Effect of polyethylene glycols on the sensitivity of the thermal lens determination of cobalt with nitrosonaphthols of various structures (nitroso-R-salt and 2-nitroso-1-naphthol)

The effect of polyethylene glycols (PEGs) with molecular weights of 2000, 6000, 13000, and 20000 on the sensitivity of the thermal lens determination of cobalt(III) using nitroso-R-salt and 2-nitroso-1-naphthol is studied. At the polymer concentration as low as 10%, the sensitivity coefficient significantly increases and the detection limit decreases by a factor of 1.5–2 with respect to the determination in an aqueous medium without PEG. This effect is accompanied by an increase in sensitivity due to an increase in the absorbance of the cobalt complexes in PEG solutions at the operating wavelength. Under optimal conditions, the detection limits for cobalt(III) are 6 and 10 ng/mL with nitroso-R-salt and 2-nitroso-1-naphthol, respectively.     

Spectrophotometric and thermal lens determination of aluminum with 3-sulfo-5-nitro-4′-diethylamino-2,2′-dihydroxyazobenzene

The conditions of the spectrophotometric and thermal lens determination of aluminum with sulfo-5-nitro-4′-diethylamino-2,2′-dihydroxyazobenzene have been compared. The limit of spectrophotometric detection of aluminum in aqueous solutions has been found to be 8 ng/mL. On the basis of the conditions of spectrophotometric determination, the conditions for thermal lens determination have been proposed (532.0 nm, exciting radiation power of 42 mW); they provide a decrease of the detection limit down to 0.6 ng/mL and an increase of the sensitivity coefficient by an order of magnitude. It has been shown that, in the case of the thermal lens determination of aluminum in water-organic mixtures (50 vol % of dimethyl sulfoxide or 30 vol % acetonitrile), the sensitivity coefficient is respectively 9.1 and 6.3-fold higher as compared with the thermal lens determination in water. As a result, the detection limits are reduced 2.5 and 10-fold, respectively. Aluminum has been determined by thermal lens spectrophotometry in Moscow’s tap water using the standard addition method, its concentration being 0.79 ± 0.07 mg/L, which is above the threshold limit value of the aluminum content of drinking water.     

Electromembrane extraction of heavy metal cations followed by capillary electrophoresis with capacitively coupled contactless conductivity detection

Electromembrane extraction (EME) was used as an off-line sample pre-treatment method for the determination of heavy metal cations in aqueous samples using CE with capacitively coupled contactless conductivity detection (CE-C(4) D). A short segment of porous polypropylene hollow fibre was penetrated with 1-octanol and 0.5%?v/v bis(2-ethylhexyl)phosphonic acid and constituted a low cost, single use, disposable supported liquid membrane, which selectively transported and pre-concentrated heavy metal cations into the fibre lumen filled with 100?mM acetic acid acceptor solution. Donor solutions were standard solutions and real samples dissolved in deionized water at neutral pH. At optimized EME conditions (penetration time, 5?s; applied voltage, 75?V; and stirring rate, 750?rpm), 15-42% recoveries of heavy metal cations were achieved for a 5?min extraction time. Repeatability of the EME pre-treatment was examined for six independent EME runs and ranged from 6.6 to 11.1%. Limits of detection for the EME-CE-C(4) D method ranged from 25 to 200?nM, resulting into one to two orders of magnitude improvement compared with CE-C(4) D without sample treatment. The developed EME sample pre-treatment procedure was applied to the analysis of heavy metal cations in tap water and powdered milk samples. Zinc in the real samples was identified and quantified in a background electrolyte solution consisting of 20?mM L-histidine and 30?mM acetic acid at pH 4.95 in about 3?min.     

