Analytica Chimica Acta (v.653, #1)

Methods and advances for monitoring neurotransmitters in vivo or for tissue analysis of neurotransmitters over the last five years are reviewed. The review is organized primarily by neurotransmitter type. Transmitter and related compounds may be monitored by either in vivo sampling coupled to analytical methods or implanted sensors. Sampling is primarily performed using microdialysis, but low-flow push–pull perfusion may offer advantages of spatial resolution while minimizing the tissue disruption associated with higher flow rates. Analytical techniques coupled to these sampling methods include liquid chromatography, capillary electrophoresis, enzyme assays, sensors, and mass spectrometry. Methods for the detection of amino acid, monoamine, neuropeptide, acetylcholine, nucleoside, and soluble gas neurotransmitters have been developed and improved upon. Advances in the speed and sensitivity of these methods have enabled improvements in temporal resolution and increased the number of compounds detectable. Similar advances have enabled improved detection at tissue samples, with a substantial emphasis on single cell and other small samples. Sensors provide excellent temporal and spatial resolution for in vivo monitoring. Advances in application to catecholamines, indoleamines, and amino acids have been prominent. Improvements in stability, sensitivity, and selectivity of the sensors have been of paramount interest.
Keywords: Neurotransmitter; Biosensor; Microdialysis;

Review: Microfluidic applications in metabolomics and metabolic profiling by James R. Kraly; Ryan E. Holcomb; Qian Guan; Charles S. Henry (23-35).
Metabolomics is an emerging area of research focused on measuring small molecules in biological samples. There are a number of different types of metabolomics, ranging from global profiling of all metabolites in a single sample to measurement of a selected group of analytes. Microfluidics and related technologies have been used in this research area with good success. The aim of this review article is to summarize the use of microfluidics in metabolomics. Direct application of microfluidics to the determination of small molecules is covered first. Next, important sample preparation methods developed for microfluidics and applicable to metabolomics are covered. Finally, a summary of metabolomic work as it relates to analysis of cellular events using microfluidics is covered.
Keywords: Microfluidics; Metabolomics; Metabolic profiling; Review;

Recent years have provided numerous new examples of applying flow-through electrochemical detectors in chemical analysis. This review, based on about 250 original research papers cited from the current analytical literature, presents their application in flow analysis and capillary electrophoretic methods. Example applications are also given for arrays of electrochemical sensors in flow analysis and electrochemical detection in microfluidic systems. Potentiometric detection with ion-selective electrodes predominates in flow analysis carried out mostly in a flow-injection system, while amperometric and conductivity detections are most commonly employed in capillary electrophoresis.
Keywords: Flow analysis; Electrochemical sensors; Electrochemical biosensors; Capillary electrophoresis; Microfluidics; Electrochemical detection;

Sampling errors can be divided into two classes, incorrect sampling and correct sampling errors. Incorrect sampling errors arise from incorrectly designed sampling equipment or procedures. Correct sampling errors are due to the heterogeneity of the material in sampling targets. Excluding the incorrect sampling errors, which can all be eliminated in practice although informed and diligent work is often needed, five factors dominate sampling variance: two factors related to material heterogeneity (analyte concentration; distributional heterogeneity) and three factors related to the sampling process itself (sample type, sample size, sampling modus). Due to highly significant interactions, a comprehensive appreciation of their combined effects is far from trivial and has in fact never been illustrated in detail. Heterogeneous materials can be well characterized by the two first factors, while all essential sampling process characteristics can be summarized by combinations of the latter three. We here present simulations based on an experimental design that varies all five factors. Within the framework of the Theory of Sampling, the empirical Total Sampling Error is a function of the fundamental sampling error and the grouping and segregation error interacting with a specific sampling process. We here illustrate absolute and relative sampling variance levels resulting from a wide array of simulated repeated samplings and express the effects by pertinent lot mean estimates and associated Root Mean Squared Errors/sampling variances, covering specific combinations of materials’ heterogeneity and typical sampling procedures as used in current science, technology and industry. Factors, levels and interactions are varied within limits selected to match realistic materials and sampling situations that mimic, e.g., sampling for genetically modified organisms; sampling of geological drill cores; sampling during off-loading 3-dimensional lots (shiploads, railroad cars, truckloads etc.) and scenarios representing a range of industrial manufacturing and production processes. A new simulation facility “SIMSAMP” is presented with selected results designed to show also the wider applicability potential. This contribution furthers a general exposé of all essential effects in the regimen covered by “correct sampling errors”, valid for all types of materials in which non-bias sampling can be achieved.
Keywords: Theory of Sampling (TOS); Process sampling; Material heterogeneity; Sampling process; Simulation; Sampling uncertainty;

