Solid-State Composite reference electrodes

Recently, a collaborative study on the Solid-State Composite reference electrodes was published in the journal of Membranes 2022, 12, 569. Slim Blidi, EACH alumnus and the first author of this publication, provides here a short overview of this novel research.

Potentiometric measurements using conventional electrodes do have some drawbacks when used. This is especially true for the conventional liquid-filled reference electrodes. They are non-robust in use and are maintenance-intensive due to the necessity of refilling the inner solution and keeping the liquid junction clog-free.

By eliminating the liquid component from the electrode structure, Solid-State Composite (SSC) reference electrodes are a viable alternative to conventional reference electrodes. Prepared by either injection molding or chemical polymerization, they are based on a silver/silver chloride reference element enclosed in a polymeric matrix (e.g. polyvinyl acetate) mixed with an inorganic salt (potassium chloride). The composite is the inorganic salt storage material while being in contact with the sample solution.

In this study, a set of the SSC reference electrodes was prepared and their performance was tested in different synthetic solutions to assess the possible influence of pH, solution composition, as well as the concentration and mobility of ions. Using potentiometry and electrochemical impedance spectroscopy methods over a period of several months, stability tests were also performed to evaluate the suitability of the studied SSC reference electrodes for continuous, prolonged, and intensive usage. Concentrations of K+, Na+, Ca2+, and Cl– ions and pH values were measured in river water samples at different temperatures using the SSC reference electrodes.

better than commercial liquid-filled reference electrodes in terms of calibration range and actual measurements. It is safe to say that SSC reference electrodes represent the future of potentiometric measurements applied to environmental samples as they are cheap to manufacture and easy to maintain.

I am extremely grateful to my supervisors at Åbo Akademi: Dr. Kim Granholm, Dr. Zekra Moussavi, and Dr. Tomasz Sokalski for their valuable guidance and moral support during the experimental work as well as to the EACH programme for this life-changing experience.

 

Slim Blidi

 

 

Quantifying acidity in heterogeneous systems: Biphasic pKa values

As a result of a recent development in our group, it is now, for the first time, possible to rigorously measure acidity of acids in biphasic systems (aqueous phase at equilibrium with a water-immiscible phase) by using biphasic pKa values (pKaow values). This work has now been published in Analytical Chemistry 2022, 94, 4059–4064.

In this work, the octanol-water biphasic pKaow values have been determined for 35 acids of various structures and chemical properties (carboxylic acids, phenols, diphenylamines, imides, different CH acids) using UV-Vis and NMR (1H, 13C, or 31P) spectrometry.

Biphasic pKa values enable quantifying acid dissociation in biphasic systems in a more realistic and rigorous way than the conventional “mono-phasic” pKa values. The latter completely ignores a large part of the picture – partitioning of the neutrals and the ions between the two phases and ion-pairing in the low-polarity phase. In contrast, pKaow values account for these effects. The ratio of the acid and its conjugate base is measured in the 1-octanol phase, using UV-Vis and/or NMR spectrometric method. The activity of H+ is measured in the aqueous phase with a conventional pH-meter. The pKaow values are obtained at different concentrations and extrapolated to zero concentration.

Biphasic systems are present in many biological and technological systems and processes: cell membranes, solvent extraction, phase-transfer catalysis, sensor membranes, etc. In all such systems, acid-base properties of the participating compounds would be best described using biphasic rather than “mono-phasic” pKa values.

Derivatization-targeted analysis of amino compounds in plant extracts by LC-MS/MS

In the recently published paper including the members of our Analytical Chemistry Chair, a method was developed to detect amino compounds with diethyl ethoxymethylenemalonate (DEEMM) derivatization in neutral loss scan mode by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Amino compounds react with DEEMM, and the derivatives lose a neutral ethanol molecule [M+H-46]+ from the parent ion upon collision-induced dissociation (CID) fragmentation. Therefore, it is possible to employ neutral loss scan (NLS) mode of the triple quadrupole mass analyzer.

The first part of the work was dedicated to the development of the method, such as testing different solvents and quenching reagents in order to obtain a cleaner chromatographic profile. Hydroxylamine was chosen as the quenching reagent, considering the retention time of its DEEMM-derivative and its solubility in the solvent employed (0.1 M HCl in 30% methanol).

