Analytica Chimica Acta (v.519, #2)

Hyphenation of column liquid chromatography and Raman spectroscopy via a liquid-core waveguide: chemometrical elimination of spectral eluent background by Reyer J Dijkstra; Hans F.M Boelens; Johan A Westerhuis; Freek Ariese; Udo A.Th Brinkman; Cees Gooijer (129-136).
In column liquid chromatography (LC) coupled to conventional Raman spectroscopy (RS) removal of the spectral background of the eluent is often demanding, because of the strong signals of the organic modifier. A new chemometrical method is proposed, called the eluent background subtraction (EBS) method, which can correct for small shape and intensity differences of the eluent spectra. The variations in the eluent spectra are modelled using principal component analysis (PCA). The PCA loading vectors are subsequently used for eluent background correction of the elution spectra of the analyte. The loading vectors are fitted under these spectra by an asymmetric least-squares method. This method was successfully applied under various experimental conditions and performed much better than conventional background correction methods. Analyte detectability was improved by (weighted) averaging of all elution spectra and smoothing via a p-spline function.
Keywords: Raman spectroscopy; Liquid chromatography; Liquid-core waveguide; Spectral background; Library fit;

A flow injection (FI) manifold based on luminol chemiluminescence (CL) detection for the determination of silicate in freshwaters is described. The molybdosilicic heteropoly acid formed by silicate and ammonium molybdate in acidic conditions generated CL response via the oxidation of luminol. The calibration graph was linear over the range 0.35–140 μg Si L−1 (r 2=0.9974) at 25 °C with relative standard deviations (n=5) in the range 1.0–3.8%. The practical limit of detection was 0.35 μg Si L−1 with a sample throughput of 80 h−1. Potential interferences in freshwaters were removed by passing the sample stream through in-line iminodiacetate chelating (to remove cations) and anion exchange (to remove phosphate) micro-columns. The method was applied to freshwater samples from the upper Tamar catchment and the results obtained (140±9 to 1493±47 μg Si L−1) were in reasonable agreement with the results obtained using a segmented flow analyser with spectrophotometric detection (182±9 to 1652±23 μg Si L−1) as the reference method.
Keywords: Flow injection analysis; Chemiluminescence; Silicate; Heteropoly acid; Freshwaters;

Progesterone is reasonably anticipated to be a human carcinogen based on sufficient evidence of carcinogenicity in experimental animals and it can be found in various surface waters which are partly used as drinking water resources. Therefore, immunoanalytical methods at a very low limit of detection (LOD) and a low limit of quantification (LOQ) are becoming more and more important for environmental analysis and especially for monitoring drinking water quality. Biosensors have suitable characteristics such as efficiency in allowing very fast, sensitive, and cost-effective detection. Here we describe a fully automated immunoassay for progesterone with a LOD in the sub-nanogram per litre range and a LOQ in the lower nanogram per litre range. In contrast to common analytical methods such as GC–MS or HPLC–MS, the biosensor used requires no sample pre-treatment and no sample pre-concentration. The basis of our sensitive assay is the antibody with a high affinity constant towards progesterone and the robust biosensor setup used.
Keywords: Progesterone; River Analyser (RIANA); Automated Water Analyser Computer Supported System (AWACSS); Immunoassay; Evanescent field; Environmental analysis;

Two kinds of polymeric pH indicators PPF (phenolphthalein-formaldehyde product) and CPF (o-cresolphthalein-formaldehyde product) immobilized cross-linked poly(vinyl alcohol) membranes (PPF–PVA and CPF–PVA) for optical intermittent determination of high basicity ([OH] = 1–8 M) based on a kinetic process were developed. In our previous work, we had demonstrated that PPF–PVA and CPF–PVA could perform the determination of high pH values from pH 10.0 to 14.0. Here the discoloring kinetic behaviors of PPF–PVA and CPF–PVA were compared with those of free phenolphthalein, o-cresolphthalein and thymolphthalein. Experimental results and theoretical analysis indicated that the response behaviors of the optodes’ membranes in concentrated NaOH solutions were diffusion-independent and still complied with the pseudo-first-order kinetics. In addition, two data analysis methods for determination were presented. One was directly based on the reduced absorbance; the other was based on the discoloring kinetic constant. It was found that the latter could perform a rapid (60 s) and reliable (relative standard deviation: 2.6%) determination for high basicity. These kinds of optodes can be used repeatedly when they are immersed in low-pH solutions to regain the protonated form after each determination.
Keywords: Optical pH sensor; Phenolphthalein; Kinetics; High basicity;

