Analytica Chimica Acta (v.431, #1)

Room temperature phosphorescence pH optosensor based on energy transfer by Wei Jun Jin; José Manuel Costa-Fernández; Alfredo Sanz-Medel (1-9).
In this paper, the development of a room temperature phosphorescence (RTP) optosensor for pH monitoring in aqueous media based on the effect of the pH on the energy transfer from a phosphor molecule (acting as a donor) to an adequate mixture of pH-sensitive dyes (acceptors) is presented. The proposed optosensor, to our knowledge the first RTP optical sensor for pH developed so far, is based on the immobilization in a solid support of a pH-insensitive phosphor molecule along with a mixture of three pH-sensitive dyes. 6-Bromo-2-naphthyl sulfate (BNS) and α-bromonaphthalene (BrN) are compared as donor molecules, because both can produce significant RTP emission in aqueous media (without deoxygenation) when retained in a polymeric resin or encapsulated in a rigid sol–gel matrix. Absorption spectra of the mixture of phenol red (PhR), bromocresol purple (BCP) and bromophenol blue (BPB) in a concentration ratio of 10−5:5×10−6:10−5  mol/l, respectively, possesses a desirable spectral overlap with the emission spectra of the donors (BNS and BrN) in a pH range around 3.5–9.2. Consequently, nonradiative energy transfer occurs from the phosphor molecules to the mixture of pH indicator dyes. An increase in the pH of the solution causes an absorption increase of the mixture of the pH indicators (acceptors) and, therefore, an increase in the amount of energy transfer occurs which brings about a decrease of the RTP emission. Thus, any changes in the absorption of the mixture of the three pH indicators are detected as measurable changes in the RTP emission of the sensing material. The proposed RTP optosensor has been evaluated for pH monitoring of different water samples.
Keywords: pH; Optosensor; Room temperature phosphorescence; Energy transfer; pH indicators; Bromo-2-naphthyl sulfate; α-Bromonaphthalene;

In co-operation with packaging material suppliers and a food manufacturer the quality of different kinds of wrapping foils for chocolate bars was investigated by using the hybrid Modular Sensor System (MOSES II). To check the reproducibility and to deduce quantitative predictions a gas chromatograph-mass spectrometer (GC/MS)-unit connected to a headspace-sampler (HSS) was used as a well established analytical reference instrumentation. All analysed samples were also qualified by a human sensory panel according to the sniff-test. The different packaging material species could be distinguished in a principal component analysis (PCA). With the aid of a principal component regression (PCR) a correlation between human and technical odour perception was carried out. The odour prediction of MOSES II was very satisfying particularly if compared to the human sensory assessments. Additionally, in order to follow the sensor fluctuations the sensor responses upon exposure to a standard sample have been monitored for 5 months. The results of these measurements and the investigations of four different MOSES II units demonstrated the long-term stability of the individual sensors and the high reproducibility from system to system.
Keywords: Quality control; Sensory evaluation; GC/MS; Gas sensor arrays; Sensor drift;

Determination of trace amounts of bisphenol F, bisphenol A and their diglycidyl ethers in wastewater by gas chromatography–mass spectrometry by José Luis Vı́lchez; Alberto Zafra; Antonio González-Casado; E. Hontoria; Monsalud del Olmo (31-40).
A simple and sensitive method for the determination of trace amounts of bisphenol F, bisphenol A and their diglycidyl ethers in wastewater is proposed. The method involves a liquid–liquid extraction (LLE) procedure using trichloromethane followed by a silylation step. Identification and quantification was performed by gas chromatography–mass spectrometry (GC–MS) using an HP1-MS column. The retention times were 6.59 and 7.02 min for BPF and BPA silylated, respectively, 8.30, 9.10 and 10.13 min for the three isomers of BFDGE and 11.12 min for BADGE. A clean-up is not necessary using SIM mode. Deuterated anthracene (2H10-anthracene) was used as an internal standard. The detection limits obtained were 0.02, 0.006, 0.13 and 0.09 μg l−1 for BPF, BPA, BFDGE and BADGE, respectively. The method was applied to the determination of these chemicals, as indicators of contamination by epoxy resins, in different types of spiked urban wastewater from Granada and Melilla (Spain) at concentration levels ranging from 0.06 to 2.50 μg l−1 for bisphenol F, 0.02 to 0.50 μg l−1 for bisphenol A, 0.45 to 50.00 μg l−1 for BFDGE and 0.30 to 50.00 μg l−1 for BADGE.
Keywords: Bisphenol F (BPF); Bisphenol A (BPA); Bisphenol A diglycidyl ether (BADGE); Bisphenol F diglycidyl ether (BFDGE); Gas chromatography–mass spectrometry; Liquid–liquid extraction; Silylation; Wastewater analysis;

