| Summary: | The increased use of silica and silicon-containing nanoparticles (Si-NP) in agricultural applications has stimulated interest in determining their potential migration in the environment and their uptake by living organisms. Understanding the fate and behavior of Si-NPs will require their accurate analysis and characterization in very complex environmental matrices. In this study, we investigated Si-NP analysis in soil using single-particle ICP-MS. A magnetic sector instrument was operated at medium resolution to overcome the impact of polyatomic interferences (e.g., <sup>14</sup>N<sup>14</sup>N and <sup>12</sup>C<sup>16</sup>O) on <sup>28</sup>Si determinations. Consequently, a size detection limit of 29 ± 3 nm (diameter of spherical SiO<sub>2</sub> NP) was achieved in Milli-Q water. Si-NP were extracted from agricultural soil using several extractants, including Ca(NO<sub>3</sub>)<sub>2</sub>, Mg(NO<sub>3</sub>)<sub>2</sub>, BaCl<sub>2</sub>, NaNO<sub>3</sub>, Na<sub>4</sub>P<sub>2</sub>O<sub>7</sub>, fulvic acid (FA) and Na<sub>2</sub>H<sub>2</sub>EDTA. The best extraction efficiency was found for Na<sub>4</sub>P<sub>2</sub>O<sub>7,</sub> for which the size distribution of Si-NP in the leachates was well preserved for at least one month. On the other hand, Ca(NO<sub>3</sub>)<sub>2</sub>, Mg(NO<sub>3</sub>)<sub>2</sub> and BaCl<sub>2</sub> were relatively less effective and generally led to particle agglomeration. A time-of-flight ICP-MS was also used to examine the nature of the extracted Si-NP on a single-particle basis. Aluminosilicates accounted for the greatest number of extracted NP (~46%), followed by NP where Si was the only detected metal (presumably SiO<sub>2</sub>, ~30%).
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