Presentation on theme: "GAS CHROMATOGRAPHY. Classification based on Mobile Phase Gas Chromatography Gas - solid Gas - liquid Stationary Phase Sample MUST be volatile at temperatures."— Presentation transcript:
Classification based on Mobile Phase Gas Chromatography Gas - solid Gas - liquid Stationary Phase Sample MUST be volatile at temperatures BELOW C Pyrolysis GC - heat solid materials to C so they decompose into gaseous products
Gas chromatography (GC), is a common type of chromatography used in analytic chemistry for separating and analyzing compounds that can be vaporized without decomposition. chromatographyanalytic chemistryseparatingvaporizeddecompositionchromatographyanalytic chemistryseparatingvaporizeddecomposition
In GC, the moving phase (mobile phase) is a carrier gas, usually an inert gas such as helium or nitrogen. gasinerthelium nitrogengasinerthelium 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. liquidpolymersolidglassmetal liquidpolymersolidglassmetal The instrument used to perform GC is called a gas chromatograph.
–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)
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. 10/10/2014 6
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) 10/10/2014 7
GLC ADVANTAGES 1.Very good separation 2.Time (analysis is short) 3.Small sample is needed - l 4.Good detection system 5.Quantitatively analyzed
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.
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 GLC DISADVANTAGES
–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
Schematic Diagram of Gas Chromatography
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. This causes each compound to elute at a different time, known as the retention time of the compound.elute The comparison of retention times is what gives GC its analytical usefulness.
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
Instrumentation for GC Carrier gas –N 2, He, H 2 InjectorColumnDetectorComputer oven
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.
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
Flow control: Flow rates are controlled by a 2 stage pressure regulator: –At the gas cylinder. –Mounted in the chromatograph. Inlet pressure psi → F = 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.
Flow rate depends on type of column – Packed column: mL/min – Capillary column: μL/min to 1 mL/min Flow rate will decrease as column T increases Flow rate will decrease as column T increases – Viscosity of carrier gas increases with T
Necessary properties Necessary properties – INERT Does not chemically interact with sample Does not chemically interact with sample – COMPATIBLE with detector No noise or explosions No noise or explosions – HIGHLY PURIFIED Impurities will degrade column and cause noise in detector Impurities will degrade column and cause noise in detector “Research grade” is very expensive, so purify a cheaper grade “Research grade” is very expensive, so purify a cheaper grade
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.
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.
Capillary columns require much smaller samples ( ul), 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.
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). volatilization –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%).
For quantitative work, more reproducible sample size are required and this can be obtained by a rotary sample valve.
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.
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 (≈ plates).
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 m in length and have an internal diameter of 2 - 4mm. They are made from glass, metals, or Teflon.
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.
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. 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.
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. These columns are flexible and can be bent into coils. They have the advantages of physical strength, flexibility and low reactivity. They have the advantages of physical strength, flexibility and low reactivity.
Properties and characteristics of GC columns PackedSCOTWCOTFSOT Length, m Inside diameter, mm Efficiency, plates/m Sample size, ng highLow Relative back pressure slowFast Relative speed Poorest→→BestChemical inertness No YesFlexible
After the components of a mixture are separated using gas chromatography, they must be detected as they exit the GC column.
A non-selective detector responds to all compounds except the carrier gas, a selective detector responds to a range of compounds with a common physical or chemical property and a specific detector responds to a single chemical compound. a selective detector responds to a range of compounds with a common physical or chemical property and a specific detector responds to a single chemical compound.
Detectors can also be grouped into concentration dependant detectors and mass flow dependant detectors. Detectors can also be grouped into concentration dependant detectors and mass flow dependant detectors. 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. -the signal is related to the rate at which solute molecules enter the detector.
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.
Characteristics of ideal detector: Adequate sensitivity. Good stability and reproducibility. A linear response to solute. A temperature range from room temp. to 400 o C. A short response time. High reliability and ease of use. Nondestructive to samples.
Detectors for GC Electron capture (ECD) Thermal conductivity (TCD) Flame ionization (FID) Fourier transform infrared (FTIR) Mass spectrometry (MS)
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.
Conditions which can be varied to accommodate a required analysis include: –Column temperature and temperature program. –Carrier gas and carrier gas flow rates. –Column's stationary phase, diameter and length. –Inlet type and flow rates. –Sample size and injection technique. –Depending on the detector installed on the GC, there may be a number of detector conditions that can also be varied.
Column temperature and temperature program:
The column(s) in a GC are contained in an oven, the temperature of which is precisely controlled electronically. The optimum column temperature is dependant upon the boiling point of the sample. As a rule of thumb, a temperature slightly above the average boiling point of the sample results in an elution time of minutes.
The rate at which a sample passes through the column is directly proportional to the temperature of the column. The higher the column temperature, the faster the sample moves through the column. However, the faster a sample moves through the column, the less it interacts with the stationary phase, and the less the analytes are separated.
If a sample has a wide boiling range, temperature is ramped either continuously or in steps to provide the desired separation. This is referred to as a temperature program.
Electronic pressure control can also be used to modify flow rate during the analysis, aiding in faster run times while keeping acceptable levels of separation.
Carrier gas selection and flow rates: The choice of carrier gas is important, with hydrogen being the most efficient and providing the best separation. However, helium has a larger range of flow rates that are comparable to hydrogen in efficiency, with the added advantage that helium is non-flammable, and works with a greater number of detectors. However, helium has a larger range of flow rates that are comparable to hydrogen in efficiency, with the added advantage that helium is non-flammable, and works with a greater number of detectors. Therefore, helium is the most common carrier gas used.
The carrier gas flow rate affects the analysis in the same way that temperature does. The higher the flow rate the faster the analysis, but the lower the separation between analytes. Selecting the flow rate is therefore the same compromise between the level of separation and length of analysis as selecting the column temperature.
With GCs made before the 1990s, carrier flow rate was controlled indirectly by controlling the carrier inlet pressure. Many modern GCs, however, electronically measure the flow rate, and electronically control the carrier gas pressure to set the flow rate. Consequently, carrier pressures and flow rates can be adjusted during the run, creating pressure/flow programs similar to temperature programs.
Detectors for GC Electron capture (ECD) –radioactive –good for X -, NO 2 - and conjugated Thermal conductivity (TCD) –change in resistance of heated wire Flame ionization (FID) –destruction of combustible sample in flame produces measurable current Fourier transform infrared (FTIR) Mass spectrometry (MS)