Bull Environ Contam Toxicol DOI 10.1007/s00128-015-1484-x

Double Preconcentration of Trace Amounts of Cadmium in Nail Samples and Measurement by Differential Pulse Voltammetry Somayeh Shahi • Mohammad Reza Nateghi

Received: 23 June 2014 / Accepted: 24 January 2015 Ó Springer Science+Business Media New York 2015

Abstract Cadion was coated on carbon powder and used as a solid phase for selective extraction and preconcentration of cadmium ions. Complexed cadmium ions were eluted from solid phase by 5 mL, nitric acid (2.0 M) with the flow rate of 2 mL min-1.The resulted solution was used for accumulation of the cadmium metal at the surface of the carbon paste electrode at -1.3 V reduction potential. Finally, cadmium was reoxidized and the differential pulse voltammogram recorded at the potential range of -0.55 to -0.2 V. Calibration graph was plotted in the concentration range of 0.5–50 lg L-1 of cadmium. Detection limit 0.06 lg L-1 was calculated based on the 3 Sb/m. The RSD was 9.13 % (n = 4) for cadmium concentration of 10 lg L-1 with preconcentration factor of 100. Method was successfully used for the determination of cadmium in finger nail samples and after spiking the samples, the recoveries were evaluated [96 %. Keywords Cadion  Cadmium  Solid phase extraction  Stripping voltammetry In recent years, determination of trace amounts of metals such as cadmium in environmental food and human samples has become more serious due to increasingly lower limits imposed on trace metal content of such samples (Stephen 2008; Beldomenico et al. 2001). Cadmium is very toxic and is known to be responsible for various diseases. While blood and other body fluids give transient concentrations, human nails provide a continuous record of elemental concentration. Fingernail growth in human is a S. Shahi  M. R. Nateghi (&) Department of Chemistry, Yazd Branch, Islamic Azad University, Yazd, Iran e-mail: [email protected]

continuous process throughout life, about 0.05–1.2 mm/ week. Further, blood and other body fluids are not suitable to analyse levels of Cd because the metal exists briefly in the medium. Moreover, nails are easier to sample, transport and store since they do not require any external conditions unlike body fluids that are prone to contamination. Therefore determination of trace amounts of cadmium in human finger nail samples is of great importance (Vance et al. 1988). Different analytical techniques such as UV-vis spectrophotometry, inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectrometry (ICP-OES), atomic absorption spectrometry (AAS), flame atomic absorption spectrometry (FAAS), atomic fluorescence spectrometry and neutron activation analysis have been developed for the determination of Cd in various samples (Lemos et al. 2010; Leopold et al. 2009; Pourreza et al. 2009; Falandysz 2012; Bagheri et al. 2012; Afkhami et al. 2006; Abdel-halim 2003; Afkhami et al. 2010; Li et al. 2002). However, most of the above methods require several time consuming manipulation steps, sophisticated instruments, special training and are expensive. In order to achieve more accurate and precise determination of trace amounts of cadmium in human samples using conventional and nonexpensive techniques, an alternative method is separation and preconcentration of cadmium in combination with a sensitive technique such as anodic stripping differential pulse voltammetry. Selective separation and preconcentration steps are needed prior to analyte determination by anodic stripping differential pulse voltammetry (ASDPV) to achieve limit of detection required for determination of cadmium at few ppb level. Because of the following advantages, (1) high preconcentration factor; (2) simple operation; (3) high recovery; (4) rapid phase separation; (5) capability for combination with different detection

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techniques; (6) low consumption of organic solvent (7) time saving and (8) cost saving (Camel 2003), solid phase extraction (SPE) (Jibrin et al. 2007; Huang et al. 2007), technique has become increasingly popular for the enrichment of metal ions prior to their determination. This study, describes a procedure based on selective enrichment by solid phase extraction of cadmium in nail sample followed by preconcentration on the surface of the carbon paste electrode (double preconcentration) and employing of the very sensitive differential pulse voltammetry technique to achieve very low limit of detection required for the determination of cadmium in human samples. Cadion (Fig. 1) (as a known and very selective ligand to the cadmium) was immobilized on the surface of activated carbon particles and used for solid phase extraction. The analytical parameters relevant to quantitative retention of cadmium on the sorbent were investigated.

