Toward use of a nano layered double hydroxide/ammonium pyrrolidine dithiocarbamate in speciation analysis: One-step dispersive solid-phase extraction of chromium species in human biological samples

Behruz Barfi; Alireza Asghari; Maryam Rajabi

doi:10.1016/j.arabjc.2017.06.002

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Original article

13 (

); 568-579

doi:

10.1016/j.arabjc.2017.06.002

Toward use of a nano layered double hydroxide/ammonium pyrrolidine dithiocarbamate in speciation analysis: One-step dispersive solid-phase extraction of chromium species in human biological samples

Behruz Barfi, Alireza Asghari^⁎, Maryam Rajabi

Department of Chemistry, Semnan University, Semnan 35195-363, Iran

⁎Corresponding author. Fax: +98 023 33654110. aasghari@semnan.ac.ir (Alireza Asghari)

Received: 2017-2-22, Accepted: 2017-6-4, Published: 2017-01-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Chromium is a controversial element with an important essentiality and toxicity. Depending on its different species, its speciation analysis in bio-origin matrices is of utmost importance. Ammonium pyrrolidine dithiocarbamate (APDC) and layered double hydroxide (LDH) nanoparticles, with the purpose of combining their outstanding performances, were served to speciate chromium ions in human biological samples. The as-obtained sorbent (nano LDH-APDC) - after characterization by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, thermal gravimetric analysis (TGA), and scanning electron microscopy (SEM) - was used as a novel pH-sensitive adsorbent in an integrated one-step dispersive solid-phase extraction (I-OS-DSPE), which combines the benefits of the air-assisted microextraction and dispersive solid-phase extraction methods. An interesting feature of the nano LDH-APDC sorbent is that it is dissolved in an aqueous solution when the pH of the solution is lower than 4. Thus the analyte elution step, as required in most of the sorbent-based extraction methods, was obviated by dissolving the sorbent in an acidic solution after extraction and separation from the sample solution. The Cr(VI) ions were first extracted, while the Cr(III) ions remained in the aqueous solution. The extract was then directly injected into a flame atomic absorption spectrometer with a micro-sampling introduction system, and the concentration of the Cr(III) ions was calculated by its subtraction from the total chromium ions present. Several variables including the pH (5), type and amount of the nanosorbent used (30 mg of nano (Zn-Al) LDH-APDC), number of extraction cycles (15 times), and elution conditions (200 µL of 6.0 mol L⁻¹ HNO₃) were investigated to achieve the maximum extraction efficiency. Under the optimum experimental conditions, the limit of detection, linear range, consumptive index, and enrichment factor for the Cr(VI) ions were 2.4 μg L⁻¹, 8.0–640 μg L⁻¹, 0.24, and 42.5 ± 1, respectively. These findings suggested that nano (Zn-Al) LDH-APDC could be regarded as a promising adsorbent for an efficient speciation of the chromium species in the human hair, nail, saliva, plasma, and urine samples.

Keywords

Nanoparticle

Speciation

Integrated

Microextraction

Bio-origin

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1 Introduction

Nowadays, there is a growing need to identify the chemical nature of the relevant element species and to determine their exact quantities (Pereira et al., 2012; Timerbaev, 2012). In many cases, one form of a metal or metalloid can be toxic, whereas the same metal when in a different form is nontoxic and even necessary for sustaining an ecosystem or the efficient functioning of a living organism (Guerrini et al., 2014). It is only over the last two decades or so that the arsenal of experimental technologies and tools of analytical chemistry have been developed to acquire the separation/detection sophistication and power required to distinguish and measure the species often occurring at trace levels and in a complex matrix environment (Galán-Cano et al., 2011; Kang et al., 2015). Among the multi-valence elements, chromium (Cr) is important in view of some health risks (Li et al., 2009). It may occur in valence states ranging from +2 to +6, from which the Cr(III) and Cr(VI) species are of primary environmental interest due to their more stability. Compounds containing Cr(VI) are believed to be responsible for most of the health problems such as chromosomal aberration, mutations, carcinogenicity, transformation in cultured cells, and a variety of DNA lesions (Zhitkovich, 2005, 2011).

In order to make the selective measurement of chromium species reliable, an analytical technique should meet a number of requirements including minor impact on the original distribution of the chromium species in the sample, good tolerance to complex matrices, practicability in terms of easy implementation, low running costs, minute sample volumes (important for certain bio-samples), high throughput, and minimum waste requirements.

To achieve the above-mentioned purposes, application of an integrated one-step dispersive solid-phase extraction (I-OS-DSPE) method applying modified nano layered double hydroxides (nano LDHs) with a selective chelating agent can be a suitable choice.

DSPE is a modified version of SPE that considerably reduces the time consumed and simplifies the extraction process. In this method, extraction is not carried out in a cartridge, column or disk but in the bulk solution, which leads to an increase in the active surface area between the analytes. Thus using a relatively less amount of adsorbent (several mg), the analytes can be efficiently extracted in a shorter time and the adsorbent containing the analytes can be easily isolated from the sample solution after extraction. In general, the DSPE method consists of two critical steps: (i) dispersion and (ii) phase separation. The first step is usually assisted by an external energy source, and therefore, a special apparatus such as ultrasonic or vortex is required. The second step is performed by centrifugation, which is very effective but makes the overall procedure time-consuming. In this sense, the development of an I-OS-DSPE method in which the use of external apparatus and centrifugation step are avoided can pave the path (Galán-Cano et al., 2011; Lasarte-Aragonés et al., 2011, 2013 ). However, to perform an I-OS-DSPE method, the nature and properties of the adsorbent are of prime importance (Barfi et al., 2015). The adsorbent used must be dissolvable, and can speciate the chromium species selectively. With a selective adsorbent, I-OS-DSPE can be applied as a convenient sample preparation method that can enrich the analytes while minimizing the matrix effects effectively.

