Direct chromium speciation in solid state materials—a GDMS approach

Abstract

Among the various elemental mass spectrometry techniques, glow discharge mass spectrometry is recognized for its ability to provide direct determination of trace elements present in solid state samples. In the present work, a pulsed glow discharge time-of-flight mass spectrometry method is developed for the direct speciation of chromium in solid state samples. The millisecond pulsed glow discharge operated with radio frequency power is a versatile ion source that provides elemental, structural and molecular information. Careful tuning of the operating parameters yields the plasma chemistry that favors cluster ion formation. Cluster ions unique to specific species permit differentiation between the trivalent and hexavalent forms of chromium, (Cr(III) and (Cr(VI)), respectively, in chromium oxide samples. Specifically, signals at 104 and 120 m/z corresponding to the Cr₂⁺ and Cr₂O⁺ cluster ions arise from the presence of Cr(III) in the sample, whereas signal at 100 m/z corresponding to the CrO₃⁺cluster ion arises from the presence of Cr(VI). The impact of glow discharge operating conditions on the appearance of these characteristic cluster ions is discussed.