Abstract
Four different methods of digestion were examined for biological, environmental and geological materials in order to determine arsenic by the method of arsine evolution-flameless atomic absorption spectrophotometry described in a previous report. The methods studied were: A) digestion with acids in a Teflon container kept in a stainless bomb at 150°C for 3 h, B) digestion with acids under reflux in a quartz flask for 3 h, C) Kjeldahl digestion with a mixture of HNO3/(1+1)H2SO4 in a quartz flask up to SO3 fuming, and D) digestion with acids in a Teflon (or conical glass) beaker with cover on a hot-plate (150°C) for 3 h. Examination of arsenic retention during digestions by using a standard solution of As(III) proved that arsenic was not lost in the methods A), B), C) and D) with a mixture of HNO3-H2SO4-HF-KMnO4, while in the method D) significant loss of arsenic was found when no KMnO4 was added. Digestion of biological samples by the method C) gave reproducible results and good recoveries in standard addition experiments. It is, however, noteworthy that the methods A) and B) gave lower values suggesting incomplete decomposition of biological materials. Methods B), C) and also D) combined with HF-treatment were examined for geological materials, and it was found that the method C) gave reasonable arsenic data although some siliceous residues remained, but in the method B) complete extraction of arsenic was difficult when diluted HCl or concentrated HNO3 was used as the extractant. The method D) employing a mixture of HNO3-H2SO4-HF-KMnO4 as the digesting solution also gave the arsenic values similar to those obtained by the method C). However, it is unfavorable in this method that a large blank value is caused by contaminations by arsenic in the reagents added and that glass and quartz wares can not be used in the subsequent procedures because of corrosions by the hydrofluoric acid. Arsenic in eleven standard reference samples were analyzed by the Kjeldahl digestion-flameless atomic absorption spectrophotometry procedures, and it was concluded that arsenic contents observed in present study were in good agreement with those certified by NBS, and those reported by IAEA and other authors.
