Enzymatic chemiluminescent assay of glucose by sequential-injection analysis with soluble enzyme and on-line sample dilution

This work reports a sequential-injection analysis (SIA) method for the enzymatic assay of glucose with soluble glucose oxidase (GOD) and on-line sample dilution with chemiluminescence (CL) detection. A zone of sample was aspirated in the holding coil of the SIA manifold and, if necessary, was diluted on-line by means of an auxiliary dilution conduit. Then, a zone of GOD was aspirated adjacent to the sample zone and a stopped-flow period was applied to allow the enzymatic reaction to proceed with production of hydrogen peroxide. Then, zones of a catalyst (Co(II) solution) and alkaline luminol were aspirated into the holding coil. Finally, the flow was reversed and the stacked zones were sent to a flow-cell located in front of a photomultiplier tube (PMT) that monitored the CL intensity. The linear dynamic range was 1 × 10⁻⁵-1 × 10⁻³ mol L⁻¹ glucose, the coefficient of variation at 8 × 10⁻⁵ mol L⁻¹ of glucose was s_r = 3.1% (n = 8), the limit of detection at the 3σ level was c_L = 1 × 10⁻⁶ mol L⁻¹ and the sampling frequency was 28 h⁻¹. With on-line dilution by a factor of 1/200, the linear range could be extended up to 0.2 mol L⁻¹ glucose. The advantages of the proposed method are the simple manifold and instrumentation used, the scope for automated on-line dilution, the low consumption of sample and reagents and the elimination of enzyme immobilisation procedures. The method was applied to the analysis of commercial drinks and honey with percent relative errors in glucose determination in the range 100 ± 6.1%.     

Determination of brominated phenols in water samples by on-line coupled isotachophoresis with capillary zone electrophoresis

A method for determination of nine brominated phenols as environmental risk compounds was developed by on-line coupled capillary isotachophoresis and capillary zone electrophoresis (ITP–CZE). For ITP step, 1 × 10⁻² mol L⁻¹ hydrochloric acid with 3 × 10⁻² mol L⁻¹ ammediol pH 9.1 was used as the leading electrolyte, and 3 × 10⁻² mol L⁻¹ β-alanine with 2 × 10⁻² mol L⁻¹ sodium hydroxide pH 10.05 was used as the terminating electrolyte. As the background electrolyte for CZE separation, 2.5 × 10⁻² mol L⁻¹ β-alanine with 2.5 × 10⁻² mol L⁻¹ lysine pH 9.6 was used. All electrolytes contained 0.05% or 0.1% (m/v) hydroxyethylcellulose to suppress the electroosmotic flow. UV detection at wavelength 220 nm was used. Detection limits in order of tens of nmol L⁻¹ were achieved. Good repeatability of migration times (less than 0.33% RSD) and good repeatability of peak areas (less than 7.19% RSD) at concentration level 5 × 10⁻⁸ mol L⁻¹ were observed. Developed ITP–CZE method was applied to determination of brominated phenols in spiked tap and river water samples.     

Development of a flow system for the determination of low concentrations of silver using Moringa oleifera seeds as biosorbent and flame atomic absorption spectrometry

In this study a method for the determination of low concentrations of silver in waters using solid-phase extraction with a flow injection analysis system and detection by flame atomic absorption spectrometry (FAAS) was developed. Moringa oleifera seeds were used as a biosorbent material. Chemical and flow variables of the on-line preconcentration system such as sample pH and flow rate, preconcentration time, eluent concentration and sorbent mass were studied. The optimum preconcentration conditions were obtained using sample pH in the range of 6.0-8.0, preconcentration time of 4 min at a flow rate of 3.5 mL min^− 1, 0.5 mol L^− 1 HNO₃ eluent at a flow rate of 4.5 mL min^− 1 and 35 mg of sorbent mass. With the optimized conditions, the preconcentration factor, precision, detection limit and sample throughput were estimated as 35 (for preconcentration of 14 mL sample), 3.8% (5.0 μg L^− 1, n = 7), 0.22 μg L^− 1 and 12 samples per hour, respectively. The developed method was successfully applied to mineral water and tap water, and accuracy was assessed through analysis of a certified reference material for water (APS-1071 NIST) and recovery tests, with recovery ranging from 94 to 101%.     

