Electrothermal vaporisation ICP-mass spectrometry (ETV-ICP-MS) for the determination and speciation of trace elements in solid samples - A review of real-life applications from the author's lab

The use of electrothermal vaporisation (ETV) from a graphite furnace as a means of sample introduction in inductively coupled plasma mass spectrometry (ICP-MS) permits the direct analysis of solid samples. A multi-step furnace temperature programme is used to separate the vaporisation of the target element(s) and of the matrix components from one another. Sometimes, a chemical modifier is used to enable a higher thermal pre-treatment temperature, by avoiding premature analyte losses (stabilisation) or promoting the selective volatilisation of matrix components. In almost all instances, accurate results can be obtained via external calibration or single standard addition using an aqueous standard solution. Absolute limits of detection are typically ~1 pg, which corresponds to 1 ng/g for a typical sample mass of 1 mg. Real-life applications carried out in the author's lab are used to illustrate the utility of this approach. These applications aim at trace element determination in industrial and environmental materials. The industrial materials analysed include different types of plastics - Carilon, polyethylene, poly(ethyleneterephtalate) and polyamide - and photo- and thermographic materials. As samples from environmental origin, plant material, animal tissue and sediments were investigated. Some applications aimed at a multi-element determination, while in other, the content of a single, but often challenging, element (e.g., Si or S) had to be measured. ETV-ICP-MS was also used in elemental speciation studies. Separation of Se-containing proteins was accomplished using polyacrylamide gel electrophoresis (PAGE). Subsequent quantification of the Se content in the protein spots was carried out using ETV-ICP-MS. As the volatilisation of methylmercury and inorganic mercury could be separated from one another with respect to time, no chromatographic or electrophoretic separation procedure was required, but ETV-ICP-MS as such sufficed for Hg speciation in fish tissue.     

Inclusion Complexes of Cyproterone Acetate with Cyclodextrins in Aqueous Solution

Cyproterone acetate (CPA) is a steroidal antiandrogen with a progestogenic activity. Given that this molecule has a very poor water solubility (2.1 g/mL), different cyclodextrins (CDs) were tested to form inclusion complexes and to increase solubility. Two different techniques were compared to study the affinity between CPA and CDs: phase-solubility studies and NMR spectroscopy. The stoichiometry and the stability constant could be determined for most complexes with the aid of phase-solubility studies. The greatest increase in solubility was achieved with the methylated -CDs, but hydroxypropylated - and -CDs also gave enhanced solubilities. ¹H-NMR studies showed a solubility increase similar to that found with phase-solubility studies. The proof of inclusion in the2,6-dimethyl--CD (DIMEB) was shown by ¹H-NMR and t-ROESY spectra.     

Electronic Structure Modulation in an Exceptionally Stable Non‐Heme Nitrosyl Iron(II) Spin‐Crossover Complex

The highly stable nitrosyl iron(II) mononuclear complex [Fe(bztpen)(NO)](PF₆)₂ (bztpen=N‐benzyl‐N,N′,N′‐tris(2‐pyridylmethyl)ethylenediamine) displays an S=1/2?S=3/2 spin crossover (SCO) behavior (T_1/2=370 K, ΔH=12.48 kJ mol^?1, ΔS=33 J K^?1 mol^?1) stemming from strong magnetic coupling between the NO radical (S=1/2) and thermally interconverted (S=0?S=2) ferrous spin states. The crystal structure of this robust complex has been investigated in the temperature range 120–420 K affording a detailed picture of how the electronic distribution of the t_2g–e_g orbitals modulates the structure of the {FeNO}⁷ bond, providing valuable magneto–structural and spectroscopic correlations and DFT analysis.     

Homodiselenacalix[4]arenes: Molecules with Unique Channelled Crystal Structures

A synthetic route towards homodiselenacalix[4]arene macrocycles is presented, based on the dynamic covalent chemistry of diselenides. The calixarene inner rim is decorated with either alkoxy or tert‐butyl ester groups. Single‐crystal X‐ray analysis of two THF solvates with methoxy and ethoxy substituents reveals the high similarity of their molecular structures and alterations on the supramolecular level. In both crystal structures, solvent channels are present and differ in both shape and capacity. Furthermore, the methoxy‐substituted macrocycle undergoes a single‐crystal‐to‐single‐crystal transformation during which the molecular structure changes its conformation from 1,3‐alternate (loaded with THF/water) to 1,2‐alternate (apohost form). Molecular modelling techniques were applied to explore the conformational and energetic behaviour of the macrocycles.     

