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Analytica Chimica Acta (v.627, #2)

Editorial Board (pp. i).

Contents (pp. iii-iv).

Analytical applications of second-order calibration methods by Verónica Gómez; María Pilar Callao (pp. 169-183).

Second-order calibration methods are gaining widespread acceptance among the analytical community mainly because: (1) a wide range of analytical instrumentation is available that enables high dimensionality data to be obtained; (2) the chemometric techniques for treating these data are highly developed; and (3) they have the so-called “second-order advantage”, i.e. they can predict the concentration of the analyte of interest even in the presence of unknown interferents. This also enables several analytes to be determined simultaneously.In this paper we describe the most common instrumental and chemometric techniques used in second-order calibration and discuss their applications since 2000. First, we introduce briefly the techniques and then we comment the applications. Given the practical nature of this paper, we have classified the techniques according to five fields of application: pharmaceuticals, biological matrices, foods, environmental matrices and synthetic matrices.

Keywords: Chemometrics; Second-order calibration; Pharmaceuticals; Biological matrices; Foods; Environmental matrices

A critical review in calibration methods for solid-phase microextraction by Gangfeng Ouyang; Janusz Pawliszyn (pp. 184-197).

Solid-phase microextraction (SPME) was developed to address the need for rapid sampling and sample preparation, both in the laboratory and on-site. Unlike traditional sample preparation methods, SPME is a non-exhaustive extraction technique in which only a small portion of the target analyte is removed from the sample matrix. Therefore, calibration of SPME for quantitative analysis is very important. In this review, we summarized the proposed SPME calibration methods and the characteristics of these methods were discussed.

Keywords: Solid-phase microextraction; Traditional calibration; Equilibrium extraction; Exhaustive extraction; Diffusion-based calibration

Evaluation of alternate lines of Fe for sequential multi-element determination of Cu, Fe, Mn and Zn in soil extracts by high-resolution continuum source flame atomic absorption spectrometry by Jorge Luiz Raposo Júnior; Silvana Ruella de Oliveira; Naíse Mary Caldas; José Anchieta Gomes Neto (pp. 198-202).

The usefulness of the secondary line at 252.744nm and the approach of side pixel registration were evaluated for the development of a method for sequential multi-element determination of Cu, Fe, Mn and Zn in soil extracts by high-resolution continuum source flame atomic absorption spectrometry (HR-CS FAAS). The influence of side pixel registration on the sensitivity and linearity was investigated by measuring at wings (248.325, 248.323, 248.321, 248.329, and 248.332nm) of the main line for Fe at 248.327nm. For the secondary line at 252.744nm or side pixel registration at 248.325nm, main lines for Cu (324.754nm), Mn (279.482nm) and Zn (213.875nm), sample flow-rate of 5.0mLmin⁻¹ and calibration by matrix matching, analytical curves in the 0.2–1.0mgL⁻¹ Cu, 1.0–20.0mgL⁻¹ Fe, 0.2–2.0mgL⁻¹ Mn, 0.1–1.0mgL⁻¹ Zn ranges were obtained with linear correlations better than 0.998. The proposed method was applied to seven soil samples and two soil reference materials (IAC 277; IAC 280). Results were in agreement at a 95% confidence level (paired t-test) with reference values. Recoveries of analytes added to soil extracts containing 0.15 and 0.30mgL⁻¹ Cu, 7.0 and 14mgL⁻¹ Fe, 0.60 and 1.20mgL⁻¹ Mn, 0.07 and 0.15mgL⁻¹ Zn, varied within the 94–99, 92–98, 93–101, and 93–103% intervals, respectively. The relative standard deviations ( n=12) were 2.7% (Cu), 1.4% (Fe – 252.744nm), 5.7% (Fe – 248.325nm), 3.2% (Mn) and 2.8% (Zn) for an extract containing 0.35mgL⁻¹ Cu, 14mgL⁻¹ Fe, 1.1mgL⁻¹ Mn and 0.12mgL⁻¹ Zn. Detection limits were 5.4μgL⁻¹ Cu, 55μgL⁻¹ Fe (252.744nm), 147μgL⁻¹ Fe (248.325nm), 3.0μgL⁻¹ Mn and 4.2μgL⁻¹ Zn.

