Plasma ashing in electrostatically shielded chamber for determination of arsenic oxides

Determination of arsenic oxides after plasma ashing of organic materials encountered with a difficulty, i.e., significantly low recovery of the oxides. A modified plasma apparatus having an electrostatically shielded chamber has been employed to overcome this difficulty: The organic materials were essentially oxidized by atomic oxygen without violent bombardments of energetic electrons and ions moving in the glow discharge region. A known amount of sodium arsenate {1530 mg As(V)} impregnated in 200 mg cellulose powder was plasma ashed and the ash residue was subjected to an ion chromatography. Nearly quantitative recovery was obtained with the modified apparatus while the recovery was approximately 60% when the electrostatic shield was eliminated. Arsenite ion was practically insensitive to the ion chromatography. However, when the arsenite-containing cellulose powder {4050 mg As(III)} was plasma ashed without the electrostatic shield, a recovery of about 60% was obtained as the form of arsenate. On the otherhand, when the same arsenite-containing cellulose powder was processed with the electrostatic shield, the recovery of arsenate was only 7%. Further oxidation of the latter ash material with hydrogen peroxide gave the quantitative recovery as the arsenate. Samples containing very low concentration of arsenate yielded somewhat low recoveries even with the use of electrostatic shield, but the recoveries were significantly raised by the addition of sodium carbonate as a fixative.