Materials and Methods All reagents used were of analytical grade. Stock standard solution of Cd2? and Pb2? (50 ppm of each) were prepared by dissolution of respecting nitrate salts and working standard solutions were freshly prepared daily by appropriate dilution. Cadion was purchased from Fluka. Sodium hydroxide was used for pH adjustment (pH = 9.3–10.8). For solid phase sorbent preparation a mixture containing 10.0 g of active carbon, 0.5 g of cadion and 50 mL of acetone was stirred at room temperature for 2 min. Then the resulted suspension solution was kept in an environment free from contamination at room temperature until the acetone evaporates and cadion is consolidated on to activated carbon. Carbon paste electrode was prepared by a mixture containing 0.2 g pure graphite powder, 0.1 g single walled carbon nanotubes (SWNTs), and 0.039 g paraffin oil, abbreviated as 59:29:12 w/w %. Carbon nanotubes were added to increase the microscopic area and also conductivity of the electrode (Afkhami et al. 2013). For making the electrode, the paste was packed under pressure into Teflon syringes and the tip of the electrode (3 mm diameter) was polished on a surface of a glossy paper to receive a shiny

Fig. 1 Structure of 1-(4-nitrophenyl)-3-(4-phenylazophenyl) triazen (cadion)

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surface. All the voltammetric measurements were carried out in an electrochemical cell housing a carbon paste as a working electrode, saturated Ag/AgCl as a reference and platinum wire as auxiliary electrode. Differential pulse voltammetry was operated using Autolab PGSTAT 302N potentiostat/galvanostat. The human finger nail samples were scraped and clean of dust particles with Triton X-100 (Ciszewski et al. 1997). This was followed by soaking the nail samples in acetone to remove external contamination (Mikasa 1988) and finally the samples were rinse with doubly distilled water, dried and stored in an oven at 150°C. Finger clippings were collected from the ten students working in our laboratory. Nail clippings were obtained from all ten fingers using stainless steel scissors. The finger nail samples (0.1 g) were totally digested with 15 mL of 2:1 mixture of concentrated nitric acid and hydrogen peroxide so the final pH was adjusted to 10.0. Enough time was spent to digest the nail sample completely. Improper digestion result in erroneous measurements particularly report of lower detection limit. Then 0.5 g of cadion coated activated carbon powder added into 500 mL sample. Solution was stirred by magnetic stirrer for 30 min. Next, the sample with flow rate of 5 mL min-1 was passed through the fused silica filter. Sorbent remained on the filter was washed with 5.0 mL nitric acid (2.0 M) by flow rate of 2 mL min-1. The final solution was examined using carbon paste electrode by anodic stripping voltammetry technique.

Results and Discussion At first, 0.5 g of solid phase sorbent was added to500 mL of blank solution without adding the analyte and the solution stirred for 30 min. Then the sorbent particles were filtered and eluted by 5 mL of 2.0 M nitric acid eluent solvent (first preconcentration step). At the next step, voltammetry was employed for investigation of the carbon paste electrode behavior in solution resulted from previous enrichment step followed by reduction and accumulation of metal ions (second preconcentration step) at -1.3 V potential (Fig. 2a). Figure 2b, shows the voltammogram obtained for the sample solution containing Cd2? at the same experimental conditions applied on the blank solution. As it is observed, the result is the same as blank solution and the relevant oxidation peak of the cadmium is not appeared. This means Cd2? ions were not accumulated on the electrode in the 2.0 mol L-1 HNO3 eluent solution/ supporting electrolyte. Because, concentrated nitric acid is an oxidizing medium, and cadmium metal produced on the surface of the electrode is reoxidized immediately by NO3through the following reaction:

Bull Environ Contam Toxicol

Fig. 2 Differential pulse voltammograms for: (a) Blank, (b) 40 lg L-1 Cd?2, (c) 40 lg L-1of Cd?2 and 200 lg L-1 Pb?2, and (d) 200 lg L-1 Pb?2. Conditions: deposition potential -1.3 V versus Ag/AgCl, deposition time 190 s, scan rate 50 mVs-1, preconcentration factor: 100

þ 2þ Cd þ 2NO þ 2NO2 þ 2H2 O 3 þ 4H ! Cd

The voltammogram obtained for the solution containing Cd2? and Pb2? is shown in Fig. 2c. As it is seen, after accumulation of cadmium at -1.3 V for 190 s the oxidation peak relevant to the cadmium is appeared at -0.45 V versus Ag/AgCl reference electrode. In order to verify that the observed peak is related to the oxidation reaction of Cd on the surface of the carbon electrode, similar experiment was carried out on the solution of Pb2? at the absence of Cd2? (Fig. 2d). Any peak was observed in the applied scanning potential range. Also for further verification the concentration of the Cd2? in the initial samples was increased (while the concentration of the lead was kept constant) and the procedure was applied on each solution and the voltammograms obtained at the same experimental conditions. Clearly, it was observed that the peak current appeared at -0.45 V increases correspondingly. Therefore, it can be attributed to the oxidation reaction of the Cd accumulated on the surface of the electrode and the presence of the lead in the measurement solution (nitric acid 2.0 M) is indispensable. It could be seen that addition of Pb2? ion certainly influences the cadmium signal and a clear peak is appeared for carbon paste electrode in 2.0 mol L-1 HNO3 supporting electrolyte. While in the absence of Pb2?, the sensitivity of the procedure to cadmium is greatly diminished. The optimized conditions for preconcentration of the investigated analyte ions on cadion modified activated carbon adsorbent were established. The pH of each solution was adjusted to values ranging from 2.0 to 10.0 with HNO3 and NaOH solutions. Quantitative recoveries ([95 %) for analyte were obtained at pH 10.0. Amount of cadion immobilized on the activated carbon was studied at the

