Lec#4_Column Efficiency_GC.ppt
- 1. Analytical Chemistry-II BS-V, CH-311 Department of Chemistry, SNS, NUST By Dr. Musammir Khan Column efficiency, Gas Chromatography
- 2. Band broadening: Efficiency (Rate theory + Plate theory) • Why the band broaden as they move through the column? • Described by Rate Theory=random walk mechanism: • Consider a single solute undergoing thousand of transfer between the stationary and mobile phase i.e. residence time in either phase is quite irregular. • Transfer from one phase to another phase require energy and molecule acquire this energy from the surrounding. • Movement through the column occur only when the molecules are in the mobile phase. • The chromatogram behave much like the Gaussian shape i.e. • Random uncertainty to be +ve or –ve, each of which has equal probability of occurrence.
- 3. Band broadening in TGA (Thermogravimetric Analysis) is not typically explained by rate theory or plate theory. Rate theory and plate theory are concepts that are commonly used to explain band broadening in chromatography, specifically in gas chromatography. In gas chromatography, band broadening occurs as solutes move through the column. Rate theory explains band broadening as a result of the random walk mechanism, where solutes undergo numerous transfers between the stationary and mobile phases with irregular residence times. This transfer requires energy and leads to band broadening. Plate theory, on the other hand, describes band broadening in terms of the efficiency of the column. It considers the concept of theoretical plates, which represent the hypothetical stages of equilibration between the solute and the stationary phase. Band broadening in plate theory is related to the number of theoretical plates and the height of each plate. In TGA, band broadening is typically not explained by rate theory or plate theory. Instead, it is influenced by factors such as the heating rate, sample size, and the thermal properties of the sample. Band broadening in TGA is often characterized by the shape and width of the thermal curve, which represents the weight loss or gain of the sample as a function of temperature.
- 4. Gaussian shape curve/chromatogram is usually obtained, which is described by standard deviation δ and variance δ2 standard deviation δ: How individual values are far from each other. variance δ2: how actual value are far from mean.
- 5. Tailing (favor mobile phase) vs. Fronting (favor stationary phase)
- 6. Column efficiency: o Quantitative measure of band broadening or the degree to which a column and other components effect the separation process. o Plate theory: Column consists of large number of theoretic plates (N), where components of a mixture gain equilibrium (between stationary and mobile phase). o Column efficiency is usually expressed in term of theoretical plate number: N = L/H [L =length of column, H = height of plate The efficiency of a column increases with plate count increase or with smaller plate height (H).
- 7. Theory of peak broadening: Column efficiency Longitudinal diffusion coefficient B: Diffusion of solute species on either side of flow from region of high conc to low conc. Stationary and mobile phase mass transfer coefficient (Cs and CM) *In case of liquid immobilized as stationary phase: Mass transfer coefficient (CS) α sq of film thickness (df)2 *In case of solid stationary phase: Mass transfer coefficient (CS) α time required for adsorption/desorption For mobile phase: For packed column: CM α square of particle diameter (dp)2 For capillary column: CM α square column inner diameter (dc)2 For packed column where flow dominate diffusion A=Eddy diffusion: Solute species make different pathways, leading to range of elution time (band broden) Van Deemter Equation
- 8. Formula for number of theoretical plates Packed column=particle diameter Dp
- 9. For packed column where flow dominate diffusion Height equivalent of theoretical plate
- 10. Variable effecting column efficiency Mobile phase flow rate: Gas vs. Liquid For liquid 10 time slower than for gaseous phase, because of lower flow rate and lower plate height i.e. the time length the mobile phase spend in contact with stationary phase.
- 11. Gas Chromatography (GC) • The components of a vaporized mixture are separated/distributed between inert mobile gaseous phase and liquid/solid mobile phase. • Used for qualitative and quantitative analysis of components in a mixture ( important industries, biomedical and forensic areas). • Unlike other chromatographic techniques, gaseous mobile phase doing only elution. • Two types: Gas-solid (GS) and gas-liquid (GL) chromatography; – GSC (solid stationary phase): Retention occur only by physical adsorption – Limited due to SEMIPERMANENT retention/severe tailing occur. – Used for smaller molecules eg gas – GLC (liquid stationary phase): partition occur between liquid immobilized on stationary phase and gaseous mobile phase – Liquid immobilized on inert solid (packed or coated on inner walls of capillary tubing) (useful = 1941 by Martin).
- 12. Instrumentation/components: • Carrier Gas System, • Sample Injection system, • Column Configurations and Column Ovens • Detectors Carrier Gas System: • Mobile phase inert carrier gas available in tank e.g. (He, Ar, N2, H2) • Flow rate can be controlled by two-stage regulators i.e. gas inlet pressure and some commercially available regulators. • A gas inlet pressure of 10-50 psi (1atm=14.6 psi) • *Volumetric flow rate (F) =25-150 mL/min packed column • *1-25 mL/min - Capillary column (low diameter~0.2-5 mm)
- 13. Instruments for Gas- Liquid Chromatography (GLC)
- 14. • he working principle of a gas chromatograph is based on the separation of components in a vaporized sample. This separation is achieved by distributing the components between a mobile gaseous phase and a stationary phase, which can be either a liquid or a solid. The stationary phase is either retained on the surface of an inert solid packing or on the walls of capillary tubing. The mobile phase, which is a gas, carries the sample through the column. As the sample moves through the column, the components are separated based on their affinity for the stationary phase. The separation is further enhanced by temperature programming, where the column temperature is increased continuously or in steps during the analysis. This allows for improved resolution and separation of the components. Various detection systems, such as mass spectrometry, can be used to identify and quantify the separated compounds.
- 15. • Sample injection: Suitably sized sample is injected through calibrated micro-syringes. • For packed analytical column = 0.1-20 µL sample required • For capillary column- 0.1-20 µL/100 i.e. smaller sample needed • Currently auto-injector and autosamplers are available • For injection volume = 0.1 µL -10-200 µL micro-syringes • Column Configurations and Column Ovens • Two types of column • *Packed column- Used in old time • *Capillary column currently used • L = 2-60 m, composition = stainless steel/silica/glass/Teflon • Inner diameter = 0.2-5 mm, coiled for fitting into oven = 10-30 cm coiled diameter • Column Temperature: Depend on B.P. of sample (T=B.P. =2-3 min eluted)
- 16. • Temperature programming improve separation as compared with ISOTHERMAL chromatogram (i.e. increasing the column temp either continuously or in steps. e.g. (a) Isothermal at 45 C. (b) Isothermal at 145 C. (c) Programmed at 30 to 180 C
- 17. Detectors Characteristics of ideal detectors: o Sensitivity~ presently 10-8 -10-15 g solute/s o Stability/reproducibility o Foolproof in hand of less skilled person o Temperature = up to 400 oC o Short response time o Universal o Non-destructive
Editor's Notes
- #8: C