Determination of nickel in hair samples by graphite furnace atomic absorption spectrometry and flow-through stripping chronopotentiometry

Abstract

Nickel in hair samples was determined by GF AAS and stripping chronopotentiometry after a dry ashing mineralisation with a mixture of nitrous oxides, oxygen and ozone. The use of common matrix modifiers brought no significant improvement in the GF AAS measurement. The detection limit, trueness and precision were found to be 0.15 mg kg⁻¹ of the hair sample, 4.3%, and 4.1%, respectively. The linear range was found to be 0.44–3.3 mg kg⁻¹. The stripping chronopotentiometric determination is based on the deposition of nickel ions from an ammonia buffer solution in a porous glassy carbon electrode followed by its stripping into an acidic solution by constant current. Copper interfered and was removed from the acidified sample solution by means of a minicolumn packed with pearl cellulose chemically modified with 8-hydroxychinolinol. Copper was trapped completely whereas nickel passed the column. The detection limit, trueness and precision were found to be 0.032 mg kg⁻¹ in the hair sample, 5.8%, and 6.1%, respectively. The linear range was found to be 0.1–20 mg kg⁻¹. Good agreement between the two methods was found for most hair samples.

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Introduction

An effective monitoring of trace elements in the environment is a highly demanded but costly task. An alternative to a complete environmental monitoring is the investigation of biological indicators reflecting possible contamination risks. Long-term exposition to contamination can successfully be monitored by analysing hair samples of inhabitants in various areas of human activity. The advantage of such samples is in the non-invasive character of sampling, ability of hair to cumulate exposition during longer periods and availability in relative large quantities.

Determination of trace elements in hair is important for monitoring the influence of the environment on humans, for epidemiological studies and for evaluation of working place exposures. This assumption is confirmed by the fact that the average trace element content in hair corresponds to their content in blood.¹

One of the species of interest in hair samples is nickel which is commonly considered toxic. It enters the environment predominantly as a consequence of its industrial use such as production and discharge of accumulators, applications in metallurgy and electroplating. Smoking is an other common source of nickel contamination. Nickel enters the living organisms from air, digestive tract and epidermis. In hair, the nickel content is in the range of 0.6–6.5 mg kg⁻¹.² Enhanced Ni content in hair of children indicated a long-term exposition to contaminated environment caused by nearby industrial plant.³

Nickel in hair samples is usually determined by nuclear activation analysis,¹ ICP OES,⁴ X-ray fluorescence⁵ and PIXE spectroscopy.⁶ Owing to low detection limits and high selectivity, nickel can advantageously be determined by electrothermal AAS by making use of slurry sampling technique.⁷

Electrochemical methods can also be used for nickel determination, e.g. adsorptive stripping voltammetry on mercury,⁸ silver amalgam,⁹ bismuth film¹⁰ and antimony film electrodes.¹¹ The electrochemical methods are based on collection of the Ni(II) dimethylglyoxime complex on the electrode surface followed by cathodic stripping of the deposit. The method is sensitive and selective enough but requires the removal of complex forming substances which interfere.

Nickel can also be determined by anodic stripping voltammetry on solid electrodes such as graphite.¹² Here, nickel is deposited as elemental nickel, then is stripped as Ni(II). Owing to the high background due to the electrode material, the detection limits were much worse than in the case of the adsorptive method.

Deposition of metals in porous carbon-based electrodes has been used for their electrochemical preconcentration in AAS¹³ and MIP OES¹⁴ methods. It was found that Ni can be deposited with high efficiency on porous carbon electrode made of crushed glassy carbon particles which have significantly lower background currents than graphite electrodes. Hence, by making use of this electrode and the stripping chronopotentiometric measurement detection limits low enough for hair samples can be expected.

The objective of this paper is to demonstrate the capacities of graphite furnace AAS and stripping chronopotentiometry with the above electrode material in the determination of low nickel concentrations in hair samples.

Experimental

The AAS measurements were done by Varian SpectrAA 300P (Techtron Pty., Limited, Mulgrave, Victoria, Australia) and AAnalyst700 (Perkin Elmer, Bodenseewerk GmbH, Germany) AAS instruments equipped with deuterium-lamp background correction. Pyrolytically coated tubes with integrated pyrolytic platforms were used. Peak areas were evaluated. The operation parameters are collected in Table 1.

Table 1 Operation parameters for the graphite furnace^a

Step no	Temperature °C	Time/s	Gas flow/L min^−1	Gas type	Read command
a HCl lamp: Ni Lumina (Perkin Elmer), wavelength: 232.0 nm, current: 4 mA, slit: 0.2 nm, sample volume: 20 µl, pyrolytically coated tube with platform, signal evaluation: peak area measurement.
1	100	8.0	3.0	Normal	No
2	140	8.0	3.0	Normal	No
3	700	17.0	3.0	Normal	No
4	2300	1.0	0	Normal	Yes
6	2600	2.0	3.0	Normal	No

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Flow-through chronopotentiometric measurements were carried out by an electrochemical analyser EcaFlow model GLP 150 (Istran, Ltd., Bratislava, Slovakia) equipped with two solenoid inert valves, a peristaltic pump and a microprocessor controlled potentiostat/galvanostat. The block diagram of the system was reported elsewhere.¹⁵ The signals were recorded and evaluated by the memory-mapping technique.¹⁶The measurement consisted of two main steps: (i) the background signal was measured first by means of a blank sample and (ii) the sample or standard solution was measured: nickel ions were collected from the ammonium sample solution on the electrode as elemental nickel. The flow was switched to the acidic carrier electrolyte to remove the sample matrix from the cell, the flow was stopped and following a quiescence period the deposit was stripped by constant current and the stripping chronopotentiogram was recorded. Finally the cell was rinsed with the carrier electrolyte. The background signal was then subtracted from the signal of the standard or sample yielding a true background corrected net signal.

A compact flow-through electrochemical cell of type 353 with Pt auxiliary, Ag/AgCl reference and E53C microporous