LDHs represent an important class of host-guest materials. They contain positively charged layers including edge-shared metal M(II) and M(III) hydroxide octahedral, with charges neutralized by the anions located in the interlayer spacing or at the edges of the lamella. Recently, owing to their low cost and excellent capability, the adsorbents based on nano LDHs have received much attention on catalysis, scavengers, adsorbents, photochemistry, electrochemistry, and pharmaceutics (Chen et al., 2011; Shao et al., 2012; Teramura et al., 2012 ). Due to the presence of large interlayer spaces and a significant number of exchangeable anions, nano LDHs have the potential to be good ion-exchangers and adsorbents. However, nano LDHs with highly variable physico-chemical properties can be synthesized by manipulating metal ions and intercalated anions and coupling with other materials. That is why nano LDHs can have widespread applications in the extraction of inorganic as well as organic analytes. Since nano LDHs can be easily dissolved in acidic solutions, after the extraction step, they can be considered as suitable adsorbents in the I-OS-DSPE method.

With chelating agents such as ammonium pyrrolidine dithiocarbamate (APDC), the Cr(III) and Cr(VI) ions can form different complexes. At different pH values, Cr(VI) ions react with APDC easily and quickly. They are reduced to Cr(III) ions, which form two different complexes, bis[-pyrrolidine-1-dithioato-S,S′]-[pyrrolidine-1-peroxydithioato-O,S]-Cr(III) and tris[pyrrolidine-1-dithioato-S,S′]-Cr(III) (Krishna et al., 2004). Both complexes can be completely extracted at two different pH values (Tunçeli and Türker, 2002).

In summary, while extensive studies have been conducted on the behavior of adsorption of anionic pollutants onto nano LDHs, little information is available on the use of these materials as adsorbents for heavy metals. In the case of this type of analyte, the nano LDH adsorbent properties should be improved by modifying the interlayer through intercalating large organic anions (organo-nano LDHs), which could provide the media for their efficient extraction.

The aim of this work was to study the possibility of using nano LDH intercalated with APDC, nano LDH-APDC, as a novel adsorbent for a simple and fast speciation of Cr(VI) ions in bio-origin samples. To the best of our knowledge, there is no report on the use of nano LDH-APDC as an adsorbent for the chromium speciation. The effects of several parameters on the extraction efficiency of Cr(VI) ions were systematically investigated and optimized, applying the I-OS-DSPE method. The nano LDH-APDC/I-OS-DSPE method was successfully applied to the determination of Cr(VI) ions in the human hair, nail, saliva, plasma, and urine samples.

2 Experimental

2.1

2.1 Standard solutions and reagents

All the reagents used were of the highest analytical reagent grade, supplied from Merck (Darmstadt, Germany). The stock standard solutions of the Cr(III) and Cr(VI) ions (1000 mg L⁻¹) were prepared, separately, by dissolving appropriate amounts of Cr(NO₃)₃ and K₂CrO₄, respectively, in deionized water. The required working standard solutions were freshly prepared by the series dilutions of the stock solutions with deionized water to the required concentrations. The chelating agent (ammonium pyrrolidine dithiocarbamate), obtained from Merck (Darmstadt, Germany), was dissolved in deionized water.

Zinc nitrate hexahydrate (Zn(NO₃)₂⋅6H₂O) (98%) and aluminum nitrate nonahydrate (Al(NO₃)₃⋅9H2O) (98%), used for the synthesis of the sorbent, were supplied from Merck (Darmstadt, Germany). The pH values for the sample solutions were adjusted with diluted nitric acid and sodium hydroxide solutions (0.1 M) to the desired ones. The laboratory glassware was kept at least overnight in 10% (v/v) nitric acid solution, and subsequently, rinsed with deionized water prior to use. Acrodisc® Minispike syringe filter PTFE membrane with pore size of 0.2 μm was obtained from Sigma–Aldrich.

2.2

2.2 Preparation of calibration standards and quality controls

A stock solution of Cr(VI) ions (10,000 µg mL⁻¹) was prepared in ultrapure water and stored at 4 °C. The working solutions were prepared by serial dilution in ultra-pure water.

Two levels of quality control (QC) materials were prepared in-house by spiking the biological samples with the working solutions. The samples were spiked with the working solutions to yield the final concentrations of 10.0, 200.0, 300.0, 400.0, 500.0, 600.0, 700.0, and 800.0 µg mL⁻¹ of Cr(VI) ions (ranging from low QC to high QC). The QC samples were stored at −20 °C.

2.3

2.3 Instrumentation

An Agilent 200 Series AA (model 240 AA) flame atomic absorption spectrometer (USA) including air–acetylene flame and simultaneous four hollow cathode lamps was applied for the evaluation and determination of the metal ions. The hollow cathode lamp of Cr was employed as the radiation source. The instrumental parameters were adjusted as wavelength, Cr, 357.9 nm (slit width of 0.2 nm) and lamp current of 10.0 mA. The pH values were adjusted using a PHS-3BW model pH-meter (Bell, Italy), supplied with a combined glass electrode. An electronic analytical balance (Shimadzu LIBROR AEU-210) was used for weighting the solid materials. Fourier transform infrared (FT-IR) spectra (4000–400 cm⁻¹), used to illustrate the chemical structure changes, were obtained on a Shimadzu FT-IR spectrometer, model 8400 (Japan), using freshly-dried standard KBr disk with a sample/KBr ratio of 1:100 by mass. Also in order to obtain a better insight into the structural properties of LDH, the X-ray diffraction (XRD) patterns were obtained for LDH with a Bruker-D8 advance X-ray powder diffractometer using a Ni-filtered Cu Kα radiation source (λ = 0.154 nm) operating at 40 kV and 30 mA. The energy dispersive X-ray fluorescence (ED-XRF) spectra were recorded on an EMAX instrument (Horiba, Japan). Analysis of the thermal behavior of the compounds before and after intercalation of the ligands into LDHs was recorded on a LINSEIS TG/DTA (STA PT 1600, Germany) thermo-gravimetric analyzer with a heating rate of 5 °C min⁻¹, from room temperature to 1000 °C. The morphological characterization of the synthesized nanosorbent was carried out by a field emission scanning electron microscope (FESEM), model Sigma (Zeiss, Germany).