The reaction mixture of β-cyclodextrin, phosphorous-pentoxide and l-glutamic acid was successfully used as chiral selector in capillary zone electrophoresis. The reaction process and the main properties of the reaction mixture was introduced in detail. The enantiomeric separation of lobeline and benzhexol were investigated. The chiral resolution was strongly influenced by the concentration of the reaction mixture, pH and the organic modifier. The two compounds studied can be enantiomerically resolved with a high resolution for lobeline (Rs=2.37) and benzhexol (Rs=2.60) in less than 14 min with no capillary treatment. A possible separation mechanism and several factors influencing the chiral separation were discussed.
Keywords: Chiral separation; Drug; Lobeline; Benzhexol; Capillary electrophoresis;

Resolution and quantification in poorly separated peaks from capillary zone electrophoresis using three-way data analysis methods by Sònia Sentellas; Javier Saurina; Santiago Hernández-Cassou; M.Teresa Galceran; Lluı́s Puignou (49-58).
This paper shows the use of three-way data analysis methods to improve resolution and quantification in poorly separated peaks from capillary zone electrophoresis (CZE). Second-order data (data matrices) obtained with a diode array detector (DAD) are analyzed. There is a large overlap in the electrophoretic time domain and the data do not have a strict trilinear structure. These shortcomings might hinder the mathematical resolution. However, three-way methods allow the pure electrophoretic profiles of the analytes to be recovered in an unknown mixture, even in the presence of uncalibrated interferences. Furthermore, the analyte can be quantified in unknown mixtures with reasonable accuracy using pure standards. As an example, ebrotidine metabolites were chosen to test the method.
Keywords: Capillary electrophoresis; Overlapping peaks; Ebrotidine metabolites; Three-way data analysis;

Facilitated transport and separation of aromatic amino acids through activated composite membranes by Juan Antonio Calzado; Cristina Palet; Manuel Valiente (59-67).
Activated composite membranes (ACM) containing bis-(2-ethyl-hexyl) phosphoric acid (DEHPA) have been found to facilitate the transport and separation of aromatic amino acids. The ACM, prepared according to a previously established procedure, were used to experimentally determine their behavior for the transport of phenylalanine (Phe), that was used as target amino acid. A general characterization of the Phe transport through DEHPA-ACM membranes including: the existence of facilitated counter transport, reproducibility and the effect of buffering the feed solution has been performed prior to a systematic study of the influence of some chemical transport parameters. In this concern, the initial Phe concentration, the nature and concentration of the stripping agent and the pH of the feed solution have been optimized (the feed phase was 0.5 mM of Phe in a 2 M NaCl solution at pH 3 and a 2 M HCl solution was used as stripping phase). Also, the influence of the DEHPA concentration in the casting solution used to prepare the membrane is established. It is found that the membrane phase controls the rate transport up to a DEHPA concentration of 1200 mM. Finally, although the affinity of the aromatic amino acids phenylalanine, tryptophan (Trp) and tyrosine (Tyr) for DEHPA is in the order Phe>Trp>Tyr, the selectivity of the DEHPA-ACM was found to be Trp>Phe>Tyr.
Keywords: Amino acid; Phenylalanine; Bis-(2-ethyl-hexyl) phosphoric acid (DEHPA); Facilitated transport; Activated composite membrane;

