Characterization and Quantification of Natural and Anthropogenic Titanium-Containing Particles Using Single-Particle ICP-TOFMS

Abstract

Titanium-containing nanoparticles (NPs) and submicrometer particles (μPs) in the environment can come from natural or anthropogenic sources. In this study, we investigate the use of single-particle inductively coupled plasma time-of-flight mass spectrometry (spICP-TOFMS) to measure and classify individual Ti-containing particles as either engineered (Ti-eng) or naturally occurring (Ti-nat) based on elemental composition and multielement mass ratios. We analyze mixtures of four Ti-containing particle types: anthropogenic food-grade TiO2 particles and particles from rutile, ilmenite, and biotite mineral samples. Through characterization of neat particle suspensions, we develop a decision-tree-based classification scheme to distinguish Ti-eng from Ti-nat particles and to classify individual Ti-nat particles by mineral type. Engineered TiO2 and rutile particles have the same major-element composition. To distinguish Ti-eng particles from rutile, we developed particle-type detection limits based on the average crustal abundance ratio of titanium to niobium. For our measurements, the average Ti mass needed to classify Ti-eng particles is 9.3 fg, which corresponds to a diameter of 211 nm for TiO2. From neat suspensions, we demonstrate classification rates of 55%, 32%, 75%, and 72% for Ti-eng, rutile, ilmenite, and biotite particles, respectively. Our classification approach minimizes false-positive classifications, with rates below 5% for all particle types. Individual Ti-eng particles can be accurately classified at the submicron size range, while the Ti-nat particles are classified in the nanoregime (diameter < 100 nm). Efficacy of our classification approach is demonstrated through the analysis of controlled mixtures of Ti-eng and Ti-nat and the analysis of natural streamwater spiked with Ti-eng particles. In control mixtures, Ti-eng particles can be measured and classified at particle-number concentrations (PNCs) 60-times lower than that of Ti-nat particles and across a PNC range of at least 3 orders of magnitude. In the streamwater sample, Ti-eng particles are classified at environmentally relevant PNCs that are 44-times lower than the background Ti-nat PNC and 2850-times lower than the total PNC.

Classification of zirconium-rich engineered and natural nano particles using single particle ICP-TOFMS

“…61 This makes the accurate quantification of ENPs a difficult task, especially when the engineered and natural particles have similar elemental compositions. 45,49 Here, we test our classification scheme of Zr-NPs, by mixing Zr-eng and Zr-nat NPs at different PNCs, measuring the NP mixtures with spICP-TOFMS, and then classifying the NPs. We provide results of our determined number concentrations of each NP type ( i.e.…”

“…The fine powder obtained from ball milling was then processed using a previously established method. 45 In short, ∼10 mg of fine powder was dispersed in 15 mL of ultrapure water (18.2 MΩ cm PURELAB flex, Elga LabWater, UK). The dispersed suspension was water bath sonicated (VWR Ultrasonic Cleaner, VWR, PA, USA) for 10 minutes, followed by 30 seconds of vortexing and 10 minutes of settling time.…”

“…39–41 spICP-TOFMS has been widely used for classification of natural and anthropogenic nanoparticles (NPs) and microparticles (μPs). In the past, spICP-TOFMS has been used to classify particles rich in titanium, 44–46 zinc, 47 cerium, 48–50 and platinum-group metals using classification strategies such as machine learning and decision trees. 51,52 Despite zirconium's abundance in the Earth's crust and the ubiquity of Zr particles in our environment, their classification by spICP-TOFMS remains largely unexplored.…”

Introducing “time-of-flight single particle investigator” (TOF-SPI): a tool for quantitative spICP-TOFMS data analysis

“…In our studies, we often use Cs as the uptake standard because it is water soluble and not typically found as a particulate in routine sample types, such as freshwater. 15 After finding droplet-derived signals, the average intensities and absolute sensitivities of all analyte elements in the calibrant microdroplets are determined; histograms of analyte signals from microdroplets are provided in Fig. 2b.…”

“…1–3 Single-particle ICP-TOFMS has been used broadly for the analysis of NPs or μPs from environmental samples, 4–7 and for the classification naturally occurring and anthropogenic particles types based on recorded multi-element compositions. 8–15 To date, many spICP-TOFMS studies rely on manufacturer-provided software to identify particle-derived signals, quantify element mass amounts per particle, and determine particle number concentrations (PNCs). While these software can be fit for purpose, 16 we have found that current software options are often not flexible enough for users to adapt analysis procedures in accordance with the newest research findings or measurement approaches.…”