Michael R. Winchester

Capabilities of single particle inductively coupled plasma mass spectrometry for the size measurement of nanoparticles: a case study on gold nanoparticles.

The increasing application of engineered nanomaterials (ENMs) in consumer and medical products has motivated the development of single-particle inductively coupled plasma mass spectrometry (spICP-MS) for characterizing nanoparticles under realistic environmental exposure conditions. Recent studies have established a set of metrological criteria and evaluated the feasibility of spICP-MS for sizing or quantifying various highly commercialized ENMs. However, less is known about the performance of spICP-MS for detecting nanoparticles with sizes greater than 80 nm. This paper presents a systematic study on spICP-MS for accurate size measurement of gold nanoparticles from 10 to 200 nm. We show that dwell time contributes significantly to the quality of data, with the optimal dwell time that limits split particle events, particle coincidences and false positives being 10 ms. A simple approach to correct for split particle events is demonstrated. We show that transient features of single particle events can be temporally resolved on a conventional quadrupole ICP-MS system using a sufficiently short dwell time (0.1 ms). We propose an intensity-size diagram for estimating the linear dynamic size range and guiding the selection of ICP-MS operating conditions. The linear dynamic size range of the ICP-MS system under standard (highest) sensitivity conditions is 10 to 70 nm but can be further extended to 200 nm by operating in less sensitive modes. Finally, the ability of spICP-MS to characterize heterogeneous forms of metal containing nanoparticles is evaluated in mixtures containing both dissolved and poly disperse nanoparticulate Au.

Real-time size discrimination and elemental analysis of gold nanoparticles using ES-DMA coupled to ICP-MS

We report the development of a hyphenated instrument with the capacity to quantitatively characterize aqueous suspended gold nanoparticles (AuNPs) based on a combination of gas-phase size separation, particle counting, and elemental analysis. A customized electrospray-differential mobility analyzer (ES-DMA) was used to achieve real-time upstream size discrimination. A condensation particle counter and inductively coupled plasma mass spectrometer (ICP-MS) were employed as downstream detectors, providing information on number density and elemental composition, respectively, of aerosolized AuNPs versus the upstream size selected by ES-DMA. A gas-exchange device was designed and optimized to improve the conversion of air flow (from the electrospray) to argon flow required to sustain the ICP-MS plasma, the key compatibility issue for instrumental hyphenation. Our work provides the proof of concept and a working prototype for utilizing this construct to successfully measure (1) number- and mass-based distributions; (2) elemental compositions of nanoparticles classified by size, where the size classification and elemental analysis are performed within a single experiment; (3) particle concentrations in both solution (before size discrimination) and aerosol (after size discrimination) phases; and (4) the number of atoms per nanoparticle or the nanoparticle density.

Emission characteristics of a pulsed, radiofrequency glow discharge atomic emission device

The instrumentation, initial observations, and operating characteristics ofa pulsed radio-frequency glow discharge atomic emission source are described. Anomalies In the temporal emission intensity wave forms for some analyze transitions are a b reported. These anomalies take the form of a well defined intensity maximum located either near the beginning of the discharge pulse or just after power termination,depending on the particular transition. Still other analyze transitions demonstrate no such temporal irregularities.Selective, gated, detection of these emission anomalies suggests possible analytical advantages in terms of instantaneous emission intensities, which may translate into improved analytical sensitivities.

Applicability of a radiofrequency powered glow discharge for the direct solids analysis of non-conducting materials by atomic emission spectrometry

Radiofrequency (r.f.) powered glow discharge atomic emission spectrometry is demonstrated to be a potentially useful technique for the elemental analysis of electrically conductive and non-conductive solids. As opposed to the direct current glow discharge, the r.f. glow discharge enables the non-conductive sample to be atomised directly from the solid state without the need for prior mixing with a conductive host matrix material. Data are reported to illustrate the dependencies of emission intensity on gas pressure and discharge power, as well as the stability of the discharge. Representative atomic emission spectra of both an electrically conductive and non-conductive sample are presented to illustrate that the r.f. glow discharge provides good atomisation efficiency for both sample types.