Immobilization of uricase on ZnO nanorods for a reagentless uric acid biosensor by Fenfen Zhang; Xiaoli Wang; Shiyun Ai; Zhengdong Sun; Qiao Wan; Ziqiang Zhu; Yuezhong Xian; Litong Jin; Katsunobu Yamamoto (155-160).
A reagentless uric acid (UA) biosensor based on uricase immobilized on ZnO nanorods was developed. Direct electrochemistry and thermal stability of immobilized uricase were studied. The ZnO nanorods derived electrode retained the enzyme bioactivity and could enhance the electron transfer between the enzyme and the electrode. This sensor showed a high thermal stability up to 85 °C and an electrocatalytic activity to the oxidation of uric acid without the presence of an electron mediator. The electrocatalytic response showed a linear dependence on the uric acid concentration ranging from 5.0 × 10−6 to 1.0 × 10−3  mol L−1 with a detection limit of 2.0 × 10−6  mol L−1 at 3σ. The apparent K M app value for the uric acid sensor was estimated to be 0.238 mM, showing a high affinity.
Keywords: Reagentless biosensor; ZnO nanorods; Uricase; Uric acid;

The present work describes the preparation and characterization of an electrodeposited DNA membrane doped with gold nanoparticles for the design of biosensors. The gold nanoparticles were deposited on the surface of DNA layer to build a hybrid device of nanoscale electrode array. The gold nanoparticles-doped DNA composite electrode was characterized by atomic force microscopy, scanning electron microscope, and electrochemistry involving electrochemical impedance spectroscopy. This electrode was successfully used for selective determination of norepinephrine (NE) in the presence of ascorbic acid (AA). The reversibility of the electrode oxidation reaction of NE is significantly improved in result of 200 mV negative shift of the voltammetric peak potential on the electrode, and a large increase in the peak current. A detection limit of 5 nM NE is obtained by using DPV in static solutions. The co-existence of a large excess of AA does not interfere with the detection. This electrode shows excellent sensitivity, good selectivity and antifouling properties.
Keywords: Gold nanoparticle; Calf-thymer DNA; Norepinephrine; Ascorbic acid;

This work reports the utility of inexpensive and disposable pencil-lead graphite as a substrate for bismuth-film electrodes (BFEs) for the simultaneous determination of Cd(II), Pb(II) and Zn(II) by square-wave anodic stripping voltammetry (SWASV). The BFE was generated in situ by depositing simultaneously the bismuth film and the metal cations by reduction at −1.4 V on the pencil graphite substrate. Then, the preconcentrated metals were oxidised by scanning the potential of the electrode from −1.4 to 0 V using a square-wave waveform. The stripping current arising from the oxidation of each metal was related to the concentration of each metal in the sample. The parameters for the simultaneous determination of the three metals were investigated with the view to apply this type of voltammetric sensor to real samples containing traces of these metals. Using the selected conditions, the limits of detection were 0.3 μg l−1 for Cd and 0.4 μg l−1 for Zn and Pb at a preconcentration time of 10 min and these values could be further decreased by the use of Nafion-covered pencil-lead BFEs. Finally, the pencil-lead BFEs were successfully applied to the determination of Pb and Zn in tap water with results in satisfactory statistical agreement with atomic absorption spectroscopy.
Keywords: Square-wave anodic stripping voltammetry; Bismuth-film electrodes; Pencil-lead graphite; Nafion;

