Obtaining number-based information such as concentration and size distribution at environmentally relevant concentrations is one of the challenges of characterizing complex nanomaterials in the environment.
While Field-Flow Fractionation combined with Inductively Coupled Plasma Mass Spectrometry (FFF-ICP-MS) has been proven to be an essential analytical technique for the characterization of environmental samples [1-3], it does not provide a direct measurement of particle number.
A new analytical method called Single Particle ICP-MS (spICP-MS) can provide number-based information for monodispersed metal and metal oxide nanoparticles at parts per trillion (ppt) concentration levels [4, 5].
Direct hyphenation of spICP-MS to the asymmetrical flow FFF (AF4) system for number- and size-based characterization of a mixture of gold nanoparticles is discussed in this article.
A Postnova AF2000 Multiflow Asymmetrical Flow Field-Flow Fractionation (AF4) system was directly interfaced to an Agilent 7900 ICP-MS system. A capillary connecting the outlet of the AF4 channel to the inlet of the ICP-MS nebulizer was used to direct the AF4 effluent into the ICP-MS nebulizer.
In the coupling experiment, a dilute mixture of 30 nm (NIST, RM8012, 27.6 nm (TEM)) and 60 nm gold nanoparticles (NIST, RM8013, 56.0 nm (TEM)) was used. The concentration of 30 nm nanoparticles in the mixture was 250 ppt, and that of 60 nm nanoparticles was 1000 ppt. The ICP-MS system was operated in the spICP-MS mode.
For the spICP-MS analysis, a sequence of 54 replicates with an analysis time of 30 seconds was constructed. There was a time delay of 14 seconds between each consecutive replicate. The sequence was started once the mixture was injected into the AF4 system.
Agilent MassHunter Workstation software was used to separately store and analyze each replicate for particle number and size. The average diameter of the replicates and the number of counted nanoparticles were compiled and graphed manually using Microsoft Excel and OriginPro.
Figure 1. Postnova AF2000 MT and Agilent 7900 ICP-MS.
Results and Discussion
The number of counted nanoparticles in each replicate was plotted against runtime, and displayed two distinct peaks, as shown in Figure 2. The first peak eluted between 12.5 and 18.3 minutes, while the second peak eluted between 18.3 and 25 minutes.
The most abundant population in the mixture comprising of 2/3 of the total number of the counted particles is represented by the first peak. Figure 2 also shows the results of the size analysis of the replicates across the peaks (red circles) obtained by AF4-spICP-MS.
The nanoparticles counted during the elution have an average diameter of 28.6 ± 0.6 nm across the first peak and of 59.5 ± 0.6 nm across the second peak. The measured average diameters matched the nominal values of the nanoparticles. Using the peak area, the number of nanoparticles present in the mixture was measured to be 6.2 × 108 for the 30 nm nanoparticles and 2.9 × 108 for the 60 nm nanoparticles.
These values are approximately 35% lower than the number of the nanoparticles present in the mixture, so they represent a recovery rate of 65% for both nanoparticle sizes. This rate matches the recovery rates for gold nanoparticles mentioned in literature .
The sample loss is non-specific as the ratio of 30 nm to 60 nm nanoparticles acquired by the method (2.13) differed by merely 1.9% from the ratio of the nanoparticles in the mixture (2.09). The underestimation of the peak area or adsorption of nanoparticles to the membrane and/or tubings could have caused the sample loss.
Volatile salts such as ammonium acetate can be added to the running carrier solution to enhance the non-specific recovery of the nanoparticles. In order to improve the peak area estimation, the number of data points can be increased by decreasing the analysis time per replicate.
Figure 2. Particle number-based fractogram of the mixture of 30 nm and 60 nm Au gold nanoparticles obtained by AF4-spICP-MS. The red circles represent the average replicate diameter measured by spICP- MS. The shown histograms representing the particle size distribution at the respective peak maxima are directly obtained from the Agilent MassHunter Workstation software.
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The data provided in this article shows the successful interfacing of two particle characterization techniques, spICP-MS and AF4, to determine the size and number of nanoparticles in a mixture. AF4 is necessary in this combination to provide sample sub-streams that are adequately purified and simplified for the spICP-MS analysis.
The combination of these two techniques will provide measurement capabilities for the characterization of complex matrices that would not be possible by either technique alone. For instance, the hyphenated technique can be used to characterize dissolved and non-dissolved metallic mixtures.
The dissolved or ionic component will be fully removed from the particulate form, due to the AF4 separation that simplifies the spICP-MS analysis of the non-dissolved component. This feature can be useful in studies such as nanoparticle toxicology, where differentiation between the dissolved and particulate components is essential
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This information has been sourced, reviewed and adapted from materials provided by Postnova Analytics
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