Fabrication and characterization of hexahistidine-tagged protein functionalized multi-walled carbon nanotubes for selective solid-phase extraction of Cu and Ni

Hexahistidine-tagged protein functionalized multi-walled carbon nanotubes (MWCNTs/6His-tagged protein) were prepared and characterized by ultraviolet-visible spectrophotometry and atomic force microscopy. Both static and dynamical adsorption experiments showed that the MWCNTs/6His-tagged protein served as good sorbent for the solid-phase extraction of Cu²⁺ and Ni²⁺. Effective on-line sorption of Cu²⁺ and Ni²⁺ on the MWCNTs/6His-tagged protein packed microcolumn was achieved in a pH range of 3.0-4.5 and 4.5-6.0, respectively. The retained Cu²⁺ and Ni²⁺ were efficiently eluted with 0.2 mol L⁻¹ imidazole-HCl solution for on-line flame atomic absorption spectrometric determination. The MWCNTs/6His-tagged protein exhibited fairly fast kinetics for the sorption of Cu²⁺ and Ni²⁺, and offered up to 20,000 and 1800 times improvement in the tolerable concentrations of co-existing ions over the MWCNTs for solid-phase extraction of Cu²⁺ and Ni²⁺, respectively. On-line solid-phase extraction at a flow rate of 5.0 mL min⁻¹ for 60 s gave an enhancement factor of 29 for Cu²⁺ and 28 for Ni²⁺, a sample throughput of 45 h⁻¹, and a detection limit (3s) of 0.31 μg L⁻¹ for Cu²⁺ and 0.63 μg L⁻¹ for Ni²⁺. The precision for 11 replicate measurements was 2.4% for 10 μg L⁻¹ Cu²⁺, and 2.5% for 15 μg L⁻¹ Ni²⁺.     

On-line molecularly imprinted solid-phase extraction for the selective spectrophotometric determination of nicotine in the urine of smokers

This work describes an on-line molecularly imprinted solid-phase extraction (MISPE) method for spectrophotometric determination of nicotine in urine samples of smokers. This method is based on manganese (VII) to manganese (VI) reduction in an alkaline medium, promoted by nicotine. Two wash solutions (1:4 (v/v) acetonitrile:sodium hydroxide - pH 11.4, and nitric acid - pH 2.5) were employed to circumvent interferences. Aqueous solutions containing nicotine plus different possible concomitants (cotinine, anabasine, norcotinine and caffeine) were tested individually. The analytical calibration curve was prepared in urine samples collected from non-smokers and spiked with nicotine standard from 1.1 to 60 μmol L⁻¹ (r² > 0.998). The limit of quantification and the analytical frequency were 1.1 μmol L⁻¹ and 11 h⁻¹, respectively. The precision, evaluated using 3, 10 and 30 μmol L⁻¹ nicotine in urine, was 10, 10 and 4% (intra-day precision) and 12, 13 and 5% (inter-day precision), respectively. Accuracy was checked through high performance liquid chromatography and the results did not present significant differences at the 95% confidence level according to the Student's t-test.     