Structural Effects in Visible‐Light‐Responsive Metal–Organic Frameworks Incorporating ortho‐Fluoroazobenzenes

The ability to control the interplay of materials with low‐energy photons is important as visible light offers several appealing features compared to ultraviolet radiation (less damaging, more selective, predominant in the solar spectrum, possibility to increase the penetration depth). Two different metal–organic frameworks (MOFs) were synthesized from the same linker bearing all‐visible ortho‐fluoroazobenzene photoswitches as pendant groups. The MOFs exhibit different architectures that strongly influence the ability of the azobenzenes to isomerize inside the voids. The framework built with Al‐based nodes has congested 1D channels that preclude efficient isomerization. As a result, local light–heat conversion can be used to alter the CO₂ adsorption capacity of the material on exposure to green light. The second framework, built with Zr nodes, provides enough room for the photoswitches to isomerize, which leads to a unique bistable photochromic MOF that readily responds to blue and green light. The superiority of green over UV irradiation was additionally demonstrated by reflectance spectroscopy and analysis of digested samples. This material offers promising perspectives for liquid‐phase applications such as light‐controlled catalysis and adsorptive separation.     

Melt Extrusion Encapsulation of Flavors: A Review

Encapsulation of flavor and aroma compounds has been largely explored in order to meet appraisal demands from consumers by improving the impact of flavor during the consumption of food products. Even though several techniques have been used for encapsulating volatile compounds, i.e., spray drying, fluidized bed coating, coacervation, and melt extrusion, those most frequently used in the food industry are spray drying and melt extrusion. In this article, the different techniques of encapsulation of flavors and fragrances in polymer-based matrices by extrusion are reviewed and partly re-defined, emphasizing the differences between the various techniques reported so far and the role of matrix types, additives, and operative conditions. Also, the role of water as a key parameter for controlled release and shelf stability of the delivery system will be discussed.     

Host-Guest Interactions between Calixarenes and Cp(2)NbCl(2)

The possible inclusion complexes of Cp₂NbCl₂ into calixarenes hosts have been investigated. The existence of a true inclusion complex in the solid state was confirmed by a combination of NMR, ab-initio calculations, thermogravimetric analysis, FTIR, Raman and PXRD. Ab-initio calculations, ¹H NMR solution and solid state ¹³C CP-MAS NMR results demonstrated that p-sulfonic calix[6]arene does form an inclusion complex with Cp₂NbCl₂. Raman spectroscopy showed, for the inclusion compound of p-sulfonic calix[6]arene-Cp₂NbCl₂, a band between 500 and 850 cm⁻¹ characteristic of Nb-O vibration. This result suggests that Nb(V) may engage in coordination with the oxygen of the sulfonate group, as part of the host-guest interaction. However, it is important to mention that the niobocene dichloride (Cp₂NbCl₂) dissolves in water and undergoes oxidation and hydrolysis processes to yield Cp₂NbCl₂(OH) species. For that reason this band does not exclude that the Nb-O band belongs to Cp₂NbCl₂(OH). Solid State ¹³C CP-MAS NMR and solution ¹H NMR spectroscopies together with ab-initio results showed that Cp₂NbCl₂ is included in the p-sulfonic calix[6]arene cavity, with both Cp rings inside the cavity. In contrast, the solution ¹H NMR results demonstrated that calix[6]arene does not form inclusion complex with Cp₂NbCl₂ in CDCl₃ solution. Cp₂NbCl₂ is not included in the calix[6]arene cavity, possibly due to the lack of sulfonate heads which promote Nb-O interactions and assist the inclusion of Cp₂NbCl₂ into the cavity.     

Speciation of copper(II) complexes in an ionic liquid based on choline chloride and in choline chloride/water mixtures

A deep-eutectic solvent with the properties of an ionic liquid is formed when choline chloride is mixed with copper(II) chloride dihydrate in a 1:2 molar ratio. EXAFS and UV-vis-near-IR optical absorption spectroscopy have been used to compare the coordination sphere of the cupric ion in this ionic liquid with that of the cupric ion in solutions of 0.1 M of CuCl(2)·2H(2)O in solvents with varying molar ratios of choline chloride and water. The EXAFS data show that species with three chloride ions and one water molecule coordinated to the cupric ion as well as species with two chloride molecules and two water molecules coordinated to the cupric ion are present in the ionic liquid. On the other hand, a fully hydrated copper(II) ion is formed in an aqueous solution free of choline chloride, and the tetrachlorocuprate(II) complex forms in aqueous choline chloride solutions with more than 50 wt % of choline chloride. In solutions with between 0 and 50 wt % of choline chloride, mixed chloro-aquo complexes occur. Upon standing at room temperature, crystals of CuCl(2)·2H(2)O and of Cu(choline)Cl(3) formed in the ionic liquid. Cu(choline)Cl(3) is the first example of a choline cation coordinating to a transition-metal ion. Crystals of [choline](3)[CuCl(4)][Cl] and of [choline](4)[Cu(4)Cl(10)O] were also synthesized from molecular or ionic liquid solvents, and their crystal structures were determined.