Phosphorus interference, elimination

The Ge(Li) gamma-ray spectrum of thermal neutron irradiated cigarette tobacco as obtained in our laboratory is shown in Fig. 8. In this case a 50 day decay period has eliminated the major portion of interferences from sodium, potassium and phosphorus activities. Assignments of photopeaks in this figure may be regarded as tentative, since half-lives of the individual peaks were not followed. As many as fifteen elements have been determined in tobacco products and biological standard kale... 

According to K. Fischer, the presence of potassium chlorate in match compositions greatly interferes with the testing for phosphorus, and it is advisable to wash the sample several times with cold water to eliminate the chlorate, before distilling. If subsUnces soluble in water are to be determined, the two tests can be comtuned. The washing with water can be avoided by using tartaric acid for the distillation instead of sulphuric acid the presence of potassium chlorate does not then influence the phosphorus test. 

The data in Tables 34-1 through 34-4 show that the accuracy obtainable in the determination of an element is greatly dependent on the nature and complexity of the substrate. Thus, the relative error in the determination of phosphorus in two phosphate rocks was 1.1 % in a synthetic mixture, it was only 0.27%. The relative error in an iron determination in a refractory was 7.8% in a manganese bronze having about the same iron content, it was only 1.8%. In this example, the limiting factor in the accuracy was not in the completion step but rather in the dissolution of the samples and the elimination of interferences. 

Interference by phosphate ion is an especially common problem. Since the phosphate ion reacts like silica to form a yellow phosphomolybdic acid, its interference must be eliminated. Numerous techniques have been proposed, either for separating the silica and phosphorus before analysis or preferentially reducing silicomolybdic acid to molybdenum blue in the presence of the phosphomolybdic acid (311-313). Snell and Snell (314) summarized the possible procedures (a) precipitating and removing phosphate as the calcium salt. (6) adjusting pH so only silica will form the yellow color, (c) destroying the yellow phosphate complex with citric, oxalic, or tartaric acids, and d) preferentially reducing the silicomolybdic acid to molybdenum blue. 

There are modifications of the above procedure which do not require the complete separation of phosphorus from silica, but depend on the preferential reduction of silicomolybdic acid to molybdenum blue in the presence of phosphomolybdic acid. In general, it is emphasized that dependable results rely on careful avoidance of contamination of the sample with silica from glassware, elimination of interference by phosphorus, and maximum sensitivity in making the colorimetric measurement (409-411). Baumann described a preferred technique for analysis of silica in blood and other biological materials (see Chapter 1, Ref. 320). 

Special procedures for eliminating interference by phosphorus in biological samples analyzed by the molybdate method were developed by McGavack, Leslie, and Kao (412) and by King (413). 

MS. Oxygen was used as the reaction gas in the collision/reaction cell to produce PO" by reacting with phosphorus in the gas phase, thereby effectively eliminating the interferences for phosphorus that are normally seen at m/z 31. 

In this methcxl the total phosphorus, in a filtered or unfiltered sample, is mineralized by evaporating the sample to fumes with perchloric acid. We have had no experimental evidence to indicate that any form of combined phosphorus in sea water can withstand this treatment without being mineralized and brought into solution. The method is taken, basically, from the procedure described by Hansen and Robinson (/. Marine Res., 12 31, 1953) but there have been several major modifications designed to give a better blank determination, to facilitate the routine use of the method, and to eliminate any interference from arsenate. 

The effect of various interfering ions on the molybdate complex must also be considered. Only a very small amount of nitrate can be tolerated, and the permissible chloride concentration depends upon the amount of HCl removed during preparation of the reducing solution (Andersson [1962a]). Molybdate complexes with sulphuric acid when the latter is present in large amounts, and under these conditions, decomposition of the silicomolybdate must occur to a certain extent. Since the heteromolybdates of arsenic and phosphorus are less stable than the corresponding silicomolybdates, interference by these elements can, to some extent, be eliminated by use of polybasic organic acids such as oxalic and tartatic, which form complexes with molybdate. Interference by phosphate, however, cannot be eliminated completely. 

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