2.4

2.4 Preparation of ammonium pyrrolidine dithiocarbamate ligand intercalated into LDHs

The ligand intercalated into Zn–Al–NO₃⁻ with a Zn²⁺/Al³⁺ molar ratio of 2:1 was synthesized via the co-precipitation method with controlled pH (pH 10), followed by hydrothermal treatment. In the present work, an aqueous solution (50 mL) containing NaOH (0.5 g) and ammonium pyrrolidine dithiocarbamate (APDC) (0.5 g) was added dropwise to a solution (100 mL) containing Zn(NO₃)₂⋅6H₂O (1.79 g) and Al(NO₃)₃⋅9H₂O (1.12 g) under nitrogen atmosphere to avoid contamination by atmospheric CO₂ with a constant vigorous stirring at room temperature for 2 h. Then the slurry obtained was treated in a hydrothermal vessel at a constant temperature of 100 °C for about 6 h. At the end, the resulting precipitate, nano (Zn-Al) LDH-APDC, was separated, washed with deionized water for several times until the pH of 7, and finally, dried at 80 °C in a vacuum oven for 18 h.

2.5

2.5 Sample preparation

Human bio-monitoring has become a valuable tool for investigation of the doses of toxic elements in human health. Blood and urine samples are the most widely used and accepted ones as matrices to diagnose exposure to toxic elements. Other biological samples such as saliva, hair, and nails have applicable advantages over the traditional ones in the estimation of the dosage of toxic elements in human body since they include easy accessibility, good stability of matrices, simple collection and transportation, lack of the requirement to special storage conditions, and repeated determinations over time (Gil and Hernández, 2015).

2.5.1

2.5.1 Human urine

The urine sample was collected from a healthy volunteer in a pre-washed polyethylene bottle. Then 2.5 mL of concentrated HNO₃ was added to 47.5 mL of the sample. The sample was filtered through a 0.45 μm pore size cellulose acetate filter, and adjusted to pH 5.0 before use. Then 10 mL of the supernatant solution was submitted to the proposed method.

2.5.2

2.5.2 Human blood

Before extraction, in order to precipitate proteins, 12 mL of the plasma sample was mixed with 0.6 mL of concentrated HNO₃. Then 2.0 mL of concentrated HCl was added to 10 mL of the deproteinized supernatant solution. The solution obtained was filtered, and after pH adjustment to 5.0, 10 mL of it was submitted to the proposed method.

2.5.3

2.5.3 Human saliva

The saliva samples were gathered for 10 min with the mouth closed, introduced into a polypropylene tube, and centrifuged at 10,000 rpm for 5 min in order to remove any sediment cellular debris. Then 10 mL of the supernatant solution was submitted to the proposed method after pH adjustment to 5.0.

2.5.4

2.5.4 Human hair and nails

In order to remove any surface dirt and grease, the human hair and nail samples were first soaked in a mixture solution with the diethyl ether:acetone:deionized water ratios of 3:1:20 (v/v/v) and 3:2:5 (v/v/v) for hair and nail, respectively, for 1 h in an ultrasonic bath, rinsed thoroughly with deionized water, and dried overnight at 110 °C. Then for sample decomposition, a 10-mL aliquot of concentrated HNO₃ was added to the accurately weighed samples (2 g). The solutions were evaporated at about 150 °C. Then 2 mL of concentrated HClO₄ or H₂O₂ was added, and the solutions were evaporated again. The residues were brought to a final volume of 50 mL with doubly distilled water, and its pH value was adjusted to 5.0 before use.

2.6

2.6 Extraction procedure

The extraction was performed by adding 30 mg of nano LDH-APDC to 10 mL of the model sample (containing 100 μg L⁻¹ of Cr(III) and Cr(VI) ions) in a 15 mL glass centrifuge tube. The pH values for the sample solutions were adjusted to 5.0 using a solution of HNO₃ (0.01 M) or NaOH (0.01 M). The mixture was rapidly withdrawn and pushed out into the tube (for 15 times, 60 s) using a gas-tight syringe needle to facilitate the process of adsorption of the metal ions onto the nanosorbent. After the extraction process, the whole volume of the sample solution and nanosorbent was aspirated into the syringe. Then the LDH nanoparticles were separated from the solution through a syringe filter, which was inserted to the tip of the syringe. Here, the use of a syringe filter eliminated the need for a centrifugation step. In this stage, in order to dissolve the nanosorbent containing the analytes, 200 μL of HNO₃ (6 mol L⁻¹) was sucked into the syringe. The plunger of the syringe was slowly moved for several times to ensure that the nanosorbent was completely dissolved. Finally, the collected eluent (100 μL) was transferred into a micro-tube, and then injected, using a micro-pipette, into a homemade micro-injection aspiration system coupled with an FAAS nebulizer to determine the concentration of Cr(VI) ions as peak areas with a 3 s integration time. The concentration of Cr(III) ions was calculated by their subtraction from the total chromium content.

2.7

2.7 Calculation of enrichment factor, absolute and relative recoveries, and consumptive indices

The enrichment factor (EF), absolute recovery (extraction recovery, ER), and relative recovery (RR) for the analytes were used as the parameters to evaluate the method. EF was calculated by Eq. (1).

(1)

EF = \frac{C_{inj}}{C_{0}}

where

C_{inj}

is the concentration of the analytes in the injected eluent (100 µL) and

C_{0}

is the initial concentration of the analytes within the sample solution.

ER was calculated by Eq. (2).

(2)

ER = \frac{n_{inj}}{n_{0}}

where

n_{inj}

is the amount of the analytes present in the extractant phase and

n_{0}

is the initial amount of the analytes within the sample solution. This type of recovery was used in the optimization process.

