Mass transfer in Gas Chromatography Done by: ENG.Nasser Badah Al-hajri 207121351 Supervisors: prof. Mohammad fahim Dr. Amal Al-kilani

Chromatography Chromatography is a process of separation of the components of a mixture. The components are separated based on their distribution between two immiscible phases The Difference in the distribution coefficient for the different components lead to separation. [C] [A] [B] Phase 1 Phase 2 A B C Mixture

Chromatographic Separation Phases: Mobile Phase (Gas / Liquid) Stationary Phase (Liquid / Solid) The Mobile Phase moves over the stationary phase Components are introduced into the mobile phase, and are carried through the stationary phase Different components interact differently with the stationary phase This leads to separation of the components

Types of Chromatography Chromatography is named based on the mobile phase: Liquid MP : Liquid Chromatography HPLC TLC, Paper Chromatography Gaseous MP : Gas Chromatography

The Stationary Phase The Stationary Phase is covalently bonded to the tube surface. The stationary phase molecules are cross-linked by covalent bonds. Some important stationary phases: Dimethyl Siloxane DB1, BP1, HP1 Polyethylene Glycol DB-WAX, CW20M Trifluoropropyl Methyl Siloxane OV210, DB-210, QF1 A more complete list is in the Training Manual.

Gas Chromatography Is a type of chromatography in which the mobile phase is a carrier gas, usually an inert gas such as helium or an unreactive gas such as nitrogen, and the stationary phase is a microscopic layer of liquid or polymer on an inert solid support, inside glass or metal tubing, called a column. Gas Chromatography is different from other forms of chromatography (HPLC, TLC, etc.) because the solutions travel through the column in a gas state.

Principle of GC Mobile Phase: A B Carrier Gas Stationary Phase The Column Mobile Phase: Carrier Gas B A Stationary Phase Compound that spends more time in the stationary phase comes out late.

GC Instrumentation Basics Waste

The Instrumentation The Carrier Gas The Injector The Column Oven The Detector Data Processing System

The Carrier Gas Should be Inert towards the Stationary Phase and the Analyte. Small Diffusion Coefficient – better separation Diffusion Coefficient  molecular mass Gas Mol. Mass Dangerous Hydrogen 2 Helium 4 Nitrogen 28 Oxygen 32 Best OK Reacts

Columns Two types of columns are used in GC: - Packed columns: are 1.5 - 10 m in length and have an internal diameter of 2 - 4 mm. The tubing is usually made of stainless steel or glass and contains a packing of finely divided, inert, solid support material (eg. diatomaceous earth) that is coated with a liquid or solid stationary phase.

- Capillary columns have a very small internal diameter, on the order of a few tenths of millimeters, and lengths between 25-60 meters are common, and it are made of fused-silica with a polyimide outer coating.

Detectors The most common are: - the flame ionization detector (FID). - the thermal conductivity detector (TCD). Both are sensitive to a wide range of components, and both work over a wide range of concentrations.

Detector (cont.) Basic Requirements: Low Noise and Drift High Sensitivity High Selectivity Good Response and Range

Noise and Drift Baseline is the signal produced by the detector when no sample is detected. Noise is the fluctuation in the baseline Low noise High noise Drift is the deviation of the baseline from the horizontal High drift Low drift

Sensitivity Sensitivity is the slope of the calibration curve. Ability of the detector to differentiate between small differences in analyte concentration. The calibration curve: Concentration Response Sensitivity is the slope of the calibration curve.

Selectivity Ability of the detector to detect only compounds of interest. Based on special properties of analytes. Selective Detectors Nitrogen Phosphorous Detector (NPD) Electron Capture Detector (ECD) Flame Photometric Detector (FPD) Non-Selective Detectors Flame Ionization Detector (FID) Thermal Conductivity Detector (TCD)

Response and Range Response of a detector: Linear Dynamic Range: Peak height x Peak width at half-max x Flow rate Injection volume Linear Dynamic Range: Concentration Response

The Chromatogram

Graph showing detector response as a function of elution time.

tr retention time: time between injection and detection of the analyte. tm = time at which an unretained analyte or mobile phase travels through the column. Adjusted retention time:for a solute is the additional time required for solute to travel the length of the column beyond the time required by unretained solvent: t’r = tr-tm

For any two components 1 and 2, the relative retention, α, is the ratio of their adjusted retention times: where

The Capacity Factor For each peak in the chromatogram, the capacity factor, k’, is defined as:

Efficiency of Separation

Resolution Solute moving through a column spreads into a Gaussian shape with standard deviation σ. Common measures of breadth are: -The width w½measured at half-height. -The width w at the baseline between tangents drawn to the steepest parts of the peak (inflection points).

Resolution (cont.) In chromatography, the resolution of two peaks from each other is defined as where Δt R or ΔV Ris the separation between peaks and wav is the average width of the two peaks.

Resolution So, separation of mixtures depends on: –width of solute peaks (want narrow) efficiency –spacing between peaks (want large spacing) selectivity

Diffusion One main cause of band spreading is diffusion. The diffusion coefficient measures the rate at which a substance moves randomly from a region of high concentration to a region of lower concentration.

