The different behaviours of dissolved silver and silver nanoparticles under ICP-MS single particle detection conditions have been used to differentiate directly between both forms of silver in aqueous samples. Suspensions containing silver nanoparticles at number concentrations below 10^9 L-1 and/or dissolved Ag(I) are introduced into the ICP-MS by conventional pneumatic nebulization and measured with a time resolution of 5 ms. Each silver nanoparticle is converted in the ICP into a packet of ions, which are detected as a single pulse, whose intensity is proportional to the number of silver atoms in the nanoparticle, whereas dissolved silver produces pulses of averaged constant intensity. The frequency plots with respect to the intensity measured for each pulse show independent distributions for dissolved silver and silver nanoparticles, whose profiles are also different (Poisson and lognormal, respectively). Size limits of detection for pure Ag nanoparticles of 18 nm, equivalent to a silver mass of 32 ag, were obtained. Number concentration limits of detection of 1 X 10^4 L-1 can be achieved. A methodological approach for identification, characterization and determination of mass and number concentration of dissolved Ag(I) and silver nanoparticles at environmentally relevant concentrations is presented

Single particle inductively coupled plasma mass spectrometry is an emergent ICPMS method for detecting, characterizing, and quantifying nanoparticles. Although the number of applications reported to date is limited, the relatively simple instrumental requirements, the low number concentration detection levels attainable, and the possibility to detect both the presence of dissolved and particulate forms of an element make this methodology very promising in the nanoscience related areas.

The metrological criteria for the implementation of the single particle inductively coupled plasma mass spectrometry (SP-ICPMS) methodology applied to nanoparticle size characterization and quantification have been investigated. The SP-ICPMS basis involves a process of counting events corresponding to individual nanoparticles, which requires (i) isolation of the contribution of the nanoparticles from that of the background/dissolved analyte, and (ii) avoiding the occurrence of multiple-nanoparticle events. A criterion based on three times the standard deviation of the continuous background (3s) was selected as the threshold for discrimination of nanoparticle events from the background. Because the detectability of nanoparticles depends on both the size and number concentration, this 3s criterion was also selected for detection of nanoparticles at the size detection limit and concentrations over the number concentration detection limit. However, at very low number concentrations, a less restrictive criterion must be used. The selection of a critical nanoparticle number concentration, based on the sample introduction and data acquisition parameters, allows the minimization of the occurrence of multiple-nanoparticle events, as well as controlling of the precision associated with the counting of nanoparticle events. Under such conditions, the standard uncertainty associated with the determination of number concentrations was 5%. The uncertainty for the determination of nanoparticle diameters was also studied, varying from 3 to 10% for diameters in the range of 100–40 nm, respectively. Reliable average number concentrations and sizes were obtained, although the number size distributions showed a significant broadening contribution due to the SP-ICPMS measurement process. The feasibility of SP-ICPMS for the implementation of the European Commission definition of "nanomaterial" was studied by analyzing commercial silver nanoparticle suspensions

The increasing demand of analytical information related to inorganic engineered nanomaterials requires the adaptation of existing techniques and methods, or the development of new ones.The challenge for the analytical sciences has been to consider the nanoparticles as a new sort of analytes, involving both chemical (composition, mass and number concentration) and physical information (e.g. size, shape, aggregation). Moreover, information about the species derived from the nanoparticles themselves and their transformations must also be supplied. Whereas techniques commonly used for nanoparticle characterization, such as light scattering techniques, show serious limitations when applied to complex samples, other well-established techniques, like electron microscopy and atomic spectrometry, can provide useful information in most cases. Furthermore, separation techniques, including flow field flow fractionation, capillary electrophoresis and hydrodynamic chromatography, are moving to the nano domain, mostly hyphenated to inductively coupled plasma mass spectrometry as element specific detector.Emerging techniques based on the detection of single nanoparticles by using ICP-MS, but also coulometry, are in their way to gain a position. Chemical sensors selective to nanoparticles are in their early stages, but they are very promising considering their portability and simplicity.Although the field is in continuous evolution, at this moment it is moving from proofs-of-2 concept in simple matrices to methods dealing with matrices of higher complexity and relevant analyte concentrations. To achieve this goal, sample preparation methods are essential to manage such complex situations. Apart from size fractionation methods, matrix digestion, extraction and concentration methods capable of preserving the nature of the nanoparticles are being developed. This review presents and discusses the state-of-the-art analytical techniques and sample preparation methods suitable for dealing with complex samples. Single-and multimethod approaches applied to solve the nanometrological challenges posed by a variety of stakeholders are also presented.

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The quality of the quantitative information in single-particle inductively coupled plasma mass spectrometry (SP-ICP-MS) depends directly on the number concentration of the nanoparticles in the sample analyzed, which is proportional to the flux of nanoparticles through the plasma. Particle number concentrations must be selected in accordance with the data acquisition frequency, to control the precision from counting statistics and the bias, which is produced by the occurrence of multiple-particle events recorded as single-particle events. With quadrupole mass spectrometers, the frequency of data acquisition is directly controlled by the dwell time. The effect of dwell times from milli- to microseconds (10 ms, 5 ms, 100 μs, and 50 μs) on the quality of the quantitative data has been studied. Working with dwell times in the millisecond range, precision figures about 5 % were achieved, whereas using microsecond dwell times, the suitable fluxes of nanoparticles are higher and precision was reduced down to 1 %; this was independent of the dwell time selected. Moreover, due to the lower occurrence of multiple-nanoparticle events, linear ranges are wider when dwell times equal to or shorter than 100 μs are used. A calculation tool is provided to determine the optimal concentration for any instrument or experimental conditions selected. On the other hand, the use of dwell times in the microsecond range reduces significantly the contribution of the background and/or the presence of dissolved species, in comparison with the use of millisecond dwell times. Although the use of dwell times equal to or shorter than 100 μs offers improved performance working in single-particle mode, the use of conventional dwell times (3-10 ms) should not be discarded, once their limitations are known.

A method for determining the size of silver nanoparticles and their quantification by asymmetric flow field-flow fractionation coupled with inductively coupled plasma mass spectrometry (ICP-MS) is proposed and was tested in consumer products. Experimental conditions were studied in detail to avoid aggregation processes or alteration of the original size distributions. Additionally, losses from sorption processes onto the channel membrane were minimized for correct quantification of the nanoparticles. Mobile phase composition, injection/focusing, and fractionation conditions were evaluated in terms of their influence on both separation resolution and recovery. The ionic strength, pH, and the presence of ionic and nonionic surfactants had a strong influence on both separation and recovery of the nanoparticles. In general, better results were obtained under those conditions that favored charge repulsions with the membrane. Recovery values of 83 ± 8% and 93 ± 4% with respect to the content of silver nanoparticles were achieved for the consumer products studied. Silver nanoparticle standards were used for size calibration of the channel. The results were compared with those obtained by photon correlation spectroscopy and images taken by transmission electron microscopy. The quantification of silver nanoparticles was performed by direct injection of ionic silver standard solutions into the ICP-MS system, integration of the corresponding peaks, and interpolation of the fractogram area. A limit of detection of 5.6 μg L(-1) silver, which corresponds to a number concentration of 1×10(12) L(-1) for nanoparticles of 10 nm, was achieved for an injection volume of 20 μL.

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