The typical operating pressure of an HPLC is sufficiently high
that it is impossible
to inject the sample in the same manner as in gas chromatography. Instead,
the sample is introduced using a loop injector
(Figure 12.28). Sampling
loops are inter- changeable, and available with
volumes ranging from
0.5 Î¼L to 2 mL.
In the load position the sampling loop is isolated
from the mobile
phase and is open
to the atmosphere. A syringe
with a capacity several times
that of the sampling
loop is used to place
the sample in the loop.
Any extra sample
beyond that needed to fill the sample loop exits through the waste line. After loading
the sample, the in-
jector is turned to the inject position. In this position
the mobile phase
is directed through the sampling loop,
and the sample
is swept onto the column.