Traditionally, liquid samples containing particulate residues were subjected to proper acid digestion before they are analyzed with inductively coupled plasma mass spectrometry, or ICP-MS — a practice that was common until recently.
Through this method, the ICP-MS data, thus recorded, indicate the bulk particle population. Degueldre demonstrated in 2003 that individual particles can also be quantitatively detected with ICP-MS and subsequently introduced the concept of single-particle (sp)ICP-MS.1
In the spICP-MS technique, the mass of recorded elements in individual particles as well as total particle number concentration is measured. The technique provides much lower detection limits (< µg particles/kg) when compared to other methods. If the shape and density are known, the size of a separate particle can be calculated from the recorded mass. A single particle generates the ICP-MS signal, which is extremely short in duration — that is, fractions of a millisecond.
A scanning mass analyzer (for example, magnetic or quadrupole sector) does not help in recording an entire sweep of elements from this fleeting signal, and measurements generally target only one or two elements inside the particles. By contrast, a time-of-flight mass analyzer, which rapidly and concurrently determines all the elements, makes it possible to determine the entire multi-element composition of particles.
Currently, spICP-MS is widely used for characterizing inorganic nanoparticles and for analyzing their effect on the environment2 as well as living systems3; however, it is not necessarily restricted to these fields. For instance, another interesting application of the spICP-MS technique is the analysis of individual nanoparticles and microparticles in ambient aerosols.4
How Does Single-Particle ICP-MS Work?
Single-particle ICP-MS analysis involves two major needs:
- A dwell/integration time of less than 2 ms is used to operate the mass analyzer and thus view individual particle detection events
- The sample’s particle number concentration is extremely low to reduce the prospect of introducing multiple particles into the ICP-MS at the same time
It is possible to use almost any kind of liquid sample introduction system, with some being more efficient for ionization and particle transport than others. Based on the MS hardware configuration, the particle suspension is often diluted to the concentration of 105–106 particles/mL. Once the number of particles in the sample is sufficiently low, only a single particle will enter the ICP at a time. After a particle reaches the plasma, it gets vaporized, atomized, and ionized, creating a cloud of elemental ions.
The ions, thus produced, are transmitted from the ICP toward the mass analyzer via a pressure-reduction interface that reconciles the difference in pressure between the low pressure (for example, 10-6 mbar) mass analyzer and the atmospheric-pressure ICP. Ions are efficiently transmitted to the mass analyzer using ion optics. Using electric and/or magnetic fields, the mass analyzer separates the ions based on their mass-to-charge ratio (m/Q) before striking a detector. The resultant data indicate the number of ions that were recorded at each m/Q, which, in turn, can be used for determining an ion’s elemental identity, and the number of ions for determining the concentration of elements.
The cloud of elemental ions — produced from a single particle in the ICP source — creates an extremely fast transient signal (signal spike), with an overall duration of just a fraction of a millisecond. Therefore, to detect these ions, the mass analyzer should be able to make an extremely rapid measurement. As mentioned before, TOF mass analyzers are capable of recording the whole mass spectrum (all m/Q values) for individual particles, unlike scanning analyzers that normally target just one or two elements.
For any recorded isotope (m/Q value), the overall ion signal seen during the duration of the transient particle signal is relative to the mass of that element inside the particle. Detected by the ICP-MS method, the frequency of particle events (transient signal spikes) is relative to the particle number concentration in the introduced liquid sample. The concentration of the sample fraction present in dissolved form is represented by continuous signal regions that lack spikes, that is, single-particle detection events.
To make sure that signals from single particles are present in the recorded mass spectral data, it is important to operate the mass analyzer with short dwell/integration times.5 When there is an increase in dwell/integration times, the number of recorded events containing the total signal of two or more consecutively sampled particles also increases, and therefore, the results are biased.
The achieved signal-to-noise ratio (SNR) is also increased when data are acquired with high-temporal resolution — co-integration of less noise (particle-free data) with the particle translates to better SNR and lower size detection limit. Size detection limits that can be achieved with the spICP-MS method are specific to isotopes and are usually in the range of 10 nm to a few hundreds of nanometers.
