Food Chemistry 132 (2012) 518–524 Contents lists available at SciVerse ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem Analytical Methods Utility of solid phase spectrophotometry for the modified determination of trace amounts of cadmium in food samples Alaa S. Amin a, Ayman A. Gouda b,c,⇑ a Chemistry Department, Faculty of Science, Benha University, Benha, Egypt Chemistry Department, Faculty of Science, Zagazig University, Zagazig, Egypt c Faculty of Community, Department of Medical Science, Umm Al-Qura University, Makkah, Saudi Arabia b a r t i c l e i n f o Article history: Received 20 July 2011 Received in revised form 2 October 2011 Accepted 10 October 2011 Available online 28 October 2011 Keywords: Cadmium determination Solid phase spectrophotometry Thiazolylazo-dyes Food samples a b s t r a c t A modified selective, highly sensitive and accurate procedure for the determination of trace amounts of cadmium which reacts with 1-(2-benzothiazolylazo)-2-hydroxy-3-naphthoic acid (BTAHNA) to give a deep violet complex with high molar absorptivity (7.05  106 L mol1 cm1, 3.92  107 L mol1 cm1, 1.78  108 L mol1 cm1, and 4.10  108 L mol1 cm1), fixed on a Dowex 1-X8 type anion-exchange resin for 10 mL, 100 mL, 500 mL, and 1000 mL, respectively. Calibration is linear over the range 0.2–3.5 lg L1 with RSD of 61.14% (n = 10). The detection and quantification limits were calculated. Increasing the sample volume can enhance the sensitivity. The method has been successfully applied for the determination of Cd(II) in food samples, water samples and some salts samples without interfering effect of various cations and anions. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Heavy metal ions are increasingly being released into the environment, leading to serious pollution, particularly as a result of industrialization. Cadmium is a very toxic element for animals and human, even at low concentrations. The International Agency for Research on Cancer classified cadmium as a human carcinogen (IARC, 1993). Due to its toxicity both to humans and animals cadmium concentration in the environment should be monitored, hence appropriate guideline values for cadmium content have been introduced; for drinking water they are as follow: WHO 3.0 lg L1 (WHO, 2006), USEPA 5.0 lg L1 (USEPA, 2003). Cadmium enters the organism primarily via the alimentary and respiratory tract. The sources of this metal are food, drinking water and air. Roughly 15,000 t of cadmium is produced worldwide each year for nickel–cadmium batteries, pigments, chemical stabilizers, metal coatings and alloys. So its usage is becoming wider and wider. However, as the levels of cadmium in geological and environmental samples are low, a preconcentrative separation and determination of trace cadmium from the natural water is ⇑ Corresponding author. Permanent address: Chemistry Department, Faculty of Science, Zagazig University, Zagazig, Egypt. Tel.: +966 542087968; fax: +20 552308213. E-mail addresses: aymanchimca@yahoo.com, aymanchimca@homail.com (A.A. Gouda). 0308-8146/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2011.10.028 essential and needs much more attention (Liu, Chang, et al., 2004; Liu, Yang, et al., 2004). One of widely used and fast emerging preconcentrative separation techniques for this purpose is the solid-phase extraction (SPE) due to the following advantages. These include: (1) higher enrichment factors; (2) absence of emulsion; (3) safety with respect to hazardous samples; (4) minimal costs due to low consumption of reagents; (5) flexibility; and (6) ease of automation (Daniel, Praveen, & Rao, 2006). An efficient solid-phase extractant should consist of a stable and insoluble porous matrix having suitable active groups (typically organic groups) that interact with heavy metal ions (Fang, Tan, & Yan, 2005). Solid phase extraction (SPE) of trace metal ions is also an important preconcentration/separation technique (Soylak, Karatepe, Elci, & Dogan, 2003; Godlewska-Zyłkiewicz, 2004). SPE has many advantages: it is a simple technique. Several analytes can be enriched and separated simultaneously. Furthermore, high preconcentration factors can be obtained by using solid phase extraction procedures. Main properties of the solid phases for solid phase extraction should be high surface area, their high purity and good sorption properties including porosity, durability, and uniform pore distribution. A large variety of efficient solid materials like Amberlite XAD resins (Tuzen & Soylak, 2004), silica gel (Sawula, 2004; Yamini, Hosseini, & Morsali, 2004), chitosan (Wang et al., 2004), benzophenone/naphthalene (Preetha & Rao, 2003), Chelex 100 (Soylak, 2004), etc. have been used for solid phase extraction of metal ions at trace levels by various researchers. Solid-phase spectrophotometry (SPS) combines the preconcentration of the species of interest on a solid matrix, usually an 519 A.S. Amin, A.A. Gouda / Food Chemistry 132 (2012) 518–524 ion-exchanger, with the aid of complexing agent and subsequent measurement of the absorbance of the complex in the solid phase. This provides an increase in selectivity and sensitivity with respect to conventional spectrophotometric method (Amin, 2002; Teixeira & Rocha, 2007). 