Diffusion (cont.) The number of moles crossing each square meter per second, called the flux, is proportional to the concentration gradient:

Broadening of Chromatographic Band by Diffusion If solute begins to move through a column in an infinitely sharp layer with m moles per unit cross-sectional area of the column and spreads by diffusion alone, then the Gaussian profile of the band is described by

The standard deviation of the band is

The Theory of Chromatography: Column Efficiency Plate theory -older; developed by Martin & Synge. Rate theory -currently in use.

Plate Theory -Martin & Synge View column as divided into a number (N) of adjacent imaginary segments called theoretical plates. within each theoretical plate complete equilibration of analytes between stationary and mobile phase occurs

Plate Theory -Martin & Synge Significance? Greater separation occurs with: –greater number of theoretical plates (N). –as plate height (H or HETP) becomes smaller. L= N×H or H= L / N where Lis the length of column, Nis the number of plates, and His the plate height

Number of plates on column: Wb –base width of the peak This equation is a measure of the efficiency of a column.

Sometimes the number of plates is measured at the bandwidth at half-height w1/2 :

Estimating the Plate Number for Asymmetric Peaks The Dorsey-Foley equation:

N can be Estimated Experimentally from a Chromatogram Knowing the number of theoretical plates and the length of the column, we can determine the HETP, height equivalent to a theoretical plate:

Effective Number of Theoretical Plates Introduced to characterize open tubular columns –uses adjusted retention volume VR’ in lieu of total retention volume VR:

Effective Number of Theoretical Plates (cont.) The Neff value is useful for comparing a packed and an open tubular column when both are used for the same separation. N and Neff are related by the expression:

Rate Theory Based on a random walk mechanism for the migration of molecules through a column. Takes into account: – mechanism of band broadening –effect of rate of elution on band shape –availability of different paths for different solute molecules to follow –diffusion of solute along length

Van Deemter Equation for Plate Height

In packed columns, all three terms contribute to band broadening In open tubular columns, A is zero In capillary electrophoresis, both A and C go to zero

The Longitudinal Diffusion Term (B/u) Longitudinal diffusion in column chromatography is a band broadening process in which solutes diffuse from the concentrated center of a zone to the more dilute regions ahead of and behind the zone center. The longitudinal diffusion term is directly proportional to the mobile-phase diffusion coefficient DM. The contribution of longitudinal diffusion is seen to be inversely proportional to the mobile phase velocity.

Mass-transfer Coefficients (Cs and CM) The need for the two mass-transfer coefficients Cs and CM arises because the equilibrium between the mobile and the stationary phase is established so slowly that a chromatographic column always operates under nonequilibrium conditions.

Longitudinal Diffusion The variance resulting from diffusion is Plate height due to longitudinal diffusion:

Longitudinal Diffusion (cont.) In packed columns, the tortuosity coefficient γis used to account for irregular diffusion patterns and is usually less than unity (γ~ 0.6), because molecular diffusivity is smaller in packed columns than in open tubes (γ= 1):

Longitudinal Diffusion (cont.) Because longitudinal diffusion in a gas is much faster than in a liquid, the optimum flow rate in gas chromatography is higher than in liquid chromatography.

Nonequilibrium (Resistance to Mass Transfer Term) This term comes from the finite time required for the solute to equilibrate between the mobile and stationary phases. Some solute is stuck in the stationary phase, but the remainder in the mobile phase moves forward resulting in spreading of the zone. The slower the flow rate, the more complete equilibration is and the less band broadening occurs

Resistance to Mass Transfer (cont.) Plate height due to finite equilibration time: where Cs describes the rate of mass transfer through the stationary phase and Cm describes the rate of mass transfer through the mobile phase. Specific equations for Cs and Cm depend on the type of chromatography.

Resistance to Mass Transfer (cont.) For gas chromatography: Mass transfer in stationary phase: Mass transfer in mobile phase:

Resistance to Mass Transfer (cont.) k’ –the capacity factor df –the thickness of stationary phase film Ds –the diffusion coefficient of solute in the stationary phase r –the column radius Dm –the diffusion coefficient of solute in the mobile phase

Resistance to Mass Transfer (cont.) Efficiency is increased by: –Decreasing stationary phase thickness –Reducing column radius –Increasing temperature

Alternative Plate Height Equation: The Knox Equation Used to compare column efficiencies Makes use of so-called reduced parameters(dimensionless quantities): –Reduced plate height: h= H/dp –Reduced velocity: v= udp/Dm

The Knox Equation (cont.) For well-packed columns of varying particle size and differing conditions, the coefficients a, b and cwill be roughly constant: e.g. a=1, b= 2, and c= 0.05 for porous particles.

Modification of the van Deemter Equation: the Giddings Equation Giddings realized that the eddy diffusion and resistance to mass transfer in the mobile phase must be treated dependently:

Required Plate Number If k2’and α are known, the required number of plates can be calculated: The Rs value is set at the 6σ level or 1.5

Required Column Length The Nreq parameter can be used to determine the length of column necessary for a separation. We know that N= L/H; thus:

Minimum Analysis Time The minimum analysis time tmin is:

The Major Objective in Chromatography The goal in chromatography is the highest possible resolution in the shortest possible time. Optimization techniques aim at choosing conditions that lead to a desired degree of resolution with a minimum expenditure of time

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