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Size and element specific determination of Ag nanoparticles in consumer spray products using asymmetric flow field flow fractionation (AF4) coupled to ICPMS

Harald Hagendorfer, Andrea Ulrich, EMPA - Eidgenössische Materialprüfungs- und Forschungsanstalt, Abteilung Analytische Chemie, Dübendorf, Switzerland;
Thomas Jocks, Wyatt Technology Europe GmbH, Dernbach, Germany

The use of nanoparticles in consumer products shows a tremendous increase over the past years. A study of the Woodrow Wilson International Centre for Scholars¹ reported that between 2005 and today the number of products based on nanoparticles increased almost twentyfold from 54 to 1015. As active compounds these products contain ZnO for surface protection, TiO2 for sun screen or Ag for anti-bacterial purposes, only to name a few applications. Furthermore, Si, C and Au are frequently used. So far, no data are available concerning the release and the behaviour of nanoparticles during the spray process. Therefore, in spite of their beneficial properties, possible risks for humans and the environment need to be thoroughly investigated. Especially the exposure pathway via the lung seems to be very critical, as could be shown in a study recently published in the Journal of Nanoparticle Research². In order to perform an appropriate and reliable risk assessment, detailed information about the quantity, size and distribution must be provided. To achieve this, the most specific and sensitive analytical tools have to be employed. In view of this, the asymmetric flow field flow fractionation (AF4) coupled to inductively coupled plasma mass spectrometry (ICPMS) could play an important future role as a tool for the determination of metallic nanoparticles in various settings. This work shows the use of this method to analyse a commercially available consumer spray product.

Material and Methods:

Table 1:  AF4 and ICPMS conditions

Wyatt Eclipse3™ AF4 Thermo Finnigan Element2 ICPMS
Carrier 18 mΩ cm DI water RF – Power 1400 W
Channel flow (Vc) 0.5 mL/min Reflected Power < 4 W
Cross flow  (Vx) 1 mL/min Plasma Gas 16 L/min
Membrane type GE PVDF 30 kD Nebulizer ESI PFA-ST @ 1 L/min
Channel and spacer 30 cm channel, small spacer, 350 μm height Spray chamber Scott double pass, quartz, cooled to 5°C
Injection volume 10 μL Integration time/scan mode 300 ms; E-Scan

To achieve a size separation of the nanoparticles the Eclipse3™ AF4-System (Wyatt Technology Europe, Dernbach, Germany) was used in combination with a metal free Shimadzu (Shimadzu Deutschland GmbH, Duisburg, Germany) HPLC pump (LC 10 AI), a degasser unit (DGU 20A3) and an auto sampler (SIL 20AC). After separation the compounds were detected using an ICPMS instrument Thermo Finnigan Element 2. AF4 was coupled directly via a capillary to the nebulizer. The parameters used for separation and detection are summarized in table 1.

Calibration: the size calibration was performed using Au nanoparticle standard material with a diameter of 10, 30 and 60 nm, respectively. Standards were derived from the NIST (National Institute for Standardization, NIST 8011, 8012, 8013). Prior to injection the standards were diluted 1:1000 using deionized water (18 mΩ cm; MiliQ, Milipore GmbH, Zug, Switzerland).

1a): Separation of the NIST Au nanoparticles using AF4 with subsequent detection by ICPMS (m/z =197).

Figure 1a shows the separation of the standard NIST Au particles (diameter 10, 30, 60 nm). The particle concentration is 50 µg Au/L. The calibration using Au nanoparticles of defined sizes enables the calculation of particle dimensions on the basis of the retention time (Figure 1 b).

1b): Integration of calibration data yields a well fitted straight line; retention time (x-axis) versus particle size (y-axis)

2): AF4 separation and ICPMS detection of a nanoscale Ag spray product (blue) with spike of 50 g Au/L, diameter 10 nm (red).

Figure 2 shows the chromatogram of a consumer spray product containing Ag nanoparticles (dilution 1:2000). The sample was spiked with Au nanoparticles (diameter 10 nm) at a concentration of 50 µg Au/L. The spray sample appears to be fairly polydisperse. A major peak (tR = 2.2 min) represents a size fraction of approximately 6 nm. A second broader peak (maximum at tR = 5.3 min) contains particles of different sizes up to 30 nm. Moreover, a very early peak (tR = 0.5 min) represents agglomerates of a size up to the µm scale, which leave the channel early due to steric elution. To evaluate these results, an orthogonal approach was chosen. The particle distribution determined by AF4-ICPMS was compared to data derived from transmission electron microscopy (TEM) and subsequent particle analysis. Both methods show strong congruency between TEM and AF4-ICPMS size distribution data as shown in Figure 3 a).

3a): Size distribution analysis data derived either by AF4-ICPMS or TEM show very similar results.

3b): TEM micrograph of Ag nanoparticles deployed in a consumer spray product.

Coupling of nanoparticle separation by AF4 to ICPMS detection results in a very sensitive method for nanoparticle analysis which has a large range of detection. The size distribution data obtained with this approach are comparable to those achieved by electron microscopy, the latter of which is much more expensive and troublesome. In this study, the analysis of a spray sample shows a polydisperse distribution of particles from approximately 6 nm to 30 nm in size. Moreover, during the spraying process, this pattern changes due to the formation of larger NP aggregates.


  1. Woodrow Wilson International Centre for Scholars, “The Project on Emerging Nanotechnologies”,
  2. Hagendorfer et. al, Size-fractionated characterization and quantification of nanoparticle release rates from a consumer spray product containing engineered nanoparticles, J Nanopart Res, in press.

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