Determination of cyclamate in artificial sweeteners and beverages using headspace single-drop microextraction and gas chromatography flame-ionisation detection

A new method for determination of cyclamate was developed using headspace single-drop microextraction (HS-SDME) and gas chromatography (GC). The method is based on the reaction of cyclamate and nitrite in the acidic media and microextraction of cyclohexene formed for subsequent determination with GC. Conditions for both derivatisation and HS-SDME have been optimised. The calibration curve for cyclamate determination showed good linearity in the range of 30–1000 μmol L⁻¹ (R² = 0.9992) and the limit of detection (S/N = 3) was estimated to be 5 μmol L⁻¹. The repeatability and reproducibility for a 200 μmol L⁻¹ of cyclamate were 4% and 4.8% (N = 5), respectively. The purposed method was successfully applied for determination of cyclamate in beverages and sweetener tablets. The average recovery of spiked samples was 97%. The results demonstrated that the developed method is simple, rapid, inexpensive, accurate and remarkably free from interference effects.     

Sensor-containing microspheres of chitosan crosslinked with 8-hydroxyquinoline-5-sulphonic acid for determination of Cu(II) in instant coffee

A novel carbon paste electrode containing chitosan microspheres for the determination of Cu(II) in instant coffee by anodic stripping voltammetry was developed. Chitosan was crosslinked with the chelating agent 8-hydroxyquinoline-5-sulphonic acid and glutaraldehyde. The microspheres of the crosslinked chitosan biopolymer were obtained by the spray drying technique and employed in the construction of the sensor. In acetate buffer solution (0.1 mol L⁻¹, pH 6.0), the calibration curve obtained was linear for concentrations of 5.0 × 10⁻⁷ to 1.4 × 10⁻⁵ mol L⁻¹ (r = 0.9990); the detection limit was 5.5 × 10⁻⁸ mol L⁻¹. The relative standard deviation (n = 8) was lower than 3.0% for solutions containing 6.0 × 10⁻⁶, 5.0 × 10⁻⁵ and 1.5 × 10⁻⁴ mol L⁻¹ of Cu(II). The method was successfully employed for determination of Cu(II) in instant coffee and the results obtained showed good agreement when compared with those using electrothermal atomic absorption spectrometry.     

Effect of modifiers for As,Cu and Pb determinations in sugar-cane spirits by GF AAS

Palladium plus magnesium nitrates with and without Ir, Ru and W were evaluated for the simultaneous determination of As, Cu and Pb in cachaça by graphite furnace atomic absorption spectrometry. For 20 μL of sample, 5 μL Pd(NO₃)₂ and 3 μL Mg(NO₃)₂ dispensed together onto the Ir-coated platform of the THGA, analytical curves in the 0–30.0 μg L⁻¹ As, 0–1.50 mg L⁻¹ Cu and 0–60.0 μg L⁻¹ Pb were built up and typical linear correlation coefficients were always better than 0.999. The limit of detection was 1.30 μg L⁻¹ As, 140 μg L⁻¹ Cu and 0.90 μg L⁻¹ Pb. As, Cu and Pb contents in 10 cachaça samples agreed with those obtained by ICP-MS. Recoveries of spiked samples varied from 96% to 106% (As), 97% to 112% (Cu) and 92% to 108% (Pb). The relative standard deviation (n = 12) was typically 2.7%, 3.3% and 1.9%.     

Determination of nitrite in food samples by anodic voltammetry using a modified electrode

A glassy carbon (GC) electrode modified with alternated layers of iron(III) tetra-(N-methyl-4-pyridyl)-porphyrin (FeT4MPyP) and copper tetrasulfonated phthalocyanine (CuTSPc) was employed for nitrite determination by differential pulse voltammetry (DPV). This modified electrode showed excellent catalytic activity for the nitrite oxidation. After optimizing the operational conditions, a linear response range from 0.5 to 7.5 μmol l⁻¹ with a low detection limit of 0.1 μmol l⁻¹ was obtained. The proposed sensor was stable with a sensitivity of 20.0 μA, 1 μmol⁻¹ and good repeatability, evaluated in terms of relative standard deviation (R.S.D. = 1.3%) for n = 10. Possible interferences from several common ions were evaluated. This sensor was applied for the voltammetric determination of nitrite in some food samples. The results were consistent with those obtained with the standard spectrophotometric procedure.     

