Rapid determination of nitrites in food using a diffuse UV-visible reflectance method.

A simple, low-cost, sensitive and selective method for the determination of trace quantities of nitrite in foods such as cheese and cured meat using diffuse ultraviolet-visible reflectance was developed. It is based on the reaction of nitrite with sulphadiazine and α-naphthol, which produces a coloured product in basic medium. The reaction is carried out directly in the measuring cell. For cheese the limit of detection (LOD), expressed as NaNO2, was estimated to be about 2.0 × 10⁻²mg l⁻¹ (2.9 × 10⁻7mol l⁻¹) in the final measuring solution and 0.17 mg kg⁻¹ in cheese (2.5 × 10⁻6mol kg⁻¹). The relative standard deviation (RSD) varied from 5% to 8% depending on the sample. For meat the LOD was estimated to be about 2.0 × 10⁻²mg l⁻¹ (2.9 × 10⁻7mol l⁻¹) in the final measuring solution and 0.13 mg kg⁻¹ in meat (1.9 × 10⁻6mol kg⁻¹). The RSD varied from 3% to 6% depending on the sample. The results of the proposed method were also compared with those obtained with the official method using the statistical Student's t-test and F-test procedures.

Flow-injection spectrophotometric determination of paraoxon by its inhibitory effect on the enzyme acetylcholinesterase.

A spectrophotometric enzymatic flow injection (FI) system for the determination of diethyl-p-nitrophenylphosphate (paraoxon) is proposed. The method was based on the determination of the acetic acid formed by the enzymatic reaction of the acetylcholinesterase, immobilized on glass beads, with the substrate acetylcholine. The acetic acid formed permeates through a PTFE membrane and is received by a solution (pH 7.0) containing the acid-base indicator Bromocresol Purple (B.C.P.), leading to a pH change and therefore to a color change. The variation of the absorbance of the solution is detected spectrophotometrically at 400 nm. The determination of paraoxon is related to its inhibitory action on the enzyme. Therefore the analytical signal is the difference between the signal that corresponds to the free and the one that corresponds to the inhibited enzyme, considering a fixed acetylcholine concentration. The correlation between the peak height and paraoxon concentration at a given acetylcholine concentration is linear in the range from 5.0 x 10(-7) mol L-1 to 5.0 x 10(-5) mol L-1 (r = 0.998) of paraoxon, with a relative estimated standard deviation (R.S.D.) of +/- 1.7% (n = 10) considering a solution containing 5.0 x 10(-6) mol L-1 of paraoxon and a solution containing 5.0 x 10(-3) mol L-1 of acetylcholine. Therefore, the quantitative limit detection is about 2.5 x 10(-7) of paraoxon (3 sigma). A 1,1'-trimethylene-bis(4-formylpyridinium bromide)dioxime (TMB-4) solution was used to reactivate the enzyme.

Determination of salicylate in blood serum by flow injection with immobilized salicylate hydroxylase.

A flow injection (FI) enzymatic system, based on the use of immobilized salicylate hydroxylase in glass beads, was developed for the determination of salicylate. Salicylate hydroxylase and nicotinamide adenine dinucleotide (NADH) are used to convert salicylate to catechol. The reaction of catechol with 4-aminophenol at high pH yields a colored product which is detected spectrophotometrically at 565 nm. Ten samples of human serum containing from 5.0 x 10(-4) to 5.0 x 10(-3) mol/L added salicylate were analyzed and the recovery was determined. Eight additional serum samples containing salicylate were analyzed by the Trinder test and the proposed method. The results obtained with the 2 methods showed good agreement by the statistical Student's t-test. The relative precision of the method is about 3.4% (RSD of the mean recovery). Considering the lowest concentration analyzed, the quantitative limit of detection is about 0.2 x 10(-5) mol/L (3 x SD). The volume of the sample used was 150 microL. The proposed method was also used to analyze medicines containing acetylsalicylic acid. The results were statistically compared with those obtained through the U.S. Pharmacopoeia procedure and showed excellent agreement.

A portable fiber-optic chemical device for the quantitative determination of carbon monoxide from automobile exhaust emissions.

