A team of scientists, including several affiliated with The Ohio State University, has made significant progress in understanding how to more accurately measure nanoparticles and microparticles using cutting-edge mass spectrometry techniques. Their findings were published in the Journal of Analytical Atomic Spectrometry and offer new insights into the limitations and challenges of single particle inductively coupled plasma-mass spectrometry (spICP-MS), particularly when analyzing particles of varying sizes.

The study was led by Byrd Center Principal Investigator Dr. John W. Olesik (research scientist and adjunct associate professor), and included two graduate students in his research group, Dr. Madeleine Lomax-Vogt (now a postdoc at the University of Vienna) and Byrd Center graduate student Lucas M. Carter, as well as Byrd Researcher Dr. Stanislav Kutuzov. Their work contributes to the growing capabilities of spICP-MS, a tool increasingly used in environmental science, materials research, and nanotechnology.

This study focuses on how different-sized particles behave during transport and detection using two types of mass spectrometers: ICP-Quadrupole MS (ICP-QMS) and ICP-Time-of-Flight MS (ICP-TOFMS). These instruments are used to measure the size and composition of tiny particles, some smaller than a virus and others large enough to be considered fine dust. spICP-TOFMS can measure hundreds of thousands of individual particles in a small volume of sample suspension (<5 mL) in minutes.

Traditionally, spICP-MS has been used to measure nanoparticles, which are smaller than 100 nanometers in diameter. The transport efficiency is the percentage of particles that make it through the instrument’s sample introduction system and then into the high temperature Ar plasma where they are vaporized, atomized, and ionized. The resulting ions are separated in the mass spectrometer based on their mass/charge to identify the elements in each particle and to measure the mass of each element in each particle.

Different sized nanoparticles and elements in solution have the same transport efficiency. However, the team found that for larger particles, known as microparticles (larger than 100 nanometers in diameter), this efficiency decreases significantly as size (and their mass) increases. This means that without adjustments, the particle number concentration (# particles/mL) of larger particles may not be measured accurately.

To study these effects, researchers measured engineered gold (Au) and silicon dioxide (SiO₂) particles ranging in size from 60 to 5000 nanometers. They discovered that for particles larger than about 600 to 800 nanometers, transport through the instrument becomes size (mass) dependent. Furthermore, particles that are too large produce huge signals that can exceed the instrument's "linear dynamic range," the range in which signal strength is proportional to particle mass, leading to inaccurate measurements of each particle’s mass (and size).  Prior to this study, particles that produced signals that were not proportional to their mass were erroneously assumed to be incompletely vaporized in the plasma.

To address the challenge of limited linear dynamic range, the researchers experimented with reducing the instrument's sensitivity. Doing so allowed them to extend the upper measurement range and confirm that even particles as large as 5000 nanometers could be fully vaporized and analyzed. However, reducing sensitivity also raised the lower size limit of what could be accurately measured, creating a tradeoff between detecting small and large particles.

This research helps understand the limitations of current instruments and shows how to better tailor measurement methods for a wide range of particle sizes. This is a step toward making nanoparticle and microparticle analysis more accurate and accessible in real-world applications.

As interest grows in understanding the effects of engineered particles in the environment and industry, this research provides a roadmap for improving how scientists collect and interpret data using spICP-MS techniques.

The team has and continues to conduct additional studies focused on detection limits and background signal interference, critical for analyzing complex samples.

This research highlights the research community's technical innovations and reflects the importance of interdisciplinary collaboration in solving complex scientific problems.

Learn more about "Challenges in measuring nanoparticles and microparticles by single particle ICP-QMS and ICP-TOFMS: size-dependent transport efficiency and limited linear dynamic range."

This article is part of the themed collection: Fast Transient Signals – Getting the most out of Multidimensional Data