Single particle inductively coupled plasma mass spectrometry for the characterization of colloidal particles in soils, sediments and sludges: comparative study of sector field and time-of-flight instruments

Abstract

Colloids and nanoparticles in solid phase environmental matrices (soils, sediments, sludges) are widely heterogenous and polydisperse, which complicates their sampling and characterization by bulk analysis techniques. Indeed, techniques based upon single particle measurements are better equipped for identifying important, but low frequency, properties or characteristics which are needed to understand the function of environmental colloids. In this study, a continuous flow extraction assisted by ultrasound was used to sample colloidal particles from several solid matrices. The high sensitivity of a sector field ICP-MS and the quasi-instantaneous, multi-isotope measurements of a time-of-flight ICP-MS were combined to enable the characterization of colloidal particles extracted from soils, sediments and sludges. Single particle (SP) analysis of the particle leachates using the sector field instrument (SP-ICP-SF-MS) led to the detection of larger numbers (up to 6800×) of Mg-, Al-, Si-, Ca-, Ti-, Fe-, and Ba-containing particles than measured by single particle time-of-flight ICP-MS (SP-ICP-ToF-MS), largely due to the different size detection limits of the techniques, i.e. ca. 16 nm by SP-ICP-SF-MS and 76 nm by SP-ICP-ToF-MS, when measuring aluminosilicates. Despite the limitation of SP-ICP-ToF-MS in detecting smaller particles, the technique was successfully used to identify mineral phases of illite, vermiculite, and smectite based on elemental ratios in the individual particles. The multi-isotope capability of the SP-ICP-ToF-MS was also used for the determination of isotopic ratios in both individual particles and bulk digested leachates. Mean ²⁰⁶Pb/²⁰⁷Pb ratios in the particles extracted from the solid phase samples deviated from measurements obtained from bulk digestions by 1.2–5.9%, indicating the potential of the SP-ICP-ToF-MS to perform such measurements. SP-ICP-SF-MS and SP-ICP-ToF-MS were complementary for obtaining insight into the composition and particle size distributions of the colloids and nanoparticles. Specifically, neither technique gave the complete particle size distribution due to their complementary size detection windows.