic solvent

Analytical Chemistry (17 decks)

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