Analytica Chimica Acta (v.460, #1)

Countercurrent supercritical fluid extraction of phenolic compounds from aqueous matrices by Per Persson; Zoran Barisic; Arieh Cohen; Lars Thörneby; Lo Gorton (1-12).
Low-molecular weight aromatic compounds were extracted from aqueous samples in a packed high-pressure stainless steel column with supercritical carbon dioxide. A countercurrent flow approach was utilised to remove several phenolic and two non-phenolic compounds with similar aromatic structures directly from aqueous standard solutions as well as a real sample (a lignocellulosic dilute-acid hydrolysate of spruce) with supercritical carbon dioxide without sample pre-treatment. Phenolics are a diverse group of compounds that vary with regard to number, position and nature of their functional groups. The main purpose of this work was to investigate the influence of substituents on the extraction efficiency during countercurrent supercritical fluid extraction (SFE). The extraction column was 1100 mm long with an inner diameter of 25 mm. It was filled with Sulzer packings to increase the contact area between the solvent and the sample. The results showed that extraction efficiencies ranging from 2 to 100% in the standard solutions and 11 to 98% in the hydrolysate could be achieved depending on the various substituents. No correlation was observed between polarity indexes and extraction efficiencies for the investigated compounds; however, a relationship between retention in RP–HPLC and extractability could be established.
Keywords: Countercurrent SFE; Phenolics; Aqueous samples; Extractability; LC–MS analysis;

On the basis of a better analytical exploitation of acid-induced cloud point approach, a systematic study on the phase behaviour of acid aqueous solutions of anionic surfactants and factors affecting anionic surfactant-mediated extractions was performed. The anionic surfactants investigated were alkylsulphonates (ASS) with alkylchain lengths comprised between 8 and 16 carbon atoms. The critical hydrochloric acid concentration (minimal acid concentration required to separation in two liquid phases) was found to increase as alkylchain length of the anionic surfactant increased from 10 to 14. Non-acid-induced liquid–liquid phase separation was observed for sodium octanesulphonate (SOS) or sodium hexadecyl sulphonate (SHS) in the hydrochloric concentration range 0–10 M. Acid aqueous solutions of sodium decylsulphonate (SDeS) and sodium dodecylsulphonate (SDoS) separated into two liquid phases at temperatures ranging between 10 and 80 °C, while temperatures >35 °C were required for sodium tetradecylsulphonate. The influence on extraction efficiency and concentrating ability of experimental variables such as hydrophobicity and concentration of surfactant, nature and concentration of analyte, hydrochloric acid concentration, time and temperature of extraction and time of equilibration and centrifugation was examined. Advantages provided by anionic surfactant-mediated extractions over the use of non-ionic surfactants (cloud point extractions) are discussed.
Keywords: Acid-induced cloud point; Alkylsulphonate surfactants; Phase behaviour; Extraction of organic compounds;

Membrane supported bilayer lipid membranes array: preparation, stability and ion-channel insertion by G Favero; A D’Annibale; L Campanella; R Santucci; T Ferri (23-34).
In the present paper we describe the preparation and characterization of reconstituted bilayer lipid membranes (BLM) supported on a polycarbonate membrane. BLMs self-assemble on the holes of the membrane, which was previously covered with Au and an octadecanethiol (ODT) self-assembled monolayer (SAM). The obtained mixed hybrid bilayer lipid membranes (MHBLM) array allows recording of relatively intense signals. The steps leading to formation of the system have been investigated, its stability determined and reproducibility evaluated. The insertion of an ion channel (Gramicidin D) and a K+ carrier (Valinomycine) was also evaluated.
Keywords: Reconstituted biological membranes; Bilayer lipid membranes; Biosensors; Ion channels; Ion carriers; Valinomycine; Gramicidin;