Keywords: Micronutrients; Soil; High-resolution continuum source flame atomic spectrometry; Side pixel registration

Recognition of phospho sugars and nucleotides with an array of boronic acid appended bipyridinium salts by Alexander Schiller; Boaz Vilozny; Ritchie A. Wessling; Bakthan Singaram (pp. 203-211).

The solution-phase sensor array of three cationic bis-boronic acid appended benzyl viologens (BBV) and the anionic fluorescent dye, 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS), is able to discriminate among five phospho sugars, four nucleotides and three neutral saccharides in aqueous buffered solution at low mM concentrations. Linear discriminant analysis, principal component analysis, and hierachical cluster analysis studies showed the “discrimination limit” (lowest analyte concentration where the discrimination is still 100%) to be 4mM. Calculated K_b and F_max/ F₀ values from binding curves of the three BBVs with1–12 were also used to perform multi-variate analyses with very good discrimination results.

Keywords: Boronic acid; Nucleotide; Phospho sugar; Sensor array; Viologen; Fluorescence

Optimization of headspace solid-phase microextraction for analysis of ethyl carbamate in alcoholic beverages using a face-centered cube central composite design by Ying Zhang; Jian Zhang (pp. 212-218).

The headspace solid-phase microextraction (HS-SPME) of ethyl carbamate from alcoholic beverages was optimized for the first time using a face-centered cube central composite design (CCD). The factors expected to influence the extraction process are discussed. Firstly, some of factors are fixed based on the opinion of expert and previous experiments, which reduce the number of factors and then avoid very complex response models and large variability. Secondly, for three remaining inexplicit factors, sample temperature, pH and %NaCl, a face-centered cube central composite design was performed and a response surface equation was derived. The statistical parameters of the derived model were r=0.974 and F=20.183. The optimum conditions were obtained using a grid method. Next, the method was analytically evaluated using the optimum conditions. The detection limit, relative standard deviation, linear range and recovery were 3μgL⁻¹, 4.3–8.6%, 10–160μgL⁻¹, and 92.8–97.5%, respectively. Finally, the method was applied to a variety of alcoholic beverages.

Keywords: Optimization; Solid-phase microextraction; Ethyl carbamate; Alcoholic beverages; Face-centered cube central composite design

Sensitive and stable monitoring of lead and cadmium in seawater using screen-printed electrode and electrochemical stripping analysis by Raquel Güell; Gemma Aragay; Clàudia Fontàs; Enriqueta Anticó; Arben Merkoçi (pp. 219-224).

Sensitive and stable monitoring of heavy metals in seawater using screen-printed electrodes (SPE) is presented. The analytical performance of SPE coupled with square wave anodic stripping voltammetry (SWASV) for the simultaneous determination of Pb and Cd in seawater samples, in the low μgL⁻¹ range, is evaluated. The stripping response for the heavy metals following 2min deposition was linear over the concentration range examined (10–2000μgL⁻¹) with detection limits of 1.8 and 2.9μgL⁻¹ for Pb and Cd, respectively. The accuracy of the method was validated by analyzing metal contents in different spiked seawater samples and comparing these results to those obtained with the well-established anodic stripping voltammetry using the hanging mercury drop electrode. Moreover, a certified reference material was also used and the results obtained were satisfactory.

Keywords: Anodic stripping voltammetry; Screen-printed electrodes; Lead; Cadmium; Seawater monitoring

Carboxyl modified multi-walled carbon nanotubes as solid-phase extraction adsorbents combined with high-performance liquid chromatography for analysis of linear alkylbenzene sulfonates by Zhuo Guan; Yuming Huang; Weidong Wang (pp. 225-231).

Recently, multi-wall carbon nanotubes (MWCNTs) as adsorbents of solid-phase extraction are attractive because they can be used for enrichment of organic compounds and metal ions at trace levels. In this study, we use the carboxyl modified multi-wall carbon nanotubes (CMMWCNTs) as adsorbents of solid-phase extraction for extraction of linear alkylbenzene sulfonates (LAS), which are widely used anion surfactant with different homologues, and detected by HPLC–UV. The effect of eluent and its volume, sample pH and flow rate, sample volume and the ultrasonic time of sample, the content of the electrolyte (NaCl) were investigated and optimized. The detection limit for LAS homologues was 0.02–0.03μgL⁻¹ with R.S.D. ( n=6) ranging from 2.04 to 10.03%. The recoveries of LAS homologues in the spiked environmental water samples ranged from 84.8 to 106.1%. The proposed method has been applied successfully to the analysis of LAS in aqueous environmental samples, which demonstrates that CMMWCNTs-based solid-phase extraction is a precision and convenient enrichment method and can be used for analysis of LAS homologues in water samples.

