Mass transfer in Gas Chromatography Done by: ENG.Nasser Badah Al-hajri 207121351 Supervisors: prof....

Mass transfer in Mass transfer in Gas ChromatographyGas Chromatography

Done by:Done by: ENG.Nasser Badah Al-hajri ENG.Nasser Badah Al-hajri207121351207121351

Supervisors:Supervisors: prof. Mohammad fahim prof. Mohammad fahim Dr. Amal Al-kilani Dr. Amal Al-kilani

ChromatographyChromatography

► ChromatographyChromatography is a process of is a process of separationseparation of the of the components of a components of a mixture.mixture.

►The components are separated based on The components are separated based on their their distributiondistribution between two immiscible between two immiscible phasesphases

AB

C

Mixture

Phase 1

Phase 2

[A]

[A]

[B]

[B]

[C]

[C]

The Difference in the distribution coefficient for the different components lead to separation.

Chromatographic SeparationChromatographic Separation

► Phases:Phases: Mobile PhaseMobile Phase (Gas / Liquid) (Gas / Liquid) Stationary PhaseStationary Phase (Liquid / Solid) (Liquid / Solid)

► The Mobile Phase moves The Mobile Phase moves overover the stationary the stationary phasephase

► ComponentsComponents are are introducedintroduced into the into the mobile mobile phasephase, and are carried through the stationary , and are carried through the stationary phasephase

► Different components Different components interact differentlyinteract differently with the with the stationary phasestationary phase

► This leads to This leads to separation of the componentsseparation of the components

Types of ChromatographyTypes of Chromatography

►Chromatography is Chromatography is namednamed based on based on the the mobile phasemobile phase:: Liquid MPLiquid MP : Liquid Chromatography : Liquid Chromatography

►HPLCHPLC►TLC, Paper ChromatographyTLC, Paper Chromatography

Gaseous MPGaseous MP : : ►Gas ChromatographyGas Chromatography

The Stationary PhaseThe Stationary Phase

► The Stationary Phase is The Stationary Phase is covalentlycovalently bonded to the bonded to the tube surface.tube surface.

► The stationary phase molecules are The stationary phase molecules are cross-linkedcross-linked by by covalent bonds.covalent bonds.

► Some important stationary phases:Some important stationary phases: Dimethyl SiloxaneDimethyl Siloxane

► DB1, BP1, HP1DB1, BP1, HP1 Polyethylene GlycolPolyethylene Glycol

► DB-WAX, CW20MDB-WAX, CW20M Trifluoropropyl Methyl SiloxaneTrifluoropropyl Methyl Siloxane

► OV210, DB-210, QF1OV210, DB-210, QF1 A more complete list is in the Training Manual.A more complete list is in the Training Manual.

Gas ChromatographyGas Chromatography

► Is a type of Is a type of chromatographychromatography in which the mobile in which the mobile phase is a carrier gas, usually an phase is a carrier gas, usually an inertinert gas such as gas such as heliumhelium or an unreactive gas such as or an unreactive gas such as nitrogennitrogen, and , and the stationary phase is a microscopic layer of liquid the stationary phase is a microscopic layer of liquid or or polymerpolymer on an inert solid support, inside glass or on an inert solid support, inside glass or metal tubing, called a column.metal tubing, called a column.

► Gas Chromatography is different from other forms Gas Chromatography is different from other forms of chromatography (of chromatography (HPLCHPLC, , TLCTLC, etc.) because the , etc.) because the

solutions travel through the column in a gas state.solutions travel through the column in a gas state.

Principle of GCPrinciple of GC

The Column

Stationary Phase

Mobile Phase:Carrier GasAB

Compound that spends more time in the stationary phase comes out late.

GC Instrumentation BasicsGC Instrumentation Basics

Waste

The InstrumentationThe Instrumentation

►The Carrier GasThe Carrier Gas►The InjectorThe Injector►The Column OvenThe Column Oven►The ColumnThe Column►The DetectorThe Detector►Data Processing SystemData Processing System

The Carrier GasThe Carrier Gas

► Should be Should be InertInert towards the towards the Stationary PhaseStationary Phase and and the the AnalyteAnalyte. .

► Small Small Diffusion CoefficientDiffusion Coefficient – – better separationbetter separation► Diffusion Coefficient Diffusion Coefficient molecular mass molecular mass

Gas Mol. Mass

Hydrogen 2

Helium 4

Nitrogen 28

Oxygen 32

Dangerous

Reacts

OK

Best

ColumnsColumns

►Two types of columns are used in GC:Two types of columns are used in GC: - - Packed columnsPacked columns: are 1.5 - 10 m in length and : are 1.5 - 10 m in length and

have an internal diameter of 2 - 4 mm. The tubing have an internal diameter of 2 - 4 mm. The tubing is usually made of stainless steel or glass and is usually made of stainless steel or glass and contains a contains a packingpacking of finely divided, inert, solid of finely divided, inert, solid support material (eg. support material (eg. diatomaceous earthdiatomaceous earth) that is ) that is coated with a liquid or solid stationary phase. coated with a liquid or solid stationary phase.

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

Detectors Detectors

► The most common are:The most common are:

- the - the flame ionization detectorflame ionization detector (FID). (FID).

- the - the thermal conductivity detectorthermal conductivity detector (TCD). (TCD).

Both are sensitive to a wide range of components, Both are sensitive to a wide range of components,

and both work over a wide range of concentrations.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

Low drift High drift

Sensitivity

► Ability of the detector to differentiate between small differences in analyte concentration.

► The calibration curve:

Concentration

Re

spo

nse

► 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

1. Nitrogen Phosphorous Detector (NPD)

2. Electron Capture Detector (ECD)

3. Flame Photometric Detector (FPD)

Non-Selective Detectors

1. Flame Ionization Detector (FID)

2. Thermal Conductivity Detector (TCD)

Response and Range

►Response of a detector:

Peak height x Peak width at half-max x Flow rateInjection volume

►Linear Dynamic Range:

Concentration

Re

spo

nse

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 / Nwhere 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 The Longitudinal Diffusion Term (B/u)Term (B/u)

► Longitudinal diffusion in column Longitudinal diffusion in column chromatography is a band broadening process chromatography is a band broadening process in which solutes diffuse from the concentrated in which solutes diffuse from the concentrated center of a zone to the more dilute regions center of a zone to the more dilute regions ahead of and behind the zone center.ahead of and behind the zone center.

► The longitudinal diffusion term is directly The longitudinal diffusion term is directly proportional to the mobile-phase diffusion proportional to the mobile-phase diffusion coefficient Dcoefficient DMM..

► The contribution of longitudinal diffusion is The contribution of longitudinal diffusion is seen to be inversely proportional to the mobile seen to be inversely proportional to the mobile phase velocity.phase velocity.

Mass-transfer Coefficients Mass-transfer Coefficients (C(Css and C and CMM))

►The need for the two mass-transfer The need for the two mass-transfer coefficients Ccoefficients Css and C and CMM arises because arises because the equilibrium between the mobile the equilibrium between the mobile and the stationary phase is and the stationary phase is established so slowly that a established so slowly that a chromatographic column always chromatographic column always operates under nonequilibrium operates under nonequilibrium conditions.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 factordf –the thickness of stationary phase

filmDs –the diffusion coefficient of solute in

the stationary phaser –the column radiusDm –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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