GAS CHROMATOGRAPHY

1. GAS CHROMATOGRAPHY

2. Classification based on Mobile Phase Gas Chromatography Pyrolysis GC -heat solid materialsto 500 - 10000Cso they decomposeinto gaseous products Gas - solid Gas - liquid Stationary Phase Sample MUST be volatile at temperatures BELOW 3500C

3. Gas chromatography (GC), is a common type of chromatography used in analytic chemistry for separating and analyzing compounds that can be vaporized without decomposition.

4. In GC, the moving phase (mobile phase) is a carrier gas, usually an inert gas such as helium or nitrogen. • The stationary phase is a microscopic layer of liquid or polymer on an inert solid support, inside a piece of glass or metal tubing called a column. • The instrument used to perform GC is called a gas chromatograph.

5. Similar to column chromatography, but differs in 3 ways: • Partitioning process carried out between Moving Gas Phase and Stationary Liquid Phase • Temperature of gas can be controlled • Concentration of compound in gas phase is a function of the vapor pressure only. • GC also known as Vapor-Phase Chromatography (VPC) and Gas-Liquid Partition Chromatography (GLPC)

6. Gas Chromatography • Low boiling point compounds have higher vapor pressures. • High boiling point compounds have lower vapor pressures requiring more energy to reach equilibrium vapor pressure, i.e., atmospheric pressure. • Boiling point increases as molecular weight increases.

7. Gas Chromatography • Gas Chromatography • Uses • Separation and analysis of organic compounds • Testing purity of compounds • Determine relative amounts of components in mixture • Compound identification • Isolation of pure compounds (microscale work)

8. GLC ADVANTAGES 1. Very good separation 2. Time (analysis is short) 3. Small sample is needed - ml 4. Good detection system 5. Quantitatively analyzed

9. Types Of Samples For GC • Volatilized Comp • Samples that can be converted to volatile compounds • The sample may be organic or inorganic, but not ionic • the molecular weight ranges from 2 to 1000 • ( polymers with high molecular weight cannot be separated.

10. GLC DISADVANTAGES Limited to volatile samples – T of column limited to ~ 380 °C – Need Pvap of analyte ~ 60 torr at that T – Analytes should have b.p. below 500 °C • Not suitable for thermally labile samples • Some samples may require intensive preparation – Samples must be soluble and not react with the column • Requires spectroscopy (usually MS) to confirm the peak identity

11. Choice of Liquid Phase • Molecular weights, functional groups, and polarities of component molecules are factors in selecting liquid phase. • Length of Column • Similar compounds require longer columns than dissimilar compounds. Isomeric mixtures often require quite long columns

12. Instrumentation

13. Schematic Diagram of Gas Chromatography

14. The gaseous compounds being analyzed interact with the walls of the column, which is coated with different stationary phases. • This causes each compound to elute at a different time, known as the retention time of the compound. • The comparison of retention times is what gives GC its analytical usefulness.

15. The time the different compounds in the sample spend in the Vapor Phase is a function of their Vapor Pressure. • The more volatile (Low Boiling Point / Higher Vapor Pressure) compounds arrive at the end of the column first and pass into the detector • The detecting cell responses are recorded on a chart, from which the components can be identified both qualitatively and quantitatively

16. Instrumentation for GC • Carrier gas • N2, He, H2 • Injector • Column • Detector • Computer oven

17. Carrier gas: • The carrier gas must be chemically inert. Commonly used gases include nitrogen, helium, argon, and carbon dioxide. • The choice of carrier gas is often dependant upon the type of detector which is used. • The carrier gas system also associated with pressure regulators and flow meters. • In addition, it contains a molecular sieve to remove water and other impurities.

19. Flow Rate of Carrier Gas • Flow rates must be precisely controlled • – Reproducible retention times, minimize detector drift • Flow rates of carrier gas: • – Linear flow rate (cm/s): u = L/tr • – Volumetric flow rate (mL/min): u (π r2) • L is length of column, tr is retention time, r is the internal radius of column

20. Flow control: • Flow rates are controlled by a 2 stage pressure regulator: • At the gas cylinder. • Mounted in the chromatograph. • Inlet pressure 10-50 psi → F = 25-150 ml/min with packed column F = 1-25 ml/min with capillary column The flow rate will be constant if the inlet pressure remains constant.

22. Flow rate depends on type of column • – Packed column: 25-100 mL/min • – Capillary column: μL/min to 1 mL/min • • Flow rate will decrease as column T increases • – Viscosity of carrier gas increases with T

23. Necessary properties • – INERT • • Does not chemically interact with sample • – COMPATIBLE with detector • • No noise or explosions • – HIGHLY PURIFIED • • Impurities will degrade column and cause noise in detector • • “Research grade” is very expensive, so purify a cheaper grade

24. Sample injection port: • The injector is a piece of hardware attached to the column head. • It provides the means to introduce a sample into a continuous flow of carrier gas. • For optimum column efficiency, the sample should not be too large, and should be introduced onto the column as vapor. • Slow injection of large samples causes band broadening and loss of resolution.

