Flow injection methods selective precipitation

Environmental samples offer a challenge to the analytical chemist because of the matrices involved. These include, among others, fresh- and seawater, sediments, marine and biological specimens, soil, and the atmosphere. For determining trace concentrations of vanadium in these complex matrices, preconcentration and separation techniques may be required prior to instrumental analysis. Hirayama et al. [14] summarize the various preconcentration and separation techniques including chelation, extraction, precipitation, coprecipitation, ion exchange in conjunction with the instrumental method of spectrometry, densitometry, flow injection, NAA, AAS, X-ray fluorescence, and inductively coupled plasma atomic emission spectrometry (ICPAES). While NAA offers great sensitivity and selectivity, its application is limited by the number of research reactors available worldwide. 

The delayed split method uses the same configuration as the dynamic split except for the addition of an on/off valve in the vent line [171-173]. With the split vent valve in the off position, the sample is injected in the normal way at a selected pressure. After a prescribed delay time the injection valve is returned to the load position and the vent valve opened, which aifects an immediate purge of the injection area. The amount of sample transferred into the column depends on the valve actuation time and the delay time. The delayed split injection techniques can be modified for solvent backflushing [171]. Immediately after the vent valve is opened a rapid negative pressure ramp is initiated. This causes a reversal of flow in the first part of the column and precipitation of the sample on the column wall. The solvent, and probably some of the sample, is backflushed out of the column and through the open vent line. The injection repeatability is similar to timed split injection. 

A potentiometric determination of saccharin was proposed by Fatibello-Filho et al. [86]. In this method, saccharin was potentiometrically measured using a silver wire coated with a mercury film as the working electrode. With this, the main difficulty was the presence of a precipitate (mercurous saccharinate) that could adsorb on tube walls and the electrode surface. To avoid these undesirable effects, a relocatable filter unit was placed before the flow-through potentiometric cell and a surfactant was added to the carrier solution (Figure 24.12). The same investigation team reported the construction and analytical evaluation of a tubular ion-selective electrode coated with an ion pair formed between saccharinate anion and toluidine blue O cation incorporated on a poly(vinyl chloride) matrix [87]. This electrode was constructed and adapted in a FIA system. The optimum experimental conditions found were an analytical path of 120 cm, an injection sample volume of 500 pL, a pH of 2.5, a flow rate of 2.3 mL/min, and a tubular electrode length of 2.5 cm. 

Up to now selective catalytic reduction has been the completely dominant method of flue gas NOx treatment. In this method ammonia is injected in the flue gas in the presence of a catalyst, commonly titanium oxide based, to reduce NO and NO2 to nitrogen and water. The catalyst can be placed in different positions in the flue gas flow, the important factor being that conditions such as the flue gas temperature are optimal (usually 300-400 C). The positions used for catalyst are high dust, where the catalyst is placed between the economiser and the air preheater low dust, with the catalyst situated after a hot gas precipitator and before the air pre-heater and tail end, with the catalyst situated after the desulphurisation plant (see Figure 1). The most widely used position worldwide is high dust, where untreated flue gas containing sulphur dioxide and particulates passes through the catalyst. 

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