Inlet Systems
Split/splitless capillary inlet
110
Van Deemter curves demonstrate advantages of using either He or H as
carrier gas. From the curves, several observations may be made:
Minima for He and H
2
occur at much higher average linear
velocities than N
2
. Thus, He, or even better, H
2
, can be used at far
higher velocities than N
2
with only small loss in efficiency. Use of H
2
or He allows shorter overall analysis times.
An additional benefit in using H
2
is that high velocities can be
achieved at relatively low column head pressures.
For any given compound, the minimum in the N
2
curve is quite
pronounced; consequently, there is only a small linear velocity range
over which column efficiency is maximized.
H , and He to a lesser extent, exhibit relatively flat minima. Both are
better suited as carrier gases when temperature•programmed
analyses are performed.
An unrelated problem with N
2
is that it is available in many grades and
is often contaminated with water and/or light hydrocarbons. He and H
2
are typically available in higher purities.
Overall, H
is the best carrier gas choice, with He nearly as good.
Initial column head pressure
In using capillary columns, linear velocity through the column must be
set. This, in turn, is controlled by pressure at the head of the column.
Pressure required to obtain a particular velocity depends primarily upon
bore (ID) and length of the particular column and oven temperature.
Hewlett•Packardfused•silicacapillary columns may be conveniently
broken into several categories based upon their bores. Table 6•1lists
suggested initial pressures for some capillary column bores and lengths.