Determination of copper in seawater based on a liquid membrane preconcentration system by Carolina Mendiguchı́a; Carlos Moreno; Manuel Garcı́a-Vargas (35-40).
A new method to determine the concentration of copper in seawater has been developed. It was based on the use of a bulk liquid membrane (BLM) system to separate and preconcentrate the copper ions, allowing the use of a very simple and common analytical technique as flame atomic absorption spectroscopy (FAAS). The liquid membrane consisted of an organic solution containing di-(2-ethylhexyl)phosphoric acid (DEHPA) in kerosene. The chemical optimization of the preconcentration system was performed by using the modified simplex method. Under optimum conditions, the preconcentration yield for real samples was 76.21%, copper preconcentration factor being 4.30. The reliability of the new method was confirmed by analyzing several real seawater samples, with copper concentrations ranging between 2.5 and 5 μg l−1, previously analyzed by a well-established technique. For these real samples, the average relative error of the determination was 9.96%, while relative standard deviation averaged 0.33%.
Keywords: Liquid membrane; Preconcentration; Copper determination; Seawater; FAAS;

Measurement of the isotopic composition and atomic weight of an isotopic reference material of platinum, IRMM-010 by C.S.J. Wolff Briche; A. Held; M. Berglund; P. De Bièvre; P.D.P. Taylor (41-47).
An isotopic reference material (IRM), IRMM-010, was prepared as a metallic platinum wire and certified for isotopic composition of platinum. The measurements were carried out with a quadrupole inductively coupled plasma mass spectrometer (ICP-MS) in peak jumping mode using a secondary electron multiplier as detector. Five synthetic isotope mixtures with certified isotope amount ratios of n(196Pt)/n(195Pt) were prepared in order to calibrate the measurements. The synthetic isotope mixtures, the natural platinum reference material as well as the isotopically enriched materials were measured in a 2% HNO3 and 2% H2O2 matrix in order to reduce memory effects during the measurements.These measurements resulted in certified isotope amount ratios and isotopic composition of a platinum reference material with natural isotopic composition. The relative abundances of the different platinum isotopes were determined as: 0.0117±0.0012 amount % of 190Pt, 0.782±0.016 amount % of 192Pt, 32.86±0.27 amount % of 194Pt, 33.77+0.16 amount % of 195Pt, 25.21+0.23 amount % of 196Pt, and 7.356±0.087 amount % of 198Pt. This lead to an atomic weight equal to 195.0844±0.0058 for this material. The purity of IRMM-010 was measured, the total impurity was found to be 140±140 μg·g−1. Uncertainties are expanded combined uncertainties expressed with a coverage factor k=2.
Keywords: Isotopic composition; Atomic weight; Platinum; Inductively coupled plasma mass spectrometry;

The application of on-line photochemistry with flow injection (FI) and liquid chromatography (LC) in conjunction with atmospheric pressure electrospray mass spectrometry (LC–APESI–MS) for the identification of similar indole derivatives is reported here. The photo-transformation of the indole compounds is strongly affected by the substituent groups on the aromatic and heterocyclic rings. Upon photolysis for 2.5 min, the mass spectrum of tryptamine (Try) which has no OH substituent on the aromatic ring does not differ greatly from that obtained without photolysis. However, after photolysis of serotonin (Ser) which has one OH group on C5 of the aromatic ring, the mass spectrum indicates the formation of dimers and higher molecular weight ions. The fragmentation pattern of 5-hydroxytryptophol (Phol) without photolysis resembles that of Ser with a base peak of m/z 160. Upon photolysis using MeOH–H2O (10/90), Phol is found to form a base peak at m/z 375 (100%) and a major peak at m/z 214 (66%) in addition to other ions with lower abundance. Melatonin (Mel) and tryptophan (Phan) upon photolysis are found to form high molecular weight ions with a relative low abundance. The mass spectrum of indole-3-acetic acid (Inaa) with on-line photolysis also shows different ions that are not formed without photolysis.
Keywords: Indoles; On-line photolysis; Flow injection; Mass spectrometry;