A new flow injection preconcentration method based on multiwalled carbon nanotubes for the ETA-AAS determination of Cd in urine

A new flow injection (FIA) procedure for the preconcentration of cadmium in urine using multiwalled carbon nanotubes (MWCNT) as sorbent and posterior electrothermal atomization atomic absorption spectrometry (ETA-AAS) Cd determination has been developed. Cadmium was retained in a column filled with previously oxidized MWCNTs and it was quantitatively eluted with a nitric acid solution. The parameters influencing the adsorption-elution process such as pH of the sample solution, amount of sorbent and flow rates of sample as well as eluent solutions have been studied. Cd concentration in the eluent was measured by ETA-AAS under the optimized conditions obtained. The results indicated the elimination of urine matrix effect as a consequence of the preconcentration process performed. Total recovery of cadmium from urine at pH 7.2 using a column with 45 mg of MWCNTs as sorbent and employing a HNO₃ 0.5 mol L⁻¹ solution for elution was attained. The detection limit obtained was 0.010 μg L⁻¹ and the preconcentration factor achieved was 3.4. The method showed adequate precision (RSD: 3.4-9.8%) and accuracy (mean recovery: 97.4-100%). The developed method was applied for the determination of cadmium in real urine samples from healthy people (in the range of 0.14-2.94 μg L⁻¹) with satisfactory results.     

Continuous flow method for the simultaneous determination of phosphate/arsenate based on their different kinetic characteristics

This paper proposes a new automated spectrophotometric method for the simultaneous determination of phosphate and arsenate without pre-treatment, which is faster, simpler, less expensive and hazardous than other well-known methods used with water samples. Such method is based on the different kinetic characteristics of complex formation of phosphate and arsenate with ammonium molybdate. A flow system was used in order to achieve good mixing and to provide precise time control. All the measurements were performed at the isosbestic point wavelength (885 nm). Chemical variables were optimized by factorial design (ammonium molybdate 0.015 mol L⁻¹, potassium antimony tartrate 1 × 10⁻⁴ mol L⁻¹, and sulphuric acid 0.7 mol L⁻¹). An appropriate linear range for both analytes (0.50-8.00 μmol L⁻¹), good inter-day reproducibility (4.9% [P] and 3.3% [P + As]) and a sample throughput of 6 h⁻¹ were obtained. The detection limits are 0.4 μmol L⁻¹ P and 0.19 μmol L⁻¹ [P + As] (3.3 Sy/x). The method was validated.     

Development of a flow system for the determination of cadmium in fuel alcohol using vermicompost as biosorbent and flame atomic absorption spectrometry

In this study a method for the determination of cadmium in fuel alcohol using solid-phase extraction with a flow injection analysis system and detection by flame atomic absorption spectrometry was developed. The sorbent material used was a vermicompost commonly used as a garden fertilizer. The chemical and flow variables of the on-line preconcentration system were optimized by means of a full factorial design. The selected factors were: sorbent mass, sample pH, buffer concentration and sample flow rate. The optimum extraction conditions were obtained using sample pH in the range of 7.3-8.3 buffered with tris(hydroxymethyl)aminomethane at 50 mmol L⁻¹, a sample flow rate of 4.5 mL min⁻¹ and 160 mg of sorbent mass. With the optimized conditions, the preconcentration factor, limit of detection and sample throughput were estimated as 32 (for preconcentration of 10 mL sample), 1.7 μg L⁻¹ and 20 samples per hour, respectively. The analytical curve was linear from 5 up to at least 50 μg L⁻¹, with a correlation coefficient of 0.998 and a relative standard deviation of 2.4% (35 μg L⁻¹, n = 7). The developed method was successfully applied to spiked fuel alcohol, and accuracy was assessed through recovery tests, with recovery ranging from 94% to 100%.     

Rapid simultaneous analysis of 1-hydroxypyrene, 2-hydroxyfluorene, 9-hydroxyphenanthrene, 1- and 2-naphthol in urine by first derivative synchronous fluorescence spectrometry using Tween-20 as a sensitizer

