gas charomatography - psau...•qualitative analysis by gas chromatography is divided into two...

Prepared by:

Dr.Elsadig H.KH.Adam

GASCHAROMATOGRAPHY

Instrumental Analysis -Dr.Elsadig H.k.Adam1

GC methods can either be:a- Gas solid chromatography GSC• The stationary phase is a solid and retention of

substances occurs as a result of adsorptionb-Gas liquid chromatography GLC• The stationary phase is a liquid supported on an inert

solid matrix.• The components of the sample have finite solubility in

the stationary phase thus they distribute themselvesbetween this phase and the gas.

• Elution is accomplished by forcing the inert carrier gasthrough the column. The rates of which the variouscomponents move along the column depends on theirtendency to dissolve in the stationary phase.


Types of samples for GC:

1- Any compound that can be volatilized withoutdecomposition, such as low molecular weighthydrocarbons, aldehydes, ketones and esters.

2- Samples that can be converted to volatilecompounds, such as fatty acids which can beconverted to methyl esters which is more volatilealso amino acids can be converted to fluoroamideester derivative.

3-The sample may be organic or inorganic, but notionic.

4-The molecular weight ranges from 2 to 1000.


Advantage of GC• High sensitivity (10-9- 10-12 gm.)• High accuracy.• High speed (short time of analysis).Limitation of GC• The sample must be volatile and thermally stable

below 4000C.• Dirty and biological samples such as blood, soil

and tissues require clean up.• GC cannot identify the compounds surely ,it must

be connected to another instrument such asmass-spectrometer for accurate identification ofcompounds.


Instrument for gas chromatographyA gas chromatographic instrument is formed of FIVE components:• Carrier gas supply with its regulator, purifier, desiccant and flow

meter.• Sample injection system.• Column and thermostating system.• Detector.• Integrator or computer data station.


1-Gas supply system:

This includes:

a-carrier gas.

b-gases used for detector.

The most commonly used carrier gases arenitrogen, helium, hydrogen and argon.

They are supplied in cylinders or produced byelectrical generators fitted with pressureregulator and manometer.


The carrier gas must be :• Pure (99.999% purity),N.B. impure gases will cause damage of the stationary

phase, noise and unidentified peaks or signals.To ensure the purity of the gases, usually the

gases are passed through filters to removeimpurities then moisture filter and lastly oxygenfilter since oxygen will destroy the stationaryphase and it gives signal in case of electroncapture detector.

• Inert( will not interact with the sample).


• Dry as traces of water at high temperature willhydrolyze the sample and the stationary phase.

• The pressure of the gas is adjusted according to itsviscosity and the length of the column, to obtain thesuitable flow rate.

The choice of the gas depends on the detector used:• For flame ionization detector, the carrier gas can be

nitrogen ,helium, argon or hydrogen. The detector gasis air and hydrogen.

• For electron capture detector the carrier gas is nitrogenand no other gases are needed for detector.

• For thermal conductivity detector low viscosity, highthermal conductivity gas is needed as carrier gas andwhich passes through the detector these are heliumand hydrogen.


Column and Thermostating System:

• The fundamental part of a gaschromatograph is the thermostating oven.

• It is the place where the chromatographiccolumn is put and the sample is separatedinto its components.

• The optimum column temperature dependsupon the boiling point of the sample and thedegree of separation required.

• There are two types of columns :Packed andCapillary.


Packed columns:

• They are fabricated from glass, metal orteflon and about two meter length and 0.5cm diameter.

• They are uniformly packed with packingmaterial consists of finely divided, uniformspherical solid inert support coated uniformlywith very thin layer of stationary liquidphase.

• The carrier gas flow ranges from 30–50ml/min.


Capillary or open tubular columns:

• These are capillary tubes made of glass orstainless steel or fused silica.

• The inner wall is coated with thin film of liquidstationary phase.

• Since there is no resistance for the gas flow asthe tube is open. The length of the tube canbe from 15 - 100 meter.

• The flow rate of carrier gas is reduced to 1ml/min.


Stationary liquid phases:Desirable properties for immobilized, liquid phase

in GLC include:1- Low volatility; ideally the boiling point of the

liquid should be at least 2000C higher than themaximum operating temperature for the column.

2- Thermally stable3-Chemically inert4-To have a reasonable retention time in the

column, a species must show some degree ofsolubility with the stationary phase.

• Generally, the polarity of the stationary shouldmatch that of the sample components. When thematch is good, the order of elution is determinedby the boiling point of the eluate.


