The well-known toxic effects of arsenic make the need for g sensitive trace analysis method imperative. Typical environmental monitoring programs for arsenic generate large numbers of samples. A wide range of other elements can be expected along with arsenic, and so the method must be adaptable to the varying compositions likely to exist with different types of materials to be tested. Therefore. the procedure must minimize the number of interfering elements. The levels of arsenic in the various samples of interest may run from tenths of a part per million to several hundred parts per million. Hence, to allow use of reasonably sized samples the method must be sensitive and linear over a range from a microgram or less up to several hundred micrograms.
Traces of arsenic are most frequently measured by the classical Gutzeit method or a modi~cation~. After appropriate digestion, the arsenic is converted to arsine and distilled from solution. In the original semiquantitative approach, the arsine is trapped on paper strips coated with a mercury(II) halide. The extent of the staining of the paper caused by the unknown is visually compared to known samples. The Marsh test is based on distillation of the arsine into a heated capillary tube. The arsine is then decomposed to give a shiny mirror of arsenic, the length of which is compared to standards. A popular Gutzeit modification gives more quantitative results. The amine is trapped in sodium hypobromite solution and determined by the molybdenum blue method. Another spectrophotometric method involves trapping arsine in silver diethyldithiocarbamate and reading The arsenic may be converted to arsenic(II1) chloride and distilled from an acid solution'. These procedures all are quite lengthy and require considerable care to eliminate interferences and especially to avoid losses of the volatile arsenic compound during the generation, distillation, and trapping.
Instrumental approaches for traces of arsenic include emission spectroscopy3, atomic absorption*, and X-ray fluorescence. The last mentioned has been used in several ways for traces of arsenic. In one procedure", after pretreatment arsenic is extracted with pyrrolidine dithiocarbamate in chloroform. The chloroform solution is then evaporated on filter paper and arsenic determined by X-ray fluorescence. The extraction is not tbo selective; lead directly interferes and large quantities of some other metals affect results by simple absorption of the arsenic Koc X-rays.
Luke6 proposed a preconcentration approach for arsenic and other elements based on precipitation with diethyldithiocarbamate with copper(I1) as a coprecipitant. The precipitate is collected on filter paper and counted b X-ray fluorescence. Complexing agents are used to remove some interfering elements. In the same report