Underestimation of chromium and zirconium in soils by hydrofluoric acid digestion and inductively coupled plasma-mass spectrometry

ABSTRACT: The concentrations of chromium (Cr) and zirconium (Zr) in 503 soil samples collected from 78 site in Japan were determined by using two utterly different methods. The first was the relatively widely used inductively coupled plasma-mass spectrometry (ICP-MS) combined with digestion with hydrofluoric acid (HF) and perchloric acid (HClO₄). The second was a non-destructive polarizing energy dispersive X-ray fluorescence spectrometric method (EDXRF) using pressed powder pellets. The Cr concentrations obtained by EDXRF were higher than those obtained by ICP-MS in ~75% of the samples. The EDXRF/ICP-MS ratios ranged from 0.48 to 9.65, and the geometric mean was 1.23. A typical example having big a difference in Cr content between the two methods was found in seven samples taken from a podzolic soil developed on sand dunes in northern Hokkaido. This sampling site is around 20 km from the northern edge of a serpentinite area that extends from north to south, forming the backbone mountain range of Hokkaido. The Cr concentrations obtained by ICP-MS for the seven samples were 27.9–52.2 mg kg⁻¹, similar to the average for all the samples, whereas those obtained by EDXRF were 101–460 mg kg⁻¹, falling within the range between the third quartile and the maximum for all the samples. It would be difficult to predict these results from the analytical data of other elements because the samples were high in silica (SiO₂) and low in magnesia (MgO), and did not show any characteristics typical of soils derived from serpentinite. X-ray adsorption near-edge structure (XANES) showed that Zr occurred as zircon (ZrSiO₄) in soils. The differences between the two methods were greater for Zr than for Cr. Zirconium concentrations obtained by EDXRF were higher than those by ICP-MS for all samples except three. The difference between the two methods was more pronounced in soils derived from granite that is widespread in western Japan. The EDXRF/ICP-MS ratios for seven soil samples from different horizons at a single sampling site ranged from 4.82 to 6.94 and were generally higher at the A horizon (Apg and Ag) and decreased with depth. Our results indicate that there is a high possibility of underestimation of Cr and Zr contents in soils for a considerable number of samples if determined with ICP-MS, and it could also overlook some important chemical characteristics of soils.