Nonionic surfactants, such as Triton X-100 and Tween-20, were shown in this study to improve the electrocatalytic activity of screen-printed carbon paste electrodes (SPCE). The electrochemical response of SPCE to hydrogen peroxide increased 8–10-fold with the modification of nonionic surfactants. In addition, the glucose biosensors fabricated from nonionic surfactant-modified SPCE exhibited 6.4–8.6-fold higher response to glucose than that fabricated from unmodified SPCE. A concentration effect is proposed for nonionic surfactant to bring neutral reactants to the surface of electrode. Moreover, nonionic surfactant-modified SPCE exhibits a capability of repetitive usage and good reproducibility (R.S.D. < 5%) in the measurement of H2O2. Interestingly, the nonionic surfactant-modified SPCE exhibited an opposite effect to ascorbic acid, a common electroactive agent, which causes interference during clinical diagnosis. The differential responses of nonionic surfactant-modified SPCE to H2O2 and ascorbic acid suggest its potential in the development of biosensors for clinical diagnosis.
Keywords: Nonionic surfactant; Screen-printed carbon paste electrode; Triton X-100; Tween-20; Electrocatalytic activity;

A recently proposed electroanalytical method, using differential pulse voltammetry (DPV) on the rotating Au-disk electrode, and electrospray ionization mass-spectrometry (ESI-MS) has been applied to study the binding of the pharmaceutical chelating agents meso-2,3-dimercaptosuccinic acid (DMSA), sodium 2,3-dimercaptopropanesulfate (DMPS) and d-penicillamine (d-Pen) with Hg2+. From the use of voltammetric titrations it was possible to obtain a detailed picture of the complexation processes at concentrations much lower than in previous studies. Predominant species were Hg(Pen)2, Hg2(DMSA)2 and Hg(DMPS)2. For Pen, Hg(Pen) was also deduced from DPV data, while Hg2(Pen)4 from ESI-MS. For DMSA and DMPS, Hg2L species were detected by DPV, and Hg2L3, Hg3L3 as well as Hg2(DMPS)2 and Hg(DMSA)2 by ESI-MS. When possible, DPV data were analyzed by multivariate curve resolution with alternating least squares (MCR–ALS).
Keywords: Mercury; meso-2,3-dimercaptosuccinic acid (DMSA); Sodium 2,3-dimercaptopropanesulfate (DMPS); d-Penicillamine (Pen); Voltammetry; Electrospray ionization mass-spectrometry;

To an iron sample solution was added polyoxyethylene-4-isononylphenoxy ether (PONPE, nonionic surfactant, average number of ethylene oxides 7.5) and the surfactant was aggregated by the addition of lithium chloride. The iron(III) matrix was collected into the condensed surfactant phase in >99.9% yields, leaving trace metals [e.g., Ti(IV), Cr(III), Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Pb(II), and Bi(III)] in the aqueous phase. After removing the surfactant phase by centrifugation, the remaining trace metals were concentrated onto an iminodiacetic acid-type chelating resin. The trace metals were desorbed with dilute nitric acid for the determination by inductively coupled plasma-mass spectrometry or graphite-furnace atomic absorption spectrometry. The proposed separation method allowed the analysis of high-purity iron metals for trace impurities at low μg g−1 to ng g−1 levels.
Keywords: Polyoxyethylene-type nonionic surfactant; Salting-out; Phase separation; Matrix-removal; Analysis of high-purity iron;

The purpose of this study was to develop and test a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) method, a fast and non-destructive method without extraction, and compare it with the standard gas chromatography (GC) method currently used. A micro-orifice uniform deposit impactor (MOUDI) was used to sample all the size distributions of the aerosol particles of essential oils to investigate the relation between size distributions and the indoor concentration distributions of ylang essential oils. Correlation coefficients for DRIFTS and GC were 0.9904, 0.9910, 0.9913, and 0.9983 for eugenol, isoeugenol, methyl ether, and eugenyl acetate, respectively. The results showed that the concentrations of the four eugenol derivatives of smoke were approximately three times higher than those of mist. Additionally, the major size distributions of aerosol were 0.19 μm and 1.8 μm for the smoke and mist methods, respectively. Because these two methods produce similar results, DRIFTS is a practical method for assessing these fragrances in aerosols.
Keywords: Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS); Gas chromatography; Size distributions; Aerosols eugenol derivatives; Cananga odorata-ylang essential oils;

