Not All Nanoparticle Analysis Challenges are Created Equal

The detection, quantitation, and characterization of nanoparticles using inductively coupled plasma–mass spectrometry (ICP-MS), and in particular using single-particle ICP-MS (SP-ICP-MS), has developed significantly in recent years. However, the difficulties involved in this type of analysis vary, depending on the composition of the nanoparticles. Martín Resano of the University of Zaragoza, together with colleagues from Ghent University, has recently developed a method for characterizing nanoparticles made from silicon dioxide (Si0₂), which are much more challenging to detect than those made from silver or gold. He recently spoke to us about this work.

You developed a new method for dealing with spectral interference in the ultratrace analysis of SiO₂ nanoparticles (1) using ICP-MS/MS. First, why was a new method needed?

For a long time, most of the work carried out on single-particle (SP)-ICP-MS had been targeting elements that were easily determined using ICP-MS, elements providing good sensitivity and not suffering from spectral overlaps, such as Ag and Au. However, there are many other types of nanoparticles (NPs) that need to be characterized as well, and now that the technique of SP-ICP-MS is more mature, it is only logical to investigate these more challenging NPs. And certainly, SiO₂ NPs are challenging. Si shows a relatively high ionization potential, a low mass, its monitoring is affected by the occurrence of severe polyatomic interferences and it is an element that brings about contamination issues. It is difficult to find a more problematic element, so it is also a good test to see what can and can´t be done with SP-ICP-MS. 

Why is ICP-MS/MS the best technique for this analysis? And specifically, why is it preferable to using sector-field ICP-MS instrumentation, which is often a good approach for dealing with spectral interferences?

Indeed, sector-field ICP-MS is well-established. However, ICP-MS/MS has brought some interesting features. First of all, a quadrupole-based device is always more robust and typically deals better with organic samples compared to a sector-field instrument. Moreover, the use of chemical reactions in gas phase makes it possible to overcome some overlaps that can’t be solved by means of sector-field ICP-MS, most notably isobaric interferences. That does not mean that ICP-MS/MS is always the best solution, but it does widen the potential of ICP-MS.

You evaluated three different modes of analysis using single-particle ICP-MS/MS: no gas, or “vented” mode; He kinetic energy discrimination (KED) mode; and chemical resolution modes using H₂, O₂, NH₃, and CH₃F. Could you briefly explain these modes, and why you wanted to consider each one for this type of analysis?

One of the advantages of using an ICP-MS/MS instrument is that you do not need to always use double mass selection (MS/MS mode). You can actually evaluate whether this is really necessary or not. Basically, you have a first quadrupole that you can use as a mass filter (resolution of 1 amu) or leave it fully open. In the latter case, the instrument will operate as a more conventional device, with a collision­–reaction cell and just one quadrupole. Then, of course, you can evaluate different gases in that cell for various purposes: collisions (for example, using He in KED mode), reactions with the interfering ions (measuring your analyte “on mass”, as in this case is done with H₂), or reactions with the analyte, with the aim of forming a new species that can be monitored free from interferences (in an approach called mass shift, for which gases like NH₃, CH₃F and O₂ are useful). The latter is a very powerful approach, but first, you can check if it is really needed. If a more straightforward approach is fit for purpose, then there is no need to complicate the method.

I must say that Eduardo Bolea-Fernández has done a whole PhD on this topic at Ghent University under the supervision of Frank Vanhaecke, Lieve Balcaen, and myself. He has published a recent review in which he explains all this in detail (2).

Which approach did you determine was best, and why?

We initially thought that using ICP-MS/MS and CH₃F as a reaction gas to form SiF⁺ would be the best approach. And such an approach worked well and provided the best LODs for trace determination of Si. But using H₂ and monitoring Si on mass actually enabled us to detect smaller NPs.

You also applied a deconvolution step. Can you briefly explain the deconvolution method and why it was used?

It is just a mathematical algorithm that enables the decomposition of overlapping peaks into their separate components. It is necessary because the background signal affecting Si in SP-ICP-MS mode is always high, despite the use of ICP-MS/MS. And that hampers the characterization of NPs (of 100 nm and below), as their distribution overlaps with the BG distribution. One of the reasons is the contamination problem I mentioned before.

What results were you able to obtain?

For trace determination of Si, a very low LOD is attained (below 15 ng L^-1). For SiO₂ NPs, it is possible to characterize them down to a diameter of 80 nm.

What is your next step in this work?

When the experimental work was carried out, the minimum dwell time for ICP-MS/MS instrumentation was 3 ms. That is rather high in comparison with other quadrupole devices. Nowadays, the situation has improved and it is possible to use only 100 µs. That helps in minimizing the background contribution, such that characterizing smaller SiO₂ NPs is already feasible. 

References

• E. Bolea-Fernandez, D. Leite, A. Rua-Ibarz, L. Balcaen, M. Aramendía, M. Resano, and F. Vanhaecke, J. Anal. At. Spectrom.32, 2140–2152 (2017). DOI: 10.1039/c7ja00138j.

• E. Bolea-Fernandez, L. Balcaen, M. Resano, and F. Vanhaecke, J. Anal. At. Spectrom.32, 1660–1679 (2017). DOI: 10.1039/c7ja00010c.

Martin Resano, PhD, of the University of Zaragoza

Martín Resano obtained his PhD in Analytical Chemistry in 1999. After conducting post-doctoral research at Ghent University in Belgium, he returned to the University of Zaragoza, where he is now a professor of analytical chemistry. There, he leads the MARTE research group, devoted to investigation of the basic capabilities of high-resolution-continuum source atomic absorption spectrometry and inductively coupled plasma–mass spectrometry for trace and isotopic analysis, with a focus on direct solid sampling, biomedical analyses, and characterization of nanoparticles. He has coauthored more than 100 publications on these topics. He currently chairs the editorial board of the Journal of Analytical Atomic Spectroscopy.