G. Kowalczyk et Je. Roberts, SOLID-STATE SI-29 NMR DETERMINATION OF CRYSTALLINE SILICA IN NATURAL IRON-OXIDE PIGMENTS, Analytica chimica acta, 286(1), 1994, pp. 25-35
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.