Keywords: Carboxyl modified multi-wall carbon nanotubes; High-performance liquid chromatography; Solid-phase extraction; Linear alkylbenzene sulfonates

Flow injection conductometric system with gas diffusion separation for the determination of Kjeldahl nitrogen in milk and chicken meat by Jaroon Junsomboon; Jaroon Jakmunee (pp. 232-238).

A simple flow injection (FI) conductometric system with gas diffusion separation was developed for the determination of Kjeldahl nitrogen (or proteins) in milk and chicken meat. The sample was digested according to the Kjeldahl standard method and the digest was diluted and directly injected into the donor stream consisting of 4M NaOH. In alkaline medium, ammonium was converted to ammonia, which diffused through the PTFE membrane to dissolve in an acceptor stream (water). Dissociation of ammonia caused a change in conductance of the acceptor solution, which was linearly proportional to the concentration of ammonium originally present in the injected solution. A conductometric flow through cell and an amplifier circuit was fabricated, which helped improve sensitivity of the conductometric detection system. With using a plumbing Teflon tape as a gas diffusion membrane and without thermostating control of the system, a linear calibration graph in range of 10–100mgL⁻¹ N-NH₄ was obtained, with detection limit of 1mgL⁻¹ and good precision (relative standard deviation of 0.3% for 11 replicate injections of 50mgL⁻¹ N-NH₄). The developed method was validated by the standard Kjeldahl distillation/titration method for the analysis of milk and chicken meat samples. The proposed system had sample throughput of 35h⁻¹ and consumed much smaller amounts of chemical than the standard method (275mg vs 17.5g of NaOH per analysis, respectively).

Keywords: Flow injection; Gas diffusion; Conductometry; Ammonium; Proteins; Milk; Chicken; Kjeldahl

Dependence of calibration sensitivity of a polysulfone/Ru(II)-Tris(4,7-diphenyl-1,10-phenanthroline)-based oxygen optical sensor on its structural parameters by Denis Badocco; Andrea Mondin; Paolo Pastore; Stefano Voltolina; Silvia Gross (pp. 239-246).

The optimum performance of an optical oxygen sensor based on polysulfone (PSF)/[Ru(II)-Tris(4,7-diphenyl-1,10-phenanthroline)] octylsulfonate (Ru(dpp)OS) was checked by carefully tuning the parameters affecting the membrane preparation. In particular, membranes having thickness ranging between 0.2 and 8.0μm with various luminophore concentrations were prepared by dip-coating and tested. The membrane thickness was controlled by tuning the solution viscosity, and was measured both by secondary ion mass spectrometry (SIMS) and by visible spectroscopy (Vis). Luminescence-quenching-based calibration was a single value of the Stern–Volmer constant (K′SV) for membranes containing up to 20mmolRu(dpp)g⁻¹PSF (1.35μm average thickness). TheK′SV value decreased for larger concentration. The highest sensitivity was obtained with membrane thickness around 1.6μm, having a response time close to 1s. Thicker membranes exhibited an emission saturation effect and were characterized by longer response time. TheK′SV behavior was interpreted on the basis of a mathematical approach accounting for the contribution of luminescence lifetime ( τ₀), oxygen diffusion coefficient (DO2) and oxygen solubility inside the membrane (sO2) establishing the role of all of them and allowing their experimental determination. Moreover, a simple experimental way to estimateK′SV without needing calibration was proposed. It was based either on the light emission asymmetry or on the percent variation of light emission on passing from pure nitrogen to pure oxygen.

Keywords: Luminescence oxygen sensors; Ru(II) complexes; Polysulfone; ASY factor

Optochemical sensing of hydrogen chloride gas using meso-tetramesitylporphyrin deposited glass plate by Palanisamy Kalimuthu; S. Abraham John (pp. 247-253).