1-(2-benzothiazolyl-azo)-2-hydroxy-3-naphthoic acid (BTAHNA) is one of the thiazolylazo reagents (Amin, 2000, 2001, 2009; Amin & El-Mossalamy, 2003; Amin & Ibrahim, 2001), it has been successfully used for spectrophotometric determination of Cd(II) (Amin, 2001), Cu(II) (Amin, 2009), Nb(III) (Amin, 2000), Ni(II) (Amin & Ibrahim, 2001) and UO2(II) (Amin & El-Mossalamy, 2003). Table 1 describes comparison of analytical performance of various spectrophotometric methods for determination of cadmium (II), while Table 2 presents comparison of detection limits of diverse instrumental techniques for the determination of cadmium (II). The goal of the present work is intended to study the possibilities of using BTAHNA as a reagent for the determination of trace Cd(II) by SPS. The optimum conditions have been established. Cd(II) reacts with BTAHNA to give a colored complex, which is easily sorbed on an anion-exchange resin and provides the basis for a relatively simple, accurate and rapid spectrophotometric method of Cd(II) at sub-lg L1 level, without a previous preconcentration step. The proposed method is free from many interferences and has been applied to the determination of Cd(II) in food samples, water samples and some salts samples. 2.2. Reagents and solutions Analytical reagent grade chemicals and doubly distilled water were used throughout the experiments. All experiments were carried out at room temperature. A known amount of cadmium acetate is dissolved in water and then diluted to 100 mL with distilled water. The stock solution is then standardized by EDTA titration (Vogel, 1978) using xylenol orange as an indicator. The working standard solutions were prepared by a suitable dilution of the stock solution. 1-(2-Benzothiazolylazo)-2-hydroxy-3-naphthoic acid (BTAHNA) of high purity used in the present investigation was easily prepared according to the procedure described previously (Amin, 2000). A stock of 1  103 mol L1 solution of BTAHNA was prepared by dissolving an appropriate amount of the reagent in a minimum amount of pure ethanol and diluting the mixture to 100 mL with ethanol. The working solution was prepared by its appropriate dilution with the same solvent. Phosphate buffer solutions of pH values ranging from 3.0 to 11.0 were prepared as recommended earlier (Britton, 1952). Dowex 1-X8 (200–400 mesh) anion-exchange resin (Aldrich) was used in the chloride form. The resin was washed several times with doubly distilled water, treated with 2.0 mol L1 HCl for 4.0 h and finally with doubly distilled water until the washing was free from chloride ions. Then, it was air-dried and stored in a polyethylene container. 2. Experimental 2.3. General procedures 2.1. Instrumentation 2.3.1. For 10 mL samples An appropriate volume of the sample containing 0.20–2.4 lg of Cd(II) was placed in a 25 mL-measuring flask with a stopper, 0.5 mL of 1  105 mol L1 BTAHNA solution and 1.0 mL of pH 8.5 phosphate buffer solution were added, the solution was made up to 10 mL (final concentration of Cd(II) was 20–240 lg L1). Finally, 50 mg of Dowex l-X8 (200–400 mesh) resin were added. The mixture was mechanically stirred for 5.0 min and the colored resin beads were collected by filtration under suction and, with the aid of a small pipette, packed into a 1.0 mm cell together with a small volume of the filtrate. The cell was centrifuged at 5000 rpm for 2.0 min. A blank solution containing all reagents except cadmium was prepared and treated in the same way as the sample. The absorbance difference between the sample and the blank, measured as described above, provided an estimation of the net absorbance. A Perkin–Elmer Lambda 12 UV–VIS spectrophotometer with a 1.0 mm quartz cell was used for all spectral measurements. A selecta desk centrifuge and an Orion research model 601 A/digital ionalyzer pH meter were used for checking pH of the solutions. A Perkin–Elmer atomic absorption spectrometry model A Analyst 300 was used for all AAS measurements. The absorbance of the BTAHNA–Cd(II) deep violet complex sorbed on the resin was measured in a 1.0 mm cell at 692 nm (corresponding to the absorption maximum of the colored complex) and 800 nm (in a region where only the resin absorbs light) against a 1.0 mm cell packed with resin equilibrated with a blank solution. The net absorbance (Ac) for the complex was obtained using the following equation (Fernandez-de Cordova, MolinaDiaz, Pascual-Reguera, & Capitan-Vallvey, 1992; Yoshimura & Waki, 1985) Ac ¼ A692  A800 ð1Þ 2.3.2. For 100 mL samples An appropriate volume containing 0.2–4.0 lg (2.0–40 lg L1) of Cd(II) was transferred into a 1 L polyethylene bottle and 0.8 mL of Table 1 Comparison of analytical performance of various spectrophotometric methods for determination of cadmium. Reagent kmax (nm) e (L mol1 cm1)  105 Remarks Ref. 1-(2-Benzothiazolylazo)-2-hydroxy-3-naphthoic acid 2-[(5-Bromo-2-pyridine)azo]-5-diethylaminopheno 2-[2-(5-Bromopyridine)azo]-5-dimethylaminophenol 