A novel chelating resin (poly-Cd(II)-DAAB-VP) was prepared by metal ion imprinted polymer (MIIP) technique. The resin was obtained by one pot reaction of Cd(II)-diazoaminobenzene-vinylpyridine with cross-linker ethyleneglycoldimethacrylate (EGDMA). Comparing with non-imprinted resin, the poly-Cd(II)-DAAB-VP has higher adsorption capacity and selectivity for Cd(II). The distribution ratio (D) values for the Cd(II)-imprinted resin show increase for Cd(II) with respect to both D values of Zn(II), Cu(II), Hg(II) and non-imprinted resin. The relatively selective factor (α r) values of Cd(II)/Cu(II), Cd(II)/Zn(II) and Cd(II)/Hg(II), are 51.2, 45.6, and 85.4, which are greater than 1. poly-Cd(II)-DAAB-VP can be used at least 20 times without considerable loss of adsorption capacity. Based on poly-Cd(II)-DAAB-VP packed columns, a highly selective solid-phase extraction (SPE) and preconcentration method for Cd(II) from aqueous solution was developed. The MIIP-SPE preconcentration procedure showed a linear calibration curve within concentration range from 0.093 to 30 μg l−1. The detection limit and quantification limit were 0.093 and 0.21 μg l−1 (3σ) for flame atomic absorption spectrometry (FAAS). The relative standard deviation of the eleven replicate determinations was 3.7% for the determination of 10 μg of Cd(II) in 100 ml water sample. Determination of Cd(II) in certified river sediment sample (GBW 08301) demonstrated that the interfering matrix had been almost removed during preconcentration. The column was good enough for Cd(II) determination in matrixes containing components with similar chemical property such as Cu(II), Zn(II) and Hg(II).
Keywords: Solid-phase extraction (SPE); Metal ion imprinted polymer (MIIP); Preconcentration; Poly-Cd(II)-DAAB-VP; Cd(II); Flame atomic absorption spectrometry (FAAS); Natural water;

Dihydroxybenzoic acid (DHB) analogues substituted at the 5-position can act as UV matrices at a wavelength of 337 nm, even when their absorption maxima are shifted past this particular wavelength.Modification of a matrix with a chiral ligand γ-(3-carboxy-4-hydroxy-anilide) (GCA) allowed it to differentiate between chiral isomers of tryptophan and also gave different intensities for glucose isomers, including structural dimers of glucose (cellobiose and maltose).An analogue that had one free hydroxyl group at the 2-OH position and a modification at the 5-position (MY10) gave protonated substance P (SP, analyte) peaks, similar in intensity to the not derivatised parent 2,5-dihydroxybenzoic acid indicating that the 5-OH position is not an important structural component.Another analogue that resembled a ‘dendrimeric’ structure of DHB (M552), also acted as a matrix, although its absorption maxima was at 552 nm suggesting the possibility of it being used at other wavelengths in addition to 337 nm.The DHB radical was complexed to a nitrone ‘spin-trap’. On complexion, the peptide (SP) peak intensity decreased. Addition of either radical initiators, such as 2,2-azobis(iso-butyronitrile) AIBN and tert-butylperoxide, or other radicals such as 2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO) gave rise to higher analyte peak intensities for [SP+Na]+.It is thought that the DHB neutral radical is an intermediary in the protonation of the analyte. The photo-fragments of DHB, specifically the m/z 137 species, may also take part in proton transfer since possible mass analogues (hydroquinone, (deoxy)benzoin) can lead to analyte enhancement. Stabilization of or an increase in the matrix radical can also lead to analyte signal enhancement.
Keywords: MALDI-TOF; MALDI-mechanisms; Radical cation; MALDI analyte peak enhancement;

