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6, 11), led to variable recoveries and low values for most elements (Table IV). Hydrofluoric acid has been used for the dissolution of geologic samples (6). There has also been a report of HF use with some biological samples, particularly when silicon analyses are needed (3). However, when HF was added to our system, unacceptable amounts of particulate matter remained after digestion of the fecal samples; there were poor recoveries of most elements except for phosphorous and zinc. Digestions seemed to be more complete with nitric acid-hydrogen peroxide or nitric-perchloric acid combinations. However, some elements, including iron and copper, were poorly recovered when these combinations were used.We have successfully used our method to digest a wide range of diet, food, and fecal samples with differing fat contents. Traditional open flask digestion procedures (9) generally require up to 20 mL of a 1:5 perchloric-nitric mixture and 8-13 h to digest a 0.5-g sample of either freeze-dried feces or diet in an open 50-mL Erlynmeyer flask on a hot plate.Compared with this open-flask, hot-plate procedure, the microwave procedure offers considerable advantages in speed and safety. Blank values are lower because less acid is required and the sample is not exposed to a hood environment for long periods of time. The closed system maintains sample integrity well and thus permits the determination of more volatile elements. In a preliminary study, we obtained 97-105% recoveries of added boron by using the closed microwave digestion method, whereas boron recoveries were nil when conventional nitric-perchloric digestion techniques (9) were used. Thus, the microwave digestion method is relatively safe, simple, and rapid with good precision and accuracy. This procedure seems to be particularly suited for providing digested samples for elemental determinations by ICP-ES.

Phosphorus Is determined directly In acid digests of copperbased alloys by using direct-current plasma emission spectrometry. The spectral overlap Interference of copper and Iron emission on the most sensitive phosphorus lines Is eliminated by Ion-exchange separation carried out In the sample flow