Analytica Chimica Acta (v.669, #1-2)

Liquid-phase microextraction approaches combined with atomic detection: A critical review by Francisco Pena-Pereira; Isela Lavilla; Carlos Bendicho (1-16).
Liquid-phase microextraction (LPME) displays unique characteristics such as excellent preconcentration capability, simplicity, low cost, sample cleanup and integration of steps. Even though LPME approaches have the potential to be combined with almost every analytical technique, their use in combination with atomic detection techniques has not been exploited until recently. A comprehensive review dealing with the applications of liquid-phase microextraction combined with atomic detection techniques is presented. Theoretical features, possible strategies for these combinations as well as the effect of key experimental parameters influencing method development are addressed. Finally, a critical comparison of the different LPME approaches in terms of enrichment factors achieved, extraction efficiency, precision, selectivity and simplicity of operation is provided.
Keywords: Review; Liquid-phase microextraction; Atomic detectors; Theoretical background; Parameter optimization; Comparison of liquid-phase microextraction modes; Applications;

Pulsating potentiometric titration technique for assay of dissolved oxygen in water at trace level by P. Sahoo; R. Ananthanarayanan; N. Malathi; M.P. Rajiniganth; N. Murali; P. Swaminathan (17-24).
A simple but high performance potentiometric titration technique using pulsating sensors has been developed for assay of dissolved oxygen (DO) in water samples down to 10.0 μg L−1 levels. The technique involves Winkler titration chemistry, commonly used for determination of dissolved oxygen in water at mg L−1 levels, with modification in methodology for accurate detection of end point even at 10.0 μg L−1 levels DO present in the sample. An indigenously built sampling cum pretreatment vessel has been deployed for collection and chemical fixing of dissolved oxygen in water samples from flowing water line without exposure to air. A potentiometric titration facility using pulsating sensors developed in-house is used to carry out titration. The power of the titration technique has been realised in estimation of very dilute solution of iodine equivalent to 10 μg L−1 O2. Finally, several water samples containing dissolved oxygen from mg L−1 to μg L−1 levels were successfully analysed with excellent reproducibility using this new technique. The precision in measurement of DO in water at 10 μg L−1 O2 level is 0.14 (n  = 5), RSD: 1.4%. Probably for the first time a potentiometric titration technique has been successfully deployed for assay of dissolved oxygen in water samples at 10 μg L−1 levels.
Keywords: Dissolved oxygen; Potentiometric titration; Pulsating sensor; Winkler titration; Voltage to frequency converter; Dissolved oxygen sampler;

A new ionic liquid-based dispersive liquid–liquid microextraction method was developed for preconcentration and determination of compounds in aqueous samples containing very high salt concentrations. This method can solve the problems associated with the limited application of the conventional IL-based DLLME in these samples. This is believed to arise from dissolving of the ionic liquids in aqueous samples with high salt content. In this method, the robustness of microextraction system against high salt concentration (up to 40%, w/v) is increased by introducing a common ion of the ionic liquid into the sample solution.The proposed method was applied satisfactorily to the preconcentration of lead and cadmium in saline samples. After preconcentration, the settled IL-phase was dissolved in 100 μL ethanol and aspirated into the flame atomic absorption spectrometer (FAAS) using a home-made microsample introduction system. Several variables affecting the microextraction efficiency were investigated and optimized. Under the optimized conditions and preconcentration of only 10 mL of sample, the enhancement factors of 273 and 311 and the detection limits of 0.6 μg L−1 and 0.03 μg L−1 were obtained for lead and cadmium, respectively. Validation of the method was performed by both an analysis of a certified reference material (CRM) and comparison of results with those obtained by ISO standard method.
Keywords: Ionic liquid-based dispersive liquid–liquid microextraction; Flame atomic absorption spectrometry; Microsample injection; Saline sample; Atom concentrator tube;

