1 Gas Chromatography Lecture 36. 2 Gas chromatography is a technique used for separation of volatile...

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Transcript of 1 Gas Chromatography Lecture 36. 2 Gas chromatography is a technique used for separation of volatile...

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1 Gas Chromatography Lecture 36 Slide 2 2 Gas chromatography is a technique used for separation of volatile substances, or substances that can be made volatile, from one another in a gaseous mixture at high temperatures. A sample containing the materials to be separated is injected into the gas chromatograph. A mobile phase (carrier gas) moves through a column that contains a wall coated or granular solid coated stationary phase. As the carrier gas flows through the column, the components of the sample come in contact with the stationary phase. The different components of the sample have different affinities for the stationary phase, which results in differential migration of solutes, thus leading to separation Slide 3 3 Martin and James introduced this separation technique in 1952, which is the latest of the major chromatograhpic techniques. However, by 1965 over 18000 publications in gas chromatography (GC) were available in the literature. This is because optimized instrumentation was feasible. Gas chromatography is good only for volatile compounds or those, which can be made volatile by suitable derivatization methods or pyrolysis. Thus, about 20% of chemicals available can be analyzed directly by GC. Slide 4 4 Gas chromatography can be used for both qualitative and quantitative analysis. Comparison of retention times can be used to identify materials in the sample by comparing retention times of peaks in a sample to retention times for standards. The same limitations for qualitative analysis discussed in Chapter 26 also apply for separations in GC. Quantitative analysis is accomplished by measurement of either peak height or peak area Slide 5 5 Gas - Solid Chromatography (GSC) The stationary phase, in this case, is a solid like silica or alumina. It is the affinity of solutes towards adsorption onto the stationary phase which determines, in part, the retention time. The mobile phase is, of course, a suitable carrier gas. This gas chromatographic technique is most useful for the separation and analysis of gases like CH 4, CO 2, CO,... etc. The use of GSC in practice is considered marginal when compared to gas liquid chromatography. Slide 6 6 Gas - Liquid Chromatography (GLC) The stationary phase is a liquid with very low volatility while the mobile phase is a suitable carrier gas. GLC is the most widely used technique for separation of volatile species. The presence of a wide variety of stationary phases with contrasting selectivities and easy column preparation add to the assets of GLC or simply GC. Slide 7 7 Instrumentation It may be wise to introduce instrumental components before proceeding further in theoretical background. This will help clarify many points, which may, otherwise, seem vague. It should also be noted that a detector will require special gas cylinders depending on the detector type utilized. The column temperature controller is simply an oven, the temperature of which can be varied or programmed Slide 8 8 Syringe Injector Detector Carrier Gas Cylinder Column To Waste or Flow Meter Flow Controller Two-Stage Regulator Slide 9 9 Slide 10 10 Three temperature zones can be identified: 1.Injector temperature, T I, where T I should allow flash vaporization of all sample components. 2.Column temperature, T c, which is adjusted as the average boiling points of sample components. 3.Detector Temperature, T D, which should exclude any possible condensation inside the detector. Generally, an intuitive equation can be used to adjust all three zones depending on the average boiling point of the sample components. This equation is formulated as: T I = T D = T c + 50 o C Slide 11 11 The Carrier Gas Unlike liquid chromatography where wide varieties of mobile phase compositions are possible, mobile phases in gas chromatography are very limited. Only slight changes between carrier gases can be identified which places real limitations to chromatographic enhancement by change or modification of carrier gases Slide 12 12 A carrier gas should have the following properties: 1.Highly pure (> 99.9%) 2.Inert so that no reaction with stationary phase or instrumental components can take place, especially at high temperatures. 3.A higher density (larger viscosity) carrier gas is preferred. 