Oxygen-flask combustion technique

The simultaneous analysis of orthophosphate, glycerol phosphates, and inositol phosphates has been achieved by spectrophotometric analysis of the molybdovanadate complexes. Also, a sensitive and selective chemiluminescent molecular emission method for the estimation of phosphorus and sulphur is described, which is based on passing solutions into a cool, reducing, nitrogen-hydrogen diffusion flame. For organic compounds it was usually necessary to prepare test solutions by an oxygen-flask combustion technique. 

Traces of chlorine have been determined in polyolefins [17] at levels between 0 and 500 ppm. The Schoniger oxygen flask combustion technique requires a 0.1 g sample and the use of a 1 litre conical flask. Chlorine-free PE foil is used to wrap the sample, which is then supported on a platinum wire attached to the flask stopper. Water is used as the absorbent. Combustion takes place at atmospheric pressure in oxygen. The chloride formed is potentiometrically titrated in nitric acid/acetone medium with 0.01 M silver nitrate solution. 

Haslam et af have adopted the oxygen flask combustion technique to the qualitative detection in polymers of nitrogen, chlorine, bromine, iodine, fluorine, phosphorus and sulphur in amounts down to 0.25%, (see Method 104). 

With a little experience the oxygen-flask combustion technique is capable of a very satisfactory degree of precision. The reproducibility available will, of course, depend upon the determination used to complete the method. As an example to indicate the precision which is obtainable the case of pheniodol may be quoted. In 24 determinations made at intervals over a period of two to three weeks, results ranging from 98 7 to 99-2 per cent with a standard deviation of 0161 were obtained. 

The oxygen-flask combustion technique is also applicable to other elements than those mentioned above. Methods for phosphorus, arsenic and boron have been described and these are discussed in a general review of the topic published by Macdonald. ... 

For an idea of some historic and more recent applications of electrochemical techniques, the reader is referred to Table 2.11 of applications providing examples of applications of electrochemical analytical techniques to elemental determinations in a variety of materials. Shearer and Morris (1970) report on the microdetermination of fluorine in organic compounds with a fluoride ion electrode following an oxygen flask combustion based on the careful work of Morris. Dabeka et al. (1979) developed a microdiffusion and fluoride-specific electrode determination of fluoride in foods. Determination of lead and cadmium in foods by anodic stripping voltammetry ... 

The present article will cover the development of the methodology since 1995, with its focus on the direction taken by development, analytical techniques coupled with oxygen flask combustion, new areas of application, and future areas of development. 

Procedures based on separation techniques such as HPLC and IC have been developed for single element analysis for the following two reasons. The first reason is to remove interferents in complicated sample matrices that can give rise to incorrect results, in particular for trace analysis in samples with a high organic content, such as the determination of total iodine in egg products. The second reason is to differentiate the total and free forms of a specific element, such as the determination of the free iodide ion and bounded iodine in food additives. The free iodide ion is determined by direct sample injection into the IC column, whereas the total iodine content is determined after oxygen flask combustion. Thus, both the free and bounded forms of iodine in food samples can be determined. 

The following three analytical techniques have been developed for multielement determination after oxygen flask combustion inductively coupled plasma (ICP)-atomic emission spectrometry (AES), IC, and RNAA. ICP-AES is mainly used for trace metal analysis and RNAA for multiple isotope determination in inorganic materials. From the summary of new procedures developed since 1995 using the oxygen flask method for elemental analysis listed in Table 1, it is clear that IC is the major analytical technique behind the recent development of multielement determination for oxygen flask combustion, in particular for organic samples. 

Ion chromatography has been successfully applied to the quantitative analysis of ions in many diverse types of industrial and environmental samples. The technique has also been valuable for microelemental analysis, e.g. for the determination of sulphur, chlorine, bromine, phosphorus and iodine as heteroatoms in solid samples. Combustion in a Schoniger oxygen flask (Section 3.31 )is a widely used method of degrading such samples, the products of combustion being absorbed in solution as anionic or cationic forms, and the solution then directly injected into the ion chromatograph. 

Combustion in an oxygen flask, commonly called the Schoniger technique, offers advantages when readily volatilized elements such as halogens, Se, S, P, B, Hg, As, or Sb are to be determined. The combustion is performed with oxygen in a sealed container and the reaction products are absorbed in a suitable solvent before the reaction vessel is opened. 

For the determination of sulfur contents of residual fuels a variety of procedures are available. The bomb (ASTM D-129, IP 61) and quartz tube (ASTM D-155, IP 63) combustion methods have long been established. Other, more rapid techniques are becoming increasingly available, including high-temperature combustion (ASTM D-1552), X-ray absorption and fluorescence methods, and the Schoniger oxygen flask procedure. 

Certainly, the best method of sample preparation is the so called combustion technique, which was derived from the Schoni-ger oxygen-flask method, and has been modified and improved in many ways. Besides manual procedures there are now semiauto-mated and fully-automated oxidizer models commercially available. In an oxygen atmosphere biological specimens labelled with H, or can be combusted to tritiated water,... 

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