DETECTORS Flame Photometric Detector - FPD
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DETECTORS Flame Photometric Detector - FPD Overview FPD detector equipped with an FID collector 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 by the compounds as they luminesce in the flame. The detector chamber must 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 second air flow directed across of the face of the PMT to prevent helium and/or hydrogen molecules from permeating the PMT’s glass window and causing malfunction. This purge air is vented to atmosphere through a short tube, coiled to prevent light from reaching the PMT. The FPD uses one of two available band pass filters to selectively detect compounds 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 more sensitive to sulfurous and phosphorous compounds than it is to hydrocarbons. Sulfur compounds like H2S or SO2 can be detected down to about 200ppb; phosphorus 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 PMTs are 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 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 available band 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...

DETECTORS Flame Photometric Detector - FPD Expected Performance FPD Noise Run (FID/FPD Combo) FPD noise averages less than 100µV from peak to peak 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: 100mL/min 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...

DETECTORS Flame Photometric Detector - FPD General Operating Procedure 1. Set the hydrogen flow to 60mL/min. This correlates to a flow of 30mL/min each for the primary and secondary hydrogen. Set the air supply to 100mL/min. The air supply tubing is T’d inside the GC so that 1030mL/min of air flows across the face of the PMT. Set the carrier gas flow between 5 and 20mL/min. 2. Use the switch on the GC’s front control panel to light the FPD flame. Sometimes the flame is difficult to light because of the hydrogen-rich atmosphere inside the FPD detector body. If you are having difficulty...

DETECTORS Flame Photometric Detector - FPD Switching Between Sulfur and Phophorus Modes The bandpass filter specified when the instrument was ordered comes installed in the FPD detector assembly. There are two options for switching between sulfur and phophorus modes. You can purchase either an additional filter or an additional PMT housing and switch them as necessary. Phosphorus wavelength filters are available under SRI part number 8670-0083; sulfur filters are available under SRI part number 8670-0082. A PMT housing with a bandpass filter of the other optional wavelength specifications...

DETECTORS Flame Photometric Detector - FPD Troubleshooting and Maintenance Changing the Photomultiplier Tube (PMT) The Photomultiplier Tube (PMT) is a consumable part, and will eventually need replacement. Additional PMTs are available under SRI part number 8670-0080. 1. Follow steps 1-4 on the “Switching Between Sulfur and Phophorus Modes” page. 2. Unscrew the stainless steel retaining nut and remove it, with the PMT and its socket, from the FPD assembly. Slide the retaining nut down the PMT amplifier lead to access the PMT. 3. Unplug the PMT from its socket. Remove the split TeflonTM...