RR was calculated by Eq. (3).

(3)

RR = \frac{C_{found - C_{real}}}{C_{added}} \times 100 %

where

C_{found}

represents the concentration of the analytes after adding a known amount of the standard to the real samples,

C_{real}

is the concentration of the analytes in the real samples, and

C_{added}

refers to a standard solution that was spiked in the real samples.

Consumptive index (CI) is defined as Eq. (4):

(4)

CI = \frac{V_{s}}{EF}

where

V_{s}

is the required volume of the sample (in mL) to achieve one unit of EF. Lower CIs mean that higher enrichments could be achieved using lower required volumes of the sample. It is an interesting parameter used to compare the methods using different sample volumes.

3 Results and discussion

3.1

3.1 Characterization of nanosorbent

A set of characterizations were performed in order to get a better insight into the structural properties of the adsorbent.

3.1.1

3.1.1 FT-IR spectra

The FT-IR spectra for (Zn-Al-NO₃)-LDH, APDC, and (Zn-Al)-LDH-APDC are shown in Fig. 1. The significant characteristics of the compounds are specified in Table 1.

IR spectra for (a) (Zn-Al-NO3)-LDH, (b) APDC, and (c) (Zn-Al)-LDH-APDC.

Table 1 Significant FT-IR bands of (Zn-Al-NO₃)-LDH, APDC, and (Zn-Al)-LDH-APDC.

(Zn-Al)-LDH	APDC	(Zn-Al)-LDH-APDC
Stretching band (OH): 3585 cm⁻¹	Stretching bands (OH): 948 cm⁻¹ and 970 cm⁻¹	Stretching bands (N-C in N-CSS): 1384–1481 cm⁻¹
Bending band (H-O-H): 1641 cm⁻¹	Stretching bands (N-C in N-CSS): 1371–1596 cm⁻¹	Aromatic stretching bands (C-N): 1006–1249 cm⁻¹
Stretching band (NO₃^-): 1367 cm⁻¹	Aromatic stretching bands (C-N): 1000–1350 cm⁻¹	Stretching bands (C-S): 945 cm⁻¹
Stretching bands (M-O) and (M-O-H): 430 cm⁻¹, 685 cm⁻¹ and 835 cm⁻¹	Aromatic stretching bands (C-H): 2893 cm⁻¹	Stretching bands (M-O) and (M-O-H): 424 cm⁻¹, 825 cm⁻¹ and 966 cm⁻¹
	Stretching bands (N-H): 3300–3650 cm⁻¹	Stretching band (C-H): 2869–2972 cm⁻¹
	Stretching bands (N-H): 3300–3650 cm⁻¹	Stretching bands (N-H): 3471 cm⁻¹

3.1.2

3.1.2 ED-XRF spectra

ED-XRF is a very powerful technique used for characterizing the structure of materials. Fig. 2 shows the ED-XRF pattern for (Zn–Al)-LDH. The characteristic reflections of the (0 0 3), (0 0 6), (0 0 9), (0 1 2), (1 1 0), and (1 1 3) planes of a crystalline LDH can be observed. In the zone close to 2θ = 60–62°, the typical doublet of the (1 1 0)–(1 1 3) planes of LDH was also observed. (Zn–Al)-LDH exhibited the characteristic reflections of hydrotalcite-like LDH, and no other crystalline phases were present, which is in agreement with the results reported by other researchers (Liu et al., 2012).

ED-XRF patterns for (a) (Zn-Al-NO3)-LDH and (b) (Zn-Al)-LDH-APDC.

The highly crystalline well-ordered layer similar to the hydrotalcite-type structure and the high purity of the samples can be justified by the appearance of symmetrical and asymmetrical diffraction peaks at low and high 2θ values, respectively. The relatively low increment of the (Zn-Al)-LDH-APDC basal spacing toward its precursor and lower intensity indicated that most anions were located on the surface of the LDH particles and some of them were intercalated near the (Zn-Al)-LDH edges in a parallel position to the layers.

3.1.3

3.1.3 FESEM pattern

With respect to the FESEM images for (Zn-Al)-LDH (Fig. 3a), basic particles have well-shaped regular structures in plate-like patterns. As it can be seen, the (Zn-Al)-LDH particles, in comparison with the (Zn-Al)-LDH-APDC particles (Fig. 3b), possess a limited-size distribution with more regular edges. In fact, due to the higher hydrophobicity and lower charges present in the plates with intercalation of the ligand, the particles in nano (Zn-Al)-LDH-APDC grow and form the slightly larger agglomeration of the LDHs platelets (Fig. 4).

FESEM images for (a) (Zn-Al-NO3)-LDH and (b) (Zn-Al)-LDH-APDC.

Particle size distribution of (a) nano (Zn-Al-NO3)-LDH and (b) nano (Zn-Al)-LDH-APDC.

3.1.4

3.1.4 TGA curves

For (Zn-Al)-LDH, the TGA curve (Fig. 5a) shows three stages of weight loss with endothermic processes. The first, second, and third stages are related to desorption of the absorbed water, dehydroxylation of the brucite-like layer, and decomposition of the interlayer anion, respectively. The thermal decomposition characteristics of the resulting product after intercalation of the ligand APDC into the (Zn-Al)-LDH structure are different from its precursor. The TGA curve for nano (Zn-Al)-LDH-APDC (Fig. 5b) shows four weight losses. The first, second, third, and fourth ones are attributed to the elimination of the adsorbed surface water, elimination of the interlayer water, dehydroxylation of the brucite-like layer, and combustion of the APDC ligand, respectively.

TGA curves for (a) APDC, (b) Zn-Al-NO3-LDH and (c) Zn-Al-APDC-LDH.

