Maria Ochsenkühn-Petropoulou

Speciation analysis of selenium using voltammetric techniques

Abstract A simple and accurate method for the separation and determination of Se(IV), selenocystine and dimethyldiselenide has been developed based on differential pulse cathodic stripping voltammetry (DPCSV) at a hanging mercury drop electrode (HMDE) and an extraction procedure for the prior separation of the Se species. Thus, selenocystine and Se(IV) were determined simultaneously in the aqueous phase using HCl as electrolyte, while the determination of dimethyldiselenide was achieved in the organic phase after extraction of the sample with dichloromethane using as electrolyte LiClO4 in absolute ethanol acidified with HCl. No significant effect on the determination of selenocystine by the presence of sulfur-cystine, even in a 300-fold greater concentration, was observed. For the optimization of the procedure several parameters of the voltammetric determination, such as the deposition time and voltage, the kind of electrolyte and the acidity of the sample as well as the kind of extractant for the effective separation of the Se species were investigated in the aqueous and organic phase. Moreover, the quantitatively transformation of the electrochemically inactive Se(VI) to Se(IV) was achieved by testing different reducing agents. The capability of the developed procedure was tested on the SRM 1633a (coal fly ash) reference material and the whole procedure was applied on a soil sample of Thermopyles Spa (Greece).

Comparison of inductively coupled plasma mass spectrometry with inductively coupled plasma atomic emission spectrometry and instrumental neutron activation analysis for the determination of rare earth elements in Greek bauxites

Abstract For the determination of rare earth elements (REE) in bauxitic materials the techniques of inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma atomic emission spectrometry (ICP-AES) and instrumental neutron activation analysis (IN AA) were compared. In the NIST (National Institute of Standards and Technology) bauxites SRM 697 Dominican, and SRM 69 b Arkansas, the concentration of some REEs were determined. With the reference bauxite BX-N of the ARNT (Association Nationale de la Recherche Technique) the precision and acccuracy of ICP-AES for the determination of REEs in bauxites was tested. Furthermore, Greek bauxites of the Parnassos-Giona area were investigated. In a comparison of the three methods it was possible to calculate from the data series the precision of each method, which showed that the tendency found in the deviations for the different REEs is in acccordance with published values. Also the limits of detection for REEs in bauxites were calculated and found to be in the same range of those in the literature.

Electrochemical speciation of organotin compounds in water and sediments. Application to sea water after ion-exchange separation

For the quantitative speciation of tributyltin, Bu3Sn+ (TBT), in the presence of dibutyltin, Bu2Sn2+ (DBT), monobutyltin, BuSn3+ (MBT), triphenyltin, Ph3Sn+ (TPT), and inorganic tin in water samples and sediments, an accurate, reproducible, simple and rapid electrochemical method was developed. After extraction of the organotin compounds with dichloromethane, TBT could be selectively determined as species by alternating current polarography directly in the organic phase without any derivatisation. The successful application of this technique could be proved by the results obtained by intercomparison exercises on TBT in water samples and sediments, organized by the Community Bureau of Reference (BCR). For the application of this technique to sea water samples a preliminary ion exchange separation of TBT from the major components of sea water was performed, achieving a detection limit for TBT in the ppt range.

Size characterization of manganese species from human serum and cerebrospinal fluid using size exclusion chromatography coupled to inductively coupled plasma mass spectrometry

Manganese, at excess concentrations, can have adverse effects on health. Mn is a neurotoxicant. The health impacts of Mn are known mostly from occupational health studies, but the exact mechanisms by which Mn can enter the brain without transferrin-receptor mediated transport is unknown. Mn speciation at the neural barriers can help to obtain more information about the pathways and carriers. This paper therefore investigates the size distribution of Mn carriers (e.g., proteins, peptides, carbonic acids) in serum before the neural barriers and in cerebrospinal fluid behind them, as a first step in characterization of the Mn carriers involved in moving Mn across the neural barriers. Serum samples showed Mn concentrations between 1.5–2.5 μg L−1 (mean 1.7 ± 0.8 μg L−1), the lower figure being fully in the physiological range, the higher being slightly elevated. The size distribution of Mn corresponding to the carriers showed a main peak in the transferrin/albumin size fitting to the known physiological ligands. However, preliminary results also indicated an increasing Mn peak at 700 Da with increasing total Mn concentration. This finding could point to excess Mn being transported by a low molecular mass (LMM) carrier. Samples of cerebrospinal fluid (CSF) showed Mn concentrations between 1.8–6.7 μg L−1 (mean 2.66 μg L−1, = 48 nM). Only sparse values have been published with which our results could be compared. The lower concentrations seem to be in the physiological range, whilst the higher ones are clearly elevated. In cerebrospinal fluid Mn was found to be nearly exclusively bound to LMM Mn carriers. The major fraction is in the range of 640–680 Da, which is similar to the LMM compound in serum and to Mn–citrate complexes suggested to be present in body fluids. Therefore, citrate was additionally determined in CSF. The concentration ranged from 420–713 μM (mean 573 μM), proving that citrate is in huge excess compared with Mn (ratio 1:12000). This finding makes a Mn–citrate complex in CSF likely.

