Theory and Instrumentation of GC Introduction

8/13/2019 Theory and Instrumentation of GC Introduction

1/13

i

Wherever you see this symbol, it is important to access the on-line courseas there is interactive material that cannot be fully shown in this referencemanual.

Theory and Instrumentation of GC

Introduction to Gas Chromatography
http://chrommunity.chromacademy.com/http://chrommunity.chromacademy.com/http://chrommunity.chromacademy.com/http://www.chromacademy.com/


2/13

Aims and Object ives

Aims

Outline a Brief History of Gas Chromatography (GC)

Compare and contrast GC with other analytical techniques primarily HighPerformance Liquid Chromatography (HPLC)

Explain the function of each major component of the GC system

Explain the terms and appearance of a typical Chromatogram

Outline the fundamental basis for separation in GC

Indicate the major advantages of GC and the application areas in which it is used

ObjectivesAt the end of this Section you should be able to:

Identify analytes suitable for GC analysis from physico-chemical data

Describe the function of the various components of a Gas Chromatograph

Explain the fundamental basis of separation in GC in terms of solubility and vapourpressure of analytes

Recognise when the use of GC might be applicable to solving analytical problems


3/13

Crawford Scientific www.chromacademy.com 2

Content

Origins of Gas Chromatography 3Why Choose Gas Chromatography? 4Why Choose Gas Chromatography? 6

GC Separation Mechanism 7The Distribut ion Constant (Partition Coefficient) (Kc) 7The Gas Chromatograph 8The GC inst rument process 8The Chromatogram 9GC Advantages & Disadvantages 10Typical GC Applications 10


4/13


Origins of Gas Chromatography

The development of GC as an analytical technique was pioneered by Martin and Synge1941; they suggested the use of gas-liquid partition chromatograms for analyticalpurposes.

When dealing with liquid-liquid partition chromatography, they predicted that the mobilephase need not be a liquid but may be a vapour. Very refined separations of volatilesubstances in a column in which a permanent gas is made to flow over gel impregnatedwith a non-volatile solvent would be much faster and thus, the columns much moreefficient and separation times much shorter.

So the concept of gas chromatography was envisioned more than fifty years ago, butunfortunately, little notice was taken of the suggestion and it was left to Martin himself andhis co-worker A. T. James to bring the concept to practical reality some years later in1951, when they published their epic paper describing the first gas chromatograph.

They demonstrated the technique by separating and quantitatively determining thecomponents of a C1-C12 fatty acid mixture. The importance of GC was recognized almostimmediately by petrochemical laboratories, which faced the challenge of analysingcomplex hydrocarbon mixtures.

An early GC separation of a fatty acid mixture by Martyn and James

Archer J.P. Mart in Richard L.M. Synge(1910-2002) (1914-1994)


5/13


Why Choose Gas Chromatography?

The two main chromatographic techniques used in modern analytical chemistry are GasChromatography (GC) and High Performance Liquid Chromatography (HPLC).

A typical HPLC chromatograph (left) and a Gas Chromatograph (right)

HPLC uses a liquid mobile phase to transport the sample components (analytes) throughthe column, which is packed with a solid stationary phase material.

Typical HPLC column (left) and GC column (right)

HPLC was first proposed by the Russian botanist Mikhail Tswett first used the termChromatography (Latin for coloured drawing) in 1906, to describe the separation thatoccurred when solutions of plant pigments were passed through columns of calciumcarbonate or alumina, using petroleum ether.

In contrast, Gas Chromatography uses a gaseous mobile phase to transport samplecomponents through either packed columns or hollow capillary columns containing thestationary phase. In most cases, GC columns have smaller internal diameter and arelonger than HPLC columns.

GC has developed into a sophisticated technique since the pioneering work of Martin and

James in 1952, and is capable of separating very complex mixtures of volatile analytes.


6/13


Schematic diagram of a typical capillary Gas Chromatograph

Gas Inlets:Gas is fed from cylinders through supply piping to the instrument. It is usual to filter gasesto ensure high gas purity and the gas supply pressure. Required gases might include:Carrier (H2, He, N2)Make-up gas (H2, He, N2)Detector fuel gas (H2& Air/Ar or Ar and CH3/N2)depending on detector type.

Injector:Here the sample is volatilised and the resulting gas entrained into the carrier streamentering the GC column.Many inlet types exist including:Split/Splitless,

Programmed Thermal Vaporising (PTV),Cool-on-column (COC) etc.The COC injector introduces the sample into the column as a liquid to avoid thermaldecomposition or improve quantitative accuracy.

Detector:The detector responds to a physico-chemical property of the analyte, amplifies thisresponse and generates an electronic signal for the data system to produce achromatogram. Many different types exist and the choice is based mainly on application,analyte chemistry and required sensitivity also on whether quantitative or qualitative datais required.

