The document discusses the Van Deemter equation, which relates the height equivalent to a theoretical plate (HETP) in gas chromatography to experimental parameters like particle diameter, diffusion coefficients, and flow rate. It explains that HETP is affected by eddy diffusion, longitudinal diffusion, and mass transfer between phases. The Van Deemter equation can be used to optimize chromatographic performance by identifying conditions that minimize band broadening like adjusting flow rates and using smaller stationary phase particles.

1. GAS CHROMATOGRAPHY
VAN DEE METER
EQUATION
PREPARED BY:
Ms. SONAM M. GANDHI
GUIDED BY:
PROF A.CENDILKUMAR
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2. INTRODUCTION
• In the mid 1950s a group of Dutch
chemicals engineers began a study of the
process that caused band broadening in
chromatography.
• They derived an expression called as the
VAN DEEMETER equation, relating the
height equivalent to a theoretical
plate(HETP) to a number of experimental
parameters , which are
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4. • Diameter of the stationary phase particles.
• The diffusion coefficients of the solute in
the stationary phase and mobile phase.
• Flow rate of the mobile phase.
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5. • Van Deemeter equation is useful in optimizing chromatographic
performance and is expressed as:
HETP = 2λdp + 2νdg + 8 {K / (1+K) 2} df2 x u
u π2 dl
• u = Velocity of carrier gas
• λ = Constant indicating packing of column
• dp = Particle diameter
• ν = Obstruction factor
• dg = diffusion coefficient of the solute molecules in carrier gas (
cm2 / sec)
• 8 / π2= Geometrical factor pertaining to the uniformity of the liquid
film thickness on the particles
• K = Capacity factor
• df = Thickness of the film of liquid phase
• dl = Diffusion coefficient of the solute in liquid phase (cm2 / sec)
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6. • Except u , all other terms in the equation
are constant so it can be simplified as
follows
HETP = A + B / u + Cu
• A = EDDY’S DIFFUSION
• B = LONGITUDINAL DIFFUSION
• C = MASS TRANSFER
• µ = VELOCITY OF THE CARRIER GAS
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7. COEFFICIENT A
• Coefficient A is called the eddy diffusion or
multiple –path coefficient and is concerned
with the different paths traveled by the
molecules of a particular solute during
their passage through the column.
• The particles of the stationary
phase, weather irregularly or spherically
shaped, are packed as tightly as
possible, and the solute molecules must
pass around them to proceed along the
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column.

8. …
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9. • Because of the large number of possible
paths , some molecules of the same kind
will reach the end of the column before
others .
• Faster molecules found in the leading
edge of the peak , and slower ones form
the tailing edge.
• The net effect of this distribution is band
broadening.
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10. • In a modern chromatographic column
,which is packed with small , uniformly
sized particles , the value of A is minimal
and the contribution of this term to
increasing the HETP is negligible.
• In GLC column , which contains no solid
particles, the value of A is zero.
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12. COEFFICENT B
 In the van dee meter equation is termed the
coefficient of longitudinal diffusion. Because
the concentration of solute is lower at the
edges of the band than in the center ,a
gradient exists and, during travel of the band
through the column, solute is diffusing
continually through the mobile phase away
from the center of the band.
 This phenomenon occurs at both the leading
& trailing edges of the peak & contributes
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further to band broadening. 12

13. FRONTING : Deformation at the beginning of the peak. It is due to saturation of
stationary phase with higher quantity of components.
TAILING: Deformation at the end of the peak. It is due to similarity of polarity for a
component towards stationary phase.
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14. • The equation predicts that the contribution
to the HETP of this is inversely
proportional to the
• mobile phase velocity
• The effect is more pronounced at low flow
rates.
• Diffusion effects are more severe in GC
than in liquid chromatography because
diffusion coefficient are several orders of
magnitude higher in the gas.
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15. • The contribution of longitudinal diffusion to
band broadening can be lessened by the
proper adjustment of flow rates.
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16. COEFFICENT C
• The co efficient of mass transfer, is
concerned with the transfer of the solute
between the two phases.
• Because the mobile phase is moving
rapidly, equilibrium between the two
phases may not be attained .
• Therefore, some solute molecules in the
mobile phase are not transferred to the
stationary phase quickly enough, and,
result are carried ahead of the center of
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the band.

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18. • Those in the stationary phase are retained
too long and ,hence, lag behind.
• In contrast to longitudinal diffusion , the
contribution to the plate height of this term
directly proportional to the flow rate; thus,
to minimize the overall effect, a
compromise in flow rate is necessary.
• Mass transfer effects also may be
lessened by using a very thin coating of
the stationary phase so that the area, in
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19. • Contact with the mobile phase is
maximized while diffusion deep in to the
stationary phase is reduced.
• An efficient GC column will have several
thousand theoretical plates , and capillary
columns will have in excess of 10,000
theoretical plates.
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20. GRAPH OF HETP
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21. Significance:
• It explains the factors responsible for band
broadening.
• It also gives the optimum flow rate of M.P
so as to get minimum HETP.
• Use full in optimizing the chromatographic
performance.
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