Quantification of Carbon Nanotubes in Environmental Matrices: Current Capabilities, Case Studies, and Future Prospects

Carbon nanotubes (CNTs) have numerous exciting potential applications and some that have reached commercialization. As such, quantitative measurements of CNTs in key environmental matrices (water, soil, sediment, and biological tissues) are needed to address concerns about their potential environmental and human health risks and to inform application development. However, standard methods for CNT quantification are not yet available. We systematically and critically review each component of the current methods for CNT quantification including CNT extraction approaches, potential biases, limits of detection, and potential for standardization. This review reveals that many of the techniques with the lowest detection limits require uncommon equipment or expertise, and thus, they are not frequently accessible. Additionally, changes to the CNTs (e.g., agglomeration) after environmental release and matrix effects can cause biases for many of the techniques, and biasing factors vary among the techniques. Five case studies are provided to illustrate how to use this information to inform responses to real-world scenarios such as monitoring potential CNT discharge into a river or ecotoxicity testing by a testing laboratory. Overall, substantial progress has been made in improving CNT quantification during the past ten years, but additional work is needed for standardization, development of extraction techniques from complex matrices, and multimethod comparisons of standard samples to reveal the comparability of techniques.

Post hoc interlaboratory comparison of single particle ICP-MS size measurements of NIST gold nanoparticle reference materials.

Single particle inductively coupled plasma-mass spectrometry (spICP-MS) is an emerging technique that enables simultaneous measurement of nanoparticle size and number quantification of metal-containing nanoparticles at realistic environmental exposure concentrations. Such measurements are needed to understand the potential environmental and human health risks of nanoparticles. Before spICP-MS can be considered a mature methodology, additional work is needed to standardize this technique including an assessment of the reliability and variability of size distribution measurements and the transferability of the technique among laboratories. This paper presents the first post hoc interlaboratory comparison study of the spICP-MS technique. Measurement results provided by six expert laboratories for two National Institute of Standards and Technology (NIST) gold nanoparticle reference materials (RM 8012 and RM 8013) were employed. The general agreement in particle size between spICP-MS measurements and measurements by six reference techniques demonstrates the reliability of spICP-MS and validates its sizing capability. However, the precision of the spICP-MS measurement was better for the larger 60 nm gold nanoparticles and evaluation of spICP-MS precision indicates substantial variability among laboratories, with lower variability between operators within laboratories. Global particle number concentration and Au mass concentration recovery were quantitative for RM 8013 but significantly lower and with a greater variability for RM 8012. Statistical analysis did not suggest an optimal dwell time, because this parameter did not significantly affect either the measured mean particle size or the ability to count nanoparticles. Finally, the spICP-MS data were often best fit with several single non-Gaussian distributions or mixtures of Gaussian distributions, rather than the more frequently used normal or log-normal distributions.

Calibration of a Condensation Particle Counter Using a NIST Traceable Method

This work presents a calibration of a commercial condensation particle counter using National Institute of Standards and Technology (NIST) traceable methods. By the nature of the metrology involved, this work also compares the measurement results of three independent techniques for measuring aerosol concentration: continuous flow condensation particle counter (CPC); aerosol electrometer (AE); and the aerosol concentration derived from microscopic particle counting. Because of the transient nature of aerosol, there are no concentration artifact standards such as exist for particle diameter standards. We employ a mobility classifier to produce a nearly monodisperse, 80 nm, polystyrene latex aerosol. The test aerosol is used as a challenge for the CPC and the AE, and is subsequently filter sampled for electron microscopy. Our test stand design incorporates a continuous CPC aerosol concentration monitor to verify the aerosol stability. The CPC determines particle concentration by single particle counting at a constant sample flow rate. The AE has been calibrated to a NIST traceable current standard. The subsequent aerosol concentration measurement is obtained by determining the electrical current produced by a charged aerosol transported to the detector by a controlled aerosol flow rate. We have NIST traceability for flow rates for all methods and a methodology to calibrate the AE to NIST traceable electrical standards. The latter provides a calibration and a determination of the uncertainty in the aerosol derived current measurement. A bias in the measurements due to multiple charged particles was observed and overcome by using an electrospray aerosol generator to produce the challenge particles. This generator was able to produce aerosol concentrations over the range of 100 particles/cm 3 to 15 000 particles/cm 3 with lower number of dimer particles (≈1%). In our work, independent measurement of aerosol concentration is obtained by quantitatively collecting samples of the airborne polystyrene latex spheres on a small pore filter material and determining the number of particles collected by electron microscopy. Electron micrograph images obtained using a field-emission scanning electron microscope are analyzed using particle counting. We found the relative uncertainty in the aerosol electrometer measurements to be in excess of 100% for particle concentrations of approximately 120 particles/cm 3 and approximately 5% for concentrations above 6000 particles/cm 3 . The uncertainty found by the microscopy method was approximately 3%.