This work examines a novel approach of coupling high-performance liquid chromatography with diode-array detection as applied to the study of the analytical performance of the main phenolic classes contained in real samples. The chromatographic behavior of the main phenolic classes (benzoic and cinnamic acids and flavan-3-ols, flavonols and flavones as subclass of flavonoids) is carefully examined using real legume samples. Extracts from lentils proved to be excellent “real sample” because they contained the most important classes of phenolics that were subjected to current analysis. Benzoic and cinnamic acids, flavan-3-ols, flavonol and flavones were definitely or provisionally identified. Robustness, extraction efficiencies and accuracy were thoroughly scrutinized to reach quality results. Extracts showed an excellent stability according to their retention times in relative standard deviation (R.S.D.≤3%) and high reproducibility in peak area (R.S.D.≤5%). High reproducibility was also observed in calibration slopes (R.S.D.≤7%). Detection limits ranged between 0.09 and 0.64 μg/ml and, thus, showed full capacity for the determination of these compounds. Accuracy was carefully evaluated by studying extraction efficiencies and recoveries from well-known phenolic standards. The high recoveries (≥95%) obtained from the well-known phenolic structures of all representative phenolic classes allowed us to evaluate the accuracy of poorly known and commercially unavailable phenolic structures. The satisfactory analytical performance of currently employed methods had justified their recruitment for the quantification of phenolics in the study samples.
Keywords: Analytical performance; Phenolic compounds; HPLC–DAD; Legumes;

The self-modeling curve resolution (SMCR) technique called orthogonal projection approach (OPA) was used to resolve a complex set of Fourier transform (FT)-Raman spectra acquired on-line during styrene/1,3-butadiene block-copolymerization. The routine application of OPA did not yield physically acceptable results and manual corrections of the erroneous parts of the styrene concentration profiles had to be employed to improve the resolved spectra and concentrations of the two monomers and polybutadiene. The failure of automatic resolution by OPA is due to the ambiguity of SMCR. This problem is convincingly illustrated in the present case wherein is shown that physically reasonable manual correction significantly improve the results. The present study demonstrates that the combination of an automatic method for curve resolution and a suitable, efficient spectroscopic set-up enables real-time monitoring of this industrially important polymer reaction better than any conventionally employed technique.
Keywords: Self-modeling curve resolution; On-line processes; Orthogonal projection approach; Polymerization; Styrene/1,3-butadiene;

Two spectrophotometric methods are described and applied to resolve ternary mixtures of the corticosteroid hydrocortisone (HYD) and the antibiotics nystatin (NYS) and oxytetracycline (OXY). The simultaneous determination of these three compounds was firstly accomplished by a derivative method using the “ratio spectrum-zero crossing derivative” and then by multivariate methods partial least squares (PLS)-1, -2 and principal component regression (PCR). Multivariate calibration methods provide, specially PLS-2 in this case, a clear example of the high resolving powder of these techniques. The two described procedures do not require any separation step. Repeatability and reproducibility studies were achieved over two series of 10 standards for each compound showing no significant differences at 95% confidence level in the four spectrophotometric methods. A comparison of the derivative and multivariate calibration results obtained in pharmaceutical formulations was performed resulting in agreement of the values obtained and the results was confirm by a high-pressure liquid chromatography (HPLC) method.
Keywords: Corticosteroids; Derivative spectrophotometry; Partial least squares; Partial component regression; Hydrocortisone; Nystatin; Oxytetracycline;

Fenitrooxon [O,O-dimethyl-O-(4-nitro-m-tolyl)phosphate] is the major metabolite of the organophosphorus insecticide fenitrothion, and 3-methyl-4-nitrophenol is its major degradation product. In the present study, we describe the development of an indirect competitive enzyme-linked immunosorbent assay (ELISA) for the detection of these compounds in water samples based on a group-specific polyclonal antiserum generated with a “bifunctional hapten”, which has two functions: the conventional function of producing an antibody against an antigen and a unique function of promoting the production of the antibodies in rabbit. For application to water samples, the influence of several factors such as organic solvent, pH, and detergent was studied. Under optimized conditions, the quantitative working range of the fenitrooxon ELISA was 0.71–27 ng ml−1 with a limit of detection (LOD) of 0.32 ng ml−1, and the fenitrooxon concentration giving 50% reduction of the maximum signal (IC50) was 4.2 ng ml−1. The quantitative working range of the 3-methyl-4-nitrophenol ELISA was 0.67–27 ng ml−1 with a LOD of 0.38 ng ml−1 and an IC50 of 3.7 ng ml−1. No significant matrix effect originating from the water sample (river water, tap water, purified water, and bottled water) was shown by addition of Tween 20 to the assay buffer. Water samples spiked with each of these compounds at 1, 5, 10, and 20 ng ml−1 were directly analyzed without extraction and clean-up by the proposed ELISA. The mean recovery was 100.9%, and the mean coefficient of variation (CV) was 7.7% for the fenitrooxon ELISA and for the 3-methyl-4-nitrophenol ELISA, the mean recovery was 97.6%, and the mean CV was 7.2%. The proposed ELISA allows precise and accurate determination of these compounds in water at such low levels.
Keywords: Group-specific ELISA; Fenitrooxon; 3-Methyl-4-nitrophenol; Bifunctional hapten; Tween 20; Water analysis;