25. Common injector types are: • Microflash vaporizer direct injector: • It involves the use of a microsyringe to inject the sample through a rubber septum into a flash vaporizer port located at the head of the column. • The temperature of the sample port is usually about 50°C higher than the boiling point of the least volatile component of the sample. • Used for packed columns, where the sample size vary from a few tenth of microliter to 20 ul.

27. Capillary columns require much smaller samples ( 10-3ul), so a sample splitter system is used to deliver only a small fraction of the injected sample to the column head, with the rest going to waste.

28. Sample splitter (Split/Splitless) injector: • The injector can be used in one of two modes; split or splitless. • a sample is introduced into a heated small chamber via a syringe through a septum (the heat facilitates volatilization of the sample). • The vaporized sample/carrier gas mixture then either sweeps entirely (splitless mode) or as portion (split mode) into the column. • In split mode, a part of the sample/carrier gas mixture in the injection chamber is exhausted through the split vent. • Split injection is preferred when working with samples with high analyte concentrations (>0.1%). • Splitless injection is best suited for trace analysis with low amount of analyte (<0.01%).

30. For quantitative work, more reproducible sample size are required and this can be obtained by a rotary sample valve.

31. Rotary sample valve: • gaseous samples in collection bottles are connected to what is most commonly a six-port switching valve. • The carrier gas flow is not interrupted while a sample can be expanded into a previously evacuated sample loop. • Upon switching, the contents of the sample loop are inserted into the carrier gas stream.

34. Column: • There are two general types of column: • Packed column • Capillary column (open tubular). • All the GC studies in the early 1950s were carried out on packed column. • In the late 1950s capillary column were constructed that much superior in speed and column efficiency (≈ 300000 plates).

36. Packed column: • Packed columns contain a finely divided, inert, solid support material coated with a thin layer of liquid stationary phase. • Most packed columns are 1.5 - 10m in length and have an internal diameter of 2 - 4mm. • They are made from glass, metals, or Teflon.

37. Capillary columns did not gain widespread until the late 1970s due to several reasons: • Small sample capacity. • Difficulties in coating the column. • Tendencies of columns to clog. • Short lifetimes of poorly prepared columns. • Fragileness of columns. • Mechanical problems in sample introduction and connection to the detector.

38. Capillary column: • Capillary columns have an internal diameter of a few tenths of a millimeter. • They were constructed of stain-less steel, aluminum, copper, plastic, or glass. • They can be one of two types; wall-coated open tubular (WCOT) or support-coated open tubular (SCOT). • Wall-coated columns consist of a capillary tube whose walls are coated with liquid stationary phase. • In support-coated columns, the inner wall of the capillary is lined with a thin layer of support material, onto which the stationary phase has been adsorbed. • SCOT columns are generally less efficient than WCOT columns, but both types are more efficient than packed columns.

40. In 1979, a new type of WCOT column appeared, the Fused Silica Open Tubular (FSOT) column. • It was drawn from specially purified silica that contains metal oxides. • These have much thinner walls than the glass capillary columns, and are given strength by an outside protective polyimide coating. • These columns are flexible and can be bent into coils. • They have the advantages of physical strength, flexibility and low reactivity.

42. Properties and characteristics of GC columns

43. GC Detectors

44. After the components of a mixture are separated using gas chromatography, they must be detected as they exit the GC column.

45. A non-selective detector responds to all compounds except the carrier gas, • a selectivedetector responds to a range of compounds with a common physical or chemical property and a specific detectorresponds to a single chemical compound.

46. Detectors can also be grouped into concentration dependant detectors and mass flow dependantdetectors. • The signal from a concentration dependant detector is related to • the concentration of solute in the detector, • does not usually destroy the sample. • Mass flow dependant detectors -usually destroy the sample -the signal is related to the rate at which solute molecules enter the detector.

47. The requirements of a GC detector depends on the separation application. • For example, one analysis might require a detector that is selective for chlorine-containing molecules, another analysis might require a detector that is non-destructive so that the analyte can be recovered for further spectroscopic analysis.

48. Characteristics of ideal detector: • Adequate sensitivity. • Good stability and reproducibility. • A linear response to solute. • A temperature range from room temp. to 400oC . • A short response time. • High reliability and ease of use. • Nondestructive to samples.

49. Detectors for GC • Electron capture (ECD) • Thermal conductivity (TCD) • Flame ionization (FID) • Fourier transform infrared (FTIR) • Mass spectrometry (MS)

50. Method development: • The method is the collection of conditions in which the GC operates for a given analysis. • Method development is the process of determining what conditions are adequate and/or ideal for the analysis required.