Page 154 - Illustrated Pocket Dictionary of Chromatography

154 PREPARATIVE (HPLC)

An HPLC precolumn is used as a sacrificial column when harsh/reactive mobile
phases are used. The packing material should match the identity of the analytical
column but can be made from much less expensive large-particle material (30–
40mm). The column is dry packed and flushed with mobile phase to remove air
before attaching to the system.

from particularly aggressive solvents (e.g, high and low pH, etc). The
premise of the precolumn is that it acts as a “sacrificial” column. The
aggressive solvent dissolves the material in the precolumn, thereby
becoming saturated with the bonded phase, hence the reason for the
bonded phase needing to match the analytical column phase. It should
be noted that the particle size is often much larger for the precolumn
because system efficiency is unaffected (it is placed before the injec-
tor). The 40- to 60-mm particles found in many solid-phase extraction
columns are acceptable. Precolumns are typically dry-packed and
then equilibrated with mobile phase off-line.

preparative (HPLC) Columns that have an ID >2cm and lengths
>25cm are used to process large samples masses and are termed
preparative columns. For preparative columns the packing is usually
≥40mm, flow rates are >10mL/min, and injection volumes are >0.1mL.

preparative thin-layer chromatography (TLC) To increase
the sample load capacity over a normal TLC plate, a preparative TLC
plate has a a thicker sorbent layer (500–2000mm vs. ~250mm). Larger
plates, height and width, may also be used. In preparative TLC the
bands are identified and the sorbent containing the band is scraped
from the plate. The analyte is then desorbed from the sorbent by
placing the sorbent is a very strong solvent. The solvent is removed
or exchanged for a long-term storage solvent.

pressure Pressure in chromatographic systems is reflective of the
resistance that the system (in particular the column) has to forced
mobile-phase flow through it. An operational diagnostic for the proper