Measurement of gold and sulfur mass fractions in l-cysteine-modified gold nanoparticles by ICP-DRC-MS after acid digestion: validation and uncertainty of results

Abstract

Measurement of mass fractions of trace element sulfur and main element gold in L-cysteine-modified gold nanoparticles was performed by inductively coupled plasma mass spectrometry with dynamic reaction cell (DRC) technology after aqua regia digestion. Sulfur concentration was measured at m/z 48 as ³²S¹⁶O⁺ in a reaction cell with O₂ to reduce the effect of polyatomic interferences at m/z 32. Following a study of the main interferences affecting the signal from ³²S¹⁶O⁺, optimization of instrument parameters by an experimental design along with an investigation of the effect of aqua regia in matrix on the signal-to-background ratio of ³²S¹⁶O⁺ were carried out to improve the detection limit for sulfur. Under the optimum experimental conditions the detection limits were 1.43 μg L⁻¹ for S and 0.033 μg L⁻¹ for Au. ⁴⁵Sc was selected as the internal standard for measurement of ³²S¹⁶O⁺. The presence of Au at high concentrations showed a negligible effect on the intensity of ³²S¹⁶O⁺ and ⁴⁵Sc⁺ under DRC mode. The method was in-house validated and the combined uncertainty was estimated according to the Guide to the Expression of Uncertainty in Measurement. Preliminary studies show that the method may be transferred to other ICP-MS systems by re-optimizing cell gas flow (e.g. 1.8 mL min⁻¹ used for NexION 300D). X-ray photoelectron spectroscopy confirmed that a self-assembled monolayer of cysteine was formed on the surface of colloidal gold nanoparticles via sulfur-to-gold bonding.