Page 63 - Illustrated Pocket Dictionary of Chromatography

58 DRY PACKING

dry packing The dry packing technique is used to pack non-wetted
packing material into columns (compare with slurry packing). Dry
packing is commonly used for packed-bed GC columns, precolumns
in HPLC, and medium- and low-pressure LC columns. To effectively
pack these columns, the column is tamped or vibrated in order to
completely settle the material.

dual-beam detector Dual-beam detectors can be dual beam in
space or time. Regardless of how the beam is split, the background
signal is compensated for and the analyte signal is continuously
corrected for the background noise. This results in lower overall
signal noise.

dual-channel detection Used when a detector cannot provide
positive compound identification/confirmation by itself. For example,
a flame ionization detector produces a retention time for identifica-
tion but in a complex mixture many compounds could coelute. To
create positive identity/confirmation, the effluent from the column can
either be split to two detectors or go through a series of nondestruc-
tive detectors. The second detector should respond to a different
characteristic of the analyte. Some detectors provide strong dual con-
firmation. For example, GC/MS provides both retention time and mass
information simultaneously.

dwell time The time required for a mixed solvent to leave the
gradient mixing chamber and arrive at the head of the column.

dwell volume The volume in an HPLC system between the
gradient mixing chamber and the head of the column. If the dwell
volume varies considerably between systems, then the analyte elution
time will be affected. This is because the actual gradient profile (at
the same flow rate) reaches the head of the column later in a system
with a larger dwell volume. In essence, this has effectively increased
the initial “hold” time for the gradient and all elution time will be
longer under these conditions.
dynamic range The range of analyte concentration that a detec-
tor gives a changing output. This does not mean that the response
must be linear. As shown in the diagram, the linear and working
ranges of the detector for the analyte are subsets of the dynamic
range.