Finally, the developed method was applied to an extract from the plant species Carduus nutans subsp. macrocephalus (Desf.) Nyman, in which 18 amino acids and 3 biogenic amines were identified for the first time in this plant species. The method can be applied to different matrices for the detection of known and unknown amino compounds.

Here you can find the 50-day free access to the article.

New publication: Classification of archaeological adhesives

Our group recently published a new article: Classification of archaeological adhesives from Eastern Europe and Urals by ATR-FT-IR spectroscopy and chemometric analysis. Archaeometry, 2021, 1–18. 

In this study, 100 adhesive samples, collected from hafting tools and ceramic containers as well as some adhesive lumps were analyzed using ATR-FT-IR in combination with PCA-based DA classification. The aim of this study was to discover the capabilities of ATR-FT-IR-DA classification as a potential screening method for the identification and grouping of different archaeological adhesive samples, and subsequently reduce the use of laborious GC-MS analysis.

100 archaeological adhesives were classified into 3 groups: birch bark tar without major additives (72), birch bark tar with additives (13) and minor/non birch bark tar samples (15). Birch bark tar containing adhesives were separated from minor/non birch bark tar samples. Samples identified as birch bark tar without major additives were further classified possibly by their location, age or cultural specific manufacturing practices. The classification results were confirmed by GC-MS analysis of 9 archaeological samples selected from three compositional groups.

The study proves that ATR-FT-IR-DA classification is a non-destructive, rapid and reliable pre-scanning method for analyzing archaeological adhesives, especially suitable for small samples. Based on the results of ATR-FT-IR spectroscopic analysis, DA classification can help further distinguish samples with different backgrounds such as sample age, initial production, environmental conditions and site-specific preservation. GC-MS analysis could be used as a supplementary/confirmatory method to investigate samples with complex components and provide archaeological DA references for future research.

The full text can be found here.

New Open Access publication: GC-MS analysis of aged oil paints

Our group recently published a new Open Access research article – Quantitative GC-MS Analysis of Artificially Aged Paints with Variable Pigment and Linseed Oil Ratios, Molecules 2021, 26 (8), 2218. 

In this freely accessible publication, seven sets of artificially aged paints prepared from a pigment (chrome oxide green, natural cinnabar, Prussian blue, red ochre, hematite + kaolinite, zinc white, or yellow ochre) and linseed oil were analysed with GC-MS. One of the main aims was to study if the pigment concentration in the paint samples affects the drying of the linseed oil. 

The palmitic acid to stearic acid ratio (P/S), azelaic acid to palmitic acid (A/P), and the relative content of dicarboxylic acids (∑D) showed, that besides the type of the pigment, also the concentration of the pigment can influence these values that are commonly used to identify the type of the oil or to characterize how dried is the sample. 

The absolute quantification of stearic acid (see figure on the right) showed that the drying of all paint sets (except for zinc white) were influenced by the pigment concentration. Therefore, this is another factor that needs to be taken into account when studying aged oil paints. 

 

 

Aminoacridines as matrix materials for the analysis of complex samples in MALDI(-)-MS

A highly interdisciplinary study ranging from computational chemistry to cultural heritage has now been published – Experimental and Computational Study of Aminoacridines as MALDI(-)-MS Matrix Materials for the Analysis of Complex Samples, J. Am. Soc. Mass Spectrom. 2021.  

9-aminoacridine (9-AA) is a well-known matrix material used for the MALD(-)-MS analysis. Among the other monoaminoacridines (AAs), only 3-AA has been tested once (in our cultural heritage workgroup) but the suitability of the other AAs was unknown.

To fill this gap, the capabilities of all five AAs were studied by analysing different materials (stearic acid, colophony resin, dyer’s madder, and a resinous sample from a 16th-century shipwreck). Also, a vast range of properties for these aminoacridines were experimentally or computationally characterized (including UV-Vis absorption and fluorescence spectra, proton transfer reactions, crystallization). 

The results demonstrated, that all the AAs are suitable for the MALDI(-)-MS analysis of these materials. Interestingly, 3-AA and 4-AA outperformed the other AAs (including the best-known 9-AA) and were the preferred matrices for the analysis of samples studied in this work.   