A colorimetric method for the quantitative determination of CO by diffuse reflectance is described. This method is based on the reduction by CO of Mo (VI) from the indicator reagent molybdosilicic acid (H8Si[Mo2O7]6). The reduction yielded a change of color from clear yellow to dark green on white disk filter chart paper wetted with reagent indicator solution. The gaseous mixture containing CO was forced to pass through this chart paper, initiating the reaction. The intensity of the color produced, measured by diffuse reflectance, was proportional to the CO concentration present in exhaust gases in the range from 0.02 to 12% volume/volume (v/v). A 650-nm light-emitting diode was used as a light source. A two-fiber-optic system carried the light from the source to the detection system, which was composed of a photodiode, an amplification circuit, and a digital display. The method was applied with success in field measurements for automobiles in the Otto cycle. In a previous paper, this method was used for the quantitative determination of exhaust emissions from diesel-fueled vehicles.

Flow injection analysis of nitrogen dioxide using a galvanic detector.

A flow injection configuration (FIA) based on a galvanic detector for the determination of nitrogen dioxide is described. The gaseous sample is directly injected into a gaseous carrier. The sample is transported toward the detector. The steady state measurements are not required to obtain the reproducible peak signals. The features of FIA are compared with that of continuous flow monitoring application. The flow injection system is simple, rapid and capable of detecting NO(2) in the range of 1-500ppm (v/v). The measuring range and sensitivity of the galvanic detector in FIA depend on the sample volume. A relative standard deviation is 2.4% (n = 10) for 200ppm (v/v) of nitrogen dioxide. The sampling frequency is about 24 h(-1).

Dual-phase gas-permeation flow-injection thermometric analysis for the determination of carbon dioxide.

A flow-injection configuration based on a dual-phase gas-permeation system from a liquid donor to a gas acceptor stream with a thermistor flow-through detector is proposed for the direct analysis of the gas in the acceptor. This system was applied for the determination of carbon dioxide (in the form of carbonate) using the following chemical reaction: CO(2)(g)+2NH(3)(g)+H(2)O(g)=(NH(4))(2)CO(3)(s), with a linear response from 1x10(-3) to 50x10(-3) mol l(-1) of CO(3)(2-). Carbon dioxide was produced in the liquid donor and permeated into the gaseous acceptor stream of air/water vapor. The detection limit is 1x10(-3) mol l(-1) of carbonate, and a sampling frequency of 60 h(-1) is achieved with a relative standard deviation of 4.1% for replicate injections. The dual-phase gas-permeation flow-injection manifold, along with the membrane and phase separations, as well as the chemical reaction, provides enhanced selectivity when compared with the system employing a liquid acceptor stream, as serious interferents in this system, for instance, acetate and formate, among others, do not interfere in the proposed system.

A possible path for mercury in biological systems: the oxidation of metallic mercury by molecular oxygen in aqueous solutions.

Metallic mercury has been assumed by several authors as not very reactive and, as a consequence, with little or no toxicity. The toxicity of this element is usually ascribed to alkyl-mercury ions considered to be formed by some microorganisms. In this work, we describe experiments that clearly show that metallic mercury can be easily oxidized by molecular oxygen in aqueous solution in the presence of species such as chloride, which complex Hg(II). The experiments were carried out using metallic mercury in NaCl aqueous solution under 'open air' (temperature and agitation rate maintained constant) and under more controlled conditions (CO2 rate bubbling, i.e. pH = 4.2; air rate bubbling, i.e., O2 constant concentration, temperature, agitation rate). The reactions were monitored spectrophotometricaly at 230 nm (HgCl2-(4)). Significative values of the concentration of Hg(II) in the form of HgCl2-(4) were soon attained in those solutions. For example, in 'open air' conditions, at 25 degrees C and [NaCl] = 30 g/l (0.51 mol/dm3), the maximal concentration of 13 ppm (6.44 x 10(-5) mol/dm3) of Hg(II) in the form of HgCl2-(4) was reached in 120 min; for [NaCl] = 5 g/l at 25 degrees C, (0.085 mol/dm3) the maximal concentration of 0.3 ppm (1.53 x 10(-6) mol/dm3) of Hg(II) in the form of HgCl2-(4) was reached in 10 min. The rate constants, kobs, of the oxidation of the metallic mercury under the studied conditions are pseudo zero-order at 25 degrees C, and under more controlled conditions have ranged from 1.0 x 10(-7) mol/min ([NaCl] = 5 g/l identical to 0.085 mol/dm3) to 20.0 x 10(-7) mol/min ([NaCl] = 300 g/l identical to 5.12 mol/dm3). The rate constant increases with temperature, up to 25 degrees C, from where kobs remains constant up to 40 degrees C. From the analysis of the experimental results it was possible to propose a mechanism of oxidation of metallic mercury by O2 in aqueous solution containing NaCl. This oxidation is proposed as a possible route for the introduction of mercury into biological systems.