616 556 555 1.14 1.39 1.41 Amin (2001) Shibata et al. (1976) Shibata et al. (1976) p-Nitrophenyldiazo aminoazobenzene 480 4.1 Triton X-100 In 50% ethanol medium Low sensitivity, extracting with trimethylbutanol Low sensitivity, with very toxic KCN as masking reagent and formaldehyde as demasking reagent o-Hydroxyphenyldiazo aminoazobenzene 2,6-Dibromo-4-nitrophenyldiazo Aminoazobenzene 2-Acetylmercaptophenyldiazo aminoazobenzene 1-(2-benzothiazolylazo)-2-hydroxy-3-naphthoic acid (BTAHNA) (SPS) 520 500 529 692 1.5 1.52 2.4 70.5 392.1 1783 4102 Low sensitivity, extracting with MIBK Many ions interfering with color reaction Ions interfering, sodium thiosulfate as masking 10 mL sample 100 mL Sample 500 mL Sample 1000 mL Sample Hsu et al. (1989) Cao and Li (1992) Liu et al. (2004) Proposed method Hsu et al. (1980) 520 A.S. Amin, A.A. Gouda / Food Chemistry 132 (2012) 518–524 Table 2 Comparison of detection limits of diverse instrumental techniques for the determination of cadmium (II). Technique Flow injection Conditions Detection limit 1 Ref. Preconcentration and flame atomic absorption spectroscopy (FAAS) Flame atomic absorption spectrometry (FAAS) Electrothermal atomic absorption spectrometry (ET AAS) Flame atomic absorption spectrometry (FAAS) 0.11 lg L Flame atomic absorption spectrometry (FAAS) 0.3 lg L1 Shabania et al. (2009) Ferreira et al. (2009) Afkhami, Madrakian, and Siampour (2006) Lemos et al. (2008) 0.14 lg L1 Zhai et al. (2007) Solid phase extraction Solid phase extraction Inductively coupled plasma atomic emission spectrometry (ICPAES) Inductively coupled plasma optical emission spectrometry (ICPOES) Flame atomic absorption spectrometry Liquid electrode plasma atomic emission spectrometric (LEP-AES) Liquid–liquid extraction Complexation Complexation flame atomic absorption spectrometry Graphite furnace atomic absorption spectrometry GF-AAS. Solid phase spectrophotometry Atomic absorption spectroscopy Atomic absorption spectrometry Cloud Point Extraction Flow injection Solid phase extraction Solid phase extraction Solid phase extraction 1  104 mol L1 BTAHNA solution and 10 mL of pH 8.5 phosphate buffer solution were added, 50 mg of Dowex l-X8 (200–400 mesh) resin were added after filling the bottle up to 100 mL. The mixture was mechanically shaken for 15 min, and treated as indicated in the above procedure. 2.3.3. For 500 mL samples An appropriate volume of sample containing 0.2–4.5 lg (0.4– 9.0 lg L1) of Cd(II) was transferred into a 1 L polyethylene bottle and 2.0 mL of 1  104 mol L1 BTAHNA solution and 40 mL of pH 8.5 phosphate buffer solution were added, 50 mg of Dowex l-X8 (200–400 mesh) resin were added after filling the bottle up to 500 mL. The mixture was mechanically shaken for 25 min and further treated as indicated in the above procedure. 2.3.4. For 1000 mL samples An appropriate volume of sample containing 0.2–3.5 lg (0.2– 3.5 lg L1) of Cd(II) was transferred into a 2 L polyethylene bottle and 3.0 mL of 1  104 mol L1 BTAHNA solution and 75 mL of pH 8.5 phosphate buffer solution were added, 50 mg of Dowex l-X8 (200–400 mesh) resin were added after filling the bottle up to 1000 mL. The stirring time was increased to 40 min. Other details were kept as above. Calibration graphs were constructed in the same way using Cd(II) solutions of known concentration. 2.4. Food samples treatment The sample was dried in a forced-draft oven at 70 °C to constant mass and then ground to a fine powder. A suitable aliquot was weighed (2.0 g dry material) into a 100 mL Claisen distilling flask, and 10 mL of HNO3 was added. After that, the flask was put into a model MDS-81D microwave oven and digested for 5.0 min at 50% power and continuously for 15 min at 100% power. Then the flask was taken out and cooled to ambient temperature before another 10 mL of HNO3 and 1.0 mL of H2O2 were added and left to stand for 20 min. The flask was placed in the microwave oven and irradiated for 40 min at 100% power. Then the flask was taken out and cooled to ambient temperature. The final 1.0 mL of HNO3 was added and again the flask was left to stand for 10 min. The final solution was neutralized to pH 8.0–9.0 with solid Na2CO3 and transferred into a 25 mL calibrated flask. The solutions were further treated as given in general procedure. Gawin et al. (2010) 1 0.014 ng mL 0.333 lg L1 1.0 ng mL1 0.18 mg L 1 0.028 mg L1 0.2 lg in 200 mL 6.0 ng g1 0.09 ng L1 53 ng L1 Puzio et al. (2008) Yaganas et al. (2008) Kagaya et al. (2010) Martinisa et al. (2009) Hata et al. (2008) Proposed method 2.5. Procedures for tobacco, green and black tea, human hair, spice and river sediment 0.25 g of tobacco sample was digested with 4.0 mL of concentrated HNO3 and 2.0 mL of concentrated H2O2 in microwave system. Blank digestions were also performed at the same conditions. After digestion, the volume was made up to 25 mL with distilled water. The procedure given above was applied to the samples. The metal concentrations in the final solutions were also determined by AAS. For the digestion of green and black tea samples, 0.25 g of tea was mixed with 6.0 mL of HNO3:H2SO4:H2O2 (1:1:1) in microwave system. After digestion, the volume was made up to 25 mL with distilled water. Blanks were prepared in the same way as the sample, but omitting the sample. The preconcentration procedure given above was applied to the samples. For the microwave digestion of human hair and a spice sample, 1.0 g of samples were digested with 4.0 mL of concentrated HNO3 and 2.0 mL of concentrated H2O2 in microwave system. After digestion, the volume was made up to 25 mL with distilled water. Blanks were prepared in the same way as the sample, but omitting the sample. The general procedure given above was applied to the samples. 