Meso-tetramesitylporphyrin (MTMP) deposited glass plate (solid state sensor) was used to sense hydrogen chloride (HCl) gas based on optochemical method. Exposure of the solid state sensor to HCl vapor results in the formation of protonated meso-tetramesitylporphyrin (PMTMP). UV–vis and fluorescence spectral techniques were used to study the protonation of MTMP in dichloromethane–methanol mixture. The optical spectra of MTMP show an intense Soret band at 418nm with a 14nm red shift upon protonation by HCl. Ab-initio calculations were carried out to visualize the effect of protonation on planarity and stability of the porphyrin ring. The solid state sensor was characterized by UV–vis spectral technique. The sensor exhibits characteristic Soret and Q bands for the deposited MTMP with slight red shift when compared to MTMP in dichloromethane. The concentration of gaseous HCl was monitored from the changes in the absorbance of Soret band of PMTMP at 452nm. The detection limit of the solid state sensor towards gaseous HCl was found to be 0.03ppm. The present solid state sensor was highly stable for several months.

Keywords: Meso-tetramesitylporphyrin; Hydrogen chloride gas; Solid state sensor; Optochemical detection; Ultraviolet–visible spectra; Fluorescence spectra

A selective colorimetric chemosensor for thiols based on intramolecular charge transfer mechanism by Yan Zeng; Guanxin Zhang; Deqing Zhang (pp. 254-257).

Compound1 as an electron donor–acceptor compound with N, N-dimethylaniline and quinone units was designed for a highly selective colorimetric determination of thiol-containing amino acids and peptides, by making use of the unique reactivity of thiol towards quinone. Compound1 shows a strong intramolecular charge transfer (ICT) band around 582nm; but, it decreased after addition of either cysteine (Cys) or glutathione (GSH). Moreover, the ICT band intensity at 582nm decreased linearly with the increasing concentrations of Cys or GSH. The interference from other amino acids can be neglected. Therefore, compound1 can be employed as a selective colorimetric visual chemosensor for thiol-containing amino acids and peptides.

Keywords: Intramolecular charge transfer; Electron donor–acceptor compound; Michael reaction; Cysteine; Glutathione

Speciation analysis of selected metals and determination of their total contents in paired serum and cerebrospinal fluid samples: An approach to investigate the permeability of the human blood-cerebrospinal fluid-barrier by Volker Nischwitz; Achim Berthele; Bernhard Michalke (pp. 258-269).

Neurodegenerative diseases like Alzheimer's disease and Parkinson's disease are gaining increasing relevance in our aging society. However, the complex multifactorial mechanisms of these diseases are not sufficiently understood yet. Several studies indicate that metal ions play an important role in the promotion of these diseases. Consequently, the transport pathways of metals and their species to the brain are of special interest. Following oral or inhalative uptake metals are absorbed and distributed via the blood stream in the body. Transport into the brain requires crossing of the neural barriers.Our study focuses on the investigation of the permeability of the blood-cerebrospinal fluid (CSF)-barrier for selected metals (Mn, Fe, Cu, Zn, Mg and Ca). For the first time paired human serum and CSF samples obtained from a neurological department were characterised for total metal concentrations and metal species. For CSF few data are available in the literature on total metal contents and applications of element speciation analysis in CSF samples are rare. In our study mean CSF/serum ratios ( n=29) were 0.7 for Mn, 0.02 for Fe, 0.02 for Cu, 0.03 for Zn, 1.3 for Mg and 0.5 for Ca. Size exclusion chromatography (SEC) online with inductively coupled plasma mass spectrometry was further developed for the size characterisation of the metal species in CSF and serum with limits of detection of 0.4μgL⁻¹ for Fe, 0.01μgL⁻¹ for Mn, 0.2μgL⁻¹ for Cu, 0.2μgL⁻¹ for Zn, 0.6μgL⁻¹ for Mg and 3.8μgL⁻¹ for Ca in the eluate from the HPLC column. Apart from Mn the application of this technique has not been published for metal speciation in CSF, yet. In the case of some Mn species it turned out that methanol, which was contained in the mobile phase of a SEC method previously published from our group on qualitative characterisation of Mn species, was interfering with the quantification. The modified method developed in this work (with NaCl but without methanol in the mobile phase; use of internal standard) allowed reliable quantification. The results clearly indicate changes in the metal species pattern due to different permeation behaviour at the blood-CSF-barrier. As part of the method validation the relative stability of complexes of albumin, transferrin and citrate with Mn, Fe, Cu and Zn was investigated.

