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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 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

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

A

van Deemter plots

19
Q

van Deemter plots

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

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

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

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

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

A

split

splitless

25
Q

what are the two types of GC column

A

packed

capillary

26
Q

packed GC columns

A

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
Q

capillary/open tubular column

A

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
Q

what does WCOT stand for

A

wall coated open tubular

29
Q

what does PLOT stand for

A

porous layer open tubular

30
Q

what does SCOT stand for

A

support coated open tubular

31
Q

GC oven settings

A

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
Q

temperature control

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
33
Q

GC temperatures

A

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
Q

name 5 types of GC detector

A

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
Q

flamed 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 is located above the flame

Quantities of the analyte down to ug-level can be detected

Good linearity

Signal is proportionate to concentration

36
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

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
Q

SEE POWERPOINT FOR…

A

Gc detector comparison

38
Q

GC positives

A

fast analysis

high efficiency - leading to high resolution

sensitive detectors

high quantitative accuracy

requires small samples

rugged and reliable techniques

39
Q

GC negatives

A

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
Q

separation fundamentals -mobile phase

A

GC – inert gas such as N2, He or H2

LC – water mixed with organic solvent

Analytical Chemistry (17 decks)

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