A novel method of first derivative synchronous fluorescence was developed for the rapid simultaneous analysis of trace 1-hydroxypyrene (1-OHP), 1-naphthol (1-NAP), 2-naphthol (2-NAP), 9-hydroxyphenanthrene (9-OHPe) and 2-hydroxyfluorene (2-OHFlu) in human urine. Only one single scan was needed for quantitative determination of five compounds simultaneously when Δλ = 10 nm was chosen. In the optimal experimental conditions, there was a linear relationship between the fluorescence intensity and the concentration of 1-OHP, 1-NAP, 2-NAP, 9-OHPe and 2-OHFlu in the range of 1.75 × 10⁻⁹ to 4.50 × 10⁻⁶ mol L⁻¹, 3.64 × 10⁻⁸ to 2.20 × 10⁻⁴ mol L⁻¹, 8.18 × 10⁻⁹ to 1.20 × 10⁻⁴ mol L⁻¹, 3.26 × 10⁻⁹ to 8.50 × 10⁻⁵ mol L⁻¹ and 4.88 × 10⁻⁹ to 5.50 × 10⁻⁶ mol L⁻¹, respectively. The limits of detection (LOD) were found to be 5.25 × 10⁻¹⁰ mol L⁻¹ for 1-OHP, 1.10 × 10⁻⁸ mol L⁻¹ for 1-NAP, 2.46 × 10⁻⁹ mol L⁻¹ for 2-NAP, 9.77 × 10⁻¹⁰ mol L⁻¹ for 9-OHPe and 1.46 × 10⁻⁹ mol L⁻¹ for 2-OHFlu. The proposed method is reliable, selective and sensitive, and has been used successfully in the determination of traces of 1-OHP, 1-NAP, 2-NAP, 9-OHPe and 2-OHFlu in human urine samples, whose results were in good agreement with those gained by the HPLC method.     

Synchronous fluorescence determination of DNA based on the interaction between methylene blue and DNA

The fluorescence intensity of methylene blue (MB) quenched by DNA in the pH range of 6.5-8.0 was studied with synchronous fluorescence technology. A novel method for detecting single-stranded and double-stranded DNA was developed. The decreased fluorescence intensity at 664 nm is in proportion to the concentration of DNA in the range of 0.28-11.0 μmol L⁻¹ for ctDNA, 0.14-8.25 μmol L⁻¹ for thermally denatured ctDNA and 0.28-8.25 μmol L⁻¹ for hsDNA. The detection limits (S/N = 3) are 0.11, 0.04 and 0.04 μmol L⁻¹, respectively. The method is rapid, selective, and the reagents are lower toxic. It has been used for the determination of DNA in synthetic samples with good satisfaction. In addition, the interaction modes between MB and ctDNA and the mechanism of the fluorescence quenching were also discussed in detail. The experimental results from absorption spectra and fluorescence polarization indicate that the possible interaction modes between MB and DNA are the electrostatic binding and the intercalation binding.     

Preliminary evaluation of monolithic column high-performance liquid chromatography with tris(2,2′-bipyridyl)ruthenium(II) chemiluminescence detection for the determination of quetiapine in human body fluids

High-performance liquid chromatography (HPLC) with tris(2,2′-bipyridyl)ruthenium(II) chemiluminescence detection methodology is reported for the determination of the atypical antipsychotic drug quetiapine and the observation of its major active and inactive metabolites in human urine and serum. The method uses a monolithic chromatographic column allowing high flow rates of 3 mL min⁻¹ enabling rapid quantification. Flow injection analysis (FIA) with tris(2,2′-bipyridyl)ruthenium(II) chemiluminescence detection and HPLC time of flight mass spectrometry (TOF-MS) were used for the determination of quetiapine in a pharmaceutical preparation to establish its suitability as a calibration standard. The limit of detection achieved with FIA was 2 × 10⁻¹¹ mol L⁻¹ in simple aqueous solution. The limits of detection achieved with HPLC were 7 × 10⁻⁸ and 2 × 10⁻¹⁰ mol L⁻¹ in urine and serum, respectively. The calibration range for FIA was between 5 × 10⁻⁹ and 1 × 10⁻⁶ mol L⁻¹. The calibration ranges for HPLC were between 1 × 10⁻⁷-1 × 10⁻⁴ and 1 × 10⁻⁸-1 × 10⁻⁴ mol L⁻¹ in urine and serum, respectively. The quetiapine concentrations in patient samples were found to be 3 × 10⁻⁶ mol L⁻¹ in urine and 7 × 10⁻⁷ mol L⁻¹ in serum. Without the need for preconcentration, the HPLC detection limits compared favourably with those in previously published methodologies. The metabolites were identified using HPLC-TOF-MS.     

