SOLID-STATE SI-29 NMR DETERMINATION OF CRYSTALLINE SILICA IN NATURAL IRON-OXIDE PIGMENTS

Synthetic iron oxide pigments have well characterized compositions. In natural iron oxide pigments, commercial products contain various amou nts of talc, kaolin, and crystalline silica in addition to the iron ox ide. The analysis for crystalline silica requires removing the iron ox ide by an acid concentration step followed by x-ray diffraction (XRD) evaluation. However some materials not removed by the acid wash may in terfere with quantitative XRD measurements. Another potential drawback of the x-ray analysis is the requirement for the particle size to be on the order of a few micrometers. In principle, nuclear magnetic reso nance (NMR) is sensitive to domain sizes down to the order of a few na nometers. We have utilized solid state NMR techniques to obtain Si-29 spectra from a number of materials to assess its viability as a quanti tative method for measuring crystalline silica in iron oxide pigments. The method employs single pulse excitation with magic angle sample sp inning (MASS). A protocol was established for quantitative silica anal ysis utilizing a reduced pulse angle of 10-degrees with a 20-min relax ation delay between acquisitions. The reduced pulse angle was used to circumvent the long silicon relaxation times (T1) for the crystalline silica species. The spectra were recorded at a measurement frequency o f 59.6 MHz with high power proton decoupling (300.1 MHz). A blind test of silica standards was used for comparison with x-ray diffraction an alyses as a test of the protocol. Quantitative determination of crysta lline silica employed tetrakis(trimethylsilyl)silane as a spin-countin g standard.