0.25 g of river sediment was digested with HCl:HNO3:H2SO4 (4:2:2) in microwave system. After digestion, the volume was made up to 25 mL with distilled water. Blanks were prepared in the same way as the sample, but omitting the sample. The general procedure given above was applied to the samples. The final volume was 5.0 mL. 2.6. Determination of cadmium (II) in water samples A choice of water samples in and around the Shobra-El-Qhema and Benha cities has been made. Each filtered environmental water sample is evaporated nearly to dryness with a mixture of 5.0 mL concentrated H2SO4 and 10 mL concentrated HNO3 in a fume cupboard and then cooled to room temperature. The residue is then heated with 10.0 mL of deionized water, in order to dissolve the salts. The solution is cooled and neutralized with dilute NH4OH. The resulting solution is filtered and quantitatively transferred into a 25 mL calibrated flask and made up to the mark with deionized water. A known aliquot of the above sample solution is taken into a 25 mL separating funnel and the cadmium content is determined as described in the general procedure. 521 A.S. Amin, A.A. Gouda / Food Chemistry 132 (2012) 518–524 2.7. Determination of cadmium ions in salt samples 2.8. Distribution measurements BTAHNA solution, buffer solution, and 50 mg of Dowex l-X8 (200–400 mesh) were added to 100 mL of aqueous solution containing 8.0 lg of Cd(II). After a 30 min equilibration, the resin beads were separated by filtration under suction. Then, the equilibrium concentration of Cd(II) in the solution was determined as described in the 100 mL procedure. The distribution ratio D was calculated from the initial and the equilibrium concentrations in the solution. 3. Results and discussions 3.1. Absorption spectra Absorption spectra in solid phase BTAHNA was fixed on an anionic resin, giving red color with a kmax = 554 nm in the resin phase, compared with kmax = 562 nm in the solution. The presence of Cd(II) ion resulted in a deep violet complex which shifted the kmax to 659–664 nm in the solution and to kmax = 692 nm in the resin phase (Fig. 1). It is evident that the sensitivity increases when the complex is sorbed on the resin. 0.9 0.8 Absorbance For the determination of analyte ions in alkaline salt samples, 3.0 g of each salt sample was dissolved in 3.0 mL of distilled water and diluted to 100 mL with distilled water. The procedure given above was applied to these solutions. The analyte ions in the final solution were also determined by atomic absorption spectrometry. 1 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 4 6 8 10 12 14 pH Fig. 2. Effect of pH on the complexation of 2.22  107 mol L1 of Cd(II) complexed with 8  105 mol L1 BTAHNA for 100 mL sample. the working pH. The absorbance is independent of the ionic strength (adjusted with the buffer solution) up to the concentration of 8  102 mol L1. At higher concentrations, the absorbance decreases quickly, as is usual in SPS studies, probably owing to the competition between the anions of the buffer for the anionic sites of the resin. Moreover, the optimum value of pH 8.5 was selected as recorded for each procedure described in the general procedures. 3.3. Effect of reagent concentration 3.2. Effect of pH pH-dependence was studied by applying the 100 mL procedure. The optimum buffer solution was investigated by examining different types of buffer (acetate, borate, phosphate, thiel, and universal) solutions. Phosphate buffer gave the best results. Moreover, optimum pH for the formation and fixation of the species is in the range of 8.3–8.7 (Fig. 2). At pH values below 6.5 or above 9.2, the absorbance decreased significantly. Hence, pH 8.5 was chosen as The absorbance was found to increase with the BTAHNA concentration. The results indicated that the maximum absorbance for the complex fixed on Dowex 1-X8 was found with 0.5, 0.8, 2.0 and 3.0 mL of 1  105 M BTAHNA for 10, 100, 500, and 1000 mL sample procedures. 3.4. Effect of shaking time The optimum stirring times were 5 min, 15 min, 25 min, and 40 min for the 10 mL, 100 mL, 500 mL, and 1000 mL procedures, respectively. The fixed complex was stable for at least 36 h after the equilibration. The complex was completely fixed on Dowex 1-X8 and the extraction coefficient constants in various volumes of the liquid phase were not altered. The sequence of (Cd(II)–BTAHNA–buffer–resin) addition gave the highest absorbance in addition to the stirring time compared with other sequences. 