Luca Flamigni

Persistence of engineered nanoparticles in a municipal solid-waste incineration plant

More than 100 million tonnes of municipal solid waste are incinerated worldwide every year. However, little is known about the fate of nanomaterials during incineration, even though the presence of engineered nanoparticles in waste is expected to grow. Here, we show that cerium oxide nanoparticles introduced into a full-scale waste incineration plant bind loosely to solid residues from the combustion process and can be efficiently removed from flue gas using current filter technology. The nanoparticles were introduced either directly onto the waste before incineration or into the gas stream exiting the furnace of an incinerator that processes 200,000 tonnes of waste per year. Nanoparticles that attached to the surface of the solid residues did not become a fixed part of the residues and did not demonstrate any physical or chemical changes. Our observations show that although it is possible to incinerate waste without releasing nanoparticles into the atmosphere, the residues to which they bind eventually end up in landfills or recovered raw materials, confirming that there is a clear environmental need to develop degradable nanoparticles.

Capabilities of inductively coupled plasma mass spectrometry for the detection of nanoparticles carried by monodisperse microdroplets

Recently, first analyses of single sub-micrometre particles, embedded in liquid droplets, by inductively coupled plasma optical emission spectrometry (ICP-OES) with a size-equivalent detection limit of several hundred nanometres were reported. To achieve lower detection limits which might allow for the analysis of particles in the nanometre size range a more sensitive technique such as mass spectrometry (MS) is required. Various modifications of particle delivery and data acquisition systems commonly used were carried out to install a setup adequate for ICP-MS detection. These modifications enabled us to supply droplets generated by a commercial microdroplet generator (droplet size: 30–40 µm) with nearly 100% efficiency and high uniformity to the ICP. Analyses were performed using both standard solutions of dissolved metals at concentrations of 1 (Ag), 2 (Au), 5 (Au), or 10 (Cu) mg L−1 and highly diluted suspensions of gold and silver nanoparticles with sizes below 110 nm. In doing so, detection efficiencies of 10−6 counts per atom could be achieved while size-related limits of quantification were found to be 21 nm and 33 nm for gold and silver, respectively. Furthermore, the advantages of utilizing microdroplet generators vs. conventional nebulizers for nanoparticle analyses by ICP-MS are discussed.

Negative mode nanostructure-initiator mass spectrometry for detection of phosphorylated metabolites

The chemical complexity of the metabolome requires the development of new detection methods to enlarge the range of compounds detectable in a biological sample. Recently, a novel matrix-free laser desorption/ionization method called nanostructure-initiator mass spectrometry (NIMS) [Northen et al., Nature 449(7165):1033–1036, 2007] was reported. Here we investigate NIMS in negative ion mode for the detection of endogenous metabolites, namely small phosphorylated molecules. 3-Aminopropyldimethylethoxysilane was found to be suitable as initiator for the analytes studied and a limit of detection in the tens of femtomoles was reached. The detection of different endogenous cell metabolites in a yeast cell extract is demonstrated.

The effect of carrier gas humidity on the vaporization of laser-produced aerosols in inductively coupled plasmas

In a recent study we reported on the discovery of an interrelationship between the kind of material sampled by laser ablation (LA) and the shape of optical emission spectrometry (OES) profiles formed by atoms along the axis of an inductively coupled plasma (ICP), which is commonly used as an atomization and ionization source for mass spectrometry (MS). These results encouraged us to resume efforts on OES-based diagnostics to investigate vaporization-related elemental fractionation effects occurring during LA-ICP-MS and to conceive strategies for their suppression. Since the quantification capabilities and sensitivity of LA-ICP-MS tend to improve when water is simultaneously added to the carrier gas, we hypothesized that a matching of analyte/material-dependent points of vaporization in the ICP may be responsible for the increased accuracy that is often observed under such conditions. In this work, the impact of water admixture was investigated by side-on OES of an ICP using a Czerny–Turner monochromator operating in 2D imaging mode. Our data indicated a superposition of calcium- and sodium-specific OES axial profiles which were separated by several millimeters when no water was supplied, thus, supporting the hypothesis previously made. Furthermore, the utilization of a micro-droplet dispenser allowed to precisely adjust the amount of water entrained into the ICP and to specify the range of relative humidity required for matching the points of vaporization. Conditions specified this way were applied to quantitative LA-ICP-MS analyses of silicate glass and brass using matrix matched and non-matrix matched calibration, respectively, whereby the former resulted in significant improvements in the accuracy as well as lower detection limits with only moderate increases of the oxide formation rate.

Diffusion- and velocity-driven spatial separation of analytes from single droplets entering an ICP off-axis

The reproducible temporal separation of ion signals generated from a single multi-element droplet, observed in previous studies, was investigated in detail in this work using an ICPTOFMS with high temporal resolution. It was shown that the signal peak intensities of individual elements temporally shift relative to each other only for droplets moving through the plasma off-axis. The magnitude of these shifts correlated with the vaporization temperatures of the analytes and depended on the radial position of the droplets as well as on the thermal properties and velocity profiles of the carrier gases of the ICP. The occurrence of the signal shifting was explained by a spatial separation of analytes already present in the vapor phase in the ICP from a yet unvaporized residue of the droplet. This separation is most likely driven by anisotropic diffusion of vaporized analytes towards the plasma axis and a radial velocity gradient. The proposed explanation is supported by modeling of the gas velocities inside the ICP and imaging of the atomic and ionic emissions produced from single droplets, whose patterns were sloping towards the center of the torch. The effects observed in these studies are important not only for the fundamental understanding of analyte–plasma interactions but have also a direct impact on the signal intensities and stability.

