5. Gas chromatography

6. Gas chromatographyAdv. Chrom.Revision1.How does a gas chromatograph separate a mixture?attraction to the column vs evaporation2.What limitation on analytes does gas chromatography have?must be volatile3.Draw a block diagram showing the essential components of a typical gas chromatograph.

Carrier gasInjection portColumnDetectorReadoutOven

Revision4.What carrier gases are commonly used in gas chromatography? What component of the chromatography system is the carrier gas?N2, He, mobile phase

5. What is the effect on elution of increasing the temperature?decreases the time in the column (retention time)

6.How is the stationary phase chosen for a given analysis, e.g. ethanol in alcoholic beverages?similar polarity to analytes

Revision7.Why is the injection port heated?to ensure the sample is vaporised immediately

8.What are the two common methods for determining the identity of compounds by GC?retention time matching, spiking

ColumnsLength(m)Diameter (mm)Stat. phasePacked(this ch.)1-33-5adsorbed into powder, filling columnCapillary(AIT)10-500.1-0.7adsorbed onto walls of hollow columnpacked column in three different components: the tubing itself, the powder or packing the liquid stationary phaseTubing & packing materialmost commonly made from stainless steelsome are glassstainless steel columns can be uncoiled and emptied of contaminated material, repacked and recoiledglass columns - packing must be blown out and in with compressed air

the powder itself is not the stationary phasean inert support material onto which the stationary phase is coatedessential properties of the support materialsmall and uniform particle sizelarge surface areaStationary phasesa liquid coated in thin layers on the support in packed columns or the walls in capillary columnsvarious requirements:low volatilitythermal stabilitychemically inert

two main types:polysiloxanes non-polar with various substituents, eg methyl, phenylpolyethylene glycols (PEGs) - polarExercise 6.1What would be a suitable column for the separation of:(a)separation of petrol componentsDB1

(b)soil extract containing DDT and other pesticidesDB5

Column preparation & conditioningsupport is coated with stationary phasemaking a solution of the liquid in a volatile solventproducing a slurry with the appropriate amount of supportevaporating the solvent offproportion of stationary phase loading (1-10%w/w)new column must be conditionedrun at maximum temperature for an extended period with the carrier gas flowingany remaining solvent or other chemicals involved in manufacture are removedcarrier gas must be flowing whenever the column is heated

Are packed columns a dying breed?capillary columns separate mixtures more rapidly and with better resolutionso why bother with packed columns?they can cope with:high levels of non-volatiles (eg salts, sugars)higher sample volumescan be repacked at little costonce a capillary column stops working, thats it

Detectorsrespond to nanogram or less quantities of analyte passing through the detector within a few secondsdesirable properties for a detector include:wide linear responsegood stability with timeresponse to all compounds Thermal conductivity (TCD)compares the thermal conductivity of the eluant gas with the carrier gassenses the change in resistance of a current-carrying filament as a result of changes in the composition of the gas flowing over itan eluting compound changes the thermal conductivity of the gas, and thus the conduction properties to the filamentuse a pair of filamentsone placed before the injection port as a referencethe second at the end of the column for the eluant gasbest carrier gas is helium (very different thermal cond.)Adv. & disadv.simplicitycheapnessresponse to all speciesnon-destructive

not very sensitiveFlame ionisation (FID)measures the electrical conductivity of the gas stream from the columnafter it passes through a hydrogen/air flamecombustible species produce ions as intermediates in the burning processions are collected by a measuring electrode placed over the flamemost sensitive to organic compounds with long C chainsdoesnt respond to non-combustibles, eg water, N2 (ideal carrier gas cheaper than He)

Adv. & disadv.very sensitive to hydrocarbons (1 ng/mL)insensitive to waterrelatively cheap ($5-10,000)

require extra gas supplies (hydrogen, air)can be hard to get litQualitative analysisRT comparisonquickerprone to temp. & flow variations

spikingmore reliableslower

Quantitative analysishow to measure the instrument response how to calibrate that response against standards

Measuring responsea peak ona computer screena roll of plain or chart paper

truest measure of the amount of compound producing that peak is the area under the peakMeasuring instrument responseelectronic integration of peak area

if not available, manual measurement of peak height is better

Class Exercise 6.2If peak area is the true measure of the amount of compound, why is peak height recommended for measurements on chart paper?

peak are normally very narrowcant measure accurately 1-3 mm

Calibrationresponses changes between analyte and from day to day

can use normal calibration standards

matrix interference is not a problem in chromatography so std addition not required

the main error in variation in injection volume, especially if done manuallyinternal standards are used to correct for this

see Chapter 8Internal standard choicenormal reasonsplusshould not overlap the analyte(s) in the chromatogramshould not extend the run time if possible

these two can be bent slightlyadjust conditions if small overlap occursextending run time may be unavoidable, so minimise

Example 6.1Ethanol to ethanoic acid

Possible Int. Stds (retention times)methanol (2) ethanol (2.5) 1-propanol (4)ethanoic acid (4.5) propanoic acid (6.5)Totally unsuitableethanoic acid and ethanol are or may be presentBestMethanol is the quickest, but is quite different to the main analyte, ethanoic acid1-propanol fits nicely between the two analytesPropanoic acid extends the run time

1-propanol would be best.

Exercise 6.2Cyclohexanol to cyclohexanone (in hexane)

Unsuitablecyclohexanol, cyclohexanone, hexane already present

Bestcyclopentanonedoesnt extend run or overlap, similar to product

Others2-hexanone 2nd choice (peak close to cyclohexanone)2-hexanol extends run & similar to minor componentExercise 6.3Main products: 2-nitrotoluene and 4-nitrotolueneMinor product: 2,4-dinitrotolueneUnreacted: toluene

should be similar to main products3-nitrotoluene is closest, but would be very hard to separatenitrobenzene still similar (1 CH3 less), lower bp, so shorter RTOptimisationthe longer a species is retained within the system, the more its peak will spread out: peak broadening

Exercise 6.4Why is peak broadening a problem?loss of resolution, difficult to measure peak area

Factors affecting elutionthe type of column (packed or capillary)the polarity of the stationary phasethe temperature of the oventhe flow rate of the carrier gas

optimum flow rate (AIT)temperature main means of improving separationeffect (temp vs time) not linearOptimum separationthe analyte peak(s) are well-resolved from other peaksall components are eluted in as short a time as possiblepeaks do not have to be completely separated

Exercise 6.5all peaks do not have to be perfectly separatedcan measure area/height accurately ( 1 is fine

Exercise 6.9remember to convert to same time units

OK