Zhiyong Guo

Determination of melamine in dairy products by an electrochemiluminescent method combined with solid-phase extraction

An electrochemiluminescence (ECL) enhancement method combined with solid-phase extraction has been developed for the determination of melamine in dairy products. It was found that melamine in a strong base solution is able to enhance the ECL of Ru(bpy)(3)(2+) at glass carbon electrode. The optimum experimental conditions for the determination of trace melamine by ECL, such as scan mode and scan rate of the applied potential, the type of buffer solutions and their pH conditions, were investigated. Under optimized conditions, the enhanced ECL intensity was linearly proportional to the logarithm of melamine concentration in the range of 0.01-1.0 ppb, and the detection limit was 0.003 ppb. The method has been successfully demonstrated to determine melamine in dairy products including liquid milk, yogurt and milk powder samples. The relative standard deviations ranging from 5.3% to 11.2% and the recoveries from 95.2% to 102.4% were acquired by this method. A possible mechanism for the ECL enhancement effect was also proposed.

Development and application of a method for analysis of phthalates in ham sausages by solid-phase extraction and gas chromatography-mass spectrometry.

A gas chromatography-mass spectrometry assay was developed and successfully applied for the determination of phthalates in ham sausage migrated from packaging film. The phthalates studied were dimethyl phthalate (DMP), diethyl phthalate (DEP), dibutyl phthalate (DBP), benzylbutyl phthalate (BBP), bis(2-ethylhexyl) phthalate (DEHP) and di-n-octyl phthalate (DNOP), with dibutyl adipate (DBA) as internal standard. The sample pre-treatments included extraction with n-hexane, solvent evaporation and reconstitution with acetonitrile before and after solid-phase extraction (SPE). The extraction and cleaning up procedure was carried out with cartridges containing dimethyl butylamine groups, which showed extraction efficiencies over 87.3%. The calibration curves obtained were linear with correlation coefficients greater than 0.99. The method proved to be accurate and precise for the six phthalates used. It was successfully applied to a study on the migration of phthalates from packaging PVC film into ham sausage.

Simultaneous determination of phthalates and adipates in human serum using gas chromatography-mass spectrometry with solid-phase extraction.

A gas chromatography-mass spectrometry assay was developed and validated for the simultaneous determination of phthalates and adipates in human serum. The phthalates and adipates studied were dimethyl phthalate, diethyl phthalate, dibutyl phthalate, benzylbutyl phthalate, di-2-ethylhexyl phthalate, di-n-octyl phthalate, diethyl adipate, dibutyl adipate, diisobutyl adipate, bis(2-butoxyethyl) adipate and di-2-ethylhexyl adipate, with diisooctyl phthalate as internal standard. The extraction and cleaning up procedure was carried out with solid-phase extraction cartridges containing dimethyl butylamine groups, which showed extraction efficiencies over 88% for each analyte and the internal standard. The calibration curves obtained were linear with correlation coefficients greater than 0.98. For all analytes, the assay gave CV% values for intra-day precision from 4.9 to 13.3% and mean accuracy values from 91.4 to 108.4%, while inter-day precision was 5.2-13.4% and mean accuracy 91.0-110.2%. The limits of detection for the assay of phthalates and adipates were in the range 0.7-4.5u2009ng/mL. The method is simple, sensitive and accurate, and allows for simultaneous determination of nanogram levels of phthalates and adipates in human serum. It was successfully applied to an investigation on the level of phthalates and adipates in a non-occupationally exposed population.

Direct electrochemistry of cytochrome c entrapped in agarose hydrogel in room temperature ionic liquids

Direct electrochemistry of cytochrome c (cyt-c) entrapped in agarose hydrogel on gold electrode (Au), edge plane pyrolytic graphite electrode (EPPGE) and glassy carbon electrode (GC) in two room temperature ionic liquids was investigated. The effects of the addition of N,N-dimethylformamide (DMF) in the agarose-cyt-c film, water concentration in ionic liquids and exterior metal ions on the electrochemical behavior of cyt-c were monitored, and electrocatalytic properties of cyt-c were also done. Results showed that a good quasi-reversible redox behavior of cyt-c could be found after adding DMF in agarose-cyt-c film, and peak shape would not change after continuously scanning for 50 cycles. In addition, a certain amount of water in hydrophilic ionic liquids is necessary to maintain electrochemical activities of cyt-c, electrochemical performance of cyt-c is the best when the water content is 5.2% and 5.8% for 1-butyl-3-methylimidazolium bromide ([Bmim][Br]) and 1-butyl-3-methylimidazolium tetrafluoroborate([Bmim][BF(4)]) respectively. However, electrochemical activities of cyt-c are inhibited by exterior metal ions. Interestingly, cyt-c entrapped in agarose hydrogel on EPPGE and GC could catalyze the electroreduction of trichloroacetic acid (TCA) and tert-butyl hydroperoxide (t-BuOOH) in [Bmim][BF(4)], but could not in [Bmim][Br]. Reasons for above-mentioned differences of electrochemical properties of cyt-c in different ionic liquids were preliminarily discussed.

Highly-sensitive ion selective electrode based on molecularly imprinted polymer particles for determination of tetracycline in aqueous samples

In this work, a highly-sensitive polymeric membrane ion selective electrode for determination of tetracycline was constructed by using molecularly imprinted polymer (MIP) particles as quasi-ionophore. The water-compatible MIP particles targeting tetracycline were synthesized with tetracycline as a template molecule, methacrylic acid as a functional monomer, ethylene glycol dimethacrylate as a cross-linker, 2,2′-azobisisobutyronitrile as an initiator and lanthanum ion as a mediator. Benefited from the distinctive performance of the quasi-ionophore and the optimized composition of the membrane and the inner filling solution, the lower detection limit of the electrode was decreased to about 1 × 10−8 mol/l. It exhibited a good electrode slope 59.8 mV/decade near the theoretical Nernstian one, with a wide linear working range from 2.0 × 10−8 to 1.0 × 10−3 mol/l. Due to the specific recognition of tetracycline by the MIP particles, the selectivity coefficients for routine interferences were less than 10−4. The fabricated electrode should be used in pH 2–4, response time of which was less than 200 s when the concentration of tetracycline was higher than 1.0 × 10−6 mol/l and no more than 30 min at the concentration of 1.0 × 10−8 mol/l. Finally, the proposed highly-sensitive ion selective electrode has been successfully applied to the determination of tetracycline in aqueous samples.

Direct electrochemistry of cytochrome c entrapped in agarose hydrogel by protein film voltammetry

Cytochrome c (cyt-c) entrapped in agarose hydrogel was electrostatically bound to the edge plane pyrolytic graphite electrode (EPPGE), and then stable agarose-cyt-c/EPPGE was prepared. Direct electrochemistry between cyt-c and the EPPGE was monitored by protein film voltammetry. The effects of ionic strength, pH and exterior substances on the direct electrochemistry, and electrocatalytic properties of cyt-c were also studied. Results show that the electrochemical behavior of heme Fe(III)/Fe(II) in cyt-c is quasireversible. Its electrochemical performance is the best when the concentration of supporting electrolyte phosphate buffer solution is 0.2 mol/l and pH is 7.0, and greatly inhibited by exterior metal ions. Catalytic reduction of oxygen could also be achieved at the agarose-cyt-c/EPPGE.