3.5. Effect of amount of resin The use of a large amount of resin (mr) lowered the absorbance. Only the amount required to fill the cell and to facilitate handling (i.e. 50 mg) was used for all measurements. The reduction of absorbance is according to the empirical equation Ac ¼ 0:0063 þ 0:047=mr ðr ¼ 0:9967Þ ð2Þ The agreement of the slope with the molar absorbance can be calculated as follows (Yoshimura & Waki, 1985). Ac ¼ ec IR C o V1000=ðmr þ V=DÞ Fig. 1. Absorption spectra of Cd(II)–BTAHNA complex (A) in solution; [Cd(II)] = 2.22  104 mol L1; [BTAHNA] = 8.0  103 mol L1; pH 8.5; (B) in the resin [Cd(II)] = 2.22  107 mol L1; [BTAHNA] = 8.0  105 mol L1; pH = 8.5. ð3Þ where ec is the molar absorptivity of the sample species in the ionexchanger phase (21.171), IR is the mean light-path length through the solid phase, Co the initial molar concentration of Cd(II), V/L the volume of the sample solution, D the distribution ratio, and mr/g the 522 A.S. Amin, A.A. Gouda / Food Chemistry 132 (2012) 518–524 mass of ion exchanger. The fraction V/D can be neglected when compared with mr being 0.125 g or higher and Eq. (4) which relates the absorbance to the mass of ion-exchanger is obtained Ac ¼ 1000ec IR C o V=mr ¼ K=mr bance increases with the sample volume (V) till 1.0 L, then the absorbance becomes independent of the sample volume at higher sample volumes (i.e. V P 1.100 L), as usual in SPS (Fernandez-de Cordova et al., 1992). In practice, the increase of sensitivity with a higher amount of sample solution can be calculated from the slope of the calibration graphs. The calculated values of the sensitivity ratio S for the samples analyzed are: S(1000/500) = 2.31; S(1000/100) = 10.48; S(1000/10) = 58.21; S(500/100) = 4.54; S(500/10) = 25.20, and S(100/10) = 5.55. The values obtained using the distribution ratio value D are 2.19, 10.35, 58.07, 4.43, 25.07, and 5.47, respectively. Detection limits of the proposed methods are similar to those obtained by other sensitive techniques such as ET-AAS, AFS and ICP-OES (Table 2) (Ferreira et al., 2009; Gawin et al., 2010; Hata et al., 2008; Kagaya et al., 2010; Lemos, Novaes, Lima, & Vieira, 2008; Martinisa, Olsinab, Altamiranoa, & Wuillouda, 2009; Puzio, Mikula, & Feist, 2008; Shabania, Dadfarniaa, Motavaselian, & Ahmadib, 2009; Yaganas, Efendioglu, & Bati, 2008; Zhai, Liu, Changa, Chena, & Huanga, 2007) and, although the time required for the analysis by these techniques is shorter than in SPS, the costs of the necessary equipments are considerably higher than the corresponding costs for the SPS technique. On the other hand, it can be stated that accuracy and precision of the proposed SPS methods are similar to those obtained by the techniques indicated above. ð4Þ where K = 1000ecIRCoV, is the slope of the graphic representation of Ac vs. 1/mr. Supposing IR = 0.1 cm, the expected value of K = 1000  21.171  0.1  2.22  107  0.100 = 0.047 which is in excellent agreement with the experimental value of 0.0473. 3.6. Fixed complex The nature of the species fixed on the resin was established at the working pH of 8.5 using the molar ratio and continuous variation methods. The plot A vs. BTAHNA to Cd(II) mole ratio, obtained by varying the BTAHNA concentration, showed an inflexion at the mole ratio of 1.0 indicating the presence of one molecule of BTAHNA in the fixed complex. Moreover, the Job method showed that the BTAHNA to Cd(II) mole ratio was 1.0. Consequently, the results indicated that the stoichiometry was 1:1 (BTAHNA:Cd(II)). The conditional formation constant (log K), calculated using the Harvey and Manning equation, applying the data obtained from the above two methods, was found to be 8.53, whereas the true constant was 8.45. 3.8. Effect of foreign ions on the extraction of the Cd(II)–BDTSC complex 3.7. Analytical data Analytical parameters are summarized in Table 3. It was verified that one of the main contributions to the relative standard deviation (RSD) comes from the variability of the ion-exchanger packing. RSD was 6.1% without centrifugation for the 100 mL sample and 10 determinations. When the cells packed with the resin phase were centrifuged for 2.0 min at 5000g before the absorbance measurements were carried out, RSD decreased to 0.93% and the absorbance value increased to about 15%. The results indicate that increasing the sample volume increases the slope of the calibration graph and so increases also the sensitivity of the proposed methods. The increase in sensitivity achieved with the proposed methods is substantial compared with the earlier spectrophotometric methods for the determination of Cd(II) (Table 1), as can be seen from the range of molar absorptivity values of these methods (Cao & Li, 1992; Hsu, Hu, & Jing, 1980; Hsu, Wang, & Yang, 1989; Shibata, Kamata, & Nakashima, 1976). The values of apparent molar absorptivity (absorbance value of the complex sorbed on the resin from an aqueous solution of Cd(II), supposing a measurement in a 10 mm optical path length cell) for the methods proposed are 7.05  