Trace Level Detection And Quantification Of Crystalline Silica In An Amorphous Silica Matrix With Natural Abundance Si-29 Nmr

A protocol for the detection of trace amounts of quartz in amorphous silica gels by NMR spectroscopy was developed and tested on commercially available samples. Using natural abundance Si-29 MAS NMR spectroscopy with CPMG acquisition and standard addition of crystalline quartz, quantitative detection of quartz concentrations down to 0.1% wt. was achieved. CPMG permitted to suppress the amorphous silica-derived signal, benefitting from the extremely long T-2 relaxation time of quartz in Si-29 and hence dramatically increasing the sensitivity. Dedicated post-processing exploiting the known CPMG spikelet frequencies allowed to probe the near-absence of quartz in commercial, 100% silica samples, enabling assessment of conformity of unknown samples to EU legislation (REACH).