3.1.5

3.1.5 FAAS evaluation

Flame atomic absorption spectrometry (FAAS) was used to assess the atomic composition of (Zn-Al)-LDH. For this purpose, 30 mg of LDH was dissolved with a few drops of concentrated HNO₃ and diluted to 50 mL with deionized water. Zn and Al analyses were then performed by FAAS after appropriate dilution with deionized water. According to the results obtained, the Zn²⁺:Al³⁺ ratio (1.95:1) is in agreement with the expectations considering the proportion of metal salt precursors used in the LDH synthesis (2:1).

3.1.6

3.1.6 ED-XRF

In order to assign and compare the chemical composition of the adsorbents before and after the intercalation of the ligand, the ED-XRF analysis was also used. The elemental mapping obtained indicated that nano (Zn-Al)-LDH consisted of the Zn, Al, N, and O elements. Furthermore, the appearance of sulfur atom in the ED-XRF spectrum of the (Zn-Al)-LDH-APDC nanosorbent could be a good testimony for the successful intercalation of the ligand in the structure of nano (Zn-Al)-LDH.

3.2

3.2 Method optimization

The method is based upon withdrawing and pushing out a mixture of the sample solution and the adsorbent in a glass tube using a single syringe. This simple action leads to an efficient dispersion of the adsorbent in the sample solution as well as the formation constant of the metal-ligand complex at a specific pH value. In this way, any parameter that can improve the phenomenon must increase the extraction of species in terms of the analytical features. In order to evaluate the method applicability and achieve the best extraction efficiency, the effective parameters including pH of the sample solution, type and amount of the adsorbent, number of extraction cycles, and elution conditions were investigated.

3.2.1

3.2.1 pH of sample solution

The pH of sample solution is one of the most significant operational parameters that plays an important role in the selective chelation reaction between the chromium ions and the chelating agent in the nano LDH-APDC/I-OS-DSPE method. An appropriate pH value not only improves the adsorption efficiency but also decreases the interference of the co-existing ions. The effect of pH on the efficiency of extraction of the Cr(VI) and Cr(III) ions was studied, and the results obtained were shown in Fig. 6. As it could be seen, while the extraction was quantitative (>90%) at pH 5.0 for Cr(VI) ions, the extraction of Cr(III) ions was negligible. The non-extractability of Cr(III) ions can be ascribed to the difficulty of displacing the coordinated water from the strongly hydrated Cr(III) ions by the APDC ligand. This makes it possible to separate Cr(VI) ions from Cr(III) ions through the control of solution pH. It should be noted that due to the high solubility of LDHs in strong acidic media (pH < 4), the study of values lower than this pH was avoided.

Effect of pH on extraction efficiency of 50 µg L−1 of Cr6+ (n = 3). Experimental conditions: “amount of nanosorbent, 30 mg; eluent, 200 µL of 6.0 mol L−1 of HNO3; number of extraction cycles, 20 cycles, 80 s”.

3.2.2

3.2.2 Type and amount of adsorbent

LDHs have relatively weak interlayer bonding and, consequently, various kinds of inorganic or organic anions could be introduced into the hydroxide interlayer by simple ion exchange reaction or surface adsorption. Primary experiments showed that (Zn–Al) LDH could be used as a precursor of nano (Zn–Al) LDH-APDC for I-OS-DSPE and speciation of chromium ions from bio-origin samples (Abdolmohammad-Zadeh et al., 2011).

Since the type of interlayer anion is important and can affect the extraction efficiency due to the resulting interlayer space, three (Zn–Al) LDHs with different interlayer anions including Cl⁻, NO₃⁻, and CO₃²⁻ were synthesized and intercalated with APDC for speciation of the chromium species. Based on the results obtained, the highest recovery was achieved in the case of the NO₃⁻ interlayer anion, and (Zn–Al-NO₃⁻) LDH was then used as a precursor of nano (Zn–Al) LDH-APDC in the further experiments.

In order to investigate the effect of the amount of adsorbent on the extraction efficiency (speciation) of the Cr(VI) ions, the extraction was conducted by adding different amounts of the adsorbent within the range of 10–50 mg. The results obtained are given in Fig. 7. It was found that the extraction efficiency gradually increased with increase in the amount of adsorbent, reached the maximum value of 30 mg, and remained constant thereafter. With increase in the amount of the adsorbent including APDC, more binding sites were available to form complexes with the Cr(VI) ions. Regarding these results, 30 mg of the adsorbent was adopted as the most favorable value for the rest of the work.

Effect of amount of nanosorbent on extraction efficiency of 50 µg L−1 of Cr6+ (n = 3). Experimental conditions: “pH of sample solution, 5; eluent, 200 µL of 6.0 mol L−1 of HNO3; number of extraction cycles, 20 cycles, 80 s”.

3.2.3

3.2.3 Number of extraction cycles

The dispersion phenomenon can accelerate the possible contact between the adsorbent and the sample solution, and accessible surface areas of the adsorbent are achieved in a shorter period of time. In this way, it is predictable that by increasing the number of extraction cycles, the recovery should also increase. However, when constant amounts of the adsorbent and sample are used, the recoveries remain constant after reaching the equilibrium status. In this way, the extraction cycles were repeated for 5–30 times. The results obtained showed that with increase in the cycles, the extraction efficiency increased till the 15th cycle and then remained constant. Hence, a 15 times extraction cycle (∼60 s) was selected in the subsequent experiments (Fig. 8).

Effect of extraction cycle on extraction efficiency of 50 µg L−1 of Cr6+ (n = 3). Experimental conditions: “amount of nanosorbent, 30 mg; pH of sample solution, 5; eluent, 200 µL of 6.0 mol L−1 of HNO3”.

3.2.4

3.2.4 Type, concentration, and volume of eluent

The dissolution of LDHs immediately after extraction by pH adjustment and releasing the adsorbed analytes into the solution is a superior advantage, leading to a great improvement in the extraction recovery and a decrease in the analysis time. Since LDHs are dissoluble in acidic solutions (pH < 4), different acids such as HNO₃, H₃PO₄, H₂SO4, CCl₃COOH, and HCl were examined as the eluent. The results obtained showed that at a prefixed volume, HNO₃ dissolved the adsorbent more effectively in a shorter time period.

