Detector Systems
Performance considerations
Contamination
Very little contamination can create serious NPD problems. Common sources include:
Columns and/or glass wool treated with H PO (phosphoric acid)
Phosphate•containingdetergents
Cyano•substitutedsilicone columns (XE•60,OV•225,etc.)
Other nitrogen•containingliquid phases
Any liquid phase deactivated for analysis of basic compounds
Fingerprints
Leak•detectionfluids
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.
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