Chromatographic Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 255

All peaks in an ideal chromatogram should be narrow, symmetrical, well-spaced
(one compound per peak) but not too far apart from each other (for a short run time).
Unfortunately, this is not always the case in practice. This is where the skilled
chromatographer who can examine the chromatogram from a trial run, and then
effectively adjust the chromatographic conditions for a better separation, is paid off.
The following terms and concepts related to separation principles are introduced.

Distribution Constant
The distribution constant (K c ) is defined as the concentration of a solute in the
stationary phase (C s ) divided by its concentration in the mobile phase (C m ):
K c ¼ C s = C m ð10 : 1Þ

The larger the K c , the more the solute will be sorbed or partitioned in the stationary
phase, and the longer it is retained on the column. K c is dependent upon the types of
stationary phase and mobile phase. Separation will be impossible if two compounds
have the same K c values. This explains why the selection of stationary phase
described in Section 10.1.2 is critical. In addition, since K c is a temperature-
dependent thermodynamic constant, temperature plays a role in separation,

particularly in GC.

Retention Factor

Retention factor (k) is defined as the ratio of the amount (not concentration) of a
solute in the stationary phase (M s ) to the amount in the mobile phase (M m ).
k ¼M s = M m ð10 : 2Þ

The larger this value, the greater the amount of a given solute in the stationary phase,
and the longer it will be retained on the column. As such, it resembles distribution
constant (K c ) and depends on both mobile-stationary phases and the temperature.
Unlike K c , however, retention factor can be readily measured from a chromatogram.
Its working definition is the ratio of the time an analyte spends in the stationary
phase (t r t m ) to the time it spends in the mobile phase (t m ).

t r t m
t m

k ¼ ð10 : 3Þ

An ideal separation of solutes in a mixture can be achieved if the k values lie in the
range of 2-10. However, the analytical time will be too long if k values are larger
than the range 20-30.

Separation Factor

Separation factor (a), defined as the ratio of distribution constants of two solutes, can
be derived in terms of the ratio of the retention factors (k) of two solutes (Eq. 10.2) or
can be measured from a chromatogram with the retention times of two solutes