Mauro Korn

Sample Preparation for the Determination of Metals in Food Samples Using Spectroanalytical Methods—A Review

Abstract The present article gives an overview of recent publications and modern techniques of sample preparation for food analysis employing atomic and inorganic mass spectrometric techniques, such as flame atomic absorption spectrometry, chemical vapor generation atomic absorption and atomic fluorescence spectrometry, graphite furnace atomic absorption spectrometry, inductively coupled plasma optical emission spectrometry, and inductively coupled plasma mass spectrometry. Among the most frequently applied sample preparation techniques for food analysis are dry ashing, usually with the addition of an ashing aid, and acid digestion, preferably with the assistance of microwave energy. Slurry preparation, particularly with the assistance of ultrasound, is increasingly used to reduce acid consumption and sample preparation time. Direct analysis of solid samples is gaining importance in the field of food analysis as it offers the highest sensitivity, avoids the use of acids and other aggressive reagents, makes possible the analysis of micro‐samples, and can be applied for fast screening analysis, e.g., of fresh meat.

A fast ultrasound-assisted extraction of Ca, Mg, Mn and Zn from vegetables

A fast and accurate method for the extraction of Ca, Mg, Mn and Zn from vegetables is proposed. Ultrasonic energy and dilute acid are used for the extraction. Optimisation conditions for the ultrasonic bath were established: 1 l of water; 25°C; 2% v/v detergent concentration; horizontal and vertical positions in the bath for sonicating samples: central and bottom, respectively. Using these conditions, the nitric acid concentration (0–1.4 mol l−1), sonication time (0–30 min) and particle size (150–63 μm) were optimised. The best conditions for extraction were: 0.14 mol l−1 HNO3, 10 min of sonication time and a particle size <75 μm. The accuracy of the proposed ultrasound-assisted extraction method was assessed by using certified reference materials, as well as wet digestion. Recoveries ranging from 96 to 102% were obtained under the extraction conditions mentioned above with different kinds of plant samples for the majority of the metals studied.

A critical evaluation of digestion procedures for coffee samples using diluted nitric acid in closed vessels for inductively coupled plasma optical emission spectrometry

The efficiency of diluted nitric acid solutions for digesting regular coffee samples was evaluated employing two closed vessel procedures: one was based on microwave-assisted heating and the other was based on conductive heating using pressurized Parr bomb. The efficiency of digestion was evaluated by determining residual carbon content (RCC) and residual acidity. The digestion was effective using both procedures, i.e. there were no solid residues after the decomposition reactions when using up to 3.5 mol L(-1) nitric acid solutions. It was demonstrated that the digestion procedures are critically dependent on reactions occurring in liquid and gas phase and that the formation of NO and its conversion to NO2 by O2 exerts a major effect in the oxidation of organic matter. These processes are more effective in closed vessels heated by microwave radiation due to the greater volume of these flasks and the temperature gradient that exists during the first step of the digestion process. The proposed model for the digestion processes in diluted nitric acid solution is corroborated by data about consumption of acid during the digestion and by measuring the pressure during the whole process.

Multicommutation in flow analysis. Part 2. Binary sampling for spectrophotometric determination of nickel, iron and chromium in steel alloys

Multicommutation flow systems for the spectrophotometric determination of nickel, iron and chromium in acid solutions of steel alloys employing a binary sampling approach are described. Dimethylglyoxime, salicylic acid and diphenylcarbazide were used as chromogenic reagents for nickel, iron and chromium, respectively. The flow networks were designed with active devices in order to provide facilities to handle reagent solutions employing a single pumping channel to propel all reagent solutions. Interference caused by iron on the nickel determination was suppressed on-line by using triethanolamine as masking agent. For sample concentrations ranging from 5 to 50 mg/l, the proposed method is characterized by a throughput of 60 determinations per hour and a relative standard deviation of about 1%. Consumptions of potassium peroxodisulphate and dimethylglyoxime solutions were 56 and 78 μl per determination, respectively. Iron and chromium were determined sequentially, with a reagent consumption of 80 μl and 26 μl, respectively. For sample concentrations ranging from 25 to 200 mg/l Fe and 20 to 60 mg/l Cr a throughput of 130 determinations per hour was achieved. Results compared well with those obtained by inductively coupled argon plasma atomic emission spectrometry.

Use of ultrasonic baths for analytical applications: a new approach for optimisation conditions

Optimisation conditions for obtaining maximum cavitation intensity in ultrasonic baths are proposed using a simple and fast method. Parameters such as water volume, temperature, detergent concentration, horizontal and vertical positions, number of tubes in the bath, sonication time and bath water substitution were studied. The results obtained for both baths studied (Neytech and Cole-Parmer) lead to the following conditions for maximum cavitation intensity: 1 L of water at room temperature, 0.2 % (v/v) of detergent, central position on the bottom of the tank. Only one tube at a time should be used inside the bath during the ultrasound application. The cavitation intensity was linear with the sonication time up to 10 minutes and the water substitution during the sonication improved reproducibility. This system using continuous water change makes possible the sonication of 6 consecutive samples, without changes in the water volume.