Direct mercury determination in aqueous slurries of environmental and biological samples by cold vapour generation–electrothermal atomic absorption spectrometry by Jorge Moreda-Piñeiro; Purificación López-Mahı́a; Soledad Muniategui-Lorenzo; Esther Fernández-Fernández; Darı́o Prada-Rodrı́guez (111-122).
Direct cold vapour generation from aqueous slurries of environmental (marine sediment, soil, coal) and biological (human hair, seafood) samples have been developed using a batch mode generation system coupled with electrothermal atomic absorption spectroscopy. The effects of several variables affecting the cold vapour generation efficiency from solid particles (hydrochloric acid and sodium tetrahydroborate concentrations, argon flow rate, acid solution volume and mean particle size) have been evaluated using a Plackett–Burman experimental design. In addition, variables affecting cold vapour trapping and atomisation efficiency on Ir-treated graphite tubes (trapping and atomisation temperatures and trapping time) have been also investigated. Atomisation and trapping temperatures, trapping time and hydrochloric acid concentration were the significant variables. The 22+star and 23+star central composite designs have been used to obtain optimum values of the variables selected. The accuracy of methods have been verified by using several certified reference materials (PACS-1, GBW-07410, NIST-1632c, CRM-397 and DORM-2). A characteristic mass of 390 pg were achieved. The detection limits of methods were in the range of 40–600 ng g−1. A particle size less than 50 μm is adequate to obtain total cold vapour generation of Hg content in the aqueous slurry particles.
Keywords: Cold vapour; Electrothermal atomisation; Aqueous slurry; Environmental and biological samples;

A new proton-selective fluorescing indicator, 3,3′,5,5′-tetramethyl-N-(9-anthrylmethyl)benzidine (TMANB) has been synthesized and applied in an optode membrane for the determination of lead ion that works on the basis of a cation-exchange mechanism. When embedded in a plasticized poly(vinyl chloride) (PVC) membrane containing lead ionophore (tert-butylcalix[4]arene-tetrakis(N,N-dimethylthioacetamide)) and a lipophilic anionic site [potassium tetrakis(4-chlorophenyl)borate], TMANB shows a significant fluorescence signal change on exposure to aqueous HCl solution containing lead ion, which exhibits the theoretically expected fluorescence response to lead ion concentration. The selectivity, response time, reproducibility and reversibility, and lifetime of the optode membrane were discussed.
Keywords: Fluorescent sensor; PVC membrane; Ion-exchange; Lead; TMANB;

Surface plasmon resonance study on enhanced refractive index change of an Ag+ ion-sensing membrane containing dithiosquarylium dye by Youngjune Hur; Kyeongsik Ock; Kyungho Kim; Sungho Jin; Yeongsoon Gal; Jaeho Kim; Sunghoon Kim; Kwangnak Koh (133-139).
In this study, we have fabricated an Ag+ ion-sensing membrane with a dithiosquarylium (DTSQ) dye containing a polymeric film. The selective sensing signal through the electrostatic interaction between the DTSQ dye and the Ag+ metal cation was effectively transduced to the refractive index (RI) change corresponding to shifts of the surface plasmon resonance (SPR) angle. In addition, a good selective Ag+ ion detection appeared in a wide concentration range from 10−4 to 10−12  M. The resonance angle shift is interpreted with Fresnel equations and Kramers–Kronig relation. In light of these calculations, the enhanced RI increase in this sensing membrane appeared to be caused by the decrease of absorption coefficient of DTSQ dye around the wavelength of SPR probe beam. These results suggest that chromogenic approaches (λ max control of Ag+ ion-sensing membrane with a DTSQ dye by appropriate molecular design) related to SPR phenomena (RI change at the wavelength of probe beam) offer a good strategy for highly sensitive metal ion detection.
Keywords: Surface plasmon resonance; Refractive index change; Dithiosquarylium dye; Metal ion detection;