Апр 25 2004

Direct Electrothermal Atomic Absorption Determination Of Arsenic, Lead And Cadmium In Some Of Foods

Опубликовано в 07:51 в категории Проблемы пищ. промышленности

DIRECT ELECTROTHERMAL ATOMIC ABSORPTION

DETERMINATION OF ARSENIC, LEAD AND CADMIUM IN SOME OF FOODS

A.N.Zaсharia*, Belgin Izgi

  • , Serif Gucer
  • , V.P.Solomienko
  • *, L.N.Arkhipova
  • *

    *       Department of Analytical Chemistry, I.I. Mechnikov Odessa National University

  •      Uludag University, Art and Sciences Faculty, Bursa Test and Analysis Laboratory

             (BUTAL), Bursa, Turkey

  • *   Odessa Standardization, Metrology and Certification Center

    Quality control of food and agricultural products including determination of heavy metals impurities is one of the main and importance stage in the general procedure of corresponding work. Unfortunately, recommended herewith analytical methods, as rule, are long-time and labor-consuming that complicates operative performing its tests.

    The electrothermal (non-flame) atomic absorption spectrophotometry (ET AAS) is on of the more effective and promising analytical method to determination of trace amounts of heavy metals including arsenic (As), lead (Pb) and cadmium (Cd) in food and agriculture products. However in the most of cases there is the problems arise in analyzing samples composed of complex and variable materials and its previously decomposition. The extremely low concentrations (0,005-0,2 ppm) of As, Pb, Cd in checking materials coupled with the spectral and chemical interferences arising from vaporization of the major salts has predicated the widespread use of sample preparation schemes designed to both pre-concentration the trace amounts of listed elements and separate them from the major interfering components prior to analysis. A number of techniques have been used for this purpose, including co-precipitation and co-crystallization, chelation-solvent extraction, chelating ion-exchange resins, electrolytic pre-concentration and other [1]. All of these methods are time consuming, often tedious and obstructed an operative checking of quality and certification listed materials.

    To eliminate or partially reduced many of this problems including the matrix interferences in analytical practice of the direct ET AAS determination Pb, Cd, As it has been perspective to utilized the L’vov platform [2] and matrix modifiers technique [3]. Unfortunately, relatively limited information has been so far reported on the usefulness it’s for dissolving some of analytical, certification and checking problems of foods.

    This report discussed the possibility of palladium nitrate mixtures with diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium nitrate and magnesium nitrates as matrix modifiers, combined with L’vov platform technique at the direct ET AAS determination of micro-quantities (> 0,002 ppm) of As, Pb and Cd in some of agricultural and food products.

    All atomic absorption measurements were made with ATI-Unicam 929 Model AAS spectrophotometer and electrothermal graphite atomizer HGA. The As, Pb, and Cd hollow-cathode lamps were used at resonance line: 193,7; 283,3 and 228,8 nm, respectively and molecular background correction was achieved with a deuterium lamp. All chemicals used were of the highest purity available and at least of analytical grade (manufactured by «MERCK KGaA», Germany). The optimum pyrolysis temperatures that was recommended for AAS determination of individual elements are given in Fig.1. At the same time typical corresponding temperature program is given in Table 1.

    About 0,5-2,0 g of the difference food and agricultural materials to be analyzed were decomposed with 10-20 ml of nitric acid (1+1) and 2 ml hydrogen peroxide (40 % v/v) during 0,5-1 h at 400oC under high pressure in autoclave.

    Table 1

    Temperature program used for the direct ET AAS determination of As, Pb and Cd in food and agricultural products with matrix modifier

    Step

    Furnace temperature

    ToC

    Time, s

    Internal gas flow

    (ml·min-1)

    Read

    ramp

    hold

    1

    100

    1

    10

    250

    2

    130

    15

    25

    250

    3

    950

    15

    25

    250

    4

    850

    1

    10

    0

    5

    2100

    0

    5

    0

    *

    6

    2500

    1

    4

    250

    Internal gas – argon (Ar). The 40 μl as total sample volume on the platform for single injection is a limit. The mixed palladium and another modifiers was prepared by mixing equal volumes of solution containing 3000 mg·l-1 of Pd(NO3)2 and 2000-2500 mg·1-1 respectively reagents.

    In the some cases (direct analysis of oils, fats and vegetable butter) metalorganic standard samples «СONOSTAN» were used. The other reference solutions were prepared daily by further dilution with 0,2 (w/v) nitric acid, which was purified by distillation in a quartz subboiling still. The effects of large mounts (103-105 fold) of Ca, Mg, Na, K, Fe, Al and its chlorides on As, Pb and Cd absorption profile were investigated. These elements were selected for examination on the basis of known interference effects for As, Pb, Cd and in accordance with their abundances in some of food, biological and agricultural materials.