Lab-on-valve (LOV) system coupled to irreversible biamperometric detection for the on-line monitoring of catechol

The analytical performance of lab-on-valve (LOV) system using irreversible biamperometry for the determination of catechol was evaluated. By integrating miniaturized electrochemical flow cell (EFC) designed and processed which is furnished with two identical polarized platinum electrodes, into the LOV unit, the lab-on-valve system combines sampling with analysis, realizing automated on-line analysis for catechol in a closed system. The biamperometric detection system was established to record the relationship between oxidation current and time by coupling the irreversible oxidation of catechol at one pretreated platinum electrode with the irreversible reduction of platinum oxide at the other pretreated platinum electrode. Factors influencing the analytical performance were optimized, including the potential difference (ΔE), buffer solution and pH, and flow variables in the LOV. A linear calibration curve was obtained within the range of 1.0 × 10⁻⁶-5.0 × 10⁻⁴ mol L⁻¹ of catechol with the detection limit (3σ) of 5.09 × 10⁻⁷ mol L⁻¹. The relative standard deviation (R.S.D.) was 2.39% for 11 successive determinations of 1 × 10⁻⁵ mol L⁻¹ catechol and the sample throughput was 35 h⁻¹. Moreover, this proposed method was applied to the analysis of catechol in beer sample, which was testified by high-performance liquid chromatography (HPLC).     

Linear polyamines as carriers in thiocyanate-selective membrane electrodes

The polyamines, octyl-[2-(2-octylamino-ethylamino)-ethyl]-amine (L¹) and octyl-{2-[2-(2-octylamino-ethylamino)-ethylamino]-ethyl}-amine (L²), have been used as anion ionophores in PVC-based membrane ion-selective electrodes. Different electrodes were prepared containing L¹, or L², and o-nitrophenyl octyl ether (NPOE) or bis(2-ethylhexyl)sebacate (DOS) as plasticizers. The response of the electrodes was tested in two different buffers, HEPES-KOH (pH 7) and MES-KOH (pH 5.6). Electrodes containing L¹ and L² with NPOE (E1 and E2, respectively) showed a Nernstian response for thiocyanate with a good response time. The detection limit, linear range and slope for electrode E1 were 3.8 × 10⁻⁶ mol dm⁻³, 1 × 10⁻⁵ to 1 × 10⁻¹ mol dm⁻³ and −57.2 mV decade⁻¹ at pH 5.6 and 4.47 × 10⁻⁶ mol dm⁻³, 1.95 × 10⁻⁵ to 1 × 10⁻¹ mol dm⁻³ and −58.1 mV decade⁻¹ at pH 7.0. For electrode E2 the detection limit, linear range and slope found were 2.63 × 10⁻⁶ mol dm⁻³, 7.94 × 10⁻⁶ to 1 × 10⁻¹ mol dm⁻³ and −58.5 mV decade⁻¹ at pH 5.6 and 1.23 × 10⁻⁵ mol dm⁻³, 7.95 × 10⁻⁵ to 1 × 10⁻¹ mol dm⁻³ and −46.0 mV decade⁻¹ at pH 7. In contrast, electrodes containing DOS as plasticizers gave only response at pH 5.6 (detection limit, linear range and slope at pH 5.6 were 3.16 × 10⁻⁵ mol dm⁻³, 1 × 10⁻⁴ to 1 × 10⁻¹ mol dm⁻³ and −52.6 mV decade⁻¹). Selectivity coefficients for different anions with respect to thiocyanate were calculated. The electrode E2 at pH 5.6 was also used for the determination of SCN⁻ by potentiometric titrations with Ag⁺ ions with good results. The electrode E2 was also used to determine concentrations of thiocyanate in biological samples.     

