Frank Vanhaecke

Size-Dependent Optical Properties of Colloidal PbS Quantum Dots

We quantitatively investigate the size-dependent optical properties of colloidal PbS nanocrystals or quantum dots (Qdots), by combining the Qdot absorbance spectra with detailed elemental analysis of the Qdot suspensions. At high energies, the molar extinction coefficient epsilon increases with the Qdot volume d(3) and agrees with theoretical calculations using the Maxwell-Garnett effective medium theory and bulk values for the Qdot dielectric function. This demonstrates that quantum confinement has no influence on epsilon in this spectral range, and it provides an accurate method to calculate the Qdot concentration. Around the band gap, epsilon only increases with d(1.3), and values are comparable to the epsilon of PbSe Qdots. The data are related to the oscillator strength f(if) of the band gap transition and results agree well with theoretical tight-binding calculations, predicting a linear dependence of f(if) on d. For both PbS and PbSe Qdots, the exciton lifetime tau is calculated from f(if). We find values ranging between 1 and 3 mus, in agreement with experimental literature data from time-resolved luminescence spectroscopy. Our results provide a thorough general framework to calculate and understand the optical properties of suspended colloidal quantum dots. Most importantly, it highlights the significance of the local field factor in these systems.

The use of internal standards in icp ms

Careful study of the matrix effect in ICP-MS showed that, in all cases studied, the magnitude of the signal suppression or enhancement depends in a regular way on the mass number. Hence, accurate correction for non-spectral interferences is only possible using an internal standard with mass number close to that of the analyte element(s). It is also shown that using an internal standard with mass number close to that of the analyte improves the precision. For both cases, the ionization energy of the internal standard seems to be of no or only secondary importance. To obtain optimal precision and accuracy, the internal standard should be selected as close in mass number as possible to that of the analyte element(s). When a number of elements over a considerable mass range are to be determined, several internal standards have to be used.

Cytotoxic effects of gold nanoparticles: a multiparametric study.

The in vitro labeling of therapeutic cells with nanoparticles (NPs) is becoming more and more common, but concerns about the possible effects of the NPs on the cultured cells are also increasing. In the present work, we evaluate the effects of poly(methacrylic acid)-coated 4 nm diameter Au NPs on a variety of sensitive and therapeutically interesting cell types (C17.2 neural progenitor cells, human umbilical vein endothelial cells, and PC12 rat pheochromocytoma cells) using a multiparametric approach. Using various NP concentrations and incubation times, we performed a stepwise analysis of the NP effects on cell viability, reactive oxygen species, cell morphology, cytoskeleton architecture, and cell functionality. The data show that higher NP concentrations (200 nM) reduce cell viability mostly through induction of reactive oxygen species, which was significantly induced at concentrations of 50 nM Au NPs or higher. At these concentrations, both actin and tubulin cytoskeleton were deformed and resulted in reduced cell proliferation and cellular differentiation. In terms of cell functionality, the NPs significantly impeded neurite outgrowth of PC12 cells up to 20 nM concentrations. At 10 nM, no significant effects on any cellular parameter could be observed. These data highlight the importance of using multiple assays to cover the broad spectrum of cell-NP interactions and to determine safe NP concentrations and put forward the described protocol as a possible template for future cell-NP interaction studies under comparable and standardized conditions.

Direct solid sampling with electrothermal vaporization/atomization: what for and how?

In this article, the advantages and the disadvantages of solid sampling-graphite-furnace-based methods are critically examined, taking into account the latest research. A discussion of the different situations for which these methods are best suited within analytical chemistry is presented, together with a general methodology intended to maximize their advantages. The topics discussed include achieving a selective atomization/vaporization of the analyte, calibration, the use of alternative working conditions, and data treatment as a function of the specific goals of the analysis. The purpose of the article is to offer a critical, but tutorial, evaluation of the possibilities of these methods, making it easier for new users to approach them and take advantage of their possibilities.

Electrothermal vaporization for sample introduction in atomic absorption, atomic emission and plasma mass spectrometry—a critical review with focus on solid sampling and slurry analysis

The aim of this work is to review and compare the present possibilities of electrothermal atomic absorption spectrometry (ETAAS), electrothermal vaporization-inductively coupled plasma-optical emission spectrometry (ETV-ICP-OES) and electrothermal vaporization-inductively coupled plasma-mass spectrometry (ETV-ICP-MS) for analysis of solid samples, slurries and complex matrixes. The paper keeps in perspective the historical evolution of these techniques, providing an overview of the most relevant advances in instrumentation, methodology and fundamentals and highlighting the most relevant applications carried out during the last decade. The benefits of continuum source-AAS, the possible impact of diode lasers on this technique, the potential of ETV-ICP-methods for fractionation/speciation studies, as well as for resolving spectral interferences, and the influence of the different types of ICP-MS instrumentation currently available on the overall performance of ETV-ICP-MS will be some of the aspects discussed in more detail.

