Inlet Systems
Split/splitless capillary inlet
Van Deemter curves demonstrate advantages of using either He or H1 as carrier gas. From the curves, several observations may be made:
C Minima for He and H2 occur at much higher average linear velocities than N2. Thus, He, or even better, H2, can be used at far higher velocities than N2 with only small loss in efficiency. Use of H2 or He allows shorter overall analysis times.
CAn additional benefit in using H2 is that high velocities can be achieved at relatively low column head pressures.
C For any given compound, the minimum in the N2 curve is quite pronounced; consequently, there is only a small linear velocity range over which column efficiency is maximized.
CH1, 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 N2 is that it is available in many grades and is often contaminated with water and/or light hydrocarbons. He and H2 are typically available in higher purities.
Overall, H1 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.
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