A novel direct immersion solid-phase microextraction (DI-SPME) method using a stainless steel wire electrochemically coated with oxidized multiwalled carbon nanotubes–polypyrrole composite (MWCNTs–PPy) was used in the gas chromatographic (GC) determination of phthalate esters in aqueous samples. The composite coating did not swell in organic solvents nor did it strip off from the substrate. It was also highly stable and extremely adherent to the surface of the steel fiber. The effects of various parameters on the efficiency of SPME process such as extraction time, extraction temperature, ionic strength, desorption time, and desorption temperature were studied. Under optimized conditions, the detection limits for the phthalate esters varied between 0.05 and 0.1 ng mL−1, the inter-day and intra-day relative standard deviations for various phthalates at 1.0 ng mL−1 concentration level (N  = 7) using a single fiber were 6.2–10.2% and 7.7–10.5, respectively. The fiber-to fiber RSD% (N  = 3) was 9.3–12.1% at 10 ng mL−1. The linear ranges varied between 0.5 and 300 ng mL−1. The method was successfully applied to the analysis of mineral water samples with the recoveries from 90 to 113%.
Keywords: Carbon nanotube; Polypyrrole; Composite polymer; Solid-phase microextraction; Phthalate esters; Gas chromatography;

Silicate-entrapped porous coatings for preparing high-efficiency solid-phase microextraction sorbents by François Breton; Maria Rowena N. Monton; Wayne M. Mullett; Janusz Pawliszyn (39-44).
We present a novel way to prepare SPME fibers using a silicate entrapment of porous particles, followed by derivatization using classical organosilane chemistry. The fibers provide a good platform for on-fiber derivatization of desired extraction phases while providing porosity necessary for high extractions capacities. The porous network was created using potassium silicate and porous silica particles. Fibers derivatized using n-butyl, n-octyl, n-octadecyl and n-triacontyl groups were shown to extract benzodiazepines successfully. The coatings were determined to have an average thickness of ca. 8 μm, as determined by a scanning electron microscope, permitting equilibrium times as fast as 2 min. The fibers also showed very good ruggedness towards a vast range of solvents and prolonged use. It was determined that greater extraction efficiencies could be obtained using triacontyl as an extraction phase. The C18 and C30 fibers were also found to provide good linearity (>0.99) for the model analytes over two orders of magnitude, with limits of detection in the sub ng mL−1 levels. C30 fibers were used to establish a correlation between structurally diverse β-blockers and their literature reported Log  P values. The C30 fibers provided a good correlation (R 2  = 0.9255) between β-blockers ranging in hydrophobicity from Log  P literature 0.16–4.15 and their respective experimentally determined Log  K spme values.
Keywords: Solid-phase microextraction; Coating procedure; Alkali silicate; Triacontyl; Organosilane chemistry; On-fiber derivatization;

Citrate-modified gold nanoparticles were found to undergo size and color transition upon interaction with alkaline earth metals. At low concentrations, metal ions coordinate with AuNPs via citrate boding inducing aggregation which results in size increase as evidenced by the decrease in their plasmon bandwidth. As the concentration increases further, color transition from red to blue is observed which is no longer attributed to aggregation but to specific ion adsorption phenomena. The response of gold nanoparticles to these changes linearly depends on cation concentration in both the UV and Vis regions, a feature that was exploited for the assessment of alkaline earth metal concentrations in water samples. Based on these findings, the modification of the surface properties of metal-coated AuNPs were investigated with regard to their potential influence on the catalytic oxidation of luminol by periodate. Interestingly, a significant amplification of the CL emission signal was recorded when metal ions were associated with AuNPs, even for chemiluminescence “inert” cations like calcium and magnesium. The analytical implications of these findings for the improvement of CL sensitivity and its potential analytical applications are also discussed.
Keywords: Alkaline earth metals; Chemiluminescence; Gold nanoparticles; Luminol; UV–vis spectra;