In theory, a higher acid concentration leads to a faster and more complete dissolution of the adsorbent, resulting in a higher extraction efficiency. The concentration of eluent was studied in the range of 2.0–10.0 mol L⁻¹. The results obtained showed that at a prefixed volume, with 6.0 mol L⁻¹ of HNO₃ the best recoveries were achieved. However, when the acid concentration was larger than 6.0 mol L⁻¹, the results obtained showed no significant improvement.

Selection of the best conditions was continued to obtain the maximum recovery with a minimal volume of the eluent. At volumes lower than 100 µL, the recovery of the Cr(VI) ions was not quantitative because of an insufficient eluent volume. The results obtained revealed that 200 µL of 6.0 mol L⁻¹ HNO₃ provided the best elution conditions for the subsequent experiments. It should be noted that half of the eluent (100 µL) was collected and injected to FAAS with a homemade micro-sampling injection system (MS-FAAS).

3.3

3.3 Analytical performance of method

The analytical characteristics of the proposed method such as linearity, limits of detection, limits of quantitation, and repeatability were calculated under the optimized conditions (Table 2). The linearity of the method was calculated by analyzing the fortified samples (from 8.0 to 640 μg L⁻¹). For each ion, the eight-point calibration curve was found to have a good linearity. Limits of detection (LODs) and limits of quantification (LOQs) were calculated as 3Sb/m and 10S_b/m, respectively, where S_b is the standard deviation of the blank and m is the slope of the calibration graph. The precision of the proposed method, as determined by the coefficient of method variations, was expressed by studying the repeatability (intra-day and inter-day precisions). The intra-day precision was evaluated by analyzing five replicates of the QC samples at three concentrations on the same day, and the inter-day precision was established by analyzing five replicates of the QC samples at three concentrations in three consecutive days. The precision studies showed a suitable relative standard deviation (RSD) for the chromium species.

Table 2 Analytical characteristics of method at optimum conditions.

Analyte	LOD (μg L⁻¹)	LDR (μg L⁻¹)	Intra-day precision	Inter-day precision	EF⁽ⁿ⁼³⁾
Cr⁶⁺	2.4	8.0–640	3.6	4.0	42.5 ± 1

Experimental conditions: “amount of nanosorbent, 30 mg; pH of sample: 5; eluent, 200 µL of 6.0 mol L⁻¹ of HNO₃; number of extraction cycles, 15 cycles, 60 s”.

3.4

3.4 Potentially interfering ions

The interference effect (synergistically and antagonistically) of the other cations and anions on the method performance was also examined. The interference was considered to occur when the measured recoveries varied more than ±5% relative to those for the target ions. In this effort, some model solutions containing 50.0 µg L⁻¹ of the standard solution of Cr(VI) were fortified with increase in the amount of potentially interfering ions, selected on the basis of their common occurrence in real samples. The results obtained indicated that the method could be applied to the real samples containing the chromium species since it was not affected by high concentrations of the alkali and alkaline earth ions and other transition metal ions (Table 3). It can be seen that the method has an excellent selectivity and is suitable for the analysis of biological samples with complex matrices.

Table 3 Influence of interfering species on recoveries of Cr⁶⁺ at optimum conditions.

Interfering ion	Concentration (mg L⁻¹)	Added as	C_Interference/C_Ion
Na⁺	500	NaCl	10,000
K⁺	500	KCl	10,000
Ca²⁺	50	CaCO₃	1000
Ba²⁺	50	BaCl₂	1000
Li⁺	500	LiNO₃	10,000
Mg²⁺	500	Mg (NO₃)₂·6H₂O	10,000
NH₄⁺	500	NH₄NO₃	10,000
Cu²⁺	50	Cu(NO₃)₂·6H₂O	1000
Zn²⁺	50	Zn(NO₃)₂·6H₂O	1000
Fe³⁺	50	FeCl₃·6H₂O	1000
Ag⁺	50	AgNO₃	1000
Mn²⁺	25	Mn(NO₃)₂·6H₂O	500
Al³⁺	500	Al(NO₃)₃·9H₂O	10,000
F⁻	500	NaF	10,000
Cl⁻	500	NaCl	10,000
Br⁻	500	NaBr	10,000
CH₃COO⁻	200	CH₃COONa	4000
SO₄²⁻	50	Na₂SO₄	100
CO₃²⁻	200	Na₂CO₃	4000
NO₃⁻	500	NaNO₃	10,000

Experimental conditions: “amount of nanosorbent: 30 mg; pH of sample: 5; eluent: 200 µL of 6.0 mol L⁻¹ of HNO₃; number of extraction cycles: 15 cycles, 60 s”.

3.5

3.5 Application of nano LDH-APDC/I-OS-DSPE to analysis of real samples

The proposed method was applied to the extractive speciation of the Cr(VI) ions in different bio-origin samples including human hair, nail, saliva, plasma, and urine samples prior to their determination using the MS-FAAS technique. For analysis of the samples, the standard addition method was used, and the analytical results were tabulated in Table 4. As it can be seen, the satisfactory agreement obtained between the added and measured amounts of the interested ions indicates the capability of the method for determination of the chromium species in different samples.

Table 4 Levels of chromium ions in bio-origin samples.

Sample	Cr⁶⁺
	Added (µg L⁻¹)	Found^a (Found-Real)^b (µg L⁻¹)	RR (%)
Hair	0.0	58.1 ± 2.3^a	–
Hair	10.0	(9.7 ± 0.3)^b	97

Nail	0.0	29.8 ± 1.1^a	–
Nail	10.0	(9.8 ± 0.3)^b	98

Saliva	0.0	22.2 ± 0.8^a	–
Saliva	10.0	(9.6 ± 0.4)^b	96

Plasma	0.0	21.8 ± 0.8^a	–
Plasma	10.0	(10.1 ± 0.4)^b	101

Urine	0.0	29.1 ± 1.1^a	–
Urine	10.0	(9.8 ± 0.4)^b	98

Experimental conditions: “amount of nanosorbent, 30 mg; pH of sample, 5; eluent: 200 µL of 6.0 mol L⁻¹ of HNO₃; number of extraction cycles, 15 cycles, 60 s”.