Rapid and selective determination of hydrogen peroxide residues in milk by batch injection analysis with amperometric detection

We report a highly rapid, precise, selective and sensitive analytical method for the determination of hydrogen peroxide in milk using a batch-injection analysis (BIA) with amperometric detection at a Prussian-blue bulk modified graphite-composite electrode. An electronic micropipette injected 100 μL aliquots of 10-fold diluted samples (high and low-fat milk) directly onto the modified electrode immersed in the BIA cell. The analytical features of our proposed method includes low RSD between injections (0.76%, n = 9), low detection limit (10 μmol L⁻¹), elevated analytical frequency (up to 80 h⁻¹) and satisfactory recovery values for spiked samples. A fresh and highly reproductive electrode surface can be easily obtained by simple mechanical polishing (RSD = 1.6%, n = 5). The storage stability of the PB-modified graphite-composite surpassed 1 year keeping equivalent performance as initially presented. The association of BIA with an improved amperometric detector provides great promise for routine monitoring of hydrogen peroxide in milk and other beverages.     

Synthesis and application of a new functionalized resin for use in an on-line,solid phase extraction system for the determination of trace elements in waters and reference cereal materials by flame atomic absorption spectrometry

The synthesis and characterization of the resin Amberlite XAD-4 functionalized with 2,6-pyridinedicarboxaldehyde and its application in an on-line system for the preconcentration of cadmium, cobalt, copper, lead and manganese prior to determination using flame atomic absorption spectrometry (FAAS) is proposed. Metal ions retained on the modified resin were eluted using 1.0 mol L⁻¹ HNO₃ solution and aspirated directly to the nebulizer–burner system of a FAAS instrument using a flow injection system. Detection limits (3σ) were determined to be 0.13 μg L⁻¹ for Cd, 0.29 μg L⁻¹ for Cu, 0.23 μg L⁻¹ for Mn, 0.58 μg L⁻¹ for Co and 2.19 μg L⁻¹ for Pb using a 10 mL of water sample loading volume. The limits of detection would be 100 times higher with units of μg kg⁻¹ for the solid samples in which their dilution ratios as (volume/weight) were 100. Enrichment factors ranged from 23.6 to 28.9 (for Co and Mn, respectively). The proposed method was successfully applied to determination of the analytes in natural water samples and certified reference materials.     

Speciation of organic and inorganic selenium in selenium-enriched rice by graphite furnace atomic absorption spectrometry after cloud point extraction

A new method was developed for the determination of organic and inorganic selenium in selenium-enriched rice by graphite furnace atomic absorption spectrometry detection after cloud point extraction. Effective separation of organic and inorganic selenium in selenium-enriched rice was achieved by sequentially extracting with water and cyclohexane. Under the optimised conditions, the limit of detection (LOD) was 0.08 μg L⁻¹, the relative standard deviation (RSD) was 2.1% (c = 10.0 μg L⁻¹, n = 11), and the enrichment factor for selenium was 82. Recoveries of inorganic selenium in the selenium-enriched rice samples were between 90.3% and 106.0%. The proposed method was successfully applied for the determination of organic and inorganic selenium as well as total selenium in selenium-enriched rice.     

A rapid and simple fluorometric method for quantifying isoeugenol in seawater and in plasma and white muscle from Australasian snapper (Pagrus auratus)

Isoeugenol residues in Australasian snapper (Pagrus auratus) white muscle, blood plasma and seawater were accurately and precisely quantified after extraction with acetonitrile using fluorometric detection (ex. 260 nm, em. 340 nm) without chromatographic separation. Isoeugenol residues in Australasian snapper (P. auratus) muscle tissue following 30 min exposure to 58.2 μmol L⁻¹ isoeugenol (ca. 20 ppm of the aquatic anaesthetic AQUI-S™) reached a maximum of 134.37 ± 8.13 μmol kg⁻¹ (±SEM; n = 6). Blood plasma isoeugenol concentrations following this harvesting regime were 253.2 ± 25.1 μmol L⁻¹. After 7 h recovery, fillet isoeugenol residues reduced to 7.89 ± 1.67 μmol kg⁻¹. Storage of fillets from fish harvested with AQUI-S™ at 3.87 ± 0.54 °C for 5 days resulted in a rate of isoeugenol decay in the fillets of 6.51 ± 1.19 μmol kg⁻¹ day⁻¹. The method reported can be used for measuring isoeugenol residues in food products or to further study the physiological and biological effects of isoeugenol in fish.     