range of 1–7 wt% and it was recognized that the sorbent consisting 5 wt% of the cadion is the most suitable for the purpose of the extraction of analyte from solution. The amount of adsorbent is another important parameter to obtain quantitative recovery. For this reason the amount of cadion modified activated carbon was optimized. The influence of the adsorbent amount was tested in the range of 0.5–3.5 g and finally 0.5 g was selected as optimum value. Because, employing the further amount of the sorbent was not necessary for complete accumulation of the analyte. Various solvents (HCl, HNO3, Acetate buffer pH = 3 and 5, concentrated NH3) were used to identify the best eluent for the sorbent.Among different eluents used, 2.0 mol L-1 of HNO3 provided higher adsorbed metal ions on the recovery and reproducibility. Therefore, this solution was chosen as an eluent for the cadmium ions. Then the concentration of the HNO3 in solution as an eluent was optimized. Results indicated that highest recoveries are obtained for analyte at 2.0 mol L-1 of HNO3 and was employed for the next studies. Subsequent experiments showed that 5.0 mL of the eluent solution was enough for elution of metal ions. The flow rate of eluent solution was also examined in the range of 1–5 mL min-1. Maximum recoveries for metal ions were obtained in the range of 2–3 mL min-1. The flow rate of 2 mL min-1 was chosen as optimum value. For application of recommended solid phase extraction to real samples, effects of some interfering ions on the recovery of cadmium ions were investigated with the optimized procedure. The tolerance limit was defined as the amount of foreign ion causing a change of \ ±10 % in the voltammetric currents. The tolerable limits of interfering ions are given in Table 1. The results show that some of the metal ions are tolerable up to 100-fold for Cd. The effect of the accumulation (deposition) potentials on the peak current of Cd oxidation was investigated. The solution resulted from preconcentration by solid phase extraction (with 100 preconcentration factor) was used for this purpose. As shown in Fig. 3, the deposition potential was changed from 0.0 to -1.6 V. The negative shift of deposition potential can clearly increase the reduction of Cd2? on the Table 1 Effect of interfering ions on recovery

Concentration (lg L-1)

Ions

1,000 1,000

Ag? Pb2?

1,000

Cu2?

1,000

Tl3?

1,000

Ni2?

1,000

Hg2?

1,000

Zn2?

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Fig. 3 Effect of deposition potential on the stripping peak current for 5 lg L-1 of Cd2? solution. Conditions: deposition time 190 s, scan rate 50 mV s-1, preconcentration factor: 100

surface of electrodeand increase the peak current. But at the deposition potentials more negative than -1.4 V the peak current was remained without considerable change. Therefore, -1.3 V was applied for the subsequent experiments. Effect of deposition time was studied in the range of 30–210 s (Fig. 4). Anodic stripping peak currentincreased by increasing the deposition time up to 210 s, above which it remains constant approximately. Therefore, a deposition time of 190 s was selected for further works. Figure 5 shows the effect of concentration of Pb2? on the oxidation signal of the cadmium at the electrode surface. The peak current increased as the concentration of Pb2? was increased, in the range of 1–200 lg L-1 and then was constant at higher concentration. More concentration of Pb2? probably occupies the cadion chelating sites at the first preconcentration step and inhibits complex formation

Fig. 4 Effect of deposition time on the stripping peak current for 5 mg L-1 of Cd2? solution. Conditions: Deposition potential -1.3 V, scan rate 50 mV s-1

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Fig. 5 Effect of concentration of lead on the stripping peak current for 10 lg L-1of Cd2?solution. Condition: deposition potential -1.3 V, deposition time; 190 s, scan rate; 50 mV s-1

of cadmium ions with ligand molecules. Therefore, 200 lg L-1 was selected as optimum concentration for Pb2? and used for further studies. Calibration graph was constructed under the optimum conditions described above using carbon paste electrode: the scan rate of 50 mV s-1, deposition potential; -1.3 V, and deposition time; 190 s. Figure 6, shows the calibration graph obtained under the optimum conditions. The limit of detection (LOD) based on three times standard deviation of the blank (3 Sb) under optimal experimental conditions was 0.06 lg L-1 (n = 9). The calibration graph was linear in the range of 0.5–50.0 lg L-1. The equation for the fitted line was Ip = 1.805 9 10-5x ? 2.589 9 10-5 (r2 = 0.996). The repeatability of the electrode was evaluated by performing four determinations with the same solutions of Cd2?. The relative standard deviation (RSD) for the response of the electrode toward a 10 lg L-1 of Cd2?

