Introduction
Field-Flow Fractionation (FFF) comprises a family of techniques, which provide high-resolution separation of sample constituents in the size range from a few nanometers up to tens of microns [1]. Its most common subtype is Asymmetrical Flow FFF (AF4). AF4 hyphenated with Multi-Angle Light Scattering (MALS) and ICP-MS has proven to be a powerful analytical tool for the characterization of nanoparticle-containing samples enabling simultaneous access to size distribution as well as size-resolved elemental composition [2, 3]. A schematic of the AF4 separation principle is shown in Figure 1.
Within the last decade, single particle ICP-MS (spICP-MS) has emerged as a fast, highly sensitive and straightforward analytical technique that provides mass-, size- and number-based information of inorganic and metal nanoparticles at ppt concentration levels [4, 5].
In this application note we compare the performance of AF4-MALS-ICP-MS and spICP-MS for the characterization of silica nanoparticles particulary focusing on the lower size limit.
Experimental
In this study, a polydisperse colloidal silica mixture comprising five different sizes ranging from approx. 10-300 nm (1 wt% each) was investigated. For spICP-MS measurements the suspension was diluted to 937 ng L-1 in ultrapure water. A single quadrupole ICP-MS (Agilent 7900 ICP-MS) was used with a dwell time of 100 μs. The nebulization efficiency was determined based on the size method and the 28Si isotope was recorded.
For AF4-MALS-ICP-MS measurements the sample was used as received. The fractionation channel was equipped with a 5 kDa RC membrane and a mixture of 0.025 % NovaChem and 0.025 mM NaCl served as eluent. The separation method comprised a linear cross flow decay of 1.5 mL min-1 for 50 minutes followed by a smooth power gradient to end the separation field. To increase the sample concentration before reaching the detectors the channel flow was divided into a 0.1 mL min-1 pure solvent flow that was discarded and a 0.4 mL min-1 sample containing detector flow using the Postnova Postnova PN1650 Smart Stream Splitting device. Also the 28Si isotope was monitored.
Results
Especially for silica nanoparticles, spICP-MS measurements are challenging due to the presence of spectral interferences coming from elements ubiquitously present in the plasma (e.g. 14N14N+ and 12C16O+) overlapping with the signal of the most abundant Si isotope at m/z = 28 [6]. As a result, the background at m/z 28 is very high which makes the detection of silica nanoparticles difficult and causes a high size detection limit. With spICP-MS one large size fraction of around 295 nm was identified as shown in Figure 2. The high ionic background signal for silica resulted in a rather high lower size detection limit of 135 nm.
In contrast the AF4 technique was able to separate the sample into five distinct size fractions (Figure 3). The MALS detector delivered the corresponding radii of gyration of 6 nm, 11 nm, 28 nm, 43 nm and 117 nm (red dots) and the ICP-MS detection identified silicon in the particles.
Conclusion
While spICP-MS can provide fast access to particle mass- and number distribution of the larger silica particle size fraction (302 nm), it struggles with the smaller size fractions (< 135 nm) due to the high ionic background present in the sample and significant polyatomic interferences generated in the plasma torch. AF4-MALS-ICP-MS is a powerful tool to overcome the size limitations that spICP-MS faces with metal oxide particles enabling accurate sizing of the investigated silica particles down to 15 nm (geometric diameter). Hyphenation of AF4 with single or even triple quadrupole spICP-MS is a promising route to further improve the size limit of spICP-MS. [7]
References
[1] C. Contado, Analytical and Bioanalytical Chemistry, 2017, 409(10), 2501-2518.
[2] S. Dubascoux, I. Le Hecho, M. Hasselöv, F. von der Kammer, M. Potin Gautier, G. Lespes, Journal of Analytical Atomic Spectrometry, 2010, 25(5), 613-623.
[3] J. Heroult, V. Nischwitz, D. Bartczak, H. Goenaga-Infante, Analytical and Bioanalytical Chemistry, 2014, 406, 3919-3927.
[4] D.M. Mitrano, B. Barber, A. Bednar, P. Westerhoff, C.P. Higgins, J.F. Ranville, Journal of Analytical Atomic Spectrometry, 2012, 27(7), 1131-1142.
[5] F. Laborda, E. Bolea, J. Jimenez-Lamana, Analytical Chemistry, 2013, 86(5), 2270-2278.
[6] T. W. May, R. H. Wiedmeyer, Atomic Spectroscopy, 1998, 19(5), 150-155.
[7] B. Hetzer, A. Burcza, V. Gräf. E. Walz, R. Greiner, Food Control, 2017, 80, 113-124.
Acknowledgements
Brant Kim from Entegris Koreo is gratefully acknowledged for providing the silica samples.
