Flame hydrogen-rich

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. 

The detector is based on the combustion of sulfur-containing compounds in a hydrogen rich air fleuie of a FID to form sulfur monoxide. The hydrogen/air flow rate ratio is the most critical parameter controlling the production of sulfur monoxide. Under optimum conditions sulfur monoxide may account for up to 20% of the sulfur species in the flame. Sulfur monoxide is a free radical and a very reactive species that is short lived however, it can be stabilized in a vacuum, and a ceramic probe under reduced pressure can be used to sample it in the flame and transfer it to... 

Tin compounds are converted to the corresponding volatile hydride (SnH4, CH3 SnH3, (CH3 )2 SnH2, and (CH3 >3 SnH) by reaction with sodium borohydride at pH 6.5 followed by separation of the hydrides and then atomic absorption spectroscopy using a hydrogen-rich hydrogen-air flame emission type detector (Sn-H band). 

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. 

Sulfur Dioxide. Both flame photometric and pulsed fluorescence methods have been applied to the continuous measurement of S02 from aircraft. In the flame photometric detector (FPD), sulfur compounds are reduced in a hydrogen-rich flame to the S2 dimer. The emission resulting from the transition of the thermally excited dimer to its ground state at 394 nm is measured by using a narrow band-pass filter and a photomultiplier tube. 

In a hydrogen-rich flame, combustion of samples containing phosphorus and/or sulfur results in the formation of chemiluminescent species which emit light characteristic of the heteroatom introduced into the flame. Selection of an interference filter with a 394- or 526-nm bandpass allows selectivities for sulfur and phosphorus respectively. Recent work by Krost and co-workers (27) found that a 690-nm filter showed selectivity for some nitrogen-containing compounds. 

Concentration Profiles. The relative fluorescence intensity profiles for OH, S2, SH, SO, and SO2 were converted to absolute number densities according to the method already outlined. Resulting concentration profiles for a rich, sulfur bearing flame are exhibited in Figure 17. H-atom densities were calculated from the measured OH concentrations and H2 and H2O equilibrium values for each flame according to Equation 6. Similar balanced radical reactions were used to calculate H2S and S concentrations 6). Although sulfur was added as H2S to this hydrogen rich flame, the dominant sulfur product at early times in the post flame gas is S02 ... 

Interpretation. The data for hydrogen rich flames is most satisfactorily explained by invoking laser induced reactions between the excited states of the alkali metal and H2O and H2 which constitute the major flame species. 

This detector burns the column eftiuent in a hydrogen-rich (reducing) flame optical emission is monitored by photomultiplier tubes through int o ce filt. A filt of394nm makes the detector almost totally specific for sitiphur, while a filter of526 nm confers selectivity for phosphorus. For example, with a 394 nm filter the sulphur phosphorus response ratio is 10 000 1, phosphorus sulphur response ratio is 10 1. 

The FPD is based on a German patent describing the emission obtained with phosphorus and sulfur compounds in a hydrogen-rich flame (64). Brody and Chaney developed this analytical method into a detector for gas chromatographic eflSuents (65) and predicted (correctly) its development in the years to come. Today, Tracor, Inc., markets it as Melpar flame photometric detector in single- and double-channel versions. 

If the hydrogen combustion reaction is conducted under conditions that result in high temperatures (flame temperature of 2488 Kelvin) and an excess of air, the excess oxygen will react with the nitrogen of the air to produce small quantities of nitric oxide. In hydrogen rich mixtures, excess hydrogen reacts with nitrogen to form trace amounts of ammonia. 

Chemiluminescence from S- or P-containing compounds is obtained by combustion in a hydrogen-rich flame. Both the excited HPO radicals formed and the Sj dimers can emit a chemiluminescence spectrum which allows the selective detection of S (eg, at 394 nm) and P (526 nm) using suitable filters and a photomultiplier tube for signal amplification [26]. This flame photometric detector shows a non-linear realtionship between concentration and output signal in the sulfur mode which in theory should be quadratic (owing to the dimer formation). However, in practive, exponential coefficients of between 1 and 2 are found. Electronic linearization of the output signal is therefore necessary. 

In contrast to the oxygen-rich flame of FIDs, the FPD uses a hydrogen-rich flame which is cooler. This enhances production of the two reactive species of interest, HPO and S2, which give off the characteristic emissions at 526 and 394 nm, respectively. The mechanisms for formation of HPO are not fully understood, however, the detector response for phosphorus containing compounds is linear. In contrast, response to sulphur containing compounds varies as the square (approximately) of the concentration. 

Flame photometric detector FPD, a selective GC detector for sulphur and phosphorus containing compounds. Separated components pass into a hydrogen-rich flame where they undergo a series of reactions to produce excited species HPO and S2. The resulting atomic emission spectrum is monitored using narrow band pass filters (526 and 394 nm, respectively) and a photomultiplier detector, sensitivity is 10 to 10 " gs . ... 

The response mechanism of the FPD to sulfur- and phosphorous-containing compounds is known superficially, even if the finer details remain obscure. In the relatively low temperature (< 1000°C) and hydrogen-rich FPD flames, sulfur-containing compounds are decomposed and interconverted by a large set of bimolecular reactions to species, such as H2S, HS, S, S2, SO and SO2 in relative proportions that depend on the instantaneous and fluctuating flame chemistry. In the presence of carbon radicals in the flame, various carbon-sulfur containing sjrecies might also be anticipated. Excited state 2 species could result from several two or three body collision reactions, such as those shown below... 

Thus hydrogen rich fuels should be avoided in purple flames. Metal fuels and charcoal would be desirable fuels, and Dechlorane should be desirable as a chlorine donor. Douda suggested that high flame temperatures might be useful because monohydroxides are probably less temperature stable than monochlorides. 

The significance of quantitative determinations of kf in such studies is limited by uncertainties concerning the relative contributions of various collision partners, M -. No systematic effort to separate these effects has been reported. In a recent study of profiles in one hydrogen-rich flame, the mean value of kf at 600 K for the particular gas composition was estimated to be 3 x 10 cm mole sec . Similarly, concentration and temperature measurements within the primary zone of a hydrogen-lean flame yielded (1000-1350 K) = 1 x W cm mole" sec, with assumed to be HgO. Both values are in reasonable agreement with those in Table 2.2, if allowance is made for the estimated influence of composition. 

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