StationaryPhases
45
10. Columnperformances
Columnperformances need to take into account vanDeemter expression, carrier gas choice,
effect of carrier gas viscosity on linear velocity and phase ratio. Column efficiency, the
resolution and sample capacityof a capillary columndependon constructionmaterial of the
column, the inner diameter and the film thickness of the stationary phase. The distribution
coefficient K
D
couldbe interpreted as a functionof chromatographic parameters. For a given
temperature columnof a tandem solute-stationaryphase, K
D
is constant. In this situation, K
D
represents the ratio between the concentration of solute in stationary phase and the
concentrationof solute in carrier gas:
K
D
=k
ǃ
=
π
/2d
f
(3)
where
ǃ
= r/2d
f
is thephase ratio, r is the radius of the column andd
f
is the film thickness of
stationaryphase.
11. Selectingstationaryphases
Using packed columns for gas chromatography needs to have a “collection” of available
columns packed with different stationary phases and variable lengths for different
purposes. The non-volatile liquid phases for packed columns have a good selectivity, but
the thermal stability as well as the difference of thickness of the film at elevated
temperatures is low and thiswill affect the inertness of stationaryphase and the efficiency
of the column.
For the best choice of a stationary phase it is important to remember the motto “like
dissolves like”. A polar phase could lead to a lower column efficiency compared to a non-
polar phase. Themaximum temperature limits decrease, as well as the operation lifetime if
operating temperature is high (Grob & Barry, 2004). The lower thermal stability of polar
phases can be avoided using a thinner film of stationary phase and a shorter column for
lower elution temperatures.
12. Installing, conditioning, exhaustingand regenerationof chromatographic
columns
On column installing it is important to select the proper ferrule, meaning that the inner
diameter of the ferrule fits to the outer diameter of the column, in order to avoid loss of
carrier gas. Table 8presents the sizeof ferrules suitable to column size (inner diameter).
Ferrules innerdiameter,mm
Columns inner diameter,mm
0.4
0.25
0.5
0.32
0.8
0.53
Table 8. Ferrules innerdiameter compatiblewith columns inner diameter