Silver Nanoparticles: Technological Advances, Societal Impacts, and Metrological Challenges

Silver nanoparticles (AgNPs) show different physical and chemical properties compared to their macroscale analogs. This is primarily due to their small size and, consequently, the exceptional surface area of these materials. Presently, advances in the synthesis, stabilization, and production of AgNPs have fostered a new generation of commercial products and intensified scientific investigation within the nanotechnology field. The use of AgNPs in commercial products is increasing and impacts on the environment and human health are largely unknown. This article discusses advances in AgNP production and presents an overview of the commercial, societal, and environmental impacts of this emerging nanoparticle (NP), and nanomaterials in general. Finally, we examine the challenges associated with AgNP characterization, discuss the importance of the development of NP reference materials (RMs) and explore their role as a metrological mechanism to improve the quality and comparability of NP measurements.

Chemical characterization of engineered nanoparticles.

Nanoparticles, particles having all three dimensions on the nanoscale (1–100 nm), are not new. They have been around since the dawn of the universe. However, interest in nanoparticles has increased explosively over the past two decades. This is especially the case for “engineered” nanoparticles, which can be defined as nanoparticles that are prepared to have certain characteristics so that they can be used for specific purposes. Though technically not nanoparticles, nanotubes and nanorods having only two dimensions on the nanoscale are often categorized and discussed together with true nanoparticles. Engineered nanoparticles are currently being used, or investigated and developed for use, in a wide range of applications spanning an array of technology sectors. Exciting examples can be found in medicine, where nanoparticles show great promise for cancer diagnosis and treatment, as well as for early detection of Alzheimer’s disease. In materials science, adding nanoparticles to plastics can make them lighter, while enhancing strength and durability. Nanoparticles are being utilized to produce selfcleaning windows, paints and coatings that resist abrasions and graffiti, as well as scratch-proof eyeglasses. In the area of environmental technologies, silver nanoparticles, known to have antimicrobial properties, are being investigated for remediation of chemical pollution. And in the area of water purification, nanoparticles are being studied for their potential to remove water contaminants selectively. The integration of nanoparticles into consumer products continues to skyrocket. However, the enormous application potential of nanoparticles is accompanied by both real and perceived risks to the environment and public health and safety. Consequently, in addition to the broad studies being conducted to find new uses for nanoparticles, research into the environmental, health, and safety (EHS) aspects of nanoparticles is being undertaken in many laboratories around the globe. At present, little is actually known concerning these EHS aspects and a very large body of knowledge remains to be discovered. Finding new uses for nanoparticles and understanding their EHS aspects require reliable methods for nanoparticle characterization. Of the types of characterization that are needed, perhaps none is more challenging than chemical characterization. In some senses, chemical characterization of nanoparticles has been performed for many years, though the terminology was different prior to the introduction of the new terms that include the prefix “nano.” For instance, researchers have been publishing papers on chemical analysis of colloidal suspensions and ultrafine particles for decades. However, somewhat different analytical problems have arisen with the advent of nanotechnology. Probably the most important example is the necessity to determine the surface compositions of engineered nanoparticles. A related example is found in the need to determine quantitatively the amount of a specific chemical component attached to the surfaces of nanoparticles relative to the amount of that same component dissolved in the solution phase of a suspension. This special issue of Analytical and Bioanalytical Chemistry is focused on chemical characterization of engiM. R. Winchester (*) Analytical Chemistry Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA e-mail: mwinchester@nist.gov

UV/visible Fourier transform spectroscopy using an inductively-coupled plasma : dual-channel noise cancellation

Abstract Although technological advances have extended the range of Fourier transform spectroscopy (FTS) into the UV/visible spectral domain, its application to spectroscopic and spectrometric problems has been hampered-relative to such applications in the infrared domain-by noise considerations. Although the technique retains high resolution, accurate wavelength registration, and simultaneous broad band coverage, the multiplex advantage present in the IR is severely compromised in the UV/visible due to the relative insignificance of detector noise. In particular, signal-carried noise distributes widely through the spectrum, degrading the dynamic range needed for many spectroscopic and analytical applications. This study demonstrates the use of complementary optical output channels in a commercial FTS to achieve up to ten-fold noise reductions for spectra acquired from an analytical inductively-coupled plasma with conventional pneumatic sample aspiration. The study also demonstrates the advisability of increasing the sampling rate of future instruments to exceed the maximum noise frequency characteristic of droplet evaporation effects.