Chemiluminescence microflow injection analysis system on a chip for the determination of nitrite in food

A new microflow injection analysis (μFIA) system on a chip for the determination of nitrite is described. The chip is produced by using two transparent poly(methylmethacrylate) (PMMA) slices measured 50 × 40 × 5 mm, and the microchannels etched by CO₂ laser are 200 μm wide and 100 μm deep with the volume of reaction area about 1.8 μL. Nitrite is sensed by the chemiluminescence (CL) reaction of luminol with ferricyanide that is the product of the reaction of ferrocyanide with nitrite in acidic medium. The syringe pump with an accurate timer controls all reagents, including the sample. The linear range of the nitrite concentration is 8–100 μg L⁻¹ and the detection limit is 4 μg L⁻¹ (S/N = 3). The proposed method has good reproducibility with the relative standard deviation 4.1% for 50 μg L⁻¹ of nitrite (n = 9) and is very sensitive and simple. It has been successfully applied to the determination of nitrite in food.     

Microfluidic chip based chemiluminescence detection of L-phenylalanine in pharmaceutical and soft drinks

A sensitive chemiluminescence (CL) method on chip coupled with microfluidic system has been reported for the determination of L-phenylalanine (L-PA). A microfluidic chip device with the detection chamber capable of fast sensing light emitted from the luminol and hydrogen peroxide CL reaction catalyzed by copper sulphate was fabricated for the determination of L-PA. The microfluidic chip was fabricated by a soft-lithographic procedure using polydimethyl siloxane (PDMS). The fabricated PDMS microfluidic chip had four inlet microchannels for introducing the sample, chemiluminescent reagent, Cu(II), and oxidant, and a 500 μm wide, 250 μm deep, and 82 mm long microchannel. The detection was based on the enhancement effect of L-PA on the CL signals of luminol–H₂O₂–Cu²⁺ system in an alkaline medium. The CL intensity of the system was enhanced linearly with the concentration of L-PA in the range of 1.5 × 10⁻⁹–1.2 × 10⁻⁷ mol L⁻¹. The limit of detection was found to be 2.4 × 10⁻¹⁰ mol L⁻¹ with the relative standard deviation of 1.8%. The presented method offers a simple, rapid and easy to handle analytical technique in terms of sensitivity, dynamic range and low detection limit for the determination of L-PA in diet soft drinks and pharmaceutical injection samples.     

Determination of Ge(IV) in rice in a mercury-coated glassy carbon electrode in the presence of catechol

Electrochemical adsorptive cathodic stripping voltammetry determination of Ge(IV) using the catechol complex on a mercury-coated (MC) electrode was prepared using a glassy carbon electrode (GCE), the peak potential of which was −0.5 V vs. Ag/AgCl on MCGCE. The various parameters of the catechol concentration, its pH, and others were optimized. The linear working ranges were obtained in the concentration of 2–700 μg L⁻¹ Ge(IV). The relative standard deviation at the Ge(IV) concentration of 50 μg L⁻¹ was 1.37% (n = 15) using the optimum condition, and the detection limit was found to be 0.6 μg L⁻¹ (8.26 × 10⁻⁹ M) (S/N = 3), with an adsorption time of 180 s. The Ge(IV) response was highly linear. This developed method was applied to Ge(IV) to determine the presence of rice grains.     

Determination of phthalate esters in wine using solid-phase extraction and gas chromatography–mass spectrometry

A method for the determination of six phthalate esters in wine samples has been developed. The phthalates were extracted from wine samples with an optimised solid-phase extraction method on C18 column and quantification was achieved via gas chromatography coupled with a mass spectrometer. The method was linear between 0.015 and 5.000 μg mL⁻¹ for DMP, DEP and DEHP and between 0.018 and 5.000 μg mL⁻¹ for iBP, DBP and BBP. The LOQs of DMP, DEP and DEH were 0.024 μg mL⁻¹ while those of iBP, DBP and BBP were 0.029 μg mL⁻¹. The intra-day method repeatability was between 10% and 15% RSD, whereas the inter-day method repeatability was between 13% and 21% RSD. A survey was performed on white and red wines (n = 62) from the market, winemakers and an experimental pilot plant. All the analysed samples were phthalate contaminated. Commercial wine showed higher detection frequency and level of total phthalate, DBP and BBP than those produced in a pilot plant. iBP and DEHP concentrations were similar in all the groups of samples. iBP concentration was higher in red wines than in white ones.     