Solvent extraction of uranium, neptunium, plutonium, americium, curium and californium ions by bis(1-phenyl-3-methyl-4-acylpyrazol-5-one) derivatives by Hideyo Takeishi; Yoshihiro Kitatsuji; Takaumi Kimura; Yoshihiro Meguro; Zenko Yoshida; Sorin Kihara (69-80).
Comprehensive study has been performed on the extraction of actinide ions of various oxidation states, An n+ (An=U, Np, Pu, Am, Cm and Cf), with bis(1-phenyl-3-methyl-4-acylpyrazol-5-one) derivatives, H2BPn (n; number of polymethylene chain=3, 4, 5, 6, 7, 8, 10 and 22). All H2BPn derivatives showed higher extraction efficiencies for An n+ than 1-phenyl-3-methyl-4-benzoylpyrazol-5-one (HPMBP) did, which is attributable to the strong multidentate interaction with An n+ or high hydrophobicity of H2BPn in comparison with HPMBP. The highest efficiency for all An n+ examined was obtained with H2BP8 or H2BP7. The extracted species were confirmed as An(BPn)(HBPn) for Am3+, Cm3+ and Cf3+, and An(BPn)2 for U4+, Np4+ and Pu4+. The extracted species of UO2 2+ depended on the length of the polymethylene chain. The species UO2(HBPn)2 was extracted with H2BP3 or H2BP4 and UO2(BPn) with H2BPn of n≥5. NpO2 + was hardly extracted from the solution of pH less than 9. On the basis of the correlation between the distribution ratio and pH the procedure for the successive separation of the actinides in nitric acid medium was developed. The recommended procedure using H2BP8 was applied to the mutual separation of UO2 2+, NpO2 +, Pu4+ and trivalent transplutonium ions.
Keywords: Solvent extraction; Bis(1-phenyl-3-methyl-4-acylpyrazol-5-one); Uranium; Transuranium elements;

A novel new method is described for the determination of zirconium based on the cathodic adsorptive stripping of the mix-polynuclear complex of zirconium–calcium–alizarin red S (ARS) at a carbon paste electrode (CPE). The second-order derivative linear scan voltammograms of the adsorbed mix-polynuclear complex of zirconium–calcium–ARS are recorded by model JP-303 polarographic analyzer from +0.20 to 0.80 V (versus SCE). Optimal conditions are found to be: a glycine (0.096 M)–potassium biphthalate (0.036 M) buffer solution (pH 4.0) containing 8.0×10−4  M Ca2+ and 2.0×10−5  M ARS; accumulation potential, +0.20 V; accumulation time, 90 s; rest time, 10 s; scan rate, 500 mV s−1. The results show that the complex can be adsorbed on the surface of the CPE, yielding one peak at −0.50 V, corresponding to reduction of ARS in the complex at the electrode. The detection limit is found to be 4.0×10−11  M with 4 min of accumulation time. The linear range is from 1.0×10−10 to 2.0×10−7  M. Application of the procedure to the determination of zirconium in the ore samples gives good results.
Keywords: Adsorption voltammetry; Carbon paste electrode; Zirconium; Mix-polynuclear complex;

A method for the trace determination of Pb2+, using a disposable 1-(2-pyridylazo)-2-naphthol (PAN) drop-coated modified screen-printed carbon electrode (PAN-SPCE), has been developed. Lead ions (Pb2+) are accumulated from ammonia buffer, on the PAN-SPCE surface by the formation of a chemical complex at open circuit. The resulting SPCE with complexed Pb2+ is then transferred to 0.1 M HCl and subjected to differential pulse anodic stripping voltammetry. Conditions were optimised with respect to the pH and concentration of the accumulation medium, preconcentration time and the mass of drop-coated PAN. A 5 min preconcentration time resulted in a limit of detection of 15 ng ml−1. The calibration plot was found bimodal with linear ranges between 0 and 700 ng ml−1 and 700–5000 ng ml−1 Pb2+. The principal metal interference was copper, but this problem was successfully overcome by the addition of iodide to the ammonia buffer prior to the accumulation step. The method was evaluated by carrying out lead determinations on spiked potable water samples; the recovery of Pb2+ was 92.5% and the coefficient of variation was found to be 4.4% (n=5) using a separate PAN-SPCE for each determination. Therefore, the performance data suggests that the method is reliable at the concentrations examined in this study; in addition, the data compared favorably with that obtained by a published method.
Keywords: Screen-printed carbon electrode; 1-(2-Pyridylazo)-2-naphthol; Differential pulse anodic stripping voltammetry; Lead; Chemically modified electrode;

