NH4NO3 extractable trace element contents of soil samples prepared for proficiency testing--a stability study

Area of Science:

• Environmental Science
• Analytical Chemistry
• Soil Science

Background:

• Accurate trace element analysis in soil is vital for environmental monitoring and regulatory compliance.
• Soil samples are used as reference materials and for proficiency testing, requiring validated stability protocols.
• Standardized extraction methods, like using 1 mol/L ammonium nitrate (NH4NO3), are employed to assess bioavailable trace elements.

Purpose of the Study:

• To evaluate the stability of extractable trace elements in soil samples under various storage conditions.
• To determine the impact of temperature and sterilization on the long-term integrity of soil reference materials.
• To establish optimal storage guidelines for soil samples intended for trace element analysis.

Main Methods:

• Soil samples were subjected to extraction with 1 mol/L NH4NO3 solution, following DIN 19730.
• Sterilized and non-sterilized samples were stored at -20°C, +4°C, +20°C, and +40°C for 18 months.
• Extractable concentrations of Cd, Cr, Cu, Ni, Pb, and Zn were quantified using ICP-AES and ETAAS at regular intervals.

Main Results:

• Trace element extractability was significantly affected by storage temperature, with higher temperatures (>+20°C) causing degradation.
• Thermal degradation of soil organic matter likely contributed to changes in metal extractability, particularly for Cr and Cu.
• Sterilization was found to be unnecessary for dry soil samples, as microbial activity was negligible.

Conclusions:

• Appropriate storage conditions, specifically avoiding temperatures above +20°C, are essential for maintaining soil sample integrity for trace element analysis.
• Dry soil samples do not require sterilization for trace element studies due to minimal microbiological impact.
• The findings provide critical data for developing robust protocols for soil reference materials and proficiency testing schemes.