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Solid-phase Extraction on Magnetic Multi-walled Carbon Nanotubes Coupled with Flame Atomic Absorption Spectrometry for Determining Lead and Cadmium in Traditional Chinese Medicine.

Byline: Gui Fang Qin, Meng Yi Jiang, Hou Shu Mei, Yao Ke Rong and Jing Jun Ma

Summary

In this study, magnetic carbon nanotubes (MCNTs) were prepared by mixing the magnetic particles and multi-walled carbon nanotubes in dispersed solutions. These MCNTs were used as adsorbents of magnetic solid-phase extraction (MSPE). By coupling MSPE with flame atomic absorption spectrometry, a rapid and sensitive method for analyzing lead and cadmium using ammonium pyrrolidine dithiocarbamate as chelating reagent was established. Under optimal conditions, calibration graphs were linear in the range of 10.0400.0 g L-1 and 10.0300.0 g L-1 with detection limit of 0.6 g L-1 and 0.5 g L-1 for Pb and Cd, respectively. A good relative standard deviation for determining 300.0 g L-1 of Pb and Cd were 3.8 and 3.4%, respectively. The proposed method was applied to analyze several traditional Chinese medicine samples with satisfactory results.

Keywords: Multi-walled carbon nanotubes; Silica-coated magnetic nanoparticles; Magnetic solid-phase extraction; Cadmium; Lead

Introduction

Lead and cadmium are highly toxic and hazardous elements even at trace levels [1]. Thus, determination of lead and cadmium in traditional chinese medicine samples is of clear significance to human health. Various instrumental methods, such as spectrometry, inductively coupled plasma-optical emission spectrometry [2], inductively coupled plasma-mass spectrometry [3], electrothermal atomic absorption spectrometry [4], and flame atomic absorption spectrometry (FAAS) [5] are widely applied for determining trace amounts of Pb and Cd. In these instrumental determinations, low concentrations of analytes and complexity of matrices are the main problems [6]. Therefore, a preconcentration/separation procedure is required, such as liquid-liquid extraction [7], liquid-phase microextraction [8], coprecipitation [9], ion exchange [10], cloud-point extraction [11], or solid-phase extraction (SPE) [12, 13], before instrumental determination of metal ions.

SPE is a routine extraction method for trace levels of contaminants in complex samples. Multi-walled carbon nanotubes (MWCNTs) also exhibited strong sorption properties toward various compounds because of their high surface area and large micropore volume; thus, MWCNTs have been used as SPE adsorbents for the separation and preconcentration of trace analytes [14, 15]. However, some unavoidable difficulties occur when CNTs are applied to extract target compounds from large volumes of liquid samples [16]. When column dynamic extraction mode is used, water samples passing through nanoparticle-packed SPE columns consume much time because of high back pressure. When static batch mode is used, centrifugal separation is inapplicable for large volumes of samples, and filtration will encounter the same problem as that of column dynamic extraction mode [17].

By combining the advantages of MWCNTs and magnetic nanoparticles to fabricate nanosized SPE adsorbents with high surface area, high chemical stability, and good magnetic separability, a new kind of magnetic nanocomposite (MNP) sorbent can be obtained. Recently, Feng et al.,. used a simple method to immobilize MWCNT sheets onto magnetic particles by simple adsorption [16]. In their work, fabricated magnetic CNTs without chemical modification were used as magnetic solid-phase extraction (MSPE) adsorbents to successfully extract PAEs from beverage, environmental water, and perfume samples. Therefore, we extend its application to inorganic analysis, and have obtained a consistent conclusion.

This study aimed to develop a MSPE method for preconcentration and determination of trace amounts of Pb and Cd from traditional chinese medicine samples. These metal ions formed stable complexes with ammonium pyrrolidine dithiocarbamate (APDC), and the formed complexes were adsorbed on Fe3O4@SiO2/MWCNTs in a batch extraction procedure. The MNPs were then collected using an external magnetic field. Afterward, the extracted heavy metal ions were washed from the surface of the adsorbent and determined simultaneously using FAAS.

