Tommaso R. I. Cataldi

Copper dispersed into polyaniline films as an amperometric sensor in alkaline solutions of amino acids and polyhydric compounds

A chemically modified electrode composed of copper microparticles dispersed into a polyaniline (PANI) film was investigated as an amperometric sensor of scarcely electroactive compounds possessing -OH and -NH2 groups. Glassy carbon was used as an electrode material and modified firstly by a PANI film, then allowed to stand in contact with a solution of copper ions, and finally, the electroreduction was done at −0.3 V. The electrochemical behaviour of the resulting modified electrode in alkaline medium was examined by cyclic voltammetry and flow-injection amperometry. Using some representative compounds, the effect of copper loading and pH on the electrode response was investigated. Constant-potential amperometric detection was applied in conjunction with anion-exchange chromatographic (AEC) separations of amino acids and carbohydrates. At an applied potential of 0.55 V vs. AgAgCl, the detection limits (SN = 3) for all analytes investigated ranged 5–15 pmol, and the linear dynamic range was three–four orders of magnitude above the detection limits. The resulting modified electrode was found to retain 95% of its initial response in flowing streams for 3 h of operating time.

Study of a nickel-catalysed glassy carbon electrode for detection of carbohydrates in liquid chromatography and flow injection analysis

Abstract A nickel-modified glassy carbon electrode was investigated. The nickel deposit was characterized by electrochemical techniques, x-ray photoelectron spectroscopy and scanning electron microscopy. The described electrode was tested as detector in liquid chromatography and flow-injection analysis and showed a promising combination of background current stability, catalytic activity, detection limits and reproducibility for the determination of carbohydrates in real samples.

Cobalt-based glassy carbon chemically modified electrode for constant-potential amperometric detection of carbohydrates in flow-injection analysis and liquid chromatography

Abstract A cobalt-based glassy carbon (Co/GC) chemically modified electrode (CME) is described for use as an amperometric detector in the flow analysis of mono-, di- and trisaccharides. The preparation of the electrode is very easy and rapid. As with copper- and nickel-based glassy carbon electrodes, the Co/GC CME allows the electrocatalytic oxidation of scarcely electroactive compounds without recourser to pulsed waveform potentials. Hence, constant-potential amperometric detection is feasible and it has been applied successfully to reducing and non-reducting sugars. The chemical nature of cobalt species on glassy carbon was investigated using x-ray photoelectron spectroscopy and the results indicate that a Co(III) oxide hydroxide is present on the glassy carbon surface. However, electrochemical detection in an alkaline mobile phase of aliphatic organic compounds seems to take place through mediation of Co(IV) species produced from the oxidation of Co(III) during anodic polarization. Carbohydrates and related polyhydroxy compounds were determined at the micromolar level with an operating potential of 0.50 V vs. Ag/AgCl after anion-exchange chromatographic separation. The detector responds linearly to an increase in glucose concentration with a linear dynamic range that extends over three orders of magnitude (0.5 μM-0.5 mM).

Enhanced stability and electrocatalytic activity of a ruthenium-modified cobalt-hexacyanoferrate film electrode

Abstract The electrochemical behaviour and electrocatalytic activity of a ruthenium-modified cobalt–hexacyanoferrate (CoHCF) thin film electrode, grown electrochemically on a glassy carbon substrate, is described. The modification, accomplished by potential cycling a freshly prepared CoHCF film electrode in acidic 50 mM NaCl solutions (pH 2–3 with HCl) containing ruthenium(III)-chloride, represents a very effective contribution to the maintenance of electrode stability. Indeed, whereas a continuous decay over a few hundred redox scans at the parent CoHCF was observed, the resulting Ru-modified CoHCF film electrode exhibits greatly improved stability and enhanced electrocatalytic activity. Fifteen thousand redox scans at 100 mV/s caused the peak current to decrease by only about 10% by virtue of the strong stability of the Ru-modified CoHCF film electrode. Such an excellent stability is likely imparted by the formation of an extended network of dinuclear [Fe, Ru] oxo-bridged, and has been interpreted by the insertion of RuO moieties within the hydrated microparticles of CoHCF. Some examples are also presented which illustrate the excellent electrocatalytic capability in acidic solutions (pH 3) of the ruthenium-modified film towards the oxidation of hydrazine, thiosulfate, and p -chlorophenol. The catalytic oxidation currents observed are proportional to the concentration of hydrazine up to about 5 mM with a sensitivity of 2.7 μA mM −1 mm −2 and correlation coefficient of 0.999.

