Alternative digestion strategy for Ti, Zr and Hf oxides: eliminating hydrofluoric acid

Group IV metal oxides have a broad impact on the environment and human health due to their diverse applications in industry, consumer products and biomedicine. However, their chemical inertness poses significant challenges for accurate quantification in biological matrices, which is essential for assessing biodistribution, toxicity, and regulatory compliance. Traditional digestion methods often rely on hydrofluoric acid (HF), a hazardous reagent requiring specialized handling and infrastructure. Here, we present an alternative, HF-free microwave assisted digestion protocol for group IV metal oxides in biological contexts, utilizing sulfuric acid/water/hydrogen peroxide mixtures to achieve complete solubilization across nano-to microscale particles. The method's efficacy was evaluated on various commercially available TiO₂, ZrO₂, and HfO₂ powders. Optimization of digestion parameters, including acid-to-peroxide ratios, temperature, and reaction time, led to recoveries exceeding 90% for all tested materials. Notably, higher temperatures and extended digestion times were required for larger particles and higher atomic number oxides, reflecting the increased metal-oxygen bond dissociation energies. The method's applicability was further demonstrated through successful quantification of spiked nanoparticles in human cancer cells and bovine liver tissue, with detection limits down to ∼1 ppb and achieving recoveries within 80-100%, maintaining sample stability over four weeks. Comparative analysis with HF-based digestion revealed comparable sensitivity and detection limits using inductively coupled plasma optical emission spectrometry (ICP-OES), with the HF-free method offering a safer and more accessible alternative for routine laboratory analysis. This validated protocol facilitates accurate quantification of group IV metal oxides in complex biological matrices, supporting preclinical and clinical studies while mitigating the risks associated with HF usage.