Experiment

A Shimadzu Model AA-6300C atomic absorption spectrometer (Shimadzu, Tokyo, Japan) equipped with hollow cathode lamps for lead and cadmium with a deuterium lamp for background correction, was used. The instrumental parameters were adjusted according to the manufacturer's recommendations. A pH3-3C digital pH meter equipped with a combined glasscalomel electrode (Hangzhou Dongxing Instrument Factory, Hangzhou, China) was used for pH adjustment.

MWCNTs were supplied by Shenzhen Nanotech Port Co. (Shenzhen, China) with greater than 93% purity, diameter between 40 and 60 nm and length of 10-20 m. Fe3O4 nanoparticles were prepared by chemical coprecipitation method [18]. Magnetic microspheres coated with silica layer (Fe3O4@SiO2) were synthesized according to previously reported methods [19, 20].

The fabrication process of magnetic MWCNTs was similar to the described procedure of Feng et al., [16]. Briefly, MWCNTs (25.0 mg) and Fe3O4@SiO2 (50.0 mg) were dispersed in DMF separately. After combining the two solutions, the MWCNTs and Fe3O4@SiO2 were dispersed homogeneously by vortexing vigorously for 1.0 min. Upon mixing, the MWCNTs and magnetic particles assembled to form magnetic MWCNTs. The resulting magnetic MWCNTs were washed with pure water and acetone in sequence, and then resuspended in 5.0 mL of water. The final concentration of the suspension solution was 15.0 mg mL-1.

Liquorice, cassia seed, dahurian angelica root and reed rhizome samples were purchased from medicinal materials market (Baoding, China). Each dried samples was homogenized in a stainless steel blender, and then weighed into a beaker. A total of 15.0 mL of concentrated HNO3 and 5.0 mL of H2O2 (30% w/w) were added, The solution was heated until transparent, and continuously evaporated until near dryness. The dissolved residue in 0.1 mol L-1 HNO3 was diluted with 50.0 mL of deionized water.

The SPE procedure was performed as follows: An aliquot of 50.0 mL of digestion solution or standard solution was prepared. APDC solution (0.5 mL, 2.0 g L-1) was added, and the pH value was adjusted to 4.0 with ammonium acetate buffer solution (0.2 mol L-1). Subsequently, 2.0 mL of the magnetic MWCNTs suspension was added to the solution, and the mixture was sonicated for 2.0 min. After equilibrium, an Nd-Fe-B strong magnet (10 mm A- 10 mm A- 50 mm) was deposited at the bottom of the bottle and the sorbents were isolated from the solution. After about 1.0 min, the solution became clear and the supernatant was decanted. Finally, 2.0 mol L-1 HNO3 was added to desorb the analytes (washed twice, 1.0 mL each time). The eluent was collected. After filtration through a 0.45 m membrane, the analyte ions in the eluent were determined by FAAS.

Results and discussion

The hydrophobic chelate of Cd and Pb formed with APDC from the aqueous phase and can easily interact with the CNTs, which increases extraction efficiency of the heavy metal ions [21]. The ligand amount is an important factor for the quantitative retentions of metal ions in SPE techniques [22]. To investigate the optimum amounts of ligand solution on the quantitative recoveries of the analyte ions on MCNTs, volume of APDC solution (2.0 g L-1) was varied from 0.0 -1.0 mL (Fig. 1). The recoveries of the analyte ions increased with increasing volume of added APDC and reached a constant value over 95% with at least 0.4 mL. The recovery values of the analytes were quantitative at the volume of ligand ranging from 0.40.6 mL. On this basis, further studies were conducted at an APDC volume of 0.5 mL. pKa value for APDC is 3.29 [23]. This value is consistent with the quantitative recovery values at the acidic pHs.

The influences of pH of the analyte solutions on the recoveries of Pb and Cd were investigated within pH 2.010.0. The quantitative recoveries (greater than 95%) for Pb and Cd were within pH 4.06.0. At the basic pH values, the recoveries were not quantitative. All subsequent studies were conducted at pH 4.0 with an ammonium acetate buffer solution.

In the present work, the Fe3O4@SiO2/MWCNTs showed an excellent capability to retain the heavy metal ions. Thus, quantitative extraction of the heavy metal ions was achieved using 30.0 mg of the Fe3O4@SiO2/MWCNTs (2.0 mL of the adsorbent suspension, the ratio of Fe3O4/SiO2 NPs and MWCNTs added was kept constant at 2:1). At higher amounts of the adsorbent, the extraction efficiency was almost constant. Therefore, in the subsequent experiments, 2.0 mL of the adsorbent suspension was added to 50.0 mL of the sample solution.

