Comparison of chemical modifiers for the determination of vanadium in water and oil samples by electrothermal atomization atomic absorption spectrometry

Abstract

Chemical modifiers (isomorphous metals and other compounds such as NH₄SCN and ascorbic acid) for the determination of vanadium are described. Ascorbic acid (100 µg) eliminated the interferences from NaCl, CaCl₂ and FeCl₃ salts. The mass of the metallic modifiers and the maximum permissible pyrolysis temperature (t_pyr) have been carefully optimized. Magnesium nitrate (20 µg), rhodium (2 µg) and platinum (2.5 µg) increased t_pyr from 1100 °C, without modifiers, to 1500, 1700 and 1600 °C, respectively. The characteristic mass without modifiers was 21.2 pg, whereas in the presence of 20 µg of Mg(NO₃)₂, 1 µg of Rh and 1 µg of Pt, the characteristic mass was 20.5 pg, 14.7 pg and 14.4 pg, respectively, measuring the peak absorbance with a moderately used graphite tube (around 100 atomization cycles). The LOD was generally 0.5 µg l^–1 without chemical modifier and the same with 1 µg of Pt. The efficiency of the above metals as chemical modifiers was tested in the analysis of oil standards diluted with isobutyl methyl ketone (IBMK). It was found that 1 µg of Pt increased the t_pyr from 1000 °C to 1400 °C and decreased the characteristic mass from 27.5 to 15.3 pg, measuring the peak absorbance with a new graphite tube. Magnesium nitrate was inadequate for this type of sample. The ageing of the graphite tube surface affected the vanadium determination. A slow drift in sensitivity appeared with increasing atomization cycles. This was controlled with periodic recalibration using integrated absorbance measurements.