Proper calibration is required to convert the recorded signal intensity to element mass and also to convert the frequency of particle events to particle number concentration. Although calibration based on reference particles can be done quite easily, it is not readily relevant because of the lack of these materials. Therefore, Pace et al.6 recommended an alternative calibration process using element standard solutions as well as a protocol for establishing the efficiency of particle transport and the efficiency of detection. Although this method is being used in several analytical laboratories, a wide range of calibration concepts has also been defined in the literature.7
The most ICP-MS-compatible solvent for single-particle analysis is ultra-pure water, which provides the best detection limits. Conversely, ultra-pure water is not viable for all systems. It is also possible to perform single-particle analysis in more intricate matrices either after diluting the sample8 or after extracting the particles.9
Multi-Element Single-Particle ICP-MS
Single-particle analysis utilizing ICP-MS with sector-field or quadrupole mass analyzers is restricted to basic systems, like single-element oxide or metal particles. This is because such mass analyzers are only capable of recording the signal of just one or two isotopes during the short time of the particle detection event. On the other hand, time-of-flight mass analyzers, like the one employed in the TOFWERK icpTOF, are capable of recording the signals of all isotopes for all individual particles.
Therefore, TOF-based instruments can define the multi-element composition of particles, apart from reporting the element mass and number concentration. This special capability of TOF-based instruments can be used for analyzing composite nanoparticles because applications like these are expanding rapidly. In addition, simple and pristine particles are usually subjected to compositional transformations after they are exposed to an adverse environment, which can possibly alter their interaction pathways and behavior. These processes can be analyzed using multi-element single-particle ICP-MS.
Due to increased and rapid production of nanoparticles, there are growing concerns with regards to their negative effect on the living system, including humans, and on the environment. Despite this fact, the effect of the concentrations of natural particles of analogous composition is greater when compared to the concentrations of engineered nanomaterials that are already released into the environment.
It is important to detect these manufactured particles to estimate their future impact. However, it is very difficult to identify a low concentration analyte in an intricate background. One potential approach recently proposed to this problem is to fingerprint individual particles using multi-element spICP-MS analysis. For instance, engineered CeO2 nanoparticles in the high background of natural Ce-containing particles in soils were successfully detected with this method.2
- Degueldre, C. and P.Y. Favarger, Colloid analysis by single particle inductively coupled plasma-mass spectroscopy: a feasibility study. Colloids Surf., A, 2003. 217(1-3): p. 137-142.
- Praetorius, A., et al., Single-particle multi-element fingerprinting (spMEF) using inductively-coupled plasma time-of-flight mass spectrometry (ICP-TOFMS) to identify engineered nanoparticles against the elevated natural background in soils. Environ. Sci.: Nano, 2017. 4(2): p. 307-314.
- Scanlan, L.D., et al., Silver Nanowire Exposure Results in Internalization and Toxicity to Daphnia magna. ACS Nano, 2013. 7(12): p. 10681-10694.
- Suzuki, Y., et al., Real-time monitoring and determination of Pb in a single airborne nanoparticle. Journal of Analytical Atomic Spectrometry, 2010. 25(7): p. 947-949.
- Hineman, A. and C. Stephan, Effect of dwell time on single particle inductively coupled plasma mass spectrometry data acquisition quality. Journal of Analytical Atomic Spectrometry, 2014. 29(7): p. 1252-1257.
- Pace, H.E., et al., Determining Transport Efficiency for the Purpose of Counting and Sizing Nanoparticles via Single Particle Inductively Coupled Plasma Mass Spectrometry. Analytical Chemistry, 2011. 83(24): p. 9361-9369.
- Gschwind, S., et al., Capabilities of inductively coupled plasma mass spectrometry for the detection of nanoparticles carried by monodisperse microdroplets. Journal of Analytical Atomic Spectrometry, 2011. 26(6): p. 1166-1174.
- Peters, R.B., et al., Development and validation of single particle ICP-MS for sizing and quantitative determination of nano-silver in chicken meat. Analytical and Bioanalytical Chemistry, 2014. 406(16): p. 3875-3885.
- Mitrano, D.M., et al., Detecting nanoparticulate silver using single-particle inductively coupled plasma-mass spectrometry. Environmental Toxicology and Chemistry, 2012. 31(1): p. 115-121.
This information has been sourced, reviewed and adapted from materials provided by TOFWERK.
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