HP 5890 Series II, Справочное руководство, Страница: 132 / 244

Detector Systems
Nitrogen-phosphorus detector (NPD)
132
Performance considerations
Contamination
Very little contamination can create serious NPD problems. Common
sources include:
C
Columns and/or glass wool treated with H
2
PO
3
(phosphoric acid)
C
Phosphate•containingdetergents
C
Cyano•substitutedsilicone columns (XE•60,OV•225,etc.)
C
Other nitrogen•containingliquid phases
C
Any liquid phase deactivated for analysis of basic compounds
C
Fingerprints
C
Leak•detectionfluids
C
Laboratory air
Contamination in the NPD can be either a positive type or a negative
type of contamination. A positive type of contamination is one that gives
a more positive offset than would normally result with a clean system.
When this happens there is a tendency to try to operate the detector with
too little power to the active element, with the final result that the
detector does not appear as sensitive as desirable (because the
temperature of the active element is actually less than normal).
If the contamination is a negative type of contamination, then there is a
tendency to
quench the reactions that give rise to the specific response
desired. This quenching can be only partial, resulting in less sensitivity
than expected, or at very high contamination levels, a complete
quenching of all signals from the detector. This latter type of quenching is
(usually) only seen on sample peaks where the apex of the peak actually
goes back down toward the baseline.