Extraction–thermal lens quantification of cobalt with Nitroso-R-Salt in two-phase aqueous systems with water-soluble polymer polyethylene glycol

A novel method is proposed for the extraction-thermal lens quantification of cobalt with Nitroso-R-Salt based on the distribution of the colored complex in a two-phase aqueous system on the basis of poly-ethylene glycol (PEG) and an ammonium sulfate solution followed by its thermal lens detection in the extract. The limit of detection is 0.3 μM (20 ng/mL); the lower limit of the analytical range is 0.7 μM (40 ng/mL); the relative standard deviation for the concentrations 1–50 μM makes 1–3% (n = 6, P = 0.95). In the determination of cobalt by spectrophotometry under the same conditions, the detection limit is 10 μM (0.6 μg/mL) and the lower limit of the analytical range is 40 μM (2.5 μg/mL). The precision of thermal lens measurements in PEG solutions is higher in comparison to that in aqueous ones because of the weaker interference of convection in aqueous solutions of PEG.     

Determination of Lysozyme by Graphene Oxide–Polyethylene Glycol-Based Fluorescence Resonance Energy Transfer

The surface of graphene oxide was modified by bis(3-aminopropyl)-terminated polyethylene glycol to produce a composite graphene–polyethylene glycol. The graphene oxide/polyethylene glycol maximum absorption peak in the ultraviolet–visible spectrum was redshifted, and transmission electron microscope images showed that graphene oxide was cleaved into small nanosheets to form graphene oxide/polyethylene glycol. The dispersibility of graphene oxide/polyethylene glycol in physiological solution was higher than for graphene oxide. The optimum composite of graphene oxide/polyethylene glycol was used as a quencher in a fluorescence resonance energy transfer aptasensor for the determination of lysozyme detection. The results showed that graphene oxide/polyethylene glycol rapidly and efficiently quenched the fluorescence of the dye-labeled aptamer. The fluorescence was recovered by adding lysozyme to the system. The aptamer fluorescence intensity exhibited a strong linear dependence on the lysozyme concentration from 50 to 300?nM, and the lysozyme detection limit was approximately 11?nM. This method was used for the determination of lysozyme in egg whites, demonstrating that this approach is a promising alternative for the determination of lysozyme.     

Anodic stripping voltammetry of antimony at unmodified carbon electrodes

Antimony is an element of significant environmental concern, yet has been neglected relative to other heavy metals in electroanalysis. As such very little research has been reported on the electroanalytical determination of antimony at unmodified carbon electrodes. In this paper we report the electrochemical determination of Sb(III) in HCl solutions using unmodified carbon substrates, with focus on non-classical carbon materials namely edge plane pyrolytic graphite (EPPG), boron doped diamond (BDD) and screen-printed electrodes (SPE). Using differential pulse anodic stripping voltammetry, EPPG was found to give a considerably greater response towards antimony than other unmodified carbon electrodes, allowing highly linear ranges in nanomolar concentrations and a detection limit of 3.9?nM in 0.25?M HCl. Furthermore, the sensitivity of the response from EPPG was 100 times greater than for glassy carbon (GC). Unmodified GC gave a comparable response to previous results using the bare substrate, and BDD gave an improved, yet still very high limit of detection of 320?nM compared to previous analysis using an iridium oxide modified BDD electrode. SPEs gave a very poor response to antimony, even at high concentrations, observing no linearity from standard additions, as well as a major interference from the ink intrinsic to the working electrode carbon material. Owing to its superior performance relative to other carbon electrodes, the EPPG electrode was subjected to further analytical testing with antimony. The response of the electrode for a 40?nM concentration of Sb(III) was reproducible with a mean peak current of 1.07?µA and variation of 8.4% (n?=?8). The effect of metals copper, bismuth and arsenic were investigated at the electrode, as they are common interferences for stripping analysis of antimony.     

Electrochemical Design of Ultrathin Palladium Coated Gold Nanoparticles as Nanostructured Catalyst for Amperometric Detection of Formaldehyde

An underpotential deposition (UPD) replacement tactic was employed to design a Pd overlayer on gold (Au) nanoparticles electrodeposited on a carbon ionic liquid electrode (CILE). Pd/Au/CILE was applied as an amperometric sensor for the determination of formaldehyde in aqueous solutions. The sensor displayed two linear ranges from 15 µM–1.4 mM and 1.4–56.7 mM of formaldehyde. The limit of detection was 3 µM of formaldehyde and the sensitivity of the sensor was 2.35 µA mM^?1, using the calibration graph in the lower range. The presence of 20 mM of formic acid and methanol and 10 mM ethanol did not interfere with the determination of formaldehyde solution.     