Experimental setup and analytical methods for the non-invasive determination of volatile organic compounds, formaldehyde and NO x in exhaled human breath by Ulrich Riess; Uwe Tegtbur; Christian Fauck; Frank Fuhrmann; Doreen Markewitz; Tunga Salthammer (53-62).
Different analytical devices were tested and evaluated for their suitability of breath gas analysis by examining the physiological parameters and chemical substances in the exhaled breath of ten healthy probands during light cycling in dependence of methanol-rich nutrition. The probands exercised under normal breathing conditions on a bicycle ergometer. Breath air was exhaled into a glass cylinder and collected under steady-state conditions. Non-invasively measured parameters were pulse rate, breath frequency, temperature, relative humidity, NO x , total volatile organic compounds (TVOCPAS), carbon dioxide (CO2), formaldehyde, methanol, acetaldehyde, acetone, isoprene and volatile organic compounds (VOCs). Methanol rich food and beverages strongly influenced the concentration of methanol and other organic substances in human breath. On the other hand, nutrition and smoking had no clear effect on the physical conditions of the probands. The proton transfer reaction mass spectrometry (PTR-MS) method was found to be very suitable for the analysis of breath gas but the m/z 31, if assigned to formaldehyde, is sensitive to interferences. The time vs. concentration curves of nitric oxide showed sudden peaks up to 120 ppb in most of the measurements. In one case a strong interference of the NO x signal was observed. The time resolved analysis of exhaled breath gas is of high capability and significance for different applications if reliable analytical techniques are used. Some compounds like nitric oxide (NO), methanol, different VOCs as well as sum parameters like TVOCPAS are especially suitable as markers. Formaldehyde, which is rapidly metabolized in the human body, could be measured reliably as a trace component by the acetylacetone (acac) method but not by PTR-MS.
Keywords: Exhaled human breath; Organic compounds; Nitric oxides; Proton transfer reaction mass spectrometry; Chemiluminescence; Photo acoustics;

Disposable electrochemical printed gold chips for the analysis of acetylcholinesterase inhibition by Vladimir Dounin; Anthony J. Veloso; Holger Schulze; Till T. Bachmann; Kagan Kerman (63-67).
The detection of trace levels of paraoxon and carbofuran was achieved utilizing differential pulse voltammetry (DPV) on gold disposable electrochemical printed (DEP) chips. The nanostructured gold surface of the chips enables highly sensitive oxidation of the thiocholine (TCh) product even in the absence of costly surface modifications. The inhibition of AChE activity at varying insecticide concentrations was detected with low detection limits of 10 ppb (36 nM) for paraoxon and 8 ppb (18 nM) for carbofuran. Fine-tuning of the experimental conditions will allow for the application of unmodified DEP gold chips for inexpensive on-field detection of AChE inhibition by various insecticides at or below the allowable concentrations set by European and North American regulation standards.
Keywords: Biosensor; Acetylcholinesterase; Paraoxon; Carbofuran; Disposable chips; Nanomaterials;

A urea biosensor based on pH-sensitive Sm2TiO5 electrolyte–insulator–semiconductor by Tung-Ming Pan; Ming-De Huang; Wan-Ying Lin; Min-Hsien Wu (68-74).
A urea biosensor based on pH-sensitive Sm2TiO5 electrolyte–insulator–semiconductor (EIS) has been described. We used X-ray diffraction, Auger electron spectroscopy, and atomic force microscopy to investigate the structural and morphological features of high-k Sm2TiO5 sensing membranes that had been subjected to annealing at different temperatures. The EIS device incorporating a high-k Sm2TiO5 sensing film that had been annealed at 900 °C exhibited good sensing characteristics, including a high sensitivity of 60.5 mV/pH (in solutions from pH 2 to 12), a small hysteresis voltage of 2.72 mV (in the pH loop 7 → 4 → 7 → 10 → 7), and a low drift rate of 1.15 mV h−1 (in the buffer solution at pH 7). The Sm2TiO5 EIS device also showed a high selective response towards H+. This improvement can be attributed to the small number of crystal defects and the large surface roughness. In addition, the urea biosensor based on pH-sensitive EIS incorporating a Sm2TiO5 sensing membrane annealed at 900 °C allowed the potentiometric analysis of urea, at concentrations ranging from 0.1 to 32 mM, with a sensitivity of 72.85 mV/purea.
Keywords: Urea; Biosensor; pH-sensitive; Sm2TiO5; Electrolyte–insulator–semiconductor;