Immobilization of single-stranded DNA (ssDNA) on chromatographic support, i.e. copolymerized particles of styrene and glycidyl methacrylate via formation of amino groups with either N-methyl-1,3-propanediamine (stationary phase 1) or hexamethylenediamine (stationary phase 2) is a promise method to separate sequence specific DNA. However, the low ligand density of nonporous particles led to nonspecific interaction of the complementary oligonucleotide DNA in the sample with the stationary phase (1 or 2). In this paper, the binding process of telomere (tel; TTAGGG) to the complementary ssDNA immobilized (AATCCC) on the two chromatographic supports was studied for the first time using extended Langmuir equation. It appeared that the nonspecific adsorption of the tel due to electrostatic interaction between the polynucleotide sample (tel) and the residual amino groups on the particle surface via amination with hexamethylenediamine (stationary phase 2) was significant and could be reduced by using a high salt (NaCl) concentration in the bulk solvent. In contrast, the nonspecific adsorption of tel was neglected in the column using DNA-immobilized particles via amination with N-methyl-1,3-propanediamine (stationary phase 1). Thus, the affinity chromatography prepared via amination by N-methyl-1,3-propanediamine was more effective for analysis of sequence-specific DNA than the one prepared via amination by hexamethylenediamine.Moreover, for the two stationary phases, increasing NaCl concentration in the bulk solvent enhanced the hybridization between tel and the complementary immobilized oligonucleotide.
Keywords: Telomeric sequence; Single-stranded DNA; Polynucleotide;

G-quartet DNA stationary phases prepared by covalent attachment of the oligonucleotides to capillary surfaces were investigated for separations of albumins of different species in open tubular capillary electrochromatography (OTCEC). Albumins were chosen as a model system of proteins that do not exhibit strong, specific binding to G-quartet DNA, in order to determine if weak interactions with the stationary phases would be useful for separation of proteins that are highly similar in structure, function, and sequence. Results were compared with results obtained using capillary zone electrophoresis with a bare capillary and OTCEC results obtained using a scrambled sequence oligonucleotide that does not form a G-quartet structure. The comparison indicates advantages associated with the G-quartet phases for protein separations.
Keywords: Albumin; G-quartet DNA; Capillary electrochromatography;

Ionic liquids as additives in high performance liquid chromatography by Xiao Xiaohua; Zhao Liang; Liu Xia; Jiang Shengxiang (207-211).
As novel solvents, ionic liquids have many applications in synthesis, catalysis and analytical separation, i.e. extraction and chromatography separation. In this paper, some amines including benzidine, benzylamine, N-ethylaniline and N,N′-dimethylaniline are separated using ionic liquids as additives for the mobile phase in high performance liquid chromatography (HPLC). The effects of the length of alkyl chain or counterions on different ionic liquids and their concentrations on the separation of these analytes are performed. The differences between ionic liquids and tetrabutylammonium bromide (TBA) on the separation of o-, m-, p-phthalic acids are compared and the results show that ionic liquids are ion-pair reagents in essence, although their hydrophobicity and hydrogen bonding also play important roles.
Keywords: Ionic liquids; Amines; Interaction mechanism; Ion-pair reagents;

A gas chromatograph–mass spectrometry (GC–MS) method has been developed for the simultaneous determination of phosphate and phthalate esters in clean room and indoor air. Dibutyl phthalate (DBP), diethylhexyl phthalate (DEHP), trimethyl phosphate (TMP) and seven other phosphate esters present in air were collected using a Waters Sep-Pak PS Air cartridge and extracted with acetone. A quantity of 4 μl of each extract was analyzed by GC–MS. Quantification limits for 2.88 m3 of air sample were 2–20 ng m−3 for phosphate esters and 100 ng m−3 for phthalate esters. These values corresponded to 10 times the standard deviation of the peak areas. The breakthrough amounts of determinants in the Sep-Pak PS Air cartridge were 100 μg for trimethyl phosphate (TMP) and ≥2600 μg for the other nine components. Analytical results for air samples collected from three office rooms ranged from 100 to 320 ng m−3 for tributyl phosphate and 350 to 780 ng m−3 for dibutyl phthalate. However, the concentration of the determinants in four newly built clean rooms were under the minimum limits for determination of phosphate and phthalate esters in air.
Keywords: GC–MS; Phosphate esters; Semiconductors; Phthalate esters; Clean room; Indoor air;

