carrier gases: helium should be used with lower velocity as compared to hydrogen
but higher ones as compared to nitrogen. This has certainly also a direct impact on
the analysis time.
General Note
Modern GC analysis uses capillary columns and measurements with temperature programs to achieve high-performance separation.
0.25 μm
0.50 μm
1.00 μm
t ime
f ilm thickness
f ilm polarity
1,2
3 4
t ime
t ime
3
4
1
2
nonpolar
polar
1: p-xylene
2: m-xylene
3: decane
4: undecane
Fig. 4.4 How film type and dimension influence the gas chromatographic separation
Table 4.1 Influence of column dimensions on chromatographic parameters
Length (L)
ID (r)
Phase thickness (d f )
Resolution
(R)
R~
ffiffiffi
L
p
R~
1
r
R~
1 ffiffiffi ffi
df
p
To double the resolution,
quadruple the length
Resolution decreases with
increased diameter
Resolution decreases
with increased
thickness
Sample
capacity
(SC)
SC~
ffiffiffi
L
p
SC~r
2
SC~d f
Longer columns have a
slightly better capacity
Capacity is exponential as
diameter increases
Sample capacity
increases with thicker
films
Retention
time (t r )
t r ~L
t r ~r
t r ~d f
Longer columns require
longer analysis times
Smaller diameter columns
allow faster analysis times
Thicker films require
longer analysis time
4.1 High Performance Chromatography: GC, HPLC
43