Use of single-collector and multi-collector ICP-mass spectrometry for isotopic analysis

This article is intended as a tutorial review on the use of single-collector and multi-collector ICPMS for isotope ratio determination. The monitoring and quantification of both induced and natural differences in the isotopic composition of target elements is covered. The capabilities of various types of ICPMS instruments for isotope ratio measurements are addressed and issues, such as the occurrence of mass discrimination and detector dead time effects and appropriate ways of correcting for the biases they give rise to are discussed. Applications relying on induced changes include elemental assay via isotope dilution, tracer experiments with stable isotopes, aiming at a more profound insight into physical processes or (bio)chemical reactions, and nuclear applications. Attention is also paid to the origin of natural variations in the isotopic composition, with focus onto the mechanisms behind the isotopic variation for those elements for which isotopic analysis can be realized using ICP-mass spectrometry, i.e. the occurrence of radiogenic nuclides formed as a result of the decay of naturally occurring and long-lived radionuclides and mass fractionation as a result of thermodynamic and kinetic isotope fractionation effects. Geochronological dating via the Rb–Sr, U, Th–Pb, and Pb–Pb methods is briefly explained and also the use of Sr and Pb isotopic analysis for provenance determination studies is covered. Subsequently, applications based on isotopic analysis of elements showing a much narrower range of variation as a result of isotope fractionation are described. Next to provenance studies, such applications include the use of isotope ratios in geochemical, environmental and biomedical studies. Although it is not the intention to comprehensively review the literature, several examples of published applications are used to illustrate the capabilities of both single-collector and multi-collector ICPMS in this context. Thereby, attention is devoted both to widely accepted applications and to more ‘exotic’ applications, aiming at an extension of the application range.

Elimination of interferences in the determination of arsenic and selenium in biological samples by inductively coupled plasma mass spectrometry

Abstract The determination of As and Se in biological samples by inductively coupled plasma mass spectrometry is well known to be degraded by spectral interferences. The resolution of quadrupole mass analysers is insufficient to resolve As+ and Se+ ions from polyatomic species such as ArCl+, ArAr+ and So+3. A study of this problem in human serum also revealed substantial non-spectral interferences on these elements occuring in the presence of organic compounds. It is shown that both problems can easily be overcome by a combination of chemical modification (addition of 4% ethanol) with nebulizer flow-rate gas adjustment. Under these conditions the use of standard additions for calibration allowed As and Se to be determined accurately in samples of biological origin. The method developed was applied to human serum and urine and for both As and Se excellent agreement with certified values was obtained.

‘Zone model’ as an explanation for signal behaviour and non-spectral interferences in inductively coupled plasma mass spectrometry

The zone model is a simplified representation of the plasma, resulting from the findings of an optimization study for a VG PlasmaQuad PQ1 inductively coupled plasma (ICP) mass spectrometer (VG Elemental, Winsford, Cheshire, UK). According to this model, for every nuclide there is a zone in the central channel of the ICP, where a maximum density of singly charged ions occurs. The position of such a zone of maximum M+ density is a function of the mass number of the nuclide and the zone can undergo a spatial displacement under the influence of an alteration of an instrumental parameter or the introduction of a different matrix. This representation not only enables an explanation of a large number of observations from the optimization study, but also allows an understanding of why both matrix induced signal suppression and enhancement were observed, why for a given matrix the extent to which the signal intensities were altered differed from day to day and finally why the extent to which a signal is influenced by the matrix was seen to be a function of the mass number of the corresponding nuclide. Although the zone model might not completely reflect the genuine physical reality in all its facets, it provides a phenomenological model for the variation of ion signals with mass number, operating parameters and matrix composition.

Inductively coupled plasma – Tandem mass spectrometry (ICP-MS/MS): A powerful and universal tool for the interference-free determination of (ultra)trace elements – A tutorial review

This paper is intended as a tutorial review on the use of inductively coupled plasma - tandem mass spectrometry (ICP-MS/MS) for the interference-free quantitative determination and isotope ratio analysis of metals and metalloids in different sample types. Attention is devoted both to the instrumentation and to some specific tools and procedures available for advanced method development. Next to the more typical reaction gases, e.g., H2, O2 and NH3, also the use of promising alternative gases, such as CH3F, is covered, and the possible reaction pathways with those reactive gases are discussed. A variety of published applications relying on the use of ICP-MS/MS are described, to illustrate the added value of tandem mass spectrometry in (ultra)trace analysis.

Precise Measurement of Isotope Ratios with a Double-Focusing Magnetic Sector ICP Mass Spectrometer

The potential of a commercially available double-focusing magnetic sector ICP mass spectrometer (Element, Finnigan MAT, Bremen, Germany) for precise isotope ratio measurement at the low-resolution setting (R = 300) was evaluated. Optimization of scanning conditions led to a relative standard deviation for a set of 10 consecutive 2 min measurements of ∼0.1% ((206)Pb(+)/(207)Pb(+)) at signal intensities of ∼200 000 counts/s (peak height). This compares favorably with the best values ever reported for quadrupole ICPMS and barely exceeds the theoretical value (counting statistics). Increasing the signal intensity to values ≥500 000 counts/s (peak height) resulted in a further reduction of the RSDs obtained (for both (25)Mg(+)/(26)Mg(+) and (206)Pb(+)/(207)Pb(+)) to typically 0.04%. These figures are remarkably better than those reported for commercially available quadrupole ICPMS systems. This improvement significantly reduces the difference between isotope ratio precision of ICPMS on one hand and those of thermal ionization mass spectrometry and plasma source multiple collector mass spectrometry on the other.