Article pubs.acs.org/ac
Simultaneous Mass Quantification of Nanoparticles of Different Composition in a Mixture by Microdroplet Generator-ICPTOFMS Olga Borovinskaya,† Sabrina Gschwind,† Bodo Hattendorf,† Martin Tanner,‡ and Detlef Günther*,† †
Department of Chemistry and Applied Biosciences, Laboratory of Inorganic Chemistry, ETH Zurich, Vladimir-Prelog-Weg 1, CH-8093 Zurich, Switzerland ‡ Tofwerk AG, Uttigenstrasse 22, CH-3600 Thun, Switzerland ABSTRACT: This work investigated the potential of a high temporal resolution inductively coupled plasma time-of-flight mass spectrometer (ICPTOFMS) in combination with a microdroplet generator (MDG) for simultaneous mass quantification of different nanoparticles (NPs) in a mixture. For this purpose, a test system containing certified Au NPs, well characterized Ag NPs, and core−shell NPs composed of an Au core and an Ag shell was employed. Thanks to the full spectra coverage and rapid simultaneous detection of the TOFMS, the element composition of individual particles can be determined. The pure Ag NPs and the core−shell NPs could be differentiated despite the same mass of Ag they contain. Calibration with monodisperse droplets consisting of standard solutions allowed for the mass quantification of NPs without the use of NP certified materials. On the basis of this mass quantification, the sizes of NPs originating from the same aqueous suspension were simultaneously determined with an accuracy of 7−12%. The size-equivalent limits of detection estimated with the 3*σ criterion were 13 nm for Au and 16 nm for Ag. Estimation of the LODs using Poisson statistics resulted in 19 and 27 nm, respectively. In addition, the 30 μs temporal resolution of the ICPTOFMS allowed studying interactions of NPs with the ICP based on their transient MS signals. The results demonstrated a difference in vaporization behavior of the core−shell NPs and solutions and indicated that vaporization of the Ag shell takes place prior to the Au core.
T
NP can be quantified using a calibration based on standards of dissolved salts.6 For the quantification of both mass and particle concentration, an accurate determination of the transport efficiency of a sample introduction system (usually a combination of a nebulizer/spray chamber) is a crucial step.10 It has been reported that the waste-collection method tends to overestimate the transport efficiency and leads to incorrect sizes and concentrations.6 Two alternative approaches to measure the transport efficiency using reference NPs of either certified size (1) or certified number concentration (2) were proposed. In approach (1), the efficiency is calculated from the signal intensities of a particle of known size and the standard solutions with the assumption that their detection efficiencies are identical. In (2), the efficiency is estimated based on the difference in the number of particles measured and initially present in the suspension. Au reference NPs were suggested as a universal system to conduct the transport efficiency measurement, which can be used to quantify NPs of any composition. So far, this approach was validated and demonstrated its feasibility only for Ag NPs.6,11 It is still unknown if the transport of particles through the system is the
hese days, nanobased materials are used in a wide range of applications related to the environment, health, nourishment, energy, and communication. According to the voluntary database established by the Project on Emerging Nanotechnologies, nanomaterials are already present in the composition or involved in the manufacture of more than 1000 different products.1 Due to extensive production and use of nanomaterials, their potential risk to the environment and humans must be carefully evaluated followed by introduction of safety regulations. Implementation of any nanospecific legislations requires the availability of appropriate and validated analytical techniques and methods, which can support quick, accurate, and comprehensive detection, characterization, and quantification of different types of nanoparticles (NPs) in various matrices. In this respect, the development of such techniques is a current challenge. Inductively coupled plasma mass spectrometry (ICPMS) has been proposed for the characterization of inorganic engineered NPs on a single particle basis.2 The so-called single particle ICPMS (sp-ICPMS) is based on the measurement of highly diluted aqueous suspensions (103−107 cm−3)3,4 and can be used to determine environmentally relevant number concentrations4−6 and size distribution of NPs,6 provided their elemental composition, shape, and density are known.7 Another interesting feature of the sp-ICPMS is the ability to distinguish particulate and dissolved forms of an analyte.8,9 The mass of an © 2014 American Chemical Society
Received: March 31, 2014 Accepted: July 11, 2014 Published: July 11, 2014 8142
dx.doi.org/10.1021/ac501150c | Anal. Chem. 2014, 86, 8142−8148
Analytical Chemistry
Article
same for all materials or the assumption of identical detection efficiencies of a particle and a highly dispersed aerosol of dissolved analyte is valid for all NP types. An alternative to sp-ICPMS, a microdroplet-based quantification approach, which can determine the mass of an NP without the use of NP standards, has been recently proposed12 and demonstrated.13 In this method, a liquid containing NPs is not aspirated in a random manner but introduced in a form of monodisperse microdroplets, which are generated by a piezoelectrically driven dispenser head at a defined frequency between 5 Hz 14 and 2 kHz. Owing to their high monodispersity, the same droplets consisting of a standard element solution are used to calibrate the instrument. The droplet size is mainly defined by the nozzle diameter of the dispenser and can be slightly adjusted by changing the actuator voltage and its pulse duration. Nozzles in size of 30, 50, and 70 μm are commercially available (Microdrop Technologies GmbH, Norderstedt, Germany). Depending on the droplet size, various systems for their efficient transport into the ICP have been proposed.13,15,16 They are all based on the use of He as carrier gas, which was shown to improve droplet transport by accelerating solvent evaporation.14,17 Measurement of the transport efficiency, which is an additional source of error,10 is not required for the mass quantification because the content of every droplet is completely transported into the ICP. The sizes of Au NPs determined using the monodisperse microdroplet generator (MDG)-ICPMS and external and internal calibration approaches were in good agreement with those obtained with asymmetric flow field-flow fractionation in combination with dynamic light scattering and transmission electron microscopy (TEM).13 The absolute particle transport through the system has still to be determined for the quantification of particle number concentration and will be discussed in another publication. The signal produced from a single particle in the ICPMS is very short (