Development of a capillary gas chromatographic procedure by J.J. Berzas; C. Guiberteau; M.J. Villaseñor; V. Rodrı́guez (219-230).
A simple and fast capillary gas chromatographic method with flame ionisation detection (FID) has been developed for the analysis of fluoxetine, fluvoxamine, citalopram, sertraline and paroxetine in their pharmaceutical preparations, using clomipramine as internal standard in order to achieve quantification. The reported method is the first screening one that allows the determination of the five selective serotonin reuptake inhibitors by GC, permitting also to avoid prederivatization for the first time and it is even a pioneering work including an extensive analytical validation and robustness treatment on placebo pharmaceutical formulations too. Optimal conditions for the quantitative gas capillary separation were investigated: column head pressure (100 kPa), injector and detector (FID) temperatures (210 and 260 °C), time and temperature for the splitless step (0.80 min and 80 °C, respectively), volume injected (2 μL) and oven temperature program, providing analysis times shorter than 7 min. Aspects such as stability of solutions, linearity, accuracy, precision, detection and quantitation limits are examined on a selected placebo in order to validate this method. Peak purity is assessed using mass-selective detection (DMS). The robustness of this method was evaluated using the Plackett–Burman fractional factorial experimental design with a matrix of 15 experiments and the statistical treatment proposed by Youden and Steinner. The scope of the validated method is proved in the analysis of almost existing pharmaceutical preparations, with recoveries between 98.5% and 102.4% with regard to their nominal contents. Finally, the proposed method could be also a very successfully option for the analysis of these SSRIs in real urine samples introducing a previous solid phase extraction-preconcentration step.
Keywords: SSRIs; Validation; Robustness; Pharmaceutical formulations; Biological samples;

The European strategy for dioxin monitoring of the food chain has defined high-resolution gas chromatography coupled to high-resolution mass spectrometry (HRGC/HRMS) method as the confirmatory method that can provide reliable and comparable results at sub-parts per trillion (ppt) level. This paper describes the first inter-laboratory study on dioxins, furans and dioxin-like PCBs by HRGC/HRMS method in animal feedingstuffs. Two different statistical approaches (ISO 5725 and Cofino’s statistics) were used for the statistical evaluation. For this particular study, the performances of the HRGC/HRMS method seem to be congener-independent in repeatability and reproducibility conditions over a concentration range covering more than four orders of magnitude. Results clearly show the effect of precision loss below 0.1 ppt level per congener in repeatability conditions and below 0.2 ppt level per congener in reproducibility conditions. LODs reported by the laboratories give median values of 0.02 ng/kg for most of the toxic congeners. Relative standard deviation between the laboratories’ mean values using upper-bound approach for TEQ calculation is 6.2%, more than twice the maximum level set at 0.75 ng TEQ/kg of product.
Keywords: Dioxins; Furans; Dioxin-like PCBs; HRGC/HRMS; Inter-laboratory study; Animal feedingstuffs;

Based on the results obtained from the inter-laboratory study in Part 1, different approaches for detection and quantification limits are evaluated and discussed. An overview of the most commonly used concepts and terminologies in analytical chemistry is presented with the aim of establishing a link between them. Whatever the method used by laboratories for detection limit assessment, the median LOD value reported for the less chlorinated PCDD/Fs (i.e. 0.02 ng/kg) is in good agreement with the values recalculated using the inter-laboratory data. For LOQ, the Eurachem approach based on a pre-established percentage of repeatability RSD appears to be suitable. The study shows that a pre-established RSDr of 20% is recommended in order to achieve an acceptable LOQ of 0.05 ng/kg per congener. The 20% value seems to be sufficiently low to get tolerable RSD close to maximum limits. Furthermore, the repeatability and the reproducibility standard deviation against parts-per-trillion congener levels has been modeled by inverse first order functions. This congener precision model provides an interesting tool to subsequently assess the performances of the method in TEQ close to regulatory limits. Finally, the paper discusses two different ways of reporting and interpreting the results to assess compliance against statutory limits.
Keywords: Dioxins; Furans; Dioxin-like PCBs; HRMS; Detection and quantification limits; Compliance and non-compliance; Measurement uncertainty;

Author Index (255-256).