Polythymine Oligonucleotide‐Modified Gold Electrode for Voltammetric Determination of Mercury(II) in Aqueous Solution

Polythymine oligonucleotide (PTO)‐modified gold electrode (PTO/Au) was developed for selective and sensitive Hg²⁺ detection in aqueous solutions. This modified electrode was prepared by self‐assembly of thiolated polythymine oligonucleotide (5′‐SH‐T₁₅‐3′) on the gold electrode via Au? S bonds, and then the surface was passivated with 1‐mercaptohexanol solution. The proposed electrode utilizes the specific binding interactions between Hg²⁺ and thymine to selectively capture Hg²⁺, thereby reducing the interference from coexistent ions. After exchanging the medium, electrochemical reduction at ?0.2 V for 60 s, voltammetric determination was performed by differential pulse voltammetry using 10 mM HEPES; pH 7.2, 1 M NaClO₄ as supporting electrolyte. This electrode showed increasing voltammetric response in the range of 0.21 nM Hg²⁺, with a relative standard deviation of 5.32% and a practical detection limit of 60 pM. Compared with the conventional stripping approach, the modified electrode exhibits good sensitivity and selectivity, and is expected to be a new type of green electrode.     

Quantification of boron in water by of spectrophotometry and thermal lens spectrometry using reaction with beryllon III

The conditions for the thermal lens quantification of boron in aqueous solutions with a detection limit of 0.3 ng/mL are found (λ = 532 nm, laser power 40 mW); this value of the detection limit is an order of magnitude lower than that attainable in conventional spectrophotometry. A 1: 1 composition of an aqueous ethylene glycol mixture is proposed, using which as a medium the detection limit for boron was reduced to 0.1 ng/mL. Using spectrophotometry and thermal lens spectrometry, boron was quantified in mineral water; the results agree with the data acquired by the reference method of inductively coupled plasma atomic emission spectrometry.     

Effect of a solvent on the parameters of the analytical signal,detection limit,and analytical range of the determination in analytical thermal lens spectrometry

The effect of the main parameters of the solvents (water, acetone, methanol, ethanol, acetonitrile, dimethyl sulfoxide, dichloromethane, toluene, and chloroform) that are most frequently used in the analytical practice on the characteristics of the thermal lens effect (increase in temperature because of optical heating and in the size of the region involved in heating) and on the detection limit and the lower determination limit of the thermal lens determination was considered, and the criteria of the selection of the medium for thermal lens experiments were discussed. It was demonstrated that the gain in these characteristics of the thermal lens determination in the given medium in comparison with the aqueous medium does not necessarily coincide with the strength of the thermooptical effect in this medium. It was demonstrated that the optimum conditions of measurements in the thermal lens detection are controlled not only by the absorption of the analyte and the reagents, but also by the intrinsic absorption of the solvent. Recommendations were given on the selection of the solvent for analytical thermal lens spectrometry.     

Thermal lens detection device

A thermal lens detection device was developed to realize an easy-to-use, portable and sensitive detector for nonfluorescent molecules. Two laser diodes (658 nm for excitation and 785 nm for probe) were made coaxial in an optical unit and were coupled to a single-mode optical fiber. On a microfluidic chip, a small holder for the optical fiber was fixed, and micro-lenses (numerical aperture of 0.2) were also integrated inside the holder. The micro-lenses were designed to realize an adequate chromatic aberration (50 μm), which was essential for sensitive thermal lens detection. Compared with conventional thermal lens detection systems which required very laborious and accurate optical alignment with the microchannel, the new device needed just attachment-detachment of the optical fiber, which was important for practical application. The lower limit of detection was 10 nM for nickel(II) phthalocyaninetetrasulfonic acid tetrasodium salt solutions (model sample), and the absorbance was 9 × 10(-6) AU. The absolute number of molecules detected was less than 200 zmol. The coefficient of variance for 5-time attachment-detachment of the optical probe was as small as 3.6%. The technical development allowed integration of the thermal lens detection devices inside a microsystem (e.g. enzyme-linked immuno-sorbent assay system), and practical microsystems were realized with sensitivities several-orders higher than absorptiometry.     