Fig. 6 Calibration graph for the solutions with different concentrations (bottom to top: 0.5, 0.9, 5.0, 10.0, 20.0, 3 40.0, 50.0 lg L-1) of cd2? in the presence of constant concentration of lead (200 ppb). Conditions: preconcentration factor 100, deposition potential -1.3 V, deposition time 190 s, scan rate 50 mV s-1

Bull Environ Contam Toxicol Table 2 Comparison of some analytical parameters of the present method and methods proposed for the determination of Cd2? LOD (lg L-1)

LDR (lg L-1)

0.8–70

b

a

References

0.40

HAF

CPE/SWASV

Elmhammedi et al. (2009)

0.38

PAR

SPE/FAAS

Ciftci (2010)

0.10

Luminol

HMDE/DPASV

Abbasi et al. (2011a, b)

Modifier

Methods

0.05–20

0.03

Nafion, PDMcT

GCE/DPASV

He et al. (2011)

0.2–30

0.01

2-Mercapto

HMDE/DPASV

Koper et al. (2012)

0.28–22

0.11

Cupferron

HMDE/DPASV

Abbasi et al. (2011a, b)

0.1–50

0.07

Bi-GCPE

CPE/SWASV

Wnsawat et al. (2012)

8-Hydroxyquinoline

HMDE/DPASV

Jugade et al. (2012)

Benzothiazole

200.0 1–25

0.5–50

0.36

Indium

GCE/SWASV

Ahandhakumar et al. (2013)

0.05 0.50

TAR DEM,8-hydroxyquinoline

SPE/FAAS SPE/ETAAS

Dias et al. (2013) Alonso et al. (2014)

0.06

Cadion

SPE/CPE/DPASV

This work

a

CPE carbon paste electrode, SWASV square-wave anodic stripping voltammetry, SPE solid phase extraction, FAAS flame atomic absorption spectrometry, HMDE Hanging mercury drop electrode, DPASV differential pulse anodic stripping voltammetry, GCE glassy carbon electrode, ETAAS electrothermal atomic absorption spectrometry, b HAF Hydroxy apatite, PAR4-(2-pyridylao) resorcinol, Luminol; 3-aminophthalhydrazide, PDMcT poly(2,5,diamercapto-1,3,4-thiadiazole, Bi-GCPE bismuth-graphene carbon paste electrode, TAR tiazolyl azo resorsinol, DEM 2-(diethylamino)ethyl metacrylate

Table 3 Determination of Cd2? in human finger nail sample by the proposed method Sample

Sample amount (g)

Nail

0.1

Nail

0.1

Added lead (lg L-1)

Peak current (A)

0

200

9.06 9 10-5

3.0

–

10

200

2.82 9 10-4

12.6

96

Added cadmium (lg L-1)

solutions was 9.3 %. For comparison some analytical parameters of the present method and methods proposed by other researchers are introduced in Table 2. As it is seen the detection limit reported for the present method is comparable and in some cases is better than previous works. To examine the reliability and accuracy of the method, fixed amount of the investigated cadmium ions was spiked into 500 mL of the finger nail samples at optimum conditions. Recoveries of the analyte ions were evaluated and the results showed the capability of the method to real samples. Real samples were extracted by cadion modified activated carbon followed by elution using 5 mL of 2.0 M nitric acid solution. ASV measurements were performed on resulted 100 times enriched sample. Analysis of finger nail sample showed that it contained trace amount of Cd2? which were well above the limit of quantification of the analytical procedure (10 Sb/m). The real sample was spiked with reference standard solution of cadmium with 200.0 lg L-1 concentration to assess the accuracy and reliability of the method. For each measurement 0.1 g finger nail sample was digested in 15 mL of Nitric acid and

Determined concentration of cadmium (lg L-1)

Recovery (%)

hydrogen peroxide and diluted to 500 mL solution. The results are given in Table 3. Table 3, shows that the average result of the three replicate analyses of the real samples obtained by the SPE/ ASV method is satisfactorily in agreement with the amount of Cd2? added. RSD of the measurement was down on the spiked sample is 7.7 %. In conclusion, this work describes a very sensitive and selective double preconcentration/separation/accumulation procedure based on solid phase extraction of cadmium in human samples using activated carbon modified by a cadion ligand followed by anodic stripping voltammetry determination. The use of cadion as the adsorbent for cadmium preconcentration possessed several advantages such as high sample cleanup, efficiency, selectivity, simplicity, high preconcentration factor and low cost. The LOD and preconcentration factor of the method is comparable or better than several of the previously reported preconcentration methods. The proposed procedure was successfully applied for determination of cadmium at low concentrations in nail samples.

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