Pico molar assay of riboflavin in human urine using voltammetry

Using a new type of DNA and carbon nano tube (CNT) mixed paste electrode using cyclic and square wave anodic stripping voltammetric (SWASV) methods, this study presents an assay of riboflavin (RF) under optimum conditions. Results of the experiment yielded a low working concentration range of nanograms with 1–10 and 10–170 ng L⁻¹ and 5–105 μg L⁻¹, at an accumulation time of 80 s in a 0.1 M H₃PO₄ electrolyte solution. A relative standard deviation of 30 μg L⁻¹ was observed at an accuracy level of 0.1164% (n = 15) under optimum conditions. The detection limit (S/N) was pegged at 0.2 ng L⁻¹ (5.31 × 10⁻¹³ mol L⁻¹ RF). The proposed method was successfully applied to an actual human urine and drug sample, and can be applied to assays of other biological samples.     

Validation of a HPLC method for simultaneous determination of main organic acids in fruits and juices

A liquid chromatographic method for fast and simultaneous determination of tartaric, malic, ascorbic and citric acids was validated for further application to fruits and juices. Moreover, the organic acids content of commercial samples of fruits and juices were evaluated, as well as the ascorbic acid stability during the storage. Determination of organic acids was carried out using a liquid chromatograph coupled to a diode array detector, with reversed phase (C₁₈ column) and isocratic elution with 0.01 mol L⁻¹ KH₂PO₄ (pH = 2.60) mobile phase. The validation parameters showed efficiency, adequate linearity, relative standard deviation values between 0.4% and 2.3% (n = 10) for repeatability and from 1.2% to 5.0% (n = 18) for reproducibility, limits of detection (LD) were between 0.03 and 3.31 μg mL⁻¹ and quantification (LQ) were between 0.10 and 11.03 μg mL⁻¹, recovery rates were between 82% and 110%, for two levels. In addition, the method is fast (10 min) and generates low and non-toxic residues. The values found for vitamin C were about 10 times above the values declared at the package. Ready to drink juices have a composition similar to the fruit, concerning to organic acids, except for the powder juice, in which only ascorbic and citric acids were found, for all tastes. After opening the package, a decrease of 14.0% and 27.0% in ascorbic acid content was observed for orange powder and ready to drink juices, respectively.     

Multi-element determination of Cu,Fe, Ni and Zn content in vegetable oils samples by high-resolution continuum source atomic absorption spectrometry and microemulsion sample preparation

The aim of this work was to evaluate the microemulsification as sample preparation procedure for determination of Cu, Fe, Ni and Zn in vegetable oils samples by High-Resolution Continuum Source Flame Atomic Absorption Spectrometry (HR-CS FAAS). Microemulsions were prepared by mixing samples with propan-1-ol and aqueous acid solution, which allowed the use of inorganic aqueous standards for the calibration. To a sample mass of 0.5 g, 100 μL of hydrochloric acid and propan-1-ol were added and the resulting mixture diluted to a final volume of 10 mL. The sample was manually shaken resulting in a visually homogeneous system. The main lines were selected for all studied metals and the detection limits (3σ, n = 10) were 0.12, 0.62, 0.58 and 0.12 mg kg⁻¹ for Cu, Fe, Ni and Zn, respectively. The relative standard deviation (RSD) ranged from 5% to 11 % in samples spiked with 0.25 and 1.5 μg mL⁻¹ of each metal, respectively. Recoveries varied from 89% to 102%. The proposed method was applied to the determination of Cu, Fe, Ni and Zn in soybean, olive and sunflower oils.     

Flow injection spectrophotometry using natural reagent from Morinda citrifolia root for determination of aluminium in tea

A flow injection (FI) spectrophotometric method with using natural reagent extracted from Morinda citrifolia root has been developed for determination of aluminium. The extract contained anthraquinone compounds which could react with Al³⁺ to form reddish complexes which had maximum absorption wavelength at 499.0 nm. The extract could be used as a reagent in FI system without further purification to obtain pure compound. A sensitive method for determination of aluminium in concentration range of 0.1-1.0 mg L⁻¹, with detection limit of 0.05 mg L⁻¹ was achieved. Relative standard deviations of 1.2% and 1.7% were obtained for the determination of 0.1 and 0.6 mg L⁻¹ Al³⁺ (n = 11). Sample throughput of 35 h⁻¹ was achieved with the consumption of 3 mL each of carrier and reagent solutions per injection. The developed method was successfully applied to tea samples, validated by the FAAS standard method. The method is simple, fast, economical and could be classified as a greener analytical method.     

