Figure 4.31
(c) A low-pressure gradient former
with three solvents.quarter of that through conventional columns. The stationary phase or packing is retained at each end by
thin stainless-steel frits or mesh disks of 2 μm porosity or less. Columns are packed by a slurry method
which involves suspending the particles of packing in a suitable solvent and 'slamming' it into the
column rapidly and at pressures in excess of 3 000 psi (200 bar). The choice of slurry solvent depends
upon the nature of the packing and many solvents have been investigated. Balanced-density slurries, in
which solvent density is matched with particle density, are sometimes used to minimize settling out of
larger particles during the packing procedure or for packings which are larger than 10 μm.
HPLC columns need more careful handling and storage than GC columns to avoid disturbance of the
packed bed. They should be kept sealed at both ends when not in use and flushed with methanol prior to
sealing.
Figure 4.32
Valve injector with external loop.