Page 156 - Illustrated Pocket Dictionary of Chromatography

156 PUMP HEAD

Shown is an HPLC dual piston pump. Note the two pump heads that house sepa-
rate pistons. The purpose of the dual piston system is to (1) allow for gradient gen-
eration and (2) produce a low-pulse flow.

pump head A finely machined block (often 316 stainless steel)
that consists of a threaded port for setting inlet and outlet check
valves, the interior chamber that “stages” mobile phase, a bored hole
that allows for passage of the piston into and out from the piston head,
and a recessed area for the piston seal. See pump.

purge and trap, P & T A dynamic gas-liquid extraction technique
that is used before analysis by GC. A sample is extracted by continu-
ously passing a constant flow gas through it. The volatile components
are purged from the sample and then are trapped on a strong adsor-
bent material. Adsorption is often enhanced by cooling the trapping
adsorbent. Once the sample is purged, then the trap is rapidly heated
and the effluent is directed onto the GC column where separation
takes place. The P & T method allows for extremely low level detec-
tion (low parts per billion to high part per trillion) and is frequently
the method of choice for environmental volatile analysis.
purifier Tank or cylinder gases can often be the source of a wide
range of impurities: oils, carbon dioxide, oxygen, water. To remove