Oxygen selective detectors

The influence of other active components, such as 1, OH, H on a semiconductor sensor, with other conditions being the same, is comparable with the influence of atomic oxygen [50]. Contribution of N and OH is proportional to their relative contents (compared to that of atomic oxygen) in the atmosphere and may become essential at altitudes lower than 60 - 70 km. The use of selective detectors excludes the influence of atomic hydrogen. Studies of adsorption of water vapours on ZnO films [50] show that their influence is negligibly small at the film temperatures below 100°C. Variations of electric conductivity of the films under the influence of water vapours and of an atomic oxygen are comparable at the ratio of their concentrations [H20]/[0] = 10" . 

Confirmation with a second column of a dissimilar polarity and a second, selective detector is necessary for the correct identification and quantitation of BTEX and oxygenated additives. And yet, even when a dissimilar column and a second PID are used in confirmation analysis, false positive detection of MTBE often takes place. 

This section has been included because it is a good example of the use of selective detectors in petroleum analysis, where the need often arises to analyze for small amounts of heterocompounds in a complex hydrocarbon matrix. Another good example is the use of the flame photometric detector for the determination of small concentrations of sulfur compounds in various plant streams. The analysis of oxygenated compounds can, however, be carried out in a totally different manner using column switching instead of the selective detector. 

The oxygen-specific detector (O-FID) uses two reactors. At first, hydrocarbons are decomposed into carbon, hydrogen and carbon monoxide at above 1300 C. CO is then converted into methane in the hydrogenation reactor and detected with the FID. With the O-FID, for example, oxygen-containing components in fuels can be selectively detected (Schneider et al., 1982). 

D 5599 Test Method for Determination of Oxygenates in Gasoline by Gas Chromatography and Oxygen Selective Flame Ionization Detector ... 

Compared with the flame ionisation detector, however, the ECD is more specialised and tends to be chosen for its selectivity which can simplify chromatograms. The ECD requires careful attention to obtain reliable results. Cleanliness is essential and the carrier gases must be very pure and dry. The two most likely impurities in these gases are water and oxygen which are sufficiently electronegative to produce a detector response and so give a noisy baseline. 

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