Keywords: Cerebrospinal fluid; Serum; Speciation; Metal; Blood-cerebrospinal fluid-barrier

Application of Fenton reaction for nanomolar determination of hydrogen peroxide in seawater by Emmanuel F. Olasehinde; Shinya Makino; Hiroaki Kondo; Kazuhiko Takeda; Hiroshi Sakugawa (pp. 270-276).

A simple and sensitive method for the determination of nanomolar levels of hydrogen peroxide (H₂O₂) in seawater has been developed and validated. This method is based on the reduction of H₂O₂ by ferrous iron in acid solution to yield hydroxyl radical (OH) which reacts with benzene to produce phenol. Phenol is separated from the reaction mixture by reversed phase high performance liquid chromatography and its fluorescence intensity signals were measured at excitation and emission of 270 and 298nm, respectively. Under optimum conditions, the calibration curve exhibited linearity in the range of (0–50)×10³nmolL⁻¹ H₂O₂. The relative standard deviations for five replicate measurements of 500 and 50nmolL⁻¹ H₂O₂ are 1.9 and 2.4%, respectively. The detection limit for H₂O₂, defined as three times the standard deviation of the lowest standard solution (5nmolL⁻¹ H₂O₂) in seawater is 4nmolL⁻¹. Interference of nitrite ion (NO₂⁻) on the fluorescence intensity of phenol was also investigated. The result indicated that the addition of 10μmolL⁻¹ NO₂⁻ to seawater samples showed no significant interference, although, the addition of 50μmolL⁻¹ NO₂⁻ to the seawater samples decreases the fluorescence intensity signals of phenol by almost 40%. Intercomparison of this method with well-accepted ( p-hydroxyphenyl) acetic acid (POHPAA)-FIA method shows excellent agreement. The proposed method has been applied on-board analysis of H₂O₂ in Seto Inland seawater samples.

Keywords: Hydrogen peroxide; Fenton reaction; Seawater; High performance liquid chromatography; Hydroxyl radical

Determination of estradiol in human serum using magnetic particles-based chemiluminescence immunoassay by Tian-Bing Xin; Shu-Xuan Liang; Xu Wang; Haifang Li; Jin-Ming Lin (pp. 277-284).

A magnetic particles (MPs)-based chemiluminescence immunoassay (CLIA) with high sensitivity, specificity, and reproducibility was proposed for the evaluation of estradiol (E₂) in human sera. The MPs coated with secondary antibody were used as dispersed solid phase for the immunoassay, and the horseradish peroxidase (HRP)–luminol–H₂O₂ chemiluminescent system with high sensitivity was chosen as the detection system. The method showed specific recognition to E₂, without cross-reaction for the major steroids, including estrone (E₁), estriol (E₃), dihydrotestosterone (DHT), androstenedione, and testosterone (T), which was commonly found in human serum. The addition of sodium trichloracetate (Na-TCA) in the enzyme buffer as a blocking agent contributed to the realization of direct analysis of E₂ in human serum without extraction. Besides, the effects of several physicochemical parameters, including the dilution ratios of E₂–6-HRP conjugate and anti-E₂ polyclonal antibody, immunoreaction time, chemiluminescent (CL) substrate volume, volume of MPs, and CL reaction time, were studied and optimized. The proposed method had a detection limit of 2.51pgmL⁻¹ with a larger working range of 15–1000pgmL⁻¹. The inter-assay and intra-assay coefficient of variation (CV) were both less than 15%. The average recoveries of three different spiked concentration samples were 93.3, 106 and 101%, respectively. The method has been successfully applied to the determination of E₂ in 105 human sera and showed a good correlation compared with the commercial radioimmunoassay (RIA) kit with a correlative coefficient of 0.9892. This method has exhibited great potential in the fabrication of diagnostic kit and could be used in the clinical analysis of E₂ in human serum.

Keywords: Magnetic particles (MPs); Chemiluminescence immunoassay (CLIA); Estradiol (E; ₂; ); Sodium trichloracetate (Na-TCA); Human serum

Erratum to “Improved sensitivity for transition metal ions by use of sulfide in the Belousov–Zhabotinskii oscillating reaction” [Anal. Chim. Acta 571 (2006) 150–155] by Jinzhang Gao; Hua Chen; Hongxia Dia; Dongyu Lv; Jie Ren; Lei Wang; Wu Yang (pp. 285-285).

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Analytica Chimica Acta (v.627, #2)