Synchronous fluorescence determination of urinary 1-hydroxypyrene, β-naphthol and 9-hydroxyphenanthrene based on the sensitizing effect of β-cyclodextrin

A novel method for the simultaneous determination of 1-hydroxypyrene (1-OHP), β-naphthol (β-NAP) and 9-hydroxyphenanthrene (9-OHPe) in human urine has been established by using synchronous fluorescence spectrometry. It was based on the fact that synchronous fluorescence spectrometry can resolve the broad-band overlapping of conventional fluorescence spectra, which arise from their similar molecular structures. Only one single scan is needed for quantitative determination of three compounds simultaneously when Δλ = 15 nm is chosen. The signals detected at these three wavelengths, 369.6, 330.0 and 358.0 nm, vary linearly when the concentration of 1-OHP, β-NAP and 9-OHPe is in the range of 2.16 × 10⁻⁸-1.50 × 10⁻⁵ mol L⁻¹, 1.20 × 10⁻⁷-1.10 × 10⁻⁵ mol L⁻¹ and 1.07 × 10⁻⁷-3.50 × 10⁻⁵ mol L⁻¹, respectively. The correlation coefficients for the standard calibration graphs were 0.994, 0.999 and 0.997 (n = 7) for 1-OHP, β-NAP and 9-OHPe, respectively. The limits of detection (LOD) for 1-OHP, β-NAP and 9-OHPe were 6.47 × 10⁻⁹ mol L⁻¹, 3.60 × 10⁻⁸ mol L⁻¹ and 3.02 × 10⁻⁸ mol L⁻¹with relative standard deviations (R.S.D.) of 4.70-6.40%, 2.80-4.20%, 3.10-4.90% (n = 6), respectively. The method described here had been applied to determine traces of 1-OHP, β-NAP and 9-OHPe in human urine, and the obtained results were in good agreement with those obtained by the HPLC method. In addition, the interaction modes between β-cyclodextrin (β-CD) and 1-OHP, β-NAP or 9-OHPe, as well as the mechanism of the fluorescence enhancement were also discussed.     

Application of sequential injection-square wave voltammetry (SI-SWV) to study the adsorption of atrazine onto a tropical soil sample

Square wave voltammetry automated by sequential injection analysis was applied to determine the Freundlich adsorption coefficients for the adsorption of atrazine onto a clay rich soil. The detection limit in soil extracts was between 0.18 and 0.48 μmol L⁻¹, depending on the medium used to prepare the extracts (0.010 mol L⁻¹ KCl, CaCl₂ or HNO₃ and 0.0050 mol L⁻¹ H₂SO₄), all of them conditioned in 40 mmol L⁻¹ Britton-Robinson buffer at pH 2.0 in presence of 0.25 mol L⁻¹ NaNO₃. Also in soil extracts the linear dynamic range was between 1.16 and 18.5 μmol L⁻¹ (0.25-4.0 μg mL⁻¹), with a sampling frequency of 190 h⁻¹. The K_f Freundlich adsorption coefficient was 3.8 ± 0.2 μmol^1−1/n Lⁿ kg⁻¹ in medium of 0.010 mol L⁻¹ KCl or CaCl₂, but increased to 7.7 ± 0.1 and 9.0 ± 0.3 μmol^1−1/n Lⁿ kg⁻¹ in 0.010 mol L⁻¹ HNO₃ and 0.0050 mol L⁻¹ H₂SO₄, respectively. The increase of K_f was related to the decrease of pH from 6.4-6.7 in KCl and CaCl₂ to 3.7-4.0 in presence of HNO₃ or H₂SO₄, which favors protonation of atrazine, facilitating electrostatic attractions with negative charges of the clay components of the soil. The 1/n parameters were between 0.76 and 0.86, indicating that the isotherms are not linear, suggesting the occurrence of chemisorption at specific adsorption sites. No statistically significant differences were observed in comparison to the adsorption coefficients obtained by HPLC. The advantage of the proposed SI-SWV method is the great saving of reagent because it does not use organic solvent as in the case of HPLC (50% (v/v) acetonitrile in the mobile phase). Additionally the start up of SI-SWV is immediate (no column conditioning necessary) and the analysis time is only 19 s.     