Sulfur-adlayer-coated gold electrode for the in vitro electrochemical detection of uric acid in urine by Md. Rezwan Miah; Muhammad Tanzirul Alam; Takeo Ohsaka (75-80).
The present article demonstrates the electrochemical oxidation of uric acid (UA) at sulfur-adlayer-coated gold (S-Au) electrode in alkaline media. At S-Au electrode, UA oxidized at a significantly lower overpotential with a higher current density as compared to the bare Au electrode. The oxidation of UA at the S-Au electrode is highly selective in the presence of the other commonly existing bio-molecules in urine. The proposed electrochemical sensor not only exhibited good reproducibility, but also showed a fast amperometric response to UA in the concentration range of 0.0025–5 mM with a low detection limit of 0.4 μM.
Keywords: Adsorption; Sulfur-adlayer; Uric acid; Enhanced selective electrooxidation;

We show for the first time that a piezoelectric-excited millimeter-sized cantilever (PEMC) sensor can detect five Cryptosporidium parvum oocysts in 25% milk in PBS background in a flow format (1 mL min−1). To improve sensitivity, a secondary antibody (murine IgM) was used to confirm the attachment of oocysts to the sensor. PEMC sensor is a resonant-mode cantilever sensor whose resonant frequency decreases when target analyte binds to its surface. The sensor was functionalized with Protein G, and then immobilized with goat polyclonal IgG anti-C. parvum for oocysts detection. In the dynamic range of 50–10,000 oocysts mL−1 the sensor response is characterized by a semi-log relationship between resonant frequency response and C. parvum oocysts concentration. In 25% milk background, binding kinetics was slower and total sensor response was lower (∼45%) than in water-like buffer.
Keywords: Piezoelectric cantilever sensor; Resonant frequency; Milk; Secondary antibody; Limit of detection;

An aptazyme-based electrochemical biosensor for the detection of adenosine by Chenhu Sun; Xueping Liu; Kejun Feng; Jianhui Jiang; Guoli Shen; Ruqin Yu (87-93).
In this work, an aptazyme-based electrochemical biosensor for the detection of adenosine is reported. Aptazyme activity was modulated by appending an “inhibitor” oligonucleotide strand containing a 32-base adenosine aptamer to the 8-17 DNAzyme. In the absence of adenosine, the DNAzyme could not form appropriate catalytic structure due to the binding with the inhibitor strand. Upon adenosine binding to the aptamer, the inhibitor strand was dissociated from the DNAzyme sequence. This allowed the DNAzyme to open and bind with the hairpin substrate, and DNAzyme activity was thereby induced, cleaving the substrate at its ribonucleotide site in the presence of Pb2+. Cleavage of the substrate yields two single-stranded products, one of which was ferrocene-tagged and acted as the signal probe. The thiolated probe modified on the gold electrode could capture the signal probe. As a result, the ferrocene (Fc) moiety was brought in close proximity to the electrode surface and the Faradaic current was observed. This electrochemical biosensor was proved to have a wide dynamic range from 5 nM to 2000 nM with a detection limit of 5 nM. The fabricated sensor is shown to exhibit high sensitivity and desirable selectivity, which might be promising for the rational construction of aptazyme-based biosensors and the determination of adenosine in clinical examination.
Keywords: DNAzyme; Aptamer; Aptazyme; Ferrocene (Fc); Electrochemical biosensor;

A new molecularly imprinted polymer material having urea molecule selective cavities was introduced. Urea was properly dissolved in acetonitrile in the presence of an acidic functional monomer. Molecularly imprinted polymers with different compositions were examined and the roper formulation was selected. It was shown that the MIP had a considerable selectivity for urea in comparison to similar compounds such as thiourea and hydroxyurea. The obtained polymer was used as an adsorber for solid phase extraction (SPE) of urea in the aqueous samples. The extracted urea was determined by using a spectrophotometric method. Different parameters of SPE were optimized and the developed procedure was used for urea determination in real samples. The calibration graph of the method was linear in the range of 0.6–8.3 μmol L−1. The detection limit was calculated to be 0.14 μmol L−1.
Keywords: Urea; Molecularly imprinted polymer; Solid phase extraction; Spectrophotometric method; Thiourea;