Binary search in flow titration employing photometric end-point detection

A binary search strategy is proposed and implemented in a continuous flow system to find the end-point titration by employing spectrophotometric detection. It takes advantage of the binary sampling process under a constant flow-rate. For this task an automated flow set-up based on solenoid valves was designed. A 386 microcomputer was employed to control the valves, and to perform data acquisition and data processing. The accuracy level can be previously settled as a software parameter. Samples with concentrations ranging within two orders of magnitude could be titrated by making use of the same flow set-up. Titration of hydrochloric acid with sodium hydroxide was performed to demonstrate the feasibility of the proposal. Titrations in triplicate, with 99.99% precision were carried out in 3 min with a solution consumption of 2 ml.

Flow-injection solid-phase spectrophotometry for the determination of zinc in pharmaceutical preparations

A flow system exploiting solid-phase spectrophotometry is proposed for the determination of zinc in pharmaceutical preparations. The chromogenic reagent 1-(2-tiazolylazo)-2-naphthol (TAN) was immobilized on C18 bonded silica loaded into a homemade flow cell with 1 mm optical path and 78 mm 2 cross section. The flow cell was designed in such a way that the sensitivity is enhanced and the attenuation of the radiation beam due to scattering and absorption by the solid material is minimized. The flow cell was placed in the spectrophotometer so that the radiation beam was focused on the overall surface containing the adsorbing material. Reagent immobilization was performed on-line and allowed to work for at least one month. Analyte reaction, retention and detection were performed simultaneously, followed by elution with hydrochloric acid. The apparent molar absorptivity was estimated as 2.0210 5 l mol ˇ1 cm ˇ1 and the procedure allowed the determination of zinc in the 0.04‐4.0 mg l ˇ1 range with a coefficient of variation of 3.3% (na10). A sample throughput of 45 determinations per hour and a detection limit of 10 m gl ˇ1 (99.7% confidence level) were achieved. The results agreed at a 95% confidence level with those found by inductively coupled plasma atomic emission spectrometry. # 1999 Elsevier Science B.V. All rights reserved.

Automatic potentiometric titration in monosegmented flow system exploiting binary search

An automatic monosegmented flow system (MSFA) based on the binary search concept to perform potentiometric titration is proposed. A tubular hydrogen ion-selective electrode without inner reference solution, consisting of a conducting epoxy cylinder machined with an axial hole coated with tridodecylamine (TDDA) was employed as a sensor. The titration procedure was implemented by exploiting the binary search approach, after each analytical cycle data were evaluated to decide the variation of the titrant volumetric fraction to be inserted for the next cycle. The flow network comprised three-way solenoid valves controlled by a microcomputer running software in QUICKBASIC 4.5. Combination of binary search and MSFA resulted in an automatic system providing possibilities to perform flow titrations without any operator assistance. The main features of the proposed system were verified by titrating hydrochloric and acetic acid solutions and the feasibility of the approach was ascertained by analyzing vinegar, coke, lemon soda, isotonic, industrial and natural orange juice samples. Results were in agreement with those obtained with a conventional potentiometric titration, and no significant difference at 95% confidence level was observed. A 1% standard deviation (n=9) in results was also observed.

Nickel and zinc determination by flow-injection solid-phase spectrophotometry exploiting different sorption rates

Flow-injection solid-phase spectrophotometry is applied for sequential determination of nickel and zinc, exploiting their different sorption rates on 1-(2-thiazolylazo)-2-naphthol (TAN) immobilized on C(18)-bonded silica. The Zn(II) sorption rate on the solid support is constant for flow rates ranging from 0.70 to 2.2 ml min(-1), but for Ni(II) the sorption rate decreases with increasing flow rate. A flow system was designed to perform sequential measurements at two different flow rates (0.85 and 1.9 ml min(-1)). The absorbance was measured at 595 nm, where both TAN-immobilized complexes showed maximum absorption. The coefficients of variation were estimated (n=10) as 1.1 and 1.7% (at 1.9 ml min(-1)) and 1.2 and 2.1% (at 0.85 ml min(-1)) for zinc and nickel, respectively. This strategy was applied to determine zinc and nickel in copper-based alloys and the results agreed with certified values at the 95% confidence level. The sample throughput was estimated as 36 h(-1).

Microwave-assisted diluted acid digestion for trace elements analysis of edible soybean products.

A new method for the decomposition of soybean based edible products (soy extract, textured soy protein, transgenic soybeans, and whole soy flour) was developed to essential (Co, Cr, Cu, Fe, Mn, Ni, Se, V, and Zn) and non-essential (As, Ba, Cd, Pb, and Sr) trace elements determination by ICP OES and ICP-MS respectively. Effects related to the concentration of HNO3 (2.1-14.5 mol L(-1)) and the use of hydrogen peroxide on the efficiency of decomposition was evaluated based on the residual carbon content (RCC). It was demonstrated that 2.1 mol L(-1) HNO3 plus 1.0 mL H2O2 was suitable for an efficient digestion, since RCC was lower than 18% and the agreement with certified values and spike recoveries were higher than 90% for all analytes. The concentrations of analytes in the samples (minimum-maximum in mgkg(-1)) were: The concentrations of analytes in the samples (minimum-maximum in mgkg(-1)) were: As (<0.007-0.040), Ba (0.064-10.6), Cd (<0.006-0.028), Co (0.012-102), Cr (0.56-5.88), Cu (6.53-13.9), Fe (24.9-126), Mn (16.4-35.2), Ni (0.74-4.78), Se (<2.90-25), Sr (2.48-20.1), Pb (<0.029-0.11), V (<0.027-20), and Zn (30.1-47.3). Soy-based foods investigated in this study presented variable composition in terms of essential and potentially toxic elements, which can be attributed to different methods of processing.