Determination of trace iron by electrothemal vaporization/low-pressure helium ICP-MS

Although ICP-MS offers a powerful multielement analytical method, the argon gas severely interferes with the determination of iron because of the formation of large background spectra, A combined method of electrothermal vaporization (ETV) and low-pressure helium-ICP-MS was therefore employed to overcome this problem. Mass spectral interferences, however, still occurred at m/z of 54 and 56, which were assigned to (ArN+)-Ar-40-N-14 and (ArO+)-Ar-40-O-16, respectively. We found that the polyatomic interferences were caused by leaks of the ambient air through the glass ETV chamber. In the present work, the chamber was newly designed and constructed from aluminum with specially prepared vacuum parts. A 5 mul volume of the sample was placed on a tungsten filament and heated at 2.6 A for 100 s to remove the solvent. After evacuating the chamber to 5 torr, the filament was heated to 2500 degreesC by the discharge of a high-capacity condenser (0.27 F) for evaporating the sample. The resulting plume was transported to the plasma torch with a stream of carrier gas for the determination of the most abundant isotope, Fe-56(+). The relative standard deviation for the signal intensity (50 pg of Fe, n = 9) was approximately 13%. The detection limit based on 3 sigma was 0.8 ng ml(-1), which was 25-times lower compared with the conventional argon ICP-MS.