Detector choices include: Flame ionisation (FID) / Electron Capture (ECD) / Flame

Photometric (FPD) / Nitrogen Phosphorous (NPD) / Thermal Conductivity (TCD) andMass Spectrometer (MS)

Data System:The data system receives the analogue signal from the detector and digitises it to form therecord of the chromatographic separation known as the Chromatogram.

The data system can also be used to perform various quantitative and qualitativeoperations on the chromatogram assisting with sample identification and quantitation.


7/13


Pneumatic controls:The gas supply is regulated to the correct pressure (or flow) and then fed to the requiredpart of the instrument. Control us usually required to regulate the gas coming into theinstrument and then to supply the various parts of the instrument. A GC fitted with a Split /Splitless inlet, capillary GC column and Flame Ionisation detector may have the followingdifferent gas specifications:

Carrier gas supply pressure / Column Inlet Pressure (column carrier gas flow) / Inlet splitflow / Detector make-up gas flow. Modern GC instruments have Electronic Pneumaticpressure controllers older instruments may have manual pressure control via regulators.

Why Choose Gas Chromatography?

The following information gives an indication of the type of sample (analyte) analysed byeither GC and HPLC and relative strengths and limitations of each technique.

GC

Samples analysed by GC must be volatile (have a significant vapour pressure

below 250oC) Derivatisation to increase volatility is possible but can be cumbersome and

introduces possible quantitativeerrors

Most GC analytes are under 500 Da Molecular Weight for volatility purposes

Highly polar analytes may be less volatile than suspected when dissolved in apolar solvent or in the presence of other polar species due to intermolecular forcessuch as hydrogen bonding.

HPLC

HPLC analysis has no volatility issues, however the analyte must be soluble in themobile phase.

HPLC can analyse samples over a wide polarity range and is able to analyse

ionic samples. Mobile phase components are selected to ensure sample solubility. HPLC has no real upper molecular weight limit and large proteins of many

thousands of Daltons may be analysed.

So under what circumstances would we chose GC to separate our samplecomponents?

Table 1. Molecular properties of selected analytes

Molecule Name Properties GC suitabili ty

Hexane C6H14(86.2Da)Boiling point: 69oC

Vapour pressure (@25oC)130kPa

Yes

Benzene C6H6(78.1Da)

Boiling point: 80.1oCVapour pressure (@25oC)

12.7kPa

Yes

Anthracene C6H14(178.1Da)Boiling point: 340oC

Vapour pressure (@25oC) n/a.kPa

No


8/13


GC Separation Mechanism

In Gas Chromatography (GC) the mobile phase is a gas and the stationary phase is eithera solid - Gas solid chromatography (GSC) or an immobilised polymeric liquid - Gas LiquidChromatography (GLC). Of the two types of GC, GLC is by far the most common as willbe seen.

The Distribution Constant (Partit ion Coefficient) (Kc)

[ ][ ]CmCs

Kc =

WhereCs refers to the concentration of analyte in the stationary phase.Cm refers to the concentration of analyte in the mobile phase.

The Distribution Coefficient measures the tendency of an analyte to be attracted to thestationary phase. Larger Kc values lead to longer retention analyte times. The value of

Kc can be controlled by the chemical nature of the stationary phase and the columntemperature.

The figure shows a typical separation process in GC. Each sample component partitionsbetween the gaseous mobile phase and liquid stationary phase (often coated onto theinner wall of a long thin capillary tube). The rate and degree of partitioning depends uponthe chemical affinity of the analyte for the stationary phase and the analyte vapourpressure which is governed by the column temperature.

The diagram shows that analyte A has a higher affinity for the mobile phase (lower Kcvalue) and therefore elutes more quickly than analyte B.

From the figure it can be seen that component A has a lower affinity for the stationaryphase and therefore is moved through the column more quickly thancomponent B, which spends more of its time in the stationary phase in this way

separation is achieved.

In GC, analyte separation is achieved by optimising the differences in stationary phaseaffinity and the relative vapour pressures of the analytes. In practice these parameters aremanipulated by changing the chemical nature of the stationary phase and the columntemperature.

i


9/13


The Gas Chromatograph

Instrumentation for Gas Chromatography has continually evolved since the inception ofthe technique in 1951 and the introduction of the first commercial systems in 1954.

Most modern commercial GC systems operate in the following way:

An inert carrier gas, such as helium, is supplied from gas cylinders to the GCwhere the pressure is regulated using manual or electronic (pneumatic) pressurecontrols.

The regulated carrier gas is supplied to the inlet and subsequently flows throughthe column and into the detector.

The sample is injected into the (usually) heated injection port where it is volatilisedand carried into the column by the carrier gas.