A novel dual-function molecularly imprinted polymer on CdTe/ZnS quantum dots for highly selective and sensitive determination of ractopamine

A novel dual-function material was synthesized by anchoring a molecularly imprinted polymer (MIP) layer on CdTe/ZnS quantum dots (QDs) using a sol–gel with surface imprinting. The material exhibited highly selective and sensitive determination of ractopamine (RAC) through spectrofluorometry and solid-phase extraction (SPE) coupled with high performance liquid chromatography (HPLC). A series of adsorption experiments revealed that the material showed high selectivity, good adsorption capacity and a fast mass transfer rate. Fluorescence from the MIP-coated QDs was more strongly quenched by RAC than that of the non-imprinted polymer, which indicated that the MIP-coated QDs acted as a fluorescence sensing material could recognize RAC. In addition, the MIP-coated QDs as a sorbent was also shown to be promising for SPE coupled with HPLC for the determination of trace RAC in feeding stuffs and pork samples. Under optimal conditions, the spectrofluorometry and SPE-HPLC methods using the MIP-coated QDs had linear ranges of 5.00 × 10⁻¹⁰–3.55 × 10⁻⁷ and 1.50 × 10⁻¹⁰–8.90 × 10⁻⁸ mol L⁻¹, respectively, with limits of detection of 1.47 × 10⁻¹⁰ and 8.30 × 10⁻¹¹ mol L⁻¹, the relative standard deviations for six repeat experiments of RAC (2.90 × 10⁻⁹ mol L⁻¹) were below 2.83% and 7.11%.     

Dual fluorescence/contactless conductivity detection for microfluidic chip

A new dual detection system for microchip is reported. Both fluorescence detector (FD) and contactless conductivity detector (CCD) were combined together and integrated on a microfluidic chip. They shared a common detection position and responded simultaneously. A blue light-emitting diode was used as excitation source and a small planar photodiode was used to collect the emitted fluorescence in fluorescence detection, which made the device more compact and portable. The coupling of the fluorescence and contactless conductivity modes at the same position of a single separation channel enhanced the detection characterization of sample and offered simultaneous detection information of both fluorescent and charged specimen. The detection conditions of the system were optimized. K⁺, Na⁺, fluorescein sodium, fluorescein isothiocyanate (FITC) and FITC-labeled amino acids were used to evaluate the performance of the dual detection system. The limits of detection (LOD) of FD for fluorescein Na⁺, FITC, FITC-labeled arginine (Arg), glycine (Gly) and phenylalanine (Phe) were 0.02 μmol L⁻¹, 0.05 μmol L⁻¹, 0.16 μmol L⁻¹, 0.15 μmol L⁻¹, 0.12 μmol L⁻¹ respectively, and the limits of detection (LOD) of CCD achieved 0.58 μmol L⁻¹ and 0.39 μmol L⁻¹ for K⁺ and Na⁺ respectively.     