Column temperature andtemperature program

• The column(s) in a GC arecontained in an oven, thetemperature of which isprecisely controlledelectronically.

• The temperature of the columncan be varied from about 50°C to250°C. It is cooler than theinjector oven, so that somecomponents of the mixture maycondense at the beginning of thecolumn.

• The rate at which a samplepasses through the column isdirectly proportional to thetemperature of the column.


• A method which holds the column at the sametemperature for the entire analysis is called"isothermal." Most methods, however, increasethe column temperature during the analysis, theinitial temperature, rate of temperature increase(the temperature "ramp"), and final temperatureare called the "temperature program."

• A temperature program allows analytes that eluteearly in the analysis to separate adequately, whileshortening the time it takes for late-elutinganalytes to pass through the column.


Detectors:

• The detector has the function to detect thepresence of chemical components in the gasflow.

• All detectors measure a relative value; asample component in the carrier gascompared to the pure carrier gas.

• This change is usually represented in the formof an electrical signal as a function of time.


A good detector should comply with thefollowing requirements:

1- Responds rapidly and reproducibly to lowconcentrations of solutes emerged from thecolumn.

2- Sensitive to very low concentrations ofsample, 10-9 to10-12 g of sample could bedetected.

3-Accurate and reliable.


The following are some types of GC detectors:1-Thermal Conductivity Detector TCD

The detector has theadvantage that it has nodestructive effect on thesample.


Most organic compounds, when pyrolyzed at the temperature ofhydrogen/air flame, produce ionic intermediates that provide amechanism by which electricity can be carried through the flame.

The FID is highly sensitive, has wide range of linear response but it is destructive for the sample.

2-Flame Ionization Detector FID:


•ECD is a selective and highly sensitive detector for molecules containingelectronegative functional groups such as halogens, peroxides, quinones, andnitro groups.

•A -ray source (a radioactive substance that emit electrons) such as Ni63.

•In the absence of the organic species, an amplifier electron current is formedwhich runs in the direction of collector electrode and is monitored as acontinuous background current.•The moment there are electro-negative components present in the carriergas, the background current is reduced because these compounds captureelectrons. The change in the background current is registered and that is thedetector signal.

3-Electron Capture Detector ECD


QUALITATIVE ANALYSIS

• Qualitative analysis by gas chromatography isdivided into two parts.

• The first is the separation of component orcomponents of interest from each others in themixture, and the second is the identification of theseparated components.

Retention Data• The retention volume, time or distance of a peak is a

qualitative property of the compound and isconstant for a given set of conditions (the sameapparatus, temperature and stationary phase).


• To identify a specific compound in the mixture, areference compound and the unknown sample areco-chromatographed under identical conditions(spiking or enrichment technique).

• The formation of one peak and the increase of itsheight indicate that the unknown may be identicalwith the reference compound, but it is not positiveproof.

• To confirm the identity of the unknown, otherstationary phases of different polarity could be alsoused and chromatography at variable temperaturesmight be carried out.


QUANTITATIVE ANALYSIS

Quantitative AnalysisA-Peak Height Measurement:• The use of the peak height as the quantitative

measurement is to be preferred to the peak areabecause of its simplicity. But this demands;

(1) The conditions of chromatography be constantallover the operation and peak width does not varyduring the set of determinations,

(2) The response of the detector must be carefullycalibrated over the concentration range of thecomponents. This is done by allowing standardsamples to be chromatographed periodically.


B-Peak Area Measurement:

• In some cases the peak width is affected withcertain factors, e.g. adsorption, overloading ofsample, long retention volumes and variation inoperating conditions like temperature.

• All these factors cause broadening of the peakand measurement of the peak height becomes oflittle importance.

• Therefore, it appears more accurate todetermine the peak area. This can be carried outas follows:


1. By means of a planimeter or counting squarenumbers on group paper.

2. Weighting the cut-out peaks if paper thicknessis uniform.

3. By means of some automatic integrating device.

4. By a geometric method or approximation suchas multiplying the peak height by the width athalf height.

• The first three methods are useful in thedetermination of asymmetrical peaks.


Calibration:

• The calibration involves the determination ofchromatograms with pure components ofsamples.

• A quantitative analysis through calibration can beachieved by plotting chromatographed increasingamounts of pure samples, against their peakareas.

• The value of the unknown concentration can bededuced from the curve


Quantitative Determination of Atropine by Area Measurement


gas charomatography - psau...•qualitative analysis by gas chromatography is divided into two...

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