The adsorption of Pb and Cd at pHless than 4.0 could be incomplete (Fig. 2). Thus, nitric acid at different concentrations and volumes was studied for eluting the retained Pb and Cd from the surface of the adsorbent. A concentration of 2.0 mol L-1 nitric acid is required to obtain quantitative elution if the eluent volume is set to 2.0 mL.

The effect of potential ions, in real samples on the recovery of 300.0 g L-1 of each of Pb and Cd standard solution in the presence of various amounts of individual interfering ions was also examined. A given species was considered to interfere if it resulted in a 5% variation of the FAAS signal. All studied ions (Mn2+, Cr3+, Fe3+, Cu2+, Zn2+, Co2+, Ni2+, and Hg2+) did not affect the absorbance in the MSPE-FAAS system when they are present in 100-fold excess. Higher concentrations of alkali and alkaline earth metals (K+, Na+, Ca2+, and Mg2+) can be tolerated. The above results indicated that the developed method is applicable to the analysis of Pb and Cd in traditional chinese medicine samples.

The limit of detection (LOD) was calculated as the ratio of thrice the standard deviation of the blank signals over the slope of the calibration curve. The LOD values of 0.6 and 0.5 g L-1 were obtained for Pb and Cd, respectively. For a sample volume of 50.0 mL, the calibration graph exhibited linearity over the range of 10.0-400.0 g L-1 for Pb and 10.0-300.0 g L-1 for Cd. The relative standard deviation (RSD) for ten replicate measurements of solutions containing 300.0 g L-1 each of Pb2+ and Cd2+ was 3.8 and 3.4%, respectively.

The proposed method was applied to determine of Pb and Cd in traditional chinese medicine samples. The results and the recovery for the spiked samples are listed in Table-1. The recoveries for the four spiked samples range within 90.0%98.5%.

Table-1: Determination of Pb and Cd in traditional chinese medicine samples using MSPE-FAAS method (n=3).

###Sample###Spiked (g L-1)###Found (g L-1)a###Recovery (%)

###Pb###Cd###Pb###Cd###Pb###Cd

###0.0###0.0###NDb###3.60.2###

###Liquorice###10.0###10.0###9.00.4###13.20.6###90.0###96.0

###20.0###20.0###19.30.8###23.31.2###96.5###98.5

###0.0###0.0###ND###ND###

###Cassia Seed###10.0###10.0###9.10.4###9.30.4###91.0###93.0

###20.0###20.0###19.40.8###19.40.9###97.0###97.0

###Dahurian###0.0###0.0###ND###6.40.3###

###Angelicaroot###10.0###10.0###9.10.4###16.10.7###91.0###97.0

###20.0###20.0###19.40.8###26.11.2###97.0###98.5

###0.0###0.0###4.10.2###ND###

###Reed Rhizome###10.0###10.0###13.50.5###9.30.4###94.0###93.0

###20.0###20.0###23.81.1###19.50.8###98.5###97.5

Conclusion

A magnetic composite of MWCNTs and Fe3O4@SiO2 was prepared by simple adsorption. The performance of magnetic CNTs for MSPE was evaluated by enriching Pb and Cd. Under optimized conditions, a rapid and sensitive method for determining Pb and Cd from traditional chinese medicine samples was established by the coupling of MSPE with FAAS. Compared with traditional SPE method, MSPE-FAAS avoids the time-consuming column passing and filtration operation. This method also shows a significant analytical potential in separating and preconcentrating target analytes from complex samples.

Acknowledgements

This project was sponsored by Hebei Province Science and Technology Support Project (No. 13227124); Science Foundation of the Department of Education of Hebei Province (No. ZD2014041); and Science Foundation of the Agricultural University of Hebei (No. LG201305).

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Author:Gui Fang Qin; Meng Yi Jiang; Hou Shu Mei; Yao Ke Rong; Jing Jun Ma
Publication:Journal of the Chemical Society of Pakistan
Article Type:Report
Date:Apr 30, 2015
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