Non-instrumental immunochemical tests for rapid ochratoxin A detection in red wine by T.Yu. Rusanova; N.V. Beloglazova; I.Yu. Goryacheva; M. Lobeau; C. Van Peteghem; S. De Saeger (97-102).
Gel-based and membrane-based flow-through immunoassay formats were investigated for rapid ochratoxin A (OTA) detection in red wine. The flow-through set-up consisted of an antibody containing gel or membrane placed at the bottom of a standard solid-phase extraction column (i.e. the flow-through column), combined with a clean-up column. Different clean-up methods were studied for red wine clarification and purification. The optimal method consisted of passing wine, diluted with an aqueous solution containing 1% polyethylene glycol (PEG 6000) and 5% sodium hydrogencarbonate, through strong anion exchange (SAX) silica. An immunoassay for OTA detection in red wine was optimized and a cut-off level at 2 μg L−1 according to EU legislation was achieved with both formats. A more significant colour difference between blank and spiked samples was observed for the gel-based assay making this superior to the membrane-based assay. The proposed rapid gel-based test was compared with a standard immunoaffinity column – high-performance liquid chromatography – fluorescent detection (IAC-HPLC-FLD) method and a good correlation of the results was obtained for naturally contaminated wine samples.
Keywords: Rapid test; Immunoassay; Wine; Ochratoxin A;

An optical reflected device using a molecularly imprinted polymer film sensor by Nan Wu; Liang Feng; Yiyong Tan; Jiming Hu (103-108).
A novel and highly selective optical sensor with molecularly imprinted polymer (MIP) film was fabricated and investigated. The optical sensor head employing a medium finesse molecularly imprinted polymer film has been fabricated and characterised. A blank polymer and formaldehyde imprinted polymer were using methacrylic acid as the functional monomer and the ethylene glycol dimethacrylate as a crosslinker. The transduction mechanism is discussed based on the changes of optical intensity of molecularly imprinted polymer film acting as an optical reflected sensor. Template molecules, which diffused into MIP, could cause film density, and refractive index change, and then induce measurable optical reflective intensity shifts. Based on the reflective intensity shifts, an optical reflection detection of formaldehyde was achieved by illuminating MIP with a laser beam. For the same MIP, the reflective intensity shift was proportional to the amount of template molecule. This optical sensor, based on an artificial recognition system, demonstrates long-time stability and resistance to harsh chemical environments. As the research moves forward gradually, we establish the possibilities of quantitative analysis primly, setting the groundwork to the synthesis of the molecular imprinted optical fiber sensor. The techniques show good reproducibility and sensitivity and will be of significant interest to the MIPcommunity.
Keywords: Optical sensor; Molecularly imprinted polymer film; Formaldehyde; Optical intensity;

Surface plasmon resonance biosensor based on water-soluble ZnO–Au nanocomposites by Liying Wang; Jian Wang; Songling Zhang; Ying Sun; Xiaonan Zhu; Yanbo Cao; Xinghua Wang; Hanqi Zhang; Daqian Song (109-115).
A wavelength modulation surface plasmon resonance biosensor based on ZnO–Au nanocomposites for the detection of human IgM was developed. Self-assembly technique has the advantages of flexibility, simplicity and the precise control of film component and was applied to the building of the sensor. The ZnO–Au nanocomposites are in a dumbbell-like shape and can be immobilized on the Au film through 1,6-hexanedithiol by covalent attachment. Meanwhile the activated ZnO nanocrystals can be used to connect protein. The biosensor based on ZnO–Au nanocomposites was used to detect human IgM. Some experimental conditions were examined and optimized. In the selected conditions, the modified biosensor exhibits a satisfactory response for human IgM in the concentration range of 0.30–20.00 μg mL−1. However, the biosensor without ZnO–Au nanocomposites shows a response for human IgM in the concentration range of 1.25–20.00 μg mL−1. Compared with the biosensor based on Au film, when the biosensor based on the ZnO–Au nanocomposites was applied, the sensitivity for determination of human IgM is significantly enhanced.
Keywords: Wavelength modulation; Surface plasmon resonance; ZnO–Au nanocomposites; Human IgM;

Microscale membrane extraction of diverse antibiotics from water by Kamilah Hylton; Manuvesh Sangwan; Somenath Mitra (116-120).
Antibiotics include a wide range of compounds that in addition to having high solubility in water may be basic, acidic or neutral. Therefore, it is anticipated that no single method would be effective in simultaneously extracting all of these compounds. In recent times, microscale membrane extraction has evolved as a viable sample preparation alternative. The two major approaches are microscale supported liquid membrane extraction (μ-SLME) and microscale liquid–liquid membrane extraction (μ-LLME). An approach that includes μ-LLME as well as μ-SLME for acidic and basic compounds is presented for the extraction and concentration of diverse antibiotics in water. Enrichment factors as high as 2700 were achieved. Relative standard deviations ranged from 1.5% to 2.0%, and detection limits were as low as 92 ng/L.
Keywords: Antibiotics; LPME; Membrane extraction;