 

 

Acidity scale in acetonitrile: 231 pKa values spanning 30 orders of magnitude

The equilibrium acidity scale (pKa scale) in acetonitrile (MeCN) has been supplemented by numerous new compounds and new ΔpKa measurements and has been published in Eur. J. Org. Chem. 2021, 1407–1419 (Open access). The pKa scale now contains altogether 231 acids – over twice more than published previously – linked by 566 ΔpKa measurements and spans between the pKa values of hydrogen iodide (2.8) and indole (32.57), covering close to 30 orders of magnitude. The acids have wide structural variety, ranging from common families – phenols (e.g. picric acid, alizarin, polyhalogenophenols), carboxylic acids (e.g. acetic, benzoic, retinoic and sorbic acid), sulfonic acids, hydrogen halides) to highly special molecules (chiral BINOL catalysts, bis(benzoxazole-2-yl)methanes, polyfluorinated compounds) and superacids (fluorinated sulfonimides, cyanoform, tetracyanopropenes).

Measurement results acquired over the last 15 years were added to the scale and new least squares treatment was carried out. The treatment yielded revised pKa values for the compounds published previously, with the root mean square difference between revised and previous values 0.04, demonstrating very good stability of the scale.

Correlation equations were developed for estimating pKa values for the studied types of compounds in water, DMSO, DMF and 1,2-dichloroethane on the basis of pKa values in acetonitrile. These equations enable predicting pKa values with an average error around or less than 1 pKa unit, which is a sufficient accuracy for many applications.

The scale is expected to be a useful tool for the widest possible research areas in organic chemistry, electrochemical power sources, catalysis, etc.

Design, synthesis and application of carbazole macrocycles in anion sensors

Our supramolecular analytical chemistry team has published an article (Beilstein J. Org. Chem. 2020, 16, 1901-1914) about developing carboxylate sensor prototypes that use macrocyclic carbazole receptors as ionophores.

Usually, reported research concludes with the demonstration of binding abilities of receptors and only suggests possible applications at a theoretical level. This work stands out from most publications by incorporating the entire workflow from receptor design to the characterisation of working sensor prototypes.

By involving this additional step, the team demonstrated the shortcoming of evaluating binding with just the receptor and analyte in solution. The predictions of selectivity can change considerably when measuring binding in an actual sensor membrane. This does not mean that such binding measurements would be obsolete, as the issue of binding anions selectively remains relevant in supramolecular chemistry.

The work was published in collaboration with the group of prof. Johan Bobacka at Åbo Akademi. The open access article was published in the Beilstein Journal of Chemistry thematic issue “Molecular Recognition” (in memory of Prof. Carsten Schmuck).

Symmetric Potentiometric Cells for the Measurement of Unified pH Values

Recently the UnipHied consortium published a new article – Symmetric Potentiometric Cells for the Measurement of Unified pH Values. Symmetry 2020, 12, 1150, where we compared the performance of different cells and instruments used to measure acidities on the unified pH scale (pHabs). The advantage of the unified pH scale is that it enables expressing the acidities of any systems – non-aqueous, solutions, colloidal systems, etc – via pHabsH2O values, which are directly comparable to the conventional pH values of the aqueous pH scale.

This is the first time unified acidities have been measured outside of our group. Nine partners from Europe – national metrology institutes and universities – have set up systems to measure pHabs and validated them with standard aqueous buffers.

The results show that the method is robust and does not depend on the cell nor the instrument (pH meter, potentiostat, or electrometer). The inherent symmetry of the cell design helps to reduce the experimental workload and improve the accuracy of the obtained results.

The next step is to measure unified acidities of non-aqueous systems, which could be used by routine laboratories to calibrate their pH electrodes.

The UnipHied project is funded from the EMPIR programme (project 17FUN09) co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme.

The secrets of two mummies have been unravelled

PLOS ONE has recently published a multidisciplinary paper on two Egyptian child mummies lead and participated by the members of our group. The team of 19 experts unravelled the secrets of two Graeco-Roman child mummies by applying most recent analytical techniques from archaeology, forensic sciences, analytical chemistry, medicine, entomology, and genetics. This is one of the most extensive multidisciplinary study of ancient mummies and the paper is free to download here.