    Diammonium hydrogen phosphate-palladium nitrate-nitric acid, as matrix modifier was suggested for Cd; magnesium nitrate-palladium nitrate - for As, and ammonium dihydrogen phosphate-magnesium nitrate-palladium nitrate - for Pb ET AAS determination in complicated matrix samples. Listed modifiers coupled with evaporation of analyte from L’vov platform has been prevented the losses of As, Pb and Cd during charring by increasing the stability and simultaneously by volatilize the matrix interference constituents prior to atomization of the analyte by increasing the volatility of the matrix.

    The thermal pretreatment curves for all of investigated elements using the proposed mixture of modifiers and without modifiers are depicted in Fig.1. The data have been blank corrected.

    Besides the stabilization to higher pretreatment temperatures the application of a modifiers and platform technique permits the use of a pyrolisis temperature of at least 900-950oC for As, Pb and Cd which should be enough to volatilization of interferences matrix components without losing the analyzed elements.

    The sensitivity obtained for direct ET AAS determination As, Pb, Cd in some of food and agricultural products (desiccated milk, wines, fats, butter, refined sunflower oil, canned juice, orchard leaves, smoked sprats in vegetable oil, meal, grain, tomato paste, powdered fruit drinks, eggs) was not significantly lower than in the aqueous reference solutions. Its indicated at the absence of major interferences. Nevertheless, in some of cases, the standard samples of appropriate materials with certificated contents of As, Pb and Cd, as well as standard additions method of analysis was accomplished. On analysis of vegetable oil, fats and agriculture butter thus used a metalorganic multielement standard sample (S-21, Lot No.21236) manufactured by CONOSTAN DIVISION (USA).

    The results obtained oneself clearly express that proposed direct ET AAS method is accurate, reproducible and suitable to routine analysis high listed materials for its simplicity, rapidity and easy operation. Its confirmed by comparison of results received with proposed ET AAS and standard solvent extraction - spectrophotometric method (Table 2).

         QA

    e

    f

    d

    a

    c

    b

    ToC

    Fig.1. Effect of thermal pretreatment and atomization temperatures on the absorbance of arsenic, lead and cadmium (QA) without palladium-consist matrix modifier (curves a, b and c) and with marked modifier (curves d, e and f).

    Table 2

    Comparison the results of As, Pb and Cd determination obtained in some of foods by ET AAS (1) and solvent extraction-spectrophotometric methods (2)

    Materials

    analysed

    Elements

    Contents of element (C+Δc) ppm

    1

    Sr1

    2

    Sr2

    1

    2

    3

    4

    5

    6

    Desiccated

    milk

    As

    0,010 + 0,002

    0,13

    0,014 + 0,002

    0,12

    Pb

    0,019 + 0,002

    0,12

    0,016 + 0,003

    0,14

    Cd

    0,0047 + 0,00006

    0,10

    0,0052 + 0,0008

    0,12

    Smoked

    sprats

    As

    0,16 + 0,03

    0,14

    0,13 + 0,02

    0,13

    Pb

    0,21 + 0,03

    0,12

    0,17 + 0,03

    0,13

    Cd

    0,046 + 0,007

    0,13

    0,045 + 0,008

    0,15

    Meal

    As

    0,041 + 0,005

    0,10

    0,037 + 0,006

    0,14

    Pb

    0,025 + 0,003

    0,11

    0,030 + 0,004

    0,12

    Cd

    0,0052 + 0,0006

    0,087

    0,0048+0,0008

    0,14

    As

    0,036 + 0,004

    0,089

    0,041 + 0,006

    0,11

    Sunflower

    oil

    Pb

    0,010 + 0,001

    0,085

    0,014 + 0,002

    0,11

    Cd

    0,0035 + 0,0003

    0,077

    0,0039+0,0007

    0,14

    Tomato

    paste

    As

    0,011 + 0,002

    0,12

    0,014 + 0,002

    0,12

    Pb

    0,028 + 0,003

    0,10

    0,024 + 0,003

    0,11

    Cd

    0,0089 + 0,0010

    0,089

    0,0083+0,0013

    0,13

    At optimizing conditions characteristic mass (gchar-the mass of elements corresponding to atomic absorption value of QA - 0,00434) for Pb, Cd and As was 20, 15 and 25 pg or 0,001-0,002 ppm, respectively. Relative standard deviation (Sr) at ET AAS determination of 0,0025 ppm Pb, Cd or As was not more then 0,15.

    Proposed method allows in 10-20 time to reduce determination procedure any of listed elements which does not exceed 20-25 min.

    The good agreement between results obtained confirms that using palladium - consist matrix modifiers combined with L’vov platform technique at the direct ET AAS determination of Pb, Cd and As is very perspective and at the same time should be true for other food and agriculture samples including their operative checking of quality and certification control.

    References

    [1]  Yu. A.Zolotov, N.M.Kuzmin, Concentration of microelements, Moscow, (1982),

           288 p. (russ.)

    [2]  G.Schlemmer, B.Welz, Spectrochim.Acta, 41B, 1157 (1986).

    [3]  N.F.Beisel, F.J.Daamen, G.R.Fuchs-Pohl, I.G.Yudelevich, Journ.Analyt.Chem.

          (russ.) 48, 1255 (1993)

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