106 L mol1 cm1, 3.92  107 L mol1 cm1, 1.78  108 L mol1 cm1, and 4.10  108 L mol1 cm1 respectively. In the SPS methods, sensitivity can be enhanced by increasing the sample volume. The increase in sensitivity can be evaluated by measuring the absorbance of the resin equilibrated with different volumes of solutions containing the same concentration of Cd(II) and proportional amounts of the other reagents. The absor- The effect of foreign ions is studied by measuring the absorbance of the reaction mixture containing 25 lg L1 of Cd(II) for 100 mL sample in the presence of different amounts of foreign ions. An error of ±3.0% in the absorbance value caused by foreign ions is considered as a tolerable limit. The interference of metal ions has been tested up to 750-fold excess. The results show that Al(III), Mn(II), W(IV), Mg(II), Pb(II), Co(II), Ca(II), La(III), Ti(IV), Th(IV), and U(VI) do not interfere. The tolerated limits for other metal ions are Fe(III) and Zr(IV) up to 400-fold excess, Cr(III) and Mo(VI) up to 100-fold excess, Cu(II), Ni(II), Ag(I), Pd(II) and Zn(II) less than 50fold excess. 1.0 mL of 5.0% citrate has been employed as a masking agent for Ni(II), Pd(II), Zn(II). The interference of copper (II) has been eliminated by using 1.0 mL of 2.0% thiosulphate as the masking agent. Ag(I) has to be removed as silver chloride, prior to the extraction of Cd(II). Anions like bromide, chloride, fluoride, iodide, nitrate, sulphate, phosphate, tartrate, citrate, thiocyanate, thiosulphate and thiourea have no effect on the extraction of Cd(II), even when they are present in 250-fold excess or more. However, EDTA, and oxalate interfere seriously. 3.9. Analytical applications The SPS procedure for cadmium ions was applied to various water samples. The results for natural water samples were given in Table 4. The proposed method has been combined with the Table 3 Analytical parameters for cadmium (II) determination using the proposed method. Parameter Slope Linear dynamic range/(lg L1) Correlation coefficient Detection limit (K = 3) (lg L1) Determination limit (K = 10) (lg L1) RSD (%) (n = 10) a b Sample volume (mL) 10 100 500 1000 6.27  103 20–240 (40–220)a 0.9996 5.80 19.2 1.14 (100)b 3.48  102 2.0–40.0 (3.0–37.5)a 0.9994 0.553 1.85 0.93 (12)b 0.158 0.4–9.0 (0.6–8.4)a 0.9992 0.119 0.395 1.02 (4)b 0.365 0.2–3.5 (0.4–3.25)a 0.9995 0.053 0.18 0.88 (2)b Evaluated by Ringbom’s method (Ringbom, 1938). Cd(II) concentration (lg L1) used for the determination of the reproducibility. A.S. Amin, A.A. Gouda / Food Chemistry 132 (2012) 518–524 Table 4 Determination of cadmium (II) in water, microwave-digested food and some salt samples. Sample Cd(II) found Water samples b (lg L1) River water (Shobra) Waste water (Benha) River water (Benha) Tap water Spring water Fortified water Lake water Bottled mineral water Microwave-digested food samples (lg g1) Human hair Tobacco Green tea Black tea Spice River sediment Rice Grain Flour Proposed method a AAS 2.01 2.51 0.91 1.68 1.41 2.34 2.85 0.85 1.95 2.55 0.93 1.64 1.45 2.54 2.82 0.88 Standard deviation RSD (%) 0.0989 0.0132 0.0197 0.0127 0.0154 0.0176 0.0142 0.0104 0.74 1.07 1.53 1.11 1.29 1.40 1.26 0.97 a b c eliminated by using 1.0 mL of 2.0% thiosulphate as the masking agent. Ag(I) has to be removed as silver chloride, prior to the extraction of cadmium (II). (3) Increasing the sample volume enhances the sensitivity. Detection and quantification limits of the 500 mL sample method are 119 ng L1 and 395 ng L1, respectively, when using 50 mg of Dowex 1-X8. For the 1000 mL sample, the detection and quantification limits are 53 ng L1 and 180 ng L1, respectively, using 50 mg of the exchanger. (4) Successful application of the proposed method to the determination of low levels of cadmium in food samples, some salts, as well as, water samples with good results. Acknowledgements 0.25 2.85 0.56 0.74 0.23 3.50 0.37 0.60 0.44 0.24 2.88 0.55 0.73 0.24 3.47 0.35 0.58 0.45 0.0126 0.0103 0.0088 0.0097 0.0118 0.0088 0.0067 0.0105 0.0091 1.50 1.27 1.01 1.23 1.42 1.28 0.99 1.30 1.17 0.40 0.42 0.089 0.74 0.32 0.33 0.132 1.07 c Salt samples Ammonium chloride (technical grade) Sodium chloride (technical grade) 523 Average of six determinations. Mean expressed as 95% tolerance limit. Mean expressed as 97% tolerance limit. microwave assisted digested samples including a human hair, a tobacco, a green and black tea, a spice and a river sediment. For this purpose, these samples were digested by closed microwave system. The results are given in Table 4. Concentrations of the investigated ions in our samples were lg g1 level. The proposed method has been applied to the direct determination of cadmium in rice, grain and flour (purchased from Benha, Egypt). The determination was performed using the standard addition calibration graph method (Table 4). The proposed SPS method is applied for the determination of Cd(II) in some salt samples. The data obtained in the analysis of some salt samples were given in Table 4. The precision shown for the samples studied is also satisfactory. Performance of the proposed method was