Insight into the retention mechanism on immobilized artificial membrane chromatography using two stationary phases

The retention behavior of sixty structurally diverse drugs on two immobilized artificial membrane (IAM) columns, IAM.PC.MG and IAM.PC.DD2 types, at two pH values, 7.4 and 5.5, was established. Extrapolated to pure aqueous phase retention factors, logkw(IAM), were determined and the role of acetonitrile as organic modifier was explored, considering the relationships with the slopes, S, of the extrapolation procedure. Good interrelations between retention factors on the two IAM stationary phases were observed, although logkw(IAM.PC.DD2) values are generally higher than logkw(IAM.PC.MG). In order to investigate the underlying retention mechanism, relationships between IAM retention factors and lipophilicity, expressed as logP or logD at pH 7.4 were established. Electrostatic interactions were considered by introducing the positively and negatively charged molecular fractions as additional parameters in the logkw(IAM)/logD relationships. The positive contribution of these fractions supported the involvement of the electrostatic interactions in the retention mechanism. Special attention was given to the retention behavior of zwitterionic compounds and for compounds with special structural characteristics.

Exploring the elution mechanism of selenium species on liquid chromatography.

In the present work, the chromatographic behavior of eight selenium species, namely selenites (Se(IV)), selenates (Se(VI)), seleno-DL-methionine (Se-Met), selenocystine (Se-Cyst), selenocystamine (Se-CM), selenourea (Se-U), dimethylselenide ((CH(3))(2) Se) and dimethyldiselenide ((CH(3))(2) Se(2)), was investigated under different stationary and mobile phase conditions, in an effort to unravel secondary interferences in their underlying elution mechanism. For this purpose, two end-capped and a polar-embedded reversed-phase stationary phases were employed using different mobile phase conditions. Retention factors (log k(w)) were compared with octanol-water distribution coefficients (log D) as well as with log k(w) values on two immobilized artificial membrane (IAM) columns and two immobilized artificial plasma proteins stationary phases, obtained in our previous work. The role of electrostatic interactions was confirmed by introducing the net charge of the investigated Se species as an additional term in the log k(w)/log D interrelation, which in most cases proved to be statistically significant. Principal component analysis of retention factors on all stationary phases and octanol-water log D values, however, showed that the elution of the investigated selenium species is mainly governed by partitioning mechanism under all different chromatographic conditions, while the pH of the mobile phase and the special column characteristics have only a minor effect.

An approach for manganese biomonitoring using a manganese carrier switch in serum from transferrin to citrate at slightly elevated manganese concentration.

After high-dose-short-term exposure (usually from occupational exposure) and even more under low-dose long term exposure (mainly environmental) manganese (Mn) biomonitoring is still problematic since these exposure scenarios are not necessarily reflected by a significant increase of total Mn in blood or serum. Usually, Mn concentrations of exposed and unexposed persons overlap and individual differentiation is often not possible. In this paper Mn speciation on a large sample size (n=180) was used in order to be able to differentiate between highly Mn-exposed or low or unexposed individuals at low total Mn concentration in serum (Mn(S)). The whole sample set consisted of three subsets from Munich, Emilia Romagna region in Italy and from Sweden. It turned out that also at low total Mn(S) concentrations a change in major Mn carriers in serum takes place from Mn-transferrin (Mn-Tf(S)) towards Mn-citrate (Mn-Cit(S)) with high statistical significance (p<0.000002). This carrier switch from Mn-Tf(S) to Mn-Cit(S) was observed between Mn(S) concentrations of 1.5μg/L to ca. 1.7μg/L. Parallel to this carrier change, for sample donors from Munich where serum and cerebrospinal fluid were available, the concentration of Mn beyond neural barriers - analysed as Mn in cerebrospinal fluid (Mn(C)) - positively correlates to Mn-Cit(S) when Mn(S) concentration was above 1.7μg/L. The correlation between Mn-Cit(S) and Mn(C) reflects the facilitated Mn transport through neural barrier by means of Mn-citrate. Regional differences in switch points from Mn-Tf(S) to Mn-Cit(S) were observed for the three sample subsets. It is currently unknown whether these differences are due to differences in location, occupation, health status or other aspects. Based on our results, Mn-Cit(S) determination was considered as a potential means for estimating the Mn load in brain and CSF, i.e., it could be used as a biomarker for Mn beyond neural barrier. For a simpler Mn-Cit(S) determination than size exclusion chromatography inductively coupled plasma mass spectrometry (SEC-ICP-MS), ultrafiltration (UF) of serum samples was tested for suitability, the latter possibly being a preferred choice for routine occupational medicine laboratories. Our results revealed that UF could be an alternative if methodical prerequisites and limitations are carefully considered. These prerequisites were determined to be a thorough cleaning procedure at a minimum Mn(S) concentration >1.5μg/L, as at lower concentrations a wide scattering of the measured concentrations in comparison to the standardized SEC-ICP-MS results were observed.