Effect of electrolytes on the sensitivity of the thermal lens determination

The effect of some electrolytes on the detection limits and sensitivity coefficients of the thermal lens determination of model compounds of different types in aqueous solutions is studied. A significant increase in the sensitivity of thermal lens determination in electrolyte solutions is shown. For example, the sensitivity coefficient is doubled in 0.6 M H₂SO₄ and 1.2 M HNO₃, regardless of the analyte nature. It is shown that the effect of electrolyte on the sensitivity of a thermal lens determination cannot be assessed a priori.     

An ion-pair reversed-phase HPLC-fluorimetric system for ultratrace determination of aluminium with salicylaldehydebenzoylhydrazone

An ion-pair HPLC fluorimetric determination of Al(III) at trace level has been developed, with salicylaldehydebenzoylhydrazone (SAB) as a precolumn reagent. The highly fluorescent AlSAB chelate (lambda(ex) 390.8 nm, lambda(em) 458.1 nm) is separated on a LiChroCART RP-18 column with an eluent consisting of 3.1 x 10(-)m tetrabutylammonium bromide, 1 x 10(4)m disodium EDTA and 5 x 10(-3)m sodium acetate in aqueous 42% w/w acetonitrile solution. The detection limit for Al is 1.5nM (40 pg/ml) in a 100-mul injection. The spectrophotometric detection limit at 390 nm is 0.3 ng/ml for 0.005 full-scale absorbance range. The selectivity is excellent and the method is useful for routine quality-control applications, such as determination of Al in tap water and in alkali pellets (LiOH, NaOH and KOH).     

Determination of glycols by HPLC with refractive index detection

A simple and fast HPLC method for the determination of glycols is described. It is characterized by a reversed-phase separation using water as eluent and a refractive index detection. The method was applied to investigate the biodegradation of glycols in a laboratory activated sludge plant and to determine the content of glycols or alcohols in detergents. The detection limit is 4 mg/l ethylene glycol or propylene glycol in an aqueous sample.     

A secondary laser cavity system for spectrophotometry of trace elements based on intracavit6y quenching and thermal lens effects

A system based on correlated cavities is described for quantitative measurements of small absorbances by means of intracavity quenching and the thermal lens effect. Theory suggesting the possibility of such measurements was confirmed experimentally. Methods for determining traces of arsenic and phosphorus in semiconductor silicon were developed; the procedures are based on the formation of reduced 12-molybdophosphoric and 12-molybdoarsenic heteropoly acids and measurement of absorbances either for aqueous solutions or for extracts into isobutanol. The total effect of intracavity quenching and thermal lensing was applied. The detection limits were 100 pg cm^?3 for arsenic and 10 pg cm^?3 for phosphorus. Layer-by-layer determination of phosphorus in silicon wafers was possible over a concentration range of 10¹⁵–10²⁰ atoms cm^?3, with a sample area of 1 cm² and layer thicknesses of 0.35–0.6 μm. The minimum detectable absorbance via the thermal lens effect was 5 × 10^?5, and that via the total effect of intracavity quenching and thermal lensing was 6 × 10^?6.     

Nanomolar Detection of Amitriptyline by Potentiometry with Ion Exchanger Based PVC Membrane ISEs

In order to lower the amount of amitriptyline released from PVC membrane and improve the sensitivity of the ion exchanger based PVC membrane ion selective electrode (ISE), 0.4% (m/m) clonidine‐tetraphenylborate was used as a electroactive component in PVC membrane of ISE for determination of amitriptyline with inner reference solution of 1 mM NaCl and 0.1 mM clonidine. Better reliability and selectivity compared with previous reports were acquired with a linear range of 10 nM?1.0 mM, the detect limit of 5.0 nM and average slope of 58.4 mV/decade. Fine reproducibility, stability and recoveries by the standard addition method were also acheived. The influence of the membrane thickness, inner reference solutions, supporting electrolyte, and the response time are discussed.     

