Single particle detection by ICP-MS: From particles via ion clouds to signals

Engineered nanomaterials (ENMs), a major product of the rapid development of nanotechnology over the last couple of decades, have found numerous applications in consumer products of daily life, yet also in medical applications and treatments, leading to significant release into the environment. Analytical methods are required both to characterize ENMs with respect to control their production, but also to investigate their biological effects and their fate in the environment.

During the last decade, inductively coupled plasma-mass spectrometry (ICP-MS) has proven to be a valuable analytical tool, providing some characteristic information. As a highly sensitive technique for multielement determination, ICP-MS can be used to provide information about the composition of such materials. Even further, it could be shown that ICP-MS can provide information about the size of nanoparticles (NPs), when used in the so called "single-particle detection" mode. In this mode, the frequency of measurements is enhanced to 1000 measurements per second or higher. When such many measurements are performed with a very short dwell time below one ms, every measurement window may access only a single particle, when highly diluted suspensions are introduced into the ICP (see figure 1).

Figure 1: Conventional steady-state and single-particle detection mode for the ICP-MS

Instead of a steady-state signal, proportional to the concentration of the analyte in a solution, under such conditions of very short dwell times, the ICP-MS generates single pulses for each particle entering the plasma, where the signal height is proportional to the number of atoms contained in the particle.

In order to fundamentally understand the process involved in single-particle signal generation and the parameters that might have an influence on the working range, detection power and accuracy, the group of researchers from the Universities of Münster and Antwerp investigated the processes taking place within the ICP. There selected the approach of both experimental ICP-MS measurements supported by plasma modelling (see figure 2).

Figure 2: Workflow of the study of the ICP-MS signal generation from the introduction of NPs to the ICP both by experimental plasma diagnostic and plasma modelling

For this purpose, a set of nanoparticles with narrow distributions around particle sizes ranging between 15 and 100 nm were synthesized and characterized by established methods. Theses particles were introduced into the ICP and a statistically significant number of short transient events was recorded by the MS being operated with a very short dwell time of 50 µs. Such short measurement time did not only allow to get a signal for each particle, but to follow the ion cloud generation and transport through the plasma into the sampler cone. From these signal profiles, the summed intensity, the maximum intensity and the signal duration were evaluated, of which all three were found to depend on particle size.

The experimental data showed an increase of the summed signal from 10 to 1661 counts for particles between 15.4 to 83.2 nm. For the same particles, the event duration (signal width) increased from 322 to 1007 µs. The simulated data from the plasma model fully supported the trends experimentally observed. Additionally, the simulated data revealed that the plasma temperature, and therefore the point of ionization of the particles within the ICP, is the same for all diameters.

The authors concluded, that their results clearly showed the limitations of unresolved single particle detection and provides information not only for an improved understanding but also might help to improve the method with respect to working range and accuracy.

AnalytikJena PlasmaQuant MS Elite ICP-MS

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last time modified: September 22, 2024