assessed using the tvalue (for accuracy) and F-test (for precision) compared with the AAS method. The mean values were obtained by a Student’s t-test and F-tests at 95% confidence limits for five degrees of freedom (Miller & Miller, 2005). The results showed that the calculated values did not exceed the theoretical values. A wider range of determination, higher accuracy, higher stability and lower time demand, are the advantage of the proposed method over other ones. 4. Conclusions The proposed method has the following characteristics: (1) BTAHNA is one of the most easily prepared high purity, sensitive, and selective spectrophotometric reagents for cadmium determination. Molar absorptivity of the chelate was found to be up to 4.10  107 L mol1 cm1 in the measured solution. (2) Most foreign ions do not interfere with the determination. 1.0 mL of 5.0% citrate has been employed as a masking agent for Ni(II), Pd(II), Zn(II). The interference of Cu(II) has been Authors wish to express their deep thanks to Prof. Dr. Raga El sheikh for helping in this research work and Zagazig University, Egypt for supporting this research work through (Support of Academic Research in Zagazig University Project, fifth stage, 2011). References Afkhami, A., Madrakian, T., & Siampour, H. (2006). Flame atomic absorption spectrometric determination of trace quantities of cadmium in water samples after cloud point extraction in Triton X-114 without added chelating agents. Journal of Hazardous Materials, 138, 269–272. Amin, A. S., & El-Mossalamy, E. H. (2003). Simple spectrophotometric method for the quantitative determination of uranium. Journal of Trace and Microprobe Techniques, 21, 637–648. Amin, A. S., & Ibrahim, M. S. (2001). Determination of nickel in biological samples spectrophotometrically involving complexation reaction with some thiazolylazo compounds. Annali di Chimica, 91, 181–189. Amin, A. S. (2002). Simple selective spectrophotometric determination of ruthenium after solid phase extraction with some quinoxaline dyes into microcrystalline-p-dichlorobenzene. Spectrochimica Acta (A), 58, 1831–1837. Amin, A. S. (2001). Spectrophotometric determination of cadmium using thiazolylazo chromogenic reagents in the presence of triton x-100: Application in environmental samples. Analytical Letters, 34, 163–175. Amin, A. S. (2000). The surfactant-sensitized analytical reaction of niobium with some thiazolylazo compounds. Microchemical Journal, 65, 261–267. Amin, A. S. (2009). Utilization of solid phase spectrophotometry for the determination of trace amounts of copper with 5-(2-benzothiazolylazo)-8hydroxyquinolene. Chemical Papers, 63, 625–634. Britton, H. T. S. (1952). Hydrogen ions (p. 1168) (4th ed.). London: Chapman and Hall. Cao, S. T., & Li, X. (1992). 1-(4-nitrophenyl)-3-(2-quinolyl) triazene as an extremely sensitive reagent for spectrophotometric determination of mercury. Talanta, 39, 1395–1398. Daniel, S., Praveen, R. S., & Rao, T. P. (2006). Ternary ion-association complex based ion imprinted polymers (IIPs) for trace determination of palladium(II) in environmental samples. Analytica Chimica Acta, 570, 79–87. Fang, G. Z., Tan, J., & Yan, X. P. (2005). An ion-imprinted functionalized silica gel sorbent prepared by a surface imprinting technique combined with a sol–gel process for selective solid-phase extraction of cadmium(II). Analytical Chemistry, 77, 1734. Fernandez-de Cordova, M. L., Molina-Diaz, A., Pascual-Reguera, M. I., & CapitanVallvey, L. F. (1992). Determination of trace amounts of cobalt at sub-lg 11 level by solid phase spectrophotometry. Analytical Letters, 25, 1961–1980. Ferreira, S. L. C., Jesus, R. M., Matos, G. D., Andrade, J. B., Bruns, R. E., Santos, W. N. L., et al. (2009). Multivariate optimization and validation of an analytical method for the determination of cadmium in wines employing ET AAS. Journal of Brazilian Chemical Society, 20, 788–794. Gawin, M., Konefał, J., Trzewik, B., Walas, S., Tobiasz, A., Mrowiec, H., et al. (2010). Preparation of a new Cd(II)-imprinted polymer and its application to determination of cadmium(II) via flow-injection-flame atomic absorption spectrometry. Talanta, 80, 1305–1310. Godlewska-Zyłkiewicz, B. (2004). Preconcentration and separation procedures for the spectrochemical determination of platinum and palladium. Microchimca Acta, 147, 189–210. Hata, N., Hieda, S., Yamada, M., Yasui, R., Kuramitz, H., & Taguchi, S. (2008). Formation of a liquid organic ion associate in aqueous solution and its application to the GF-AAS determination of trace cadmium in environmental water as a complex with 2-(5-bromo-2-pyridylazo)-5-(N-propyl-Nsulfopropylamino) phenol. Analytical Science, 24, 925–928. Hsu, C., Hu, C., & Jing, J. (1980). Spectrophotometric determination of micro amounts of cadmium in waste water with cadion and triton X-100. Talanta, 27, 676–678. 