Determination of polyethylene glycols by precipitation with iodine

Two methods for the determination of polyethylene glycols (PEGs) in aqueous solution by precipitation with iodine have been developed. For PEGs with molecular weight 4 x 10(3)-2 x 10(4) the excess of iodine is titrated with thiosulphate, and for PEGs with average m.w. > 2 x 10(4) turbidimetric measurement is used. Both methods are relatively simple and give accurate and reproducible results.     

Selective determination of ammonium ions by high-speed ion-exclusion chromatography on a weakly basic anion-exchange resin column

This paper describes an ion-exclusion chromatographic system for the rapid and selective determination of ammonium ion. The optimized ion-exclusion chromatographic system was established with a polymethacrylate-based weakly basic anion-exchange resin column (TSKgel DEAE-5PW) as the separation column, an aqueous solution containing 0.05 mM tetramethylammonium hydroxide (pH 9.10) as eluent with conductimetric detection for the analyte determination. Under the optimum chromatographic conditions, ammonium ion was determined within 2.3 min with a detection limit (S/N=3) better than 0.125 microM. Ammonium ion in rain and river waters was precisely determined using this ion-exclusion chromatographic system.     

Determination of Adsorbates on the Surface of Polymer with Low Absorption Capacity by Thermal Lens Spectrometry

Thermal lens spectrometry in a coaxial configuration is used for the direct determination of adsorbates on a planar surface of polyethylene terephthalate (PET). A possibility of the direct measurement of the rate of adsorption from solutions and the determination of the parameters of the adsorbed layer is demonstrated by the example of an investigation of the adsorption of iron(II) tris(1,10-phenantrolinate) on a PET surface. The adsorption isotherm of iron(II) tris(1,10-phenantrolinate) on the PET surface is described by the Langmuir equation and is linear in the concentration range in solution from 0.02 to 0.7 mM. The method for calculating the thermal perturbation in surface-absorbing solids was used to interpret the results of the adsorption study, and a possibility of determining iron(II) tris(1,10-phenantrolinate) on the surface at a level smaller than a monolayer was shown. Thermal lens spectrometry enables the determination of the absorption of the surface layer at a level up to 5 × 10^–5 absorbance units, which corresponds to the surface concentration of iron(II) tris(1,10-phenanthrolinate) 2 × 10^–13 mol/cm². Using the example of the adsorption of 4-(2-pyridylazo) resorcinol on the PET surface, it is demonstrated that, in the case of strong absorption of the surface layer, the thermal destruction of substance and the deformation of the substrate may occur. A local increase in temperature in the layer is also confirmed by theoretical calculations.     

Rapid and highly sensitive determination of low-molecular-weight carbonyl compounds in drinking water and natural water by preconcentration HPLC with 2,4-dinitrophenylhydrazine.

The aim of this research was to develop a simple procedure for a highly sensitive determination of low-molecular-weight (LMW) carbonyl compounds in drinking water and natural water. We employed a preconcentration HPLC system with 2,4-dinitrophenylhydrazine (DNPH) for the determination of LMW carbonyl compounds. A C-18 reverse-phase preconcentration column was used instead of a sample loop at the sample injection valve. A 0.1 - 5.0 mL portion of the derivatized sample solution was injected with a gas-tight syringe, and a 15% acetonitrile aqueous solution was pushed through the preconcentration column to remove the unreacted excess DNPH, which caused serious interference in the determination of formaldehyde. The detection limits were 1 - 3 nM with a relative standard deviation of 2 - 5% for 20 nM standard solutions (n = 5). The calibration curves were essentially unaffected by coexisting sea salts. Applications to commercial mineral water, tap water, river water, pond water and seawater are presented.