In comparison with the other extraction methods (Table 5), the nano LDH-APDC/I-OS-DSPE method has some advantages including more eco-friendly compatibilities, simplicity and fastness, perform in fewer steps, making the easier automation, elimination of the separation step (elution of the analyte from the adsorbent), lower consumptive indices and higher enrichment factors when equal volumes of the samples are considered. This provides comparable or even better LODs than the other methods.

Table 5 Comparison of nano LDH-APDC/I-OS-DSPE of Cr(VI) with other published methods.

Sorbent/determination technique	Sample	LOD (µg L⁻¹)	EF (sample volume)	CI	Extraction time (min)	References
Naphthalene/FAAS	Tannery effluents	0.5	100 (1000 mL)	10	>15	Krishna et al. (2004)
Amberlite XAD-16/FAAS	Tap water	45	25 (250 mL)	10	>72	Tunçeli and Türker (2002)
Chitin/spectrophotometry	Natural water	50	100 (100 mL)	1	∼15	Hoshi et al. (1998)
Alumina/spectrophotometry	Electroplating wastewater	5	25 (250 mL)	10	>55	Rajesh et al. (2007)
C18 bonded phase silica SPE disks/FAAS	River water	15	45 (250 mL)	5.5	>25	Saber Tehrani et al. (2004)
Chromosorb-108 resin/FAAS	Tobacco, coffee, soil	5	71 (500 mL)	7	–	Tuzen and Soylak (2006)
Funaria/FAAS	Groundwater	145	20 (200 mL)	10	>100	Krishna et al. (2005)
Amberlite/FAAS	Tap and mineral waters	7.7	75 (150 mL)	2	>40	Narin et al. (2008)
Nano-(Zn-Al) LDH/FAAS	Hair, Nail, Saliva, Plasma, urine	2.4	42.5 (10 mL)	0.24	1.5	Present work

4 Conclusions

The nano LDH-APDC/I-OS-DSPE method was successfully applied to the speciation of chromium species from biological samples prior to their determination using micro-sampling flame atomic absorption spectrometry without matrix interferences. The most important features of the method are as follows:

Elimination of the need for an additional elution step, through dissolving the adsorbent in an acidic solution.
A maximum extraction in a short period of time, due to the nano-sized structure of the sorbent.
Elimination of the time-consuming centrifugation step using a nano-filter.
Minimum consumption of organic solvents.
Performance in a short period of time, due to fast filtering through a nanofilter.
Opening a new horizon to the automation and integration of the dispersive micro-solid phase extraction method, due to the elimination of elution step using dissolvable nanosorbents.

Nano LDH-APDC/I-OS-DSPE is an excellent selective method for the speciation of chromium ions as well as its low cost performance, low limits of detection, acceptable repeatabilities, and suitable linearity range. Analysis of the real samples showed that it could successfully be applied to complex matrices such as urine, plasma, saliva, hair, and nail.

Acknowledgment

The authors would like to thank the Semnan University Research Council for the financial support of this work.

References

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Tunçeli A., Türker A.R., . Speciation of Cr (III) and Cr (VI) in water after preconcentration of its 1,5-diphenylcarbazone complex on amberlite XAD-16 resin and determination by FAAS. Talanta. 2002;57:1199-1204.
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Show Sections

[1] Abdolmohammad-Zadeh H., Rezvani Z., Sadeghi G., Zorufi E., . Layered double hydroxides: a novel nano-sorbent for solid-phase extraction. Anal. Chim. Acta. 2011;685:212-219.
[Google Scholar]

[2] Barfi B., Asghari A., Rajabi M., Sabzalian S., Khanalipoor F., Behzad M., . Optimized syringe-assisted dispersive micro solid phase extraction coupled with microsampling flame atomic absorption spectrometry for the simple and fast determination of potentially toxic metals in fruit juice and bio-fluid samples. RSC Adv.. 2015;5:31930-31941.
[Google Scholar]

[3] Chen C., Gunawan P., Xu R., . Self-assembled Fe₃O₄-layered double hydroxide colloidal nanohybrids with excellent performance for treatment of organic dyes in water. J. Mater. Chem.. 2011;21:1218-1225.
[Google Scholar]

[4] Galán-Cano F., Lucena R., Cárdenas S., Valcárcel M., . Direct coupling of dispersive micro-solid phase extraction and thermal desorption for sensitive gas chromatographic analysis. Anal. Methods. 2011;3:991-995.
[Google Scholar]

[5] Gil F., Hernández A., . Toxicological importance of human biomonitoring of metallic and metalloid elements in different biological samples. Food Chem. Toxicol.. 2015;80:287-297.
[Google Scholar]

[6] Guerrini L., Rodriguez-Loureiro I., Correa-Duarte M.A., Lee Y.H., Ling X.Y., de Abajo F.J.G., Alvarez-Puebla R.A., . Chemical speciation of heavy metals by surface-enhanced Raman scattering spectroscopy: identification and quantification of inorganic-and methyl-mercury in water. Nanoscale. 2014;6:8368-8375.
[Google Scholar]

[7] Hoshi S., Konuma K., Sugawara K., Uto M., Akatsuka K., . The simple and rapid spectrophotometric determination of trace chromium (VI) after preconcentration as its colored complex on chitin. Talanta. 1998;47:659-663.
[Google Scholar]

[8] Kang L., Gao Z., Huang W., Jin M., Wang Q., . Nanocarrier-mediated co-delivery of chemotherapeutic drugs and gene agents for cancer treatment. Acta Pharm. Sin. B. 2015;5:169-175.
[Google Scholar]

