Presented by: Anvita Jadhav

M.Pharm (IP)

Gas Chromatography Gas chromatography is a common type of

chromatography used in analytical chemistry forseparating and analyzing the compounds that can bevaporized without decomposition.

It has two types

• Mobile Phase : Gas

• Stationary Phase: SolidGSC

• Mobile Phase : Gas

• Stationary Phase : LiquidGLC

The distribution of an analyte between stationary and mobilephase is expressed by the distribution constant K.

K = Cs/Cm

Cs = concentration of a component in the stationary phaseCm = concentration of a component in the mobile phase

In case of GSC, the interaction of solutes with the stationaryphase is in the form of their adsorption on it & this adsorption isnon-linear.

This does not keep the ratio of the concentration of a solute inthe stationary phase (Cs) to that in the mobile phase (Cm)constant.

In case of the GLC ratio of the concentration of a solute in thestationary phase (Cs) to that in the mobile phase (Cm) constant.

Isotherm for Linear-NonidealGLC

Isotherm for Nonlinear-NonidealGSC

The isotherm is a graphical representation of the distribution constant K

CS = concentration in stationary phase;CG = concentration in mobile phase at equilibrium.

Plate Theory The theory assumes that the column is divided into a

number of zones called theoretical plates.

At each plate equilibrium of the solute between themobile phase & the stationary is assumed to take place.

The partitioning of a solute between the phases takesplate at each theoretical plate.

Thus, the number theoretical plates in the column isused as a measure of efficiency of the column toseparate the components from each other

The number of theoretical plates can be determined by

where, n = no. of theoretical plates

VR = retention time

W = base width of the peak

HETP value can be determined by,

Plate theory disregards the kinetics of mass transfer;therefore, it reveals little about the factors influencingHETP values.

The resulting behavior of the plate column is calculated onthe assumption that the distribution coefficient remainsunaffected by the presence of other solutes and that thedistribution isotherm is linear.

The diffusion of solute in the mobile phase from one plateto another is also neglected.

Plate Theory

Discrete Flow Model

Continuous Flow Model

Discrete-Flow Model The assumptions in this model are

(a)

• All the mobile phase moves from one segment tothe next segment at the end of a discrete interval

(b)

• The sample molecules are always in equilibriumwith the mobile and stationary phases

Continuous-Flow Model The assumptions in this model are

(a)

• The mobile and stationary phases remain in equilibrium throughout the separation

(b)

• The mobile phase flows from one segment to the next segment at a constant rate

(c)• Perfect mixing takes place in all segments

Rate Theory It was introduced by Van Deemter.

It describes the effect of an elution band as well as itstime of elution.

Van Deemter equation describes the relation of theheight of a theoretical plate H and the average linearvelocity of the mobile phase.

Van Deemter Equation

H = height of a theoretical plate

u = average linear velocity of the mobile phase

A = eddy diffusion term

B = longitudinal or ordinary diffusion term

C = nonequilibrium or resistance to mass transfer term

Eddy Diffusion The A term refers to band broadening caused by

dispersion (multi-pathway) effects (Eddy diffusion)

A = 2λdp

λ = correction factor for the irregularity of the columnpacking

dp = average particle diameter.

In this case the spaces along the column are not uniform.

When a sample migrates down the column, each molecule “sees”different paths and each path is of a different length.

Some molecules take the longer paths and others take theshorter paths.

There are also variations in the velocities of the mobile phasewithin these pathways.

The overall result is that some molecules lag behind the center ofthe zone, whereas others move ahead of the zone.

Longitudinal Diffusion The B term represents band broadening by

longitudinal diffusion, the molecular diffusion both inand against the flow direction:

B = 2γDG

γ = labyrinth factor of the pore channels (0<γ <1)

DG = diffusion coefficient of the analyte in the gasphase

This process results when there exists a region of highconcentration and a region of low concentration.

The migration is from the higher to the lowerconcentration region in the axial direction of the column.

Diffusion occurs on the molecular level, resulting frommovement of molecules after collision

The diffusion is about 100–1,000-fold faster in gases than inliquids, therefore B terms shows higher impact in GC thanin LC.

Mass transfer under non equilibrium The C terms refers to the mass transfer between stationary and

mobile phase.

As the zone of solute continues to migrate down the column, it isconstantly bringing an ever-changing concentration profile incontact with the next part of the column. This effect results indifferent rates of equilibration along the column.

Thus theoretical plate in the column is constantly attempting toequilibrate with a variable concentration zone in the mobile phase.

At one time the zone attempts to equilibrate with a lowconcentration in the mobile phase, and then at another time with ahigh concentration.

These overall processes result in nonequilibrium at each theoreticalplate.

The rapid mass transfer depends on the factors originating from the stationary phase as well as the mobile phase The term ‘C’ in Van Deemter equation is therefore, the sum of Cs & CM.

The stationary phase contribution (Cs) to the plate height H, due to the mass transfer under nonequilibrium condition, is given by,

q = configuration factor

r = a constant dependent upon the relative rate of migration of a solute & the mobile phase,

d = thickness of the stationary phase

Ds = diffusion coefficient of a solute in the stationary phase.

The mobile phase contribution (CM) to the plateheight H, due to the mass transfer undernonequilibrium conditions, is given by,

DG = diffusion coefficient in the gas phase

dp = average particle diameter

ω = obstruction factor for packed bed

Van Deemter PlotThe term ‘A’ is independent

of flow rate of the mobile

phase

The term B/u decreases

drastically in the beginning

with increase in the flow rate

of mobile phase. Increase in

the flow rate beyond

particular value, leads to slow

decrease in the value of B/u.

The term Cu increases with

increse in the flow rate