The sample is separated inside the column - usually a long silica based columnwith small internal diameter. The sample separates by differential partition of theanalytes between the mobile and stationary phases, based on relative vapourpressure and solubility in the immobilised liquid stationary phase.

On elution from the column, the carrier gas and analytes pass into a detector,which responds to some physico-chemical property of the analyte and generatesan electronic signal measuring the amount of analyte present.

The data system then produces an integrated chromatogram.

The GC instrument process

The sample is injected into the inlet where it is volatilised and a representativeportion is carried onto the column by the carrier gas.

The sample components are separated by differential portioning in the stationaryand mobile phases.

The separated sample components elute from the column into the detector wheresome physico-chemical parameter is detected and a signal produced.

This signal is then amplified and sent to the data system where the chromatogramis electronically constructed.


10/13


The Chromatogram

As the components elute from the column they pass into a detector where somephysico-chemical property of the analyte produces a response from the detector. Thisresponse is amplified and plotted against time giving rise to a chromatogram.

Typical chromatogram

Components (such as the injection solvent) that are not retained within the column eluteat the dead time or hold up time tM. There are various ways of measuring this parameterusing unretained compounds such as methane or hexane.

Those compounds (analytes and sample components) that are retained elute as

approximately Gaussian shaped peaks later in the chromatogram. Retention timesprovide the qualitativeaspect of the chromatogram and the retention time of a compoundalways will be the same under identical chromatographic conditions. The chromatographicpeak height or peak area is related to the quantity of analyte. For determination of theactual amount of the compound, the area or height is compared against standards ofknown concentration.


11/13


GC Advantages & Disadvantages

Gas chromatography has several important advantages which are listed opposite.

GC techniques produce fast analyses because of the highly efficient nature of theseparations achieved this will be studied later on. It can be argued that modern GC

produces the fastest separations of all chromatographic techniques. A column has beenproduced to separate 970 components within a reasonable analysis time - proving thatvery complex separations may be carried out using GC.

By using a combination of oven temperature and stationary phase chemistry (polarity)very difficult separations may also be carried out including separations of chiral andother positional isomers.

GC is excellent for quantitativeanalysis with a range of sensitive and linear detectors tochoose from.

GC is however limited to the analysis of volatile samples. Some highly polar analytes canbe derivatisedto impart a degree of volatility, but this process can be difficult and may

incur quantitative errors.

A practical upper temperature limit for conventional GC columns is around 350-380oC.Analyte boiling points rarely exceed 400oC in GC analysis and the upper Molecular Weightis usually around 500 Da.

Advantages

Fast analysis.

High efficiency leading high resolution.

Sensitive detectors (ppb).

Non destructive enabling coupling to Mass Spectrometers (MS) an instrumentthat measures the masses of ind ividual mo lecules that have been converted

into ions, i.e., molecules that have been electrically charged. High quantitative accuracy (


12/13


High temperature applications using specially designed columns are able to analyserelatively non-volatile substances and Cool-on-Column injection techniques allow thesampling of moderately thermally labile materials.Purge and trap and headspace autosampling techniques are now well established and areable to desorb or extract samples collected in the most inhospitable of environments, suchas the emission stacks of industrial plants.

Detector technology for GC is able to detect very small amounts of pesticides for example,from environmental samples and GC-MS techniques allow structural elucidation of eventhe most complex analytes.

Click on each of the application areas opposite to get more specific details on applicationsby industrial sector.

Pharmaceutical In the pharmaceutical industry GC is used to analyse residual solvents inboth raw materials (drug substance) and finished products (drug product).Biopharmaceutical applications include urine drugs screens forbarbiturates and underivatised drugs ethylene oxide in sterilized products

as sutures.

Foods/Flavours/Fragrances The food industry uses GC for a wide variety of applications includingquality testing and solvents testing. The Flavours and Fragrancesindustries use GC for quality testing and fingerprinting of fragrances forcharacterization.

PetrochemicalGC applications include natural gas analysis or refineries, gasolinecharacterization and fraction quantitation, aromatics in benzene, etc.Geochemical applications include mapping of oil reserves and tracing ofreservoir etc.

EnvironmentalEnvironmental GC applications include detection of pollutants such aspesticides, fungicides, herbicides, purgeable aromatics, etc.Industrial environmental protection applications include stack and wasteemissions as well as water discharges.

Chemical/IndustrialChemical/Industrial uses include determination of product content,determination of purity, monitoring production process, etc GCs are usedto detect organic acids, alcohols, amines, esters, and solvents.


13/13
http://chrommunity.chromacademy.com/

Date post:	04-Jun-2018
Category:	Documents
Upload:	augur886
View:	229 times
Download:	0 times

Theory and Instrumentation of GC Introduction

Documents