Factors affecting Height Equivalent to theoretical Plates explained

1. VAN-DEEMTER EQUATION
Dr. Tambe V.S.
PES Modern College of Pharmacy (for Ladies), Moshi

2. Band Broadening Processes
Initial Time, t1 Time, t2
N=
𝐿
𝐻
Number of theoretical plates =
𝐿𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑐𝑜𝑙𝑢𝑚𝑛/𝐻𝑒𝑖𝑔ℎ𝑡 𝑒𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 𝑡𝑜 𝑜𝑛𝑒 𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑝𝑙𝑎𝑡𝑒 𝐻𝐸𝑇𝑃

3. Types of Band Broadening Processes
Non column broadening/ extra column band
broadening
Dispersion of analyte in-
• Dead volumes of system (injector, connection
between injector and column, connection
between column and detector)
• Can be minimized by reducing dead volume
On column broadening
(Explained by Van Deemter equation)

5. A: Eddys Diffusion/ Multiple path effect

7. Inhomogeneity's of flow velocities and pathlengths
around packed particles
A = λ dp
λ is packing factor, function of packing uniformity
and the column geometry
Column should be packed uniformly
dp: particle size, should be small and uniform with
narrow particle size distribution

8. B: Molecular or Longitudinal diffusion
Tendency of molecules to migrate from the concentrated part of the
band to the dilute region on either side.
B dominates at low velocity, as diffusion increases with time, solute gets
enough time to diffuse in both directions
It can occur in MP and SP
D gas ≥ 104 D liquid
More important for gas as compared to liquid

9. B: Molecular or Longitudinal diffusion
ϒ is obstruction factor, hindrance to free molecular diffusion offered
by particles or bed structure, Offered by packed column (0.7 in packed
column, 1 for open column)
In solution, molecule has equal probability of diffusion in any
direction. In packed column, the solid packing material may restrict
the ability of solute to diffuse in one particular direction, thereby
hindering longitudinal diffusion.
If the SP is packed uniformly and tightly, there is more obstruction
provided to the diffusing particles, reducing band width and
increasing column efficiency
Tm is interparticle tortuosity factor (How tortuous path MP takes)
𝐵 = 2ϒ 𝐷𝑚/𝑇𝑚

10. C : Resistance To Mass Transfer Under Non Equilibrium Conditions
Equilibrium of analyte between stationary and mobile phase is not instantaneous,
it takes finite time, which is barely given in chromatography

11. Cs: Resistance to mass transfer at solute to stationary phase interface
𝐶𝑠 =
𝐷𝑓
𝐷𝑠
Df: In partition chromatography, solid support is coated with liquid SP.
Lesser is the thickness of SP, lesser will be resistance to mass transfer at
SP. (but also reduces capacity of column)
Ds: Diffusion coefficient of solute in stationary phase, it should be more
to achieve fast equilibrium
To increase the Diffusion coefficient
• Non-viscous liquid stationary phases should to used
• Increase in column temperature to increase diffusion
Resistance to mass
transfer should be low

12. At low flow rate enough time is available for mass transfer of
solute under equilibrium conditions

13. Cm: Resistance to mass transfer of solute between adjacent stream
lines of mobile phase
𝐶𝑚 =
𝑑𝑃2
𝐷m
dp: Particle Size for packed column, and column diameter for open
tubular column (smaller should be diameter of column)
Smaller is the particle size of the stationary phase, lesser will be
resistance to mass transfer.
Dm: Diffusion coefficient of solute in mobile phase, it should be more to
achieve fast equilibrium
To increase the Diffusion coefficient-
• Non-viscous liquid mobile phases should to used
• Increase in column temperature to increase diffusion
Resistance to mass
transfer should be low

14. GC: CM < CS
HPLC: CM = CS

15. Optimum Mobile Phase Velocity

16. Effect of particle size on H

17. SELECTION OF CARRIER GAS AND OPTIMUM FLOW RATE

18. Van-Deemter equation is used to compare performance of
various stationary phases and mobile phases
Comparison of performance of SP/ Columns
• Run the chromatograms using a single SP for a particular analyte by
varying flow rates
• Calculate the number of theoretical plates using the equation
• Calculate H using the formula: H = L/N
• Plot H versus Flow rate
• Repeat for other Columns

19. Thank you…