Varian 450-GC, Руководство пользователя

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Detectors – Pulsed Flame Photometric Detector (PFPD)
Page: 368 User Manual 450-GC Varian, Inc.
Optimizing the PFPD
The PFPD is optimized to maximize:
•
Detectivity of Sulfur or Phosphorus.
•
Selectivity of Sulfur and Phosphorus to Carbon or each other.
•
Analytical throughput (shorten total analysis time).
Detectivity
Detectivity is the minimum detectable quantity (MDQ) of Sulfur or Phosphorus
(usually expressed as picograms of Sulfur (or Phosphorus) per second.
Detectivity is the preferred method of expressing the sensitivity of a detection
method because it includes a consideration of the Signal-to-Noise ratio.
Sensitivity alone is insufficient in that the sensitivity of the PFPD can be
increased simply by increasing the voltage of the photomultiplier tube, but the
noise increases as the signal increases, yielding little enhancement in the
minimum amount detectable.
Selectivity
An important question when optimizing the selectivity of the PFPD is, "Selective
relative to what?" In some situations, the Sulfur or Phosphorus response is
best optimized relative to Carbon (the principal background emission in the air-
hydrogen flame). At other times, it may be more important to optimize the
Phosphorus response relative to the Sulfur response, as when Phosphorus is
analyzed in certain plant tissue extracts that are high in Sulfur content.
Throughput
The additional selectivity of the PFPD can result in shorter analysis times and
thereby greater sample throughput. As an example, by optimizing the sulfur
response relative to hydrocarbons, the PFPD may detect fewer peaks thereby
simplifying the chromatogram and increasing the chromatographic resolution.
With greater resolution, the chromatography can be adjusted such that the
peaks of interest elute faster thereby decreasing the analysis time. In addition,
when hydrogen is used as the carrier gas, the PFPD can be operated at higher
flow rates without greatly affecting the detectivity of the method. Higher carrier
gas flow rates translates into shorter analysis times and greater sample
throughput.