Visualization, velocimetry, and mass spectrometric analysis of engineered and laser-produced particles passing through inductively coupled plasma sources

Velocities of particles passing through the load coil region of an inductively coupled plasma (ICP) attached to a quadrupole mass spectrometer (MS) were measured by particle image velocimetry (PIV). Particles were produced either by laser ablation (LA) of solid targets or from drying analyte-spiked microdroplets ejected by a piezoelectrically actuated quartz capillary. For instance, velocities determined under conditions typically applied to LA-ICP-MS analyses were found to range between 10 and 20 m s−1, depending on the axial position. Our data, furthermore, evidence significant changes of the gas velocity upon modifications of the ICP operating conditions such as plasma power, gas flow rate, and torch injector diameter if helium is admixed in excess of >50% of the total gas flow passing through the injector. For instance, an increase of the ICP RF power from 800 to 1600 W resulted in particle velocity gradients up to 15 m s−1 kW−1 measured after the third turn of the RF-coil. Temporal changes in velocity, i.e. particle accelerations over the axis of the load coil region were specified to 300–1000 m s−2. In addition, ICP-MS analyses of laser-produced aerosols carried out at constant volumetric flow rates but reduced injector diameters made signal intensities of elements such as Y, Ce, or U drop by up to two orders of magnitude suggesting incomplete particle evaporation as well as notably different aerosol penetration depths. Sensitivities measured in this case turned out to correlate with boiling points of the respective oxides rather than the element-specific ionization potentials commonly observed. The mechanisms controlling gas velocity and sensitivity variations are discussed and consequences on LA-ICP-MS analyses are drawn.

Occurrence of gas flow rotational motion inside the ICP torch: a computational and experimental study

An inductively coupled plasma, connected to the sampling cone of a mass spectrometer, is computationally investigated. The occurrence of rotational motion of the auxiliary and carrier gas flows is studied. The effects of operating parameters, i.e., applied power and gas flow rates, as well as geometrical parameters, i.e., sampler orifice diameter and injector inlet diameter, are investigated. Our calculations predict that at higher applied power the auxiliary and carrier gas flows inside the torch move more forward to the sampling cone, which is validated experimentally for the auxiliary gas flow, by means of an Elan 6000 ICP-MS. Furthermore, an increase of the gas flow rates can also modify the occurrence of rotational motion. This is especially true for the carrier gas flow rate, which has a more pronounced effect to reduce the backward motion than the flow rates of the auxiliary and cooling gas. Moreover, a larger sampler orifice (e.g., 2 mm instead of 1 mm) reduces the backward flow of the auxiliary gas path lines. Finally, according to our model, an injector inlet of 2 mm diameter causes more rotations in the carrier gas flow than an injector inlet diameter of 1.5 mm, which can be avoided again by changing the operating parameters.

Accelerated evaporation of microdroplets at ambient conditions for the on-line analysis of nanoparticles by inductively-coupled plasma mass spectrometry

The helium-assisted evaporation of pure, saline, and nanoparticle (NP)-containing microdroplets (∅droplet = 40–50 μm) at ambient conditions was studied. To quantify the liquid-to-gas mass transfer water droplets experience during evaporation, their sizes were monitored in different sections of a custom-made transport system, corresponding to increasing residence times after production. Drying times specified this way turned out to be approximately three times faster than values achievable by heavier gases such as argon which is commonly chosen as the carrier gas for subsequent NP analysis by inductively-coupled plasma mass spectrometry (ICP-MS). Furthermore, residues of saline droplets doped with well-defined amounts of an acidified calcium standard solution were classified by light scattering (LS) and scanning electron microscopy (SEM) indicating nearly complete desolvation over transport distances of a few tens of centimeters if helium or argon–helium mixtures were supplied. Mass transfer rates along droplet trajectories simulated on the basis of computational fluid dynamics (CFD) were found to be consistent with measured ones which proved the applicability of the evaporation model used, thus allowing an optimization of user-defined transport systems without the need for a time-consuming adaptation by trial-and-error. The analytical capabilities of helium-assisted evaporation as an alternative to conventional approaches employing argon-only-based set-ups operated at elevated temperatures were, in addition, demonstrated by ICP-quadruple (Q)MS of gold NPs. Therefore, NP-containing droplets were delivered through an either horizontally or vertically arranged transport assembly both resulting in over-all throughputs of 90 to 100%.

Element analysis of small and even smaller objects by ICPMS and LA-ICPMS.

Inductively coupled plasma mass spectrometry is increasingly used for non-traditional applications such as the analysis of solids at high spatial resolution when combined with laser ablation or the analysis of engineered nanoparticles. This report highlights recent projects and discusses the potentials and limitations these techniques offer. High-resolution laser ablation instrumentation allows element imaging at the μm-scale and can, therefore, be applied to, e.g., the mapping of metal isotope-labeled antibodies in biological tissues. Despite these advancements, the quantitative analysis of laser-produced aerosols is still a major concern. Here, the accuracy of analysis was found to strongly depend on particle size distribution but also on the morphology and composition of particles. In order to achieve a controlled supply of nanoparticles for analysis by inductively coupled plasma mass spectrometry, a dedicated microdroplet injection system was developed and characterized. This system allows a reproducible injection of single nanoparticles together with internal standards to determine their mass and composition.