Determination of sugars and alditols in food samples by HPAEC with integrated pulsed amperometric detection using alkaline eluents containing barium or strontium ions

Abstract The addition, in mobile phase, of alkaline-earth divalent cations such as Sr(II) and Ba(II) is shown to be a versatile means of improving the separation of alditols and carbohydrates, which are conventionally determined by high-performance anion-exchange chromatography (HPAEC) coupled with integrated pulsed amperometry using gold working electrodes. It is believed that Ba(II) or Sr(II) ions are effective both in forming solution complexes with sugar analytes, and ensuring thoroughly carbonate-free alkaline mobile phase flowing through the column. As a result, substantial benefits can be obtained in terms of peak shape, attainable column efficiency, and detection sensitivity. This work describes determination of alditols (myo-inositol, d -sorbitol, and d -mannitol), and carbohydrates (glucose, fructose, and sucrose) in some common fruits and fruit products (apricot, plum, watermelon, lemon, mandarin, grape, and dietetic cherry jam) and vegetable foodstuffs (cauliflower, cucumber, fennel, tomato, turnip, and celery). The free sugar contents in fruit juices were determined directly, without pretreatment or derivatization, following a sample extraction. While myo-inositol was found in many fruits, fruit products and vegetables examined, no presence of xylitol was detected.

On the ability of ruthenium to stabilize polynuclear hexacyanometallate film electrodes

Abstract A novel procedure that yields extremely stable ruthenium-modified inorganic film electrodes is presented. The following prussian blue (PB) analogues were investigated: indium(III)-hexacyanoferrate (InHCF), nickel(II)-hexacyanoferrate (NiHCF), cobalt(II)-hexacyanoferrate (CoHCF), and iron(III)-hexacyanorutenate also known as ruthenium purple (RP). The process includes electrochemical film deposition and subsequent voltammetric conditioning in a Ru(III)-containing solution at pH 2. Electrochemical and spectroscopic properties of each resulting modified film are better defined, redox and switching rates improved, and stability strongly enhanced. XPS supports the conclusions that the incorporation of ruthenium imparts an extensive crosslinking between hydrate microparticles through dinuclear [Fe, Ru] oxo ( O ) and cyanide ( CN ) bridges.

Amperometric determination of underivatized amino acids at a nickel-modified gold electrode by anion-exchange chromatography

A nickel-based composite electrode obtained by anodic electrodeposition of nickel (III) oxyhydroxide film on the gold electrode substrate was characterized as an amperometric sensor and successfully applied to the determination of underivatised amino acids in flow-through systems. The electrodeposition of nickel oxyhydroxide films was obtained by cycling a gold electrode between 0.0 V and +1.0 V vs. a saturated calomel electrode in a 80 microM Ni2+ solution buffered at pH 10 with NaHCO3/Na2CO3. The resulting Au-Ni composite electrode exhibits good stability in alkaline medium and can be used as an amperometric sensor of underivatised amino acids at a fixed applied potential (+0.55 V vs. Ag/AgCl). The detection limits (S/N=3) for all investigated compounds ranged between 5 and 30 pmol injected, while the linear ranges spanned over two or three orders of magnitude. The contents of several free amino acids in two sample cheeses from different brands were evaluated by calibration graphs.

