Hydrogen flame emission detection

More recent flame photometric methods rely on direct measurement of the phosphorus emission. If organophosphorus compounds are injected into a hydrogen flame, a continuous emission is obtained in the 490-650 nm region. A broad band system, with an intensity maximum at 526 nm, is superimposed on this background139 it is attributed to the HPO species formed in the flame. An early determination of phosphorus at 0.01-0.04 m concentrations was based on examination of the continuous emission standard and sample solutions were injected into the burner and the intensities were measured at 540 nm the calibration graph was linear down to the detection limit of 10 4 M phosphorus sodium or calcium, if present in the sample, interfered with the results140. 

The most popular thermionic detector (TID) is the nitrogen-phosphorus detector (NPD). The NPD is specific for compounds containing nitrogen or phosphorus. The detector uses a thermionic emission source in the form of a bead or cylinder composed of a ceramic material impregnated with an alkyl-metal. The sample impinges on the electrically heated and now molten potassium and rubidium metal salts of the active element. Samples which contain N or P are ionized and the resulting current measured. In this mode, the detector is usually operated at 600-800°C with hydrogen flows about 10 times less than those used for flame-ionization detection (FID). 

The amount of thermal radiation (heat) emitted from a hydrogen flame is low and is hard to detect by feeling (low emissivity). Most commercially available combustible gas detectors can be calibrated for hydrogen detection. Typically alarms from these sensors are set by the manufacturer between 10%-50% of the lower flammability limit (TFT) of hydrogen to avoid the presence of an unwanted flammable envi ronment. 

Gas chromatography [15], flame photometry [9] and sometimes flame photometry in combination with gas chromatography are used for the detection of sulphur dioxide and other gaseous sulphur compounds. The gas is burned in a hydrogen flame, and the sulphur emission line at 394 nm is measured. 

Here, the emission occurs in the blue with maxima at 384 and 394 nm. The chemiluminescent intensity is proportional to the concentration of the excited sulfur dimer. Similarly, combustion of phosphorus compounds in a hydrogen flame gives emission due to HPO at, 526 nm. Linear working curves over four decades of concentration are reported. Both of these flame chemiluminescence techniques have been employed for detection of sulfur and phosphorus species in the effluent from gas chromatographic columns. 

Several other methods have been reported for the analysis of tetraalkylleads in gasoline using gas chromatography. The methods of detection included an electron capture detector >a flame-emission detector, and a hydrogen-rich flame ionization detector. An atomic-absorption spectrometer has been used as a detector for GC391. 

Tin compounds are converted to the corresponding volatile hydride (SnHi, CHj, SnHj, (CH3)2 SnHj and (CH3)3SnH) by reaction with sodium borohydride at pH6.5 followed by separation of the hydrides by gas chromatography and then detection by atomic absorption spectroscopy using a hydrogen-rich hydrogen-air flame emission type detector (Sn-H band). The apparatus used is shown in Figs. 15.9-15.11. 

Total sulfur in air, most of which is sulfur dioxide, can be measured by burning the sample in a hydrogen-rich flame and measuring the blue chemiluminescent emission from sulfur atom combination to excited S2 (313). Concentrations of about 0.01 ppm can be detected. 

It solely operates on the principle of photon emission. If P- or S-containing hydrocarbons are ignited in a hydrogen-rich flame, it gives rise to chemiluminescent species spontaneously which may subsequently be detected by a suitably photomultiplier device. Hence, FPD is regarded as a specific detector for P- or S-containing compounds. 

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