A Novel Way of Sampling Particles Containing Platinum Group Elements from Automobile Catalysts

A new sampling system was developed in order to be applied directly at the exhaust pipe of vehicles combined with an exhaust gas analyzer during the standard New European Driving Cycle (NEDC). In that way a quantitative sampling of Pt, Pd, and Rh (Platinum Group Elements, PGEs) in particle form on filters and gas monitoring could be achieved simultaneously, applying the sampling system on a variety of vehicles of different age and engine capacity. The samples were examined by SEM-EDX for morphological and qualitative analysis of the particles. By using an appropriate digestion procedure and the analytical techniques of ICP-MS and GFAAS, concentrations were found in the range of 0.35–87 ng/km for Pt, 3.3–437 ng/km for Pd, and 0.9–72 ng/km for Rh. Furthermore the investigation showed that fresh catalysts as well as engines with larger capacity emitted higher amounts of PGEs in comparison to older catalysts and smaller capacity engines. According to the results of this research the palladium emissions were dominant in comparison to the platinum ones. This way of sampling might prove advantageous in testing new technology catalysts through their PGEs emission.

Biomimetic chromatographic analysis of selenium species: Application for the estimation of their pharmacokinetic properties

AbstractThe retention behavior of selenites, selenates, seleno-dl-methionine, selenocystine, selenocystamine, selenourea, dimethyl selenide, and dimethyl diselenide was investigated by means of biomimetic liquid chromatography. For this purpose, two immobilized artificial membrane (IAM) columns, namely, IAM.PC.DD2 and IAM.PC.MG, and two immobilized plasma protein columns, human serum albumin (HSA) and α1-acid glycoprotein (AGP) columns, were employed using different mobile phase conditions in respect to pH and buffer composition. In general, satisfactory interrelations between retention factors obtained with the two IAM stationary phases and HSA/AGP columns were obtained. Large differences were observed between biomimetic retention factors and octanol–water logD values, since the latter fail to describe electrostatic interactions. In contrast, despite the column diversity, the net retention outcome on all four biomimetic columns was quite similar, especially in the presence of phosphate-buffered saline, which by its effective shielding alleviates the differences between the stationary phases. Of the two IAM columns, IAM.PC.DD2 showed better performance when compared with HSA and AGP columns as well as to octanol–water partitioning. Biomimetic chromatographic indices were further used to estimate the percentage of human oral absorption and plasma protein binding of the eight selenium species investigated, according to equations previously reported in the literature. The estimated values of human oral absorption imply moderate absorption only for dimethyl diselenide, which also may exhibit considerable plasma protein binding. Moderate affinity for plasma proteins should also be expected for dimethyl selenide and selenocystamine. FigureBiomimetic chromatography in estimating pharmacokinetic properties of Se species

Investigation of the chromatographic behaviour of some selenium species--comparison with their octanol-water partitioning.

The retention behaviour of selenites (Se(IV)), selenates (Se(VI)), seleno-dl-methionine (Se-Met), selenocystine (Se-Cyst), selenocystamine (Se-CM) and selenourea (Se-U) was investigated using a Discovery end-capped reversed-phase column as stationary phase and different mobile phase conditions. Extrapolated to 100% aqueous mobile phase retention factors (logk(w)) of the investigated Se species, determined using different methanol fractions (phi) as organic modifier, were compared with the corresponding actual values. The proper operation of this column even at 100% aqueous phase proved to be valuable for the accurate determination of logk(w) values of Se-CM and Se-Cyst, presenting a convex curvature logk=f(phi) at low MeOH fractions, often neglected in the extrapolation procedure. The effect of the presence of n-decylamine as well as saturation of the mobile phase with n-octanol was also studied. For ampholytic Se-Met and Se-Cyst the effect of n-decylamine in retention reflected the predominance of zwitterionic nature in the case of Se-Met in contrary to the non-zwitterionic species found in the case of Se-Cyst, in accordance with our previous findings concerning partitioning experiments in the n-octanol/water system. Finally, an attempt was made to correlate logk(w) values with the logarithm of n-octanol/water distribution coefficient, logD, of the investigated Se species and an indicative logD value of Se-U was derived.