Determination of glycoalkaloids and relative aglycones by nonaqueous capillary electrophoresis coupled with electrospray ionization-ion trap mass spectrometry

Glycoalkaloids are naturally occurring nitrogen‐containing compounds present in many species of the family Solanaceae, including cultivated and wild potatoes (Solanum spp.), tomatoes (Lycopersicon spp.), etc. These compounds have pharmacological and toxicological effects on humans due to their significant anticholinesterase activity and disruption of cell membranes. Herein is reported the development of a capillary electrophoresis (CE) method using nonaqueous (NA) separation solutions in combination with ion trap mass spectrometry (MS and MS/MS) detection for the identification and quantification of glycoalkaloids and their relative aglycones. A mixture 90:10 v/v of MeCN‐MeOH containing 50 mM ammonium acetate and 1.2 M acetic acid (applied voltage of 25.5 kV) was selected as a good compromise for the separation and detection of these compounds. The electrospray MS measurements were carried out in the positive ionization mode using a coaxial sheath liquid, methanol‐water (1:1) with 1% of acetic acid at a flow rate of 2.5 νL/min. Under optimized experimental conditions, the predominant ion was the protonated molecular ion ([M+H]+) of solanidine (m/z = 398), tomatidine (m/z = 416), chaconine (m/z = 852), solanine (m/z = 868), and tomatine (m/z = 1034). MS/MS experiments were carried out systematically by changing the relative collisional energy and monitoring the intensities of the fragment ions that were not high enough to allow better quantification than with the mother ions. The method was used for analyzing glycoalkaloids in potato extracts.

Optimizing separation conditions for riboflavin, flavin mononucleotide and flavin adenine dinucleotide in capillary zone electrophoresis with laser-induced fluorescence detection

A method was developed for the quantitative determination of riboflavin, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), using free solution capillary zone electrophoresis in uncoated fused-silica capillaries with laser-induced fluorescence (LIF) detection. Various factors influencing the separation and detection of flavin vitamers were investigated, including pH (5.5-10.5), concentration and nature of the run buffer (phosphate, borate and carbonate), applied voltage (15-30 kV), temperature (15-30 degrees C) and injection time. Optimal resolution and detection were obtained with a pH 9.8, 30 mM aqueous phosphate buffer at 15 degrees C and 30 kV of applied voltage. LIF detection was obtained with a He-Cd laser source using an excitation wavelength at 442 nm and lambda(em) > or = 515 nm. Riboflavin could be determined in the concentration ranges 0.5-350 microg/l with a rather low detection limit (LOD) down to 50 amol. The LODs of FAD and FMN were slightly higher, 300 and 350 amol, respectively. Combined with a simple clean-up procedure, the practical utility of this method is illustrated by the measurements of flavin derivates in foods and beverages, such as wines, milk, yoghurt and raw eggs.

Determination of elemental compositions by gas chromatography/time‐of‐flight mass spectrometry using chemical and electron ionization

Many metabolomic applications use gas chromatography/mass spectrometry (GC/MS) under standard 70 eV electron ionization (EI) parameters. However, the abundance of molecular ions is often extremely low, impeding the calculation of elemental compositions for the identification of unknown compounds. On changing the beam-steering voltage of the ion source, the relative abundances of molecular ions at 70 eV EI were increased up to ten-fold for alkanes, fatty acid methyl esters and trimethylsilylated metabolites, concomitant with 2-fold absolute increases in ion intensities. We have compared the abundance, mass accuracy and isotope ratio accuracy of molecular species in EI with those in chemical ionization (CI) with methane as reagent gas under high-mass tuning. Thirty-three peaks of a diverse set of trimethylsilylated metabolites were analyzed in triplicate, resulting in 342 ion species ([M+H](+), [M-CH(3)](+) for CI and [M](+.), [M-CH(3)](+.) for EI). On average, CI yielded 8-fold more intense molecular species than EI. Using internal recalibration, average mass errors of 1.8 +/- 1.6 mm/z units and isotope ratio errors of 2.3 +/- 2.0% (A+1/A ratio) and 1.7 +/- 1.8% (A+2/A ratio) were obtained. When constraining lists of calculated elemental compositions by chemical and heuristic rules using the Seven Golden Rules algorithm and PubChem queries, the correct formula was retrieved as top hit in 60% of the cases and within the top-3 hits in 80% of the cases.