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DETECTORS Flame Photometric Detector - FPD

Overview
The Flame Photometric Detector is similar to the FID in that the sample exits the analytical column into a hydrogen diffusion flame. Where the FID measures ions produced by organic compounds during combustion, the FPD analyzes the spectrum of light emitted FPD/FID by the compounds as they luminesce in the flame. The detector detector chambermust be light tight so that only light from the flame will be “seen” by the photomultiplier tube (PMT) and analyzed. The FPD uses a second hydrogen flow to purge the optical path between the PMT and the hydrogen diffusion flame. This second hydrogen flow helps to augment the FPD sensitivity by making the flame hydrogen rich. The FPD also uses a FPD detector equipped with an FID collector secondair flow directed across of the face of FID collector the PMT to prevent helium and/or hydrogen Exhaust electrode molecules from permeating the PMT’s glass tube Ignitor window and causing malfunction. This purge air is vented to atmosphere through a short tube, PMT coiled to prevent light from reaching the PMT. housing Bandpass The FPD uses one of two available band pass air purge filter and outletfilters to selectively detect compounds PMT containing sulfur or phosphorus. Compounds containing phosphorus are detectable with the 526nm filter, which is yellow on one side. The 394nm filter (blue on one side) allows detection of sulfur-containing compounds. While not completely selective, the FPD is 100,000 times Hydrogen & more sensitive to sulfurous and phosphorous air supplies compoundsthan it is to hydrocarbons. Sulfur compounds like H2S or SO2 can be detected Exhaust down to about 200ppb; phosphorus cap nut compounds can be detected down to 10ppb. To detect phosphorus and sulfur at the same time, the Dual FPD, featuring two PMTs, may be used. The single or dual FPD may be equipped with an FID collector electrode and electrometer which will detect the hydrocarbon peaks as the PMTsare responding to the sulfur and phophorus compounds. Because the hydrogen-rich flame required for optimum sulfur and phosphorus detection is not optimum for the best hydrocarbon response, the FID in combination FID collector electrode with the FPD is less sensitive than a pure FID response.

DETECTORS Flame Photometric Detector - FPD

Theory of Operation
The FPD uses one of two availableband pass filters over a photomultiplier tube (PMT) to selectively detect compounds containing sulfur or phosphorus as they combust in the hydrogen flame. When compounds are burned in the FPD flame, they emit photons of distinct wavelengths. Only those photons that are within the frequency range of the filter specifications can pass through the filter to the PMT. The PMT converts the photons it“sees” through the bandpass filter to an analog signal, which is acquired by the Peak Simple data system.

Simplified FPD Schematic
Exhaust tube

Jet Column effluent
Photons

Ignitor blade Ignitor

Ferrule Bandpass filter over PMT window (394nm for sulfur, 526nm for phosphorus) Purge air outlet

Primary H2 inlet to jet Secondary H2 inlet into PMT housing Purge air inlet Stainless steelretaining nut

Air inlet

H2

Rubber o-ring

Photomultiplier tube (PMT) Photomultiplier signal cable to amplifier

Split TeflonTM ferrule 10-pin socket

DETECTORS Flame Photometric Detector - FPD Expected Performance

FPD Noise Run (FID/FPD Combo)
Column: 15m MXT-1 Carrier: Helium @ 10mL/min FPD gain: HIGH FPD temp: 150oC FPD PMT volts: -400 FPD H2: 60mL/min (30mL/min for each of thetwo hydrogen flows) FPD air: 100mL/min

FPD noise averages less than 100µV from peak to peak

FPD Sulfur (FID/FPD Combo)
Sample: 1cc 10ppm H2S in 3 replicate injections to demonstrate the consistency of the FPD response Column: 15m MXT-1 Carrier: Helium @ 10mL/min FPD gain: HIGH; FPD temp: 150oC FPD PMT volts: -400 FPD H2: 60mL/min (30mL/min for each of the two hydrogen flows) FPD air:...
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