The voltammetric behaviour of five nitro-substituted aromatic compounds, such as nitrobenzene, 2-, 3-, 4-nitrophenol and 2,4-dinitrophenol, was investigated, and a method was developed for the simultaneous determination of these compounds, based on their reduction at a hanging mercury drop electrode (HMDE). It was found that by applying the differential pulse voltammetric (DPV) approach in a Britton–Robinson buffer of pH 5.4, nitrobenzene, 2-, 3- and 4-nitrophenol had well defined voltammetric reduction waves with peak potentials at −344, −284, −292 and −376 mV, respectively. However, 2,4-dinitrophenol had two peaks with potentials at −240 and −364 mV under the same conditions. For each compound linear calibration graphs were obtained in the general concentration range of 0.05–3.0 mg l−1. In mixtures of the five compounds, serious overlapping of voltammetric peaks was observed, and with conventional analytical methodology pre-separation steps would be required. In this study, chemometrics methods of data analysis, such as partial least squares (PLS), principal component regression (PCR) and classical least squares (CLS), were applied to resolve the overlapped voltammograms. Orthogonal experimental design was used for construction of the training sets containing the five aromatic compounds in the concentration range of 0.1–2.6 mg l−1. Five significant factors or principal components were modelled for prediction by the PLS and PCR methods, respectively. The percent of relative prediction error (RPE) was similar and acceptable for both methods being approximately ±10%. The percent of recoveries were within ±10% of the target value. The CLS method performed poorly (RPET=38%). The developed method was then applied to the analysis of these nitro-substituted aromatic compounds in field samples with similar satisfactory results.
Keywords: Nitrobenzene; 2-Nitrophenol; 3-Nitrophenol; 4-Nitrophenol; 2,4-Dinitrophenol; Chemometrics; Differential pulse voltammetry;

Use of circular dichroism and artificial neural networks for the kinetic-spectrophotometric resolution of enantiomers by Marcelo Blanco; Jordi Coello; Hortensia Iturriaga; Santiago Maspoch; Marta Porcel (115-123).
The circular dichroism (CD) technique was used to resolve 1-phenylethylamine enantiomers by their differential rate of reaction under non-pseudo first-order kinetic conditions with the chiral feagent (−)-citronellal. The same reaction was also monitored under pseudo first-order conditions using UV–VIS spectrophotometry and the results provided by the two techniques were compared by using principal component regression (PCR), partial least-squares regression (PLSR) and artificial neural networks (ANNs) for multivariate calibration. The best results were obtained by compressing the data matrix provided by the CD technique with principal component analysis (PCA) and using the scores of the principal components as input for the ANN. The relative standard error (R.S.E.) of prediction thus obtained was about 3% for both enantiomers.
Keywords: Circular dichroism; Artificial neural networks; Kinetic-spectrophotometric methods; Enantiomers; 1-Phenylethylamine;

A characterization of the effect of sample movement on spectral response using an FT-NIR spectrometer is presented. The data were evaluated by means of both multivariate data analysis and visual inspection of interferograms and spectra. A fiber-optic probe was used to collect spectra from model samples such as paper, plastic and fine and coarse powders. During controlled movement at velocities between 0.046 and 0.73 m s−1, it was found that sample movement causes features not normally present in the interferograms. However, it is shown that after transformation to frequency domain single-beam spectra these artifacts are present well outside the near infrared spectral range (10,000–4000 cm−1). The implication of these results is that FT-NIR is a suitable technique for process analysis of samples moving with low or moderate velocities.
Keywords: FT-NIR spectrometer; Interferograms; Fiber-optic probes;