524 A.S. Amin, A.A. Gouda / Food Chemistry 132 (2012) 518–524 Hsu, C. G., Wang, W., & Yang, L. (1989). Spectrophotometric and first-derivative spectrophotometric determination of cadmium with o-hydroxybenzenediazoaminoazobenzene (HDAA). Mikrochimca Acta I, 313–320. International Agency for Research on Cancer (IARC) (1993). In Proceedings of the meeting of the IARC working group on beryllium, cadmium, mercury and exposures in the glass manufacturing industry, Scandinavian Journal of Work Environment and Health 19, 360–374. Kagaya, S., nakada, S., Inoue, Y., Kamichatani, W., Yanai, H., Saito, M., et al. (2010). Determination of cadmium in water samples by liquid electrode plasma atomic emission spectrometry after solid phase extraction using a mini cartridge packed with chelate resin immobilizing carboxymethylated pentaethylenehexamine. Analytical Science, 26, 515–518. Lemos, V. A., Novaes, C. G., Lima, A. S., & Vieira, D. R. (2008). Flow injection preconcentration system using a new functionalized resin for determination of cadmium and nickel in tobacco samples. Journal of Hazardous Materials, 155, 128–134. Liu, Y., Chang, X., Wang, S., Guo, Y., Din, B., & Meng, S. (2004). Solid-phase extraction and preconcentration of cadmium(II) in aqueous solution with Cd(II)-imprinted resin (poly-Cd(II)-DAAB-VP) packed columns. Analytical Chimica Acta, 519, 173–179. Liu, Y., Yang, D., Chang, X., Guo, Y., Din, B., & Meng, S. (2004). Direct spectrophotometric determination of trace cadmium(II) in food samples with 2-acetylmercaptophenyldiazoaminoazobenzene (AMPDAA). Microchimica Acta, 147, 265–271. Martinisa, E. M., Olsinab, R. A., Altamiranoa, J. C., & Wuillouda, R. G. (2009). On-line ionic liquid-based preconcentration system coupled to flame atomic absorption spectrometry for trace cadmium determination in plastic food packaging materials. Talanta, 78, 857–862. Miller, J. N., & Miller, J. C. (2005). Statistics and chemometrics for analytical chemistry (5th ed.). Harlow: Pearson/Prentice Hall. Preetha, C. R., & Rao, T. P. (2003). Preparation of 1-(2-pyridylazo)-2-naphthol functionalized benzophenone/naphthalene and their uses in solid phase extractive preconcentration/separation of uranium(VI). Radiochimica Acta, 91(5), 247–252. Puzio, B., Mikula, B., & Feist, B. (2008). Preconcentration of Cd(II) and Cu(II) by solid phase extraction on cyanopropyl modified silica columns using 1,10phenanthroline as the complexing agent. Microchimica Acta, 160, 197–201. Ringbom, A. (1938). Über die Genauigkeit der colorimetrischen Analysenmethoden I. Fresenius Journal of Analytical Chemistry, 115, 332–343. Sawula, G. M. (2004). On-site preconcentration and trace metal ions determination in the okavango delta water system, botswana. Talanta, 64, 80–86. Shabania, A. M. H., Dadfarniaa, S., Motavaselian, F., & Ahmadib, S. H. (2009). Separation and preconcentration of cadmium ions using octadecyl silica membrane disks modified by methyltrioctylammonium chloride. Journal of Hazardous Materials, 162, 373–377. Shibata, S., Kamata, E., & Nakashima, R. (1976). 2-[2-(5-Bromopyridyl)azo]-5dimethylaminophenol; a new sensitive reagent for cadmium. Analytica Chimica Acta, 82, 169–174. Soylak, M. (2004). Solid phase extraction of Cu(II), Pb(II), Fe(III), Co(II), and Cr(III) on chelex-100 column prior to their flame atomic absorption spectrometric determinations. Analytical Letters, 37, 1203–1217. Soylak, M., Karatepe, A. U., Elci, L., & Dogan, M. (2003). Column preconcentration/ separation and atomic absorption spectrometric determinations of some heavy metals in table salt samples using amberlite XAD-1180. Turk. J. Chem, 27, 235–242. Teixeira, L. S. G., & Rocha, F. R. P. (2007). A green analytical procedure for sensitive and selective determination of iron in water samples by flow-injection solidphase spectrophotometry. Talanta, 71, 1507–1511. Tuzen, M., & Soylak, M. (2004). Column system using diaion HP-2MG for determination of some metal ions by flame atomic absorption spectrometry. Analytica Chimica Acta, 504, 325–334. Unites States Environmental Protection Agency (2003). National primary drinking water standards, June 2003. Vogel, A. I. (1978). A text book of quantitative inorganic analysis (4th ed.). London: Longmann, Green. Wang, R. M., Xie, X., Wang, J. Q., Pan, S. J., Wang, Y. P., & Xia, C. G. (2004). Preparation and adsorption properties of modified chitosan. Polymer Advanced Technology, 15, 52–54. World Health Organization (2006). Guidelines for drinking-water quality (3rd ed., Vol. 1). Geneva: Recommendations. Yaganas, M., Efendioglu, A., & Bati, B. (2008). Solid phase extraction of cadmium in edible oils using zinc piperazinedithiocarbamate and its determination by flame atomic absorption spectrometry. Turkish Journal of Chemistry, 32, 431–440. Yamini, Y., Hosseini, M. H., & Morsali, A. (2004). Solid phase extraction and flame atomic absorption spectrometric determination of trace amounts of zinc and cobalt ions in water samples. Microchimica Acta, 146, 67–721. Yoshimura, K., & Waki, H. (1985). Ion-exchanger phase absorptiometry for trace analysis. Talanta, 32, 345–352. Zhai, Y., Liu, Y., Changa, X., Chena, S., & Huanga, X. (2007). Selective solid-phase extraction of trace cadmium(II) with an ionic imprinted polymer prepared from a dual-ligand monomer. Analytica Chimica Acta, 593, 123–128.