4.Compatible with the detector since some detectors require the use of a specific carrier gas. 5.A cheap and available carrier gas is an advantage. Slide 13 13 Longitudinal Diffusion Term This is an important factor contributing to band broadening which is a function of the diffusivity of the solute in the gaseous mobile phase as well as the molecular diffusion of the carrier gas itself. H L = K D M /V Where; D M is the diffusion coefficient of solute in the carrier gas. This term can be minimized when mobile phases of low diffusion, i.e. high density, are used in conjunction with higher flow rates. Slide 14 14 The same van Deemter equation as in LC can be written for GC where: H = A + B/V + CV The optimum carrier gas velocity is given by the derivative of van Deemter equation V opt = { B/C } 1/2 However, the obtained velocity is much greater than that obtained in LC. Slide 15 15 The carrier gas pressure ranges from 10-50 psi. Higher pressures potentially increase compression possibility while very low pressures result in large band broadening due to diffusion. Depending on the column dimensions, flow rates from 1-150 mL/min are reported. Conventional analytical columns (1/8) usually use flow rates in the range from 20-50 mL/min while capillary columns use flow rates from 1-5 mL/min depending on the dimensions and nature of column. In most cases, a selection between helium and nitrogen is made as these two gases are the most versatile and common carrier gases in GC. Slide 16 16 Gas Chromatography Lecture 37 Slide 17 17 Injectors Septum type injectors are the most common. These are composed of a glass tube where vaporization of the sample takes place. The sample is introduced into the injector through a self-sealing silicone rubber septum. The carrier gas flows through the injector carrying vaporized solutes. The temperature of the injector should be adjusted so that flash vaporization of all solutes occurs. If the temperature of the injector is not high enough (at least 50 degrees above highest boiling component), band broadening will take place. Slide 18 18 Carrier Gas Syringe Vaporization Chamber To Column Septum Slide 19 19 Slide 20 20 Slide 21 21 Slide 22 22 Slide 23 23 Slide 24 24 Slide 25 25 Slide 26 26 Column Configurations and Ovens The column in chromatography is undoubtedly the heart of the technique. A column can either be a packed or open tubular. Traditionally, packed columns were most common but fast developments in open tubular techniques and reported advantages in terms of efficiency and speed may make open tubular columns the best choice in the near future. Packed columns are relatively short (~2meters) while open tubular columns may be as long as 30-100 meters Slide 27 27 Packed columns are made of stainless steel or glass while open tubular columns are usually made of fused silica. The temperature of the column is adjusted so that it is close to the average boiling point of the sample mixture. However, temperature programming is used very often to achieve better separations. The temperature of the column is assumed to be the same as the oven which houses the column. The oven temperature should be stable and easily changed in order to obtain reproducible results. Slide 28 28 Detection Systems Several detectors are available for use in GC. Each detector has its own characteristics and features as well as drawbacks. Properties of an ideal detector include: 1.High sensitivity 2.Minimum drift 3.Wide dynamic range 4.Operational temperatures up to 400 o C. 5.Fast response time 6.Same response factor for all solutes 7.Good reliability (no fooling) 8.Nondestructive 9.Responds to all solutes (universal) Slide 29 29 a. Thermal Conductivity Detector (TCD) This is a nondestructive detector which is used for the separation and collection of solutes to further perform some other experiments on each purely separated component. The heart of the detector is a heated filament which is cooled by helium carrier gas. Any solute passes across the filament will not cool it as much as helium does because helium has the highest thermal conductivity. This results in an increase in the temperature of the filament which is related to concentration. The detector is simple, nondestructive, and universal but is not very sensitive and is flow rate sensitive. Slide 30 30 Slide 31 31 Slide 32 32 Note that gases should always be flowing through the detector including just before, and few minutes after, the operation of the detector. Otherwise, the filament will melt. Also, keep away any oxygen since oxygen will oxidize the filament and results in its destruction. Remember that TCD characteristics include: 1.Rugged 2.Wide dynamic range (10 5 ) 3.Nondestructive 4.Insensitive (10 -8 g/s) 5.Flow rate sensitive Slide 33 33 b. Flame Ionization Detector (FID) This is one of the most sensitive and reliable destructive detectors. Separate two gas cylinders, one for fuel and the other for O 2 or air are used in the ignition of the flame of the FID. The fuel is usually hydrogen gas. The flow rate of air and hydrogen should be carefully adjusted in order to successfully ignite the flame. Slide 34 34 Slide 35 35 Slide 36 36 The FID detector is a mass sensitive detector where solutes are ionized in the flame and electrons emitted are attracted by a positive electrode, where a current is obtained. The FID detector is not responsive to air, water, carbon disulfide. This is an extremely important advantage where volatile solutes present in water matrix can be easily analyzed without any pretreatment. Slide 37 37 Remember that FID characteristics include: Rugged Sensitive (10 -13 g/s) Wide dynamic