Determination of resveratrol in grains,hulls and leaves of common and tartary buckwheat by HPLC with electrochemical detection at carbon paste electrode

A reverse-phase high-performance liquid chromatographic method for the determination of trans-resveratrol with spectrophotometric detection (306 nm) and amperometric detection at carbon paste electrode (E = + 1,2 V) was developed and tested on real samples of grains, hulls and leaves of six varieties of common buckwheat (Fagopyrum esculentum Möench) and two varieties of tartary buckwheat (Fagopyrum tataricum (L.) Gaertn.). Optimal conditions for the determination of trans-resveratrol were as follows: column Kromasil C-18 (7 μm), 125 × 4 mm; mobile phase acetonitrile: diluted BR buffer pH 7 (50:50, 30:70 for grains and hulls and 20:80 for leaves); flow rate 1 ml min⁻¹. Under these conditions, the limit of detection of trans-resveratrol (L_D) was 3.5 × 10⁻⁸ mol l⁻¹ (R² = 0.9986) for electrochemical detection and 3.2 × 10⁻⁸ mol l⁻¹ (R² = 0.9993) for spectrophotometric detection.     

Feasibility and application of liquid–liquid extraction combined with gas chromatography–mass spectrometry for the analysis of phenolic acids from grape polyphenols degraded by human faecal microbiota

In this study the feasibility of a LLE–GC–EI-MS method for the analysis of 43 phenolic acids belonging to different chemical structure families which have been described in the literature as microbial-derived metabolites after consumption of dietary polyphenols was proved. In addition, the method was applied for the characterisation of phenolic metabolites resulting from the incubation, in anaerobic conditions, of a commercial grape seed extract (GSE) and their corresponding flavan-3-ol monomeric (GSE-M) and oligomeric (GSE-O) fractions with human faeces from healthy volunteers (n = 3). The method showed average values of repeatability and reproducibility of 5.0% and 6.3%, respectively, adequate and low detection (1.8–30.8 μg L⁻¹) and quantification limits (6.0–102.8 μg L⁻¹) and good recovery values (95%, as average value). A total of 27 phenolic acids were identified in the faecal solutions after incubation with the grape seed extracts. In general, faecal samples incubated with GSE and GSE-M (monomeric fraction) yield a higher formation of phenolic acids compared to the samples incubated with the oligomer fraction (GSE-O).     

Amperometric detection of ascorbic acid in honey using ascorbate oxidase immobilised on amberlite IRA-743

A differential amperometric method for the specific determination of ascorbic acid in honey was developed by association of a flow injection analysis (FIA) system and a tubular reactor containing the ascorbate oxidase enzyme immobilised. A gold electrode modified by electrochemical deposition of palladium was employed as working electrode. Ascorbic acid was quantified in seven samples of commercial honeys using a potential of +0.60 V vs. Ag/AgCl_(sat). The linear dynamic range in ascorbic acid extends from 1 to 50 μmol L⁻¹, at pH 7.0. At flow rate of 1.5 mL min⁻¹ and injecting 250 μL sample volumes, a sampling frequency of 180 determinations per hour is afforded. The detection and quantification limit of this method are 0.14 and 0.49 μmol L⁻¹, respectively. The samples analyses were compared with the volumetric method, and showed an excellent correlation between the methods.     

Spectrophotometric determination of iodate in iodised salt by flow injection analysis

A simple and sensitive flow injection (FI) method for the determination of iodate is proposed. The method is based on the reaction of iodate with hydroxylamine in acidic solution. Sulfanilamide is diazotised by the nascent nitrite and the diazonium ion produced is then coupled with N-(1-naphthyl)ethyenediamine in hydrochloric acid medium to form an azo dye which is measured spectrophotometrically. The calibration graph for iodate is linear in the range of 0.1–30 mg L⁻¹ with a correlation coefficient of 0.9992. The limit of detection and relative standard deviation are 0.02 mg L⁻¹ and 1.2% (5 mg L⁻¹, n = 8). The method has been applied to the determination of iodate in table salts and accuracy was assessed through recovery experiments and independent analysis by a conventional titrimetric method.