[9] Krishna M.B., Chandrasekaran K., Rao S.V., Karunasagar D., Arunachalam J., . Speciation of Cr (III) and Cr (VI) in waters using immobilized moss and determination by ICP-MS and FAAS. Talanta. 2005;65:135-143.
[Google Scholar]

[10] Krishna P.G., Gladis J.M., Rambabu U., Rao T.P., Naidu G., . Preconcentrative separation of chromium (VI) species from chromium (III) by coprecipitation of its ethyl xanthate complex onto naphthalene. Talanta. 2004;63:541-546.
[Google Scholar]

[11] Lasarte-Aragonés G., Lucena R., Cárdenas S., Valcárcel M., . Effervescence-assisted dispersive micro-solid phase extraction. J. Chromatogr. A. 2011;1218:9128-9134.
[Google Scholar]

[12] Lasarte-Aragonés G., Lucena R., Cárdenas S., Valcárcel M., . Effervescence-assisted carbon nanotubes dispersion for the micro-solid-phase extraction of triazine herbicides from environmental waters. Anal. Bioanal. Chem.. 2013;405:3269-3277.
[Google Scholar]

[13] Li Z., Chang X., Hu Z., Huang X., Zou X., Wu Q., Nie R., . Zincon-modified activated carbon for solid-phase extraction and preconcentration of trace lead and chromium from environmental samples. J. Hazard. Mater.. 2009;166:133-137.
[Google Scholar]

[14] Liu Y.-L., Zhou J.-B., Zhao R.-S., Chen X.-F., . Using Zn/Al layered double hydroxide as a novel solid-phase extraction adsorbent to extract polycyclic aromatic hydrocarbons at trace levels in water samples prior to the determination of gas chromatography–mass spectrometry. Anal. Bioanal. Chem.. 2012;404:1603-1610.
[Google Scholar]

[15] Narin I., Kars A., Soylak M., . A novel solid phase extraction procedure on Amberlite XAD-1180 for speciation of Cr (III), Cr (VI) and total chromium in environmental and pharmaceutical samples. J. Hazard. Mater.. 2008;150:453-458.
[Google Scholar]

[16] Pereira C.D., Techy J.G., Ganzarolli E.M., Quináia S.P., . Chromium fractionation and speciation in natural waters. J. Environ. Monit.. 2012;14:1559-1564.
[Google Scholar]

[17] Rajesh N., Deepthi B., Subramaniam A., . Solid phase extraction of chromium (VI) from aqueous solutions by adsorption of its ion-association complex with cetyltrimethylammoniumbromide on an alumina column. J. Hazard. Mater.. 2007;144:464-469.
[Google Scholar]

[18] Saber Tehrani M., Afshar Ebrahimi A., Rastegar F., . Chromium speciation by surfactant assisted solid-phase extraction and flame atomic absorption spectrometric detection. Anal. Chim.. 2004;94:429-435.
[Google Scholar]

[19] Shao M., Ning F., Zhao J., Wei M., Evans D.G., Duan X., . Preparation of Fe₃O₄@ SiO₂@ layered double hydroxide core–shell microspheres for magnetic separation of proteins. J. Am. Chem. Soc.. 2012;134:1071-1077.
[Google Scholar]

[20] Teramura K., Iguchi S., Mizuno Y., Shishido T., Tanaka T., . Photocatalytic conversion of CO₂ in water over layered double hydroxides. Angew. Chem.. 2012;124:8132-8135.
[Google Scholar]

[21] Timerbaev A.R., . Element speciation analysis using capillary electrophoresis: twenty years of development and applications. Chem. Rev.. 2012;113:778-812.
[Google Scholar]

[22] Tunçeli A., Türker A.R., . Speciation of Cr (III) and Cr (VI) in water after preconcentration of its 1,5-diphenylcarbazone complex on amberlite XAD-16 resin and determination by FAAS. Talanta. 2002;57:1199-1204.
[Google Scholar]

[23] Tuzen M., Soylak M., . Chromium speciation in environmental samples by solid phase extraction on Chromosorb 108. J. Hazard. Mater.. 2006;129:266-273.
[Google Scholar]

[24] Zhitkovich A., . Importance of chromium-DNA adducts in mutagenicity and toxicity of chromium (VI) Chem. Res. Toxicol.. 2005;18:3-11.
[Google Scholar]

[25] Zhitkovich A., . Chromium in drinking water: sources, metabolism, and cancer risks. Chem. Res. Toxicol.. 2011;24:1617-1629.
[Google Scholar]

Toward use of a nano layered double hydroxide/ammonium pyrrolidine dithiocarbamate in speciation analysis: One-step dispersive solid-phase extraction of chromium species in human biological samples

Abstract

Keywords

Nanoparticle

Speciation

Integrated

Microextraction

Bio-origin

1 Introduction

2 Experimental

2.1 Standard solutions and reagents

2.2 Preparation of calibration standards and quality controls

2.3 Instrumentation

2.4 Preparation of ammonium pyrrolidine dithiocarbamate ligand intercalated into LDHs

2.5 Sample preparation

2.5.1 Human urine

2.5.2 Human blood

2.5.3 Human saliva

2.5.4 Human hair and nails

2.6 Extraction procedure

2.7 Calculation of enrichment factor, absolute and relative recoveries, and consumptive indices

3 Results and discussion

3.1 Characterization of nanosorbent

3.1.1 FT-IR spectra

3.1.2 ED-XRF spectra

3.1.3 FESEM pattern

3.1.4 TGA curves

3.1.5 FAAS evaluation

3.1.6 ED-XRF

3.2 Method optimization

3.2.1 pH of sample solution

3.2.2 Type and amount of adsorbent

3.2.3 Number of extraction cycles

3.2.4 Type, concentration, and volume of eluent

3.3 Analytical performance of method

3.4 Potentially interfering ions

3.5 Application of nano LDH-APDC/I-OS-DSPE to analysis of real samples

4 Conclusions

Acknowledgment

References

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