Page 79 - Practical book PC406-MNU-2025

MANSOURA NATIONAL UNIVERSIY
PHARM D- CLINICAL PHARMACY LEVEL II INSTRUNENTAL ANALYSIS (PC 406)

The effectiveness of chromatographic column in separating two solutes depends
on several factors.

These factors can be explored and summarized through the following part:
1) The Capacity Factor ( K') is widely used to describe the migration rates of

solutes on columns.

 It is defined as:

   t ( t ) t 
t
For solute A, K'  R A M and K'  R B M for solute B.
A
t M B t M
Ideally, separations are performed under conditions in which the capacity

factors for the solutes in a mixture lie in the range between 1 and 5.

2) The selectivity factors (α) of a column for the two solutes A and B in a mixture is
defined as:
t
K'    t
  B  R B M
K' A    t M
t
A
R
where K' B is the caacity factor of the more retained solute B, and K' A is that for the
more rapidly eluted solute A.

 By this definition, α is always greater than unity.

 The larger its value, the more selective the stationary phase and more easier

the separation will be.

Efficiency of Chromatographic Columns

The efficiency of chromatographic column refers to the amount of band broading

that occurs when a compound passes through the column.

Two related terms are widely used as quantitative measures of chromatographic

column efficiency.

Plate Height, H, (is also known as the height equivalent of a theoretical plate

(HETP), and Number of theoretical plates (N).

The two are related by the equation.

N = L / H (where L is the column length, usually in centimeters).

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