Determination of glycogen by Rayleigh light scattering by Shu Zhen Zhang; Feng Lin Zhao; Ke An Li; Shen Yang Tong (133-139).
Glycogen in aqueous solution displays intense light scattering. Study by dynamic laser light scattering showed that glycogen in aqueous solution exists as particles, with average hydrodynamics radii (R h) ca. 22 nm for rabbit liver glycogen (RL gly) and 18 nm for oyster glycogen (O gly) and the particle size distribution width is narrow. Based on this, a method for glycogen determination based on Rayleigh light scattering using a conventional spectrofluorometer is developed. The light scattering intensity (LSI) is well proportional to the concentration of RL gly over the wide range of 0.8–4000 μg ml−1 and other two kinds of glycogen were also studied. The effect of factors such as temperature, ionic strength, pH, holding time and solvent were studied in detail and only organic solvents affect the LSI. There was little or no interference from monosaccharides or disaccharides, β-cyclodextrin, some proteins and metal ions. This method has many advantages such as extremely simplicity, stability, accuracy, reproducibility and wide linear range. Six synthetic samples were analyzed with satisfactory results.
Keywords: Glycogen; Rayleigh light scattering;

Liquid chromatographic separation and detection with catalytic analysis was investigated for highly sensitive and automated fractional determination of vanadium(IV) and -(V). Chromatographic separation between these species was performed with a mixture (pH 3.0) of ethylenediamine and tartaric acid as the eluent and a separation column containing sulphonate groups. The eluent was introduced to an air-segmented continuous flow analysis system based on the catalytic reaction with Bindschedler’s green leuco base. The detection limit of this method was 0.2 nmol dm−3 for both vanadium(IV) and -(V).
Keywords: Liquid chromatography; Catalytic analysis; Vanadium; Bindschedler’s green leuco base; Fractional determination;

A stopped-flow method for the determination of thiocyanates and thiosulphates by means of iodine-azide reaction has been elaborated. Spectrophotometric detection was applied to monitor the decrease of the absorbance of the iodine–starch complex at 595 nm within a few seconds. It was found that the rate of decrease near zero reaction time is related to the concentration of the sulphur compounds. The initial-rate method for pseudo-first-order reaction conditions has been successfully developed for the determination of traces of thiocyanates and thiosulphates. Under the optimum reaction conditions established for stopped-flow determination, the linear calibration range depends on the concentration of iodine in the iodine-azide solution applied and for thiocyanate is from 0.03 to 15 μg ml−1 (R.S.D is from 3.6 to 1.2%) and for thiosulphate is from 0.05 to 11 μg ml−1 (R.S.D. is from 2.7 to 1.8%).
Keywords: Thiosulphate; Thiocyanate; Stopped-flow; Iodine-azide;

Comparative study of different permanently-treated graphite tubes for the determination of As, Sb, and Se in natural waters by hydride generation-electrothermal atomic absorption spectrometry by Jorge Moreda-Piñeiro; Carmen Moscoso-Pérez; Purificación López-Mahı́a; Soledad Muniategui-Lorenzo; Esther Fernández-Fernández; Darı́o Prada-Rodrı́guez (157-165).
Methods for the direct trace determination of As, Sb, and Se in natural waters involving hydride generation, trapping, and atomization of the hydride in a graphite furnace have been developed. Graphite tubes permanently treated with uranium, tantalum, lanthanum oxide, niobium, beryllium oxide, chromium oxide, tantalum carbide, chromium carbide, and vanadium carbide were investigated for the in situ pre-concentration of the hydrides studied. The results obtained were compared with those achieved by iridium, tungstate and zirconium-treated graphite tubes (GT). Trapping on Zr-treated graphite tubes offers the best analytical performances for the hydrides studied, with characteristic masses of 45, 17 and 90 pg for As, Sb, and Se, respectively. In addition, the use of U-treated GTs offers adequate trapping efficiency for arsine; for stibine, the use of Cr3C2-, Cr2O3-, TaC-, VC- and U-treated GTs gives analytical performances similar to the use of Ir- and W-treated tubes. The accuracy was studied using TM-24, 1643d, CASS-3, and SLRS-4 reference materials. The 25 samples could be analyzed per hour.
Keywords: Hydride generation; Electrothermal atomization; In situ trapping; Permanently-treated graphite tubes; Atomic absorption spectrometry;