Simultaneous determination of N-acetyl-p-aminophenol and p-aminophenol with poly(3,4-ethylenedioxythiophene) modified glassy carbon electrode

A sensitive and selective method was developed for the determination of N-acetyl-p-aminophenol (APAP) and p-aminophenol (PAP) using poly(3,4-ethylenedioxythiophene) (PEDOT)-modified glassy carbon electrode (GCE). Cyclic voltammetry and differential pulse voltammetry were used to investigate the electrochemical reaction of APAP and PAP at the modified electrode. Both APAP and PAP showed quasireversible redox reactions with formal potentials of 367 mV and 101 mV (vs. Ag/AgCl), respectively, in phosphate buffer solution of pH 7.0. The significant peak potential difference (266 mV) between APAP and PAP enabled the simultaneous determination both species based on differential pulse voltammetry. The voltammetric responses gave linear ranges of 1.0 × 10⁻⁶-1.0 × 10⁻⁴ mol L⁻¹ and 4.0 × 10⁻⁶-3.2 × 10⁻⁴ mol L⁻¹, with detection limits of 4.0 × 10⁻⁷ mol L⁻¹ and 1.2 × 10⁻⁶ mol L⁻¹ for APAP and PAP, respectively. The method was successfully applied for the determination of APAP and PAP in pharmaceutical formulations and biological samples.     

Simultaneous extraction and preconcentration of uranium and thorium in aqueous samples by new modified mesoporous silica prior to inductively coupled plasma optical emission spectrometry determination

A new synthesized modified mesoporous silica (MCM-41) using 5-nitro-2-furaldehyde (fural) was applied as an effective sorbent for the solid phase extraction of uranium(VI) and thorium(IV) ions from aqueous solution for the measurement by inductively coupled plasma optical emission spectrometry (ICP OES). The influences of some analytical parameters on the quantitative recoveries of the analyte ions were investigated in batch method. Under optimal conditions, the analyte ions were sorbed by the sorbent at pH 5.5 and then eluted with 1.0 mL of 1.0 mol L⁻¹ HNO₃. The preconcentration factor was 100 for a 100 mL sample volume. The limits of detection (LOD) obtained for uranium(VI) and thorium(IV) were 0.3 μg L⁻¹. The maximum sorption capacity of the modified MCM-41 was found to be 47 and 49 mg g⁻¹ for uranium(VI) and thorium(IV), respectively. The sorbent exhibited good stability, reusability, high adsorption capacity and fast rate of equilibrium for sorption/desorption of uranium and thorium ions. The applicability of the synthesized sorbent was examined using CRM and real water samples.     

Flow injection analysis of ultratrace orthophosphate in seawater with solid-phase enrichment and luminol chemiluminescence detection

Solid-phase extraction technique had been applied to extract molybdophosphoric heteropoly acid (MoP) paired with cetyltrimethylammonium bromide (CTAB) from seawater matrix using C18 sorbent. Chemiluminescence emission could be generated via MoP reaction with alkaline luminol. Based on these, a novel on-line solid-phase extraction method coupled with flow injection (FI) analysis and luminol chemiluminescence detection had been established to determine ultratrace orthophosphate in seawater. The MoP-CTAB compound could be efficiently extracted on an in-line Sep-Pak C18 cartridge, and rapidly eluted by 0.3 mol l⁻¹ sulphuric acid-ethanol solution. Then the compound was reduced by luminol to produce chemiluminescence light, which could be detected using a luminescence analyzer. Experimental parameters were optimized using a univariate experimental design. Using artificial seawater with salinity of 35 as a matrix, the standard curve with a linear range between 0.005 and 0.194 μmol l⁻¹ had been obtained, and the recovery and the detection limit of the proposed method were found to be 92.5% and 0.002 μmol l⁻¹, respectively. The relative standard deviation (R.S.D.), which was determined over eight hour, was 4.66% (n = 7) for the artificial seawater at a concentration of 0.097 μmol l⁻¹ orthophosphate. Si of 200 μmol l⁻¹ would not interfere with the detection of 0.012 μmol l⁻¹ orthophosphate compound. Three typical seawater samples were analyzed using both the proposed method and the magnesium hydroxide-induced coprecipitation (MAGIC) method, and the results of the two methods showed no significant difference using the t test. Compared to the MAGIC method, the proposed method was more sensitive, time saving and easy for on-line analysis.