Determination of the Nanoscale Silica Mass Fraction by AF4/ICP-MS with Isotope Dilution Analysis Using ²⁹Si-Enriched...

A methodology based on the use of asymmetrical flow field-flow fractionation (AF4) coupled to ICP-MS with size fraction-targeted isotope dilution analysis (IDA) has been developed, validated, and applied for the first time to determine the mass fraction of nanoscale silica (SiO₂). For this purpose, ²⁹Si-enriched SiO₂ nanoparticles, to be used as an IDA spike/internal standard, were synthesized and characterized in-house. Double IDA was used to quantify an aqueous suspension of Stöber silica particles of similar characteristics to those of the ²⁹SiO₂ nanoparticle (NP) spike using a representative test material of natural Si isotopic composition as the calibrant. For fumed SiO₂ NP in a highly complex food matrix, a methodology based on single IDA with AF4/ICP-MS using the same ²⁹SiO₂ NP spike was developed and validated. Relative expanded measurement uncertainties (k = 2) of 4% (double IDA) and 8% (single IDA) were achieved for nanoscale silica mass fractions of 5143 and 107 mg kg^-1 in water suspension and food matrix, respectively. To assess the accuracy of AF4/ICP-IDMS for the characterization of SiO₂ NP in a food matrix, standard addition measurements on samples spiked with Aerosil AF200, also in-house characterized for Si mass fraction, were undertaken, with an average recovery of 95.6 ± 4.1% (RSD, n = 3) obtained. The particle-specific IDA data obtained for both SiO₂ NP-containing samples were also compared with that of post-AF4 channel external calibration using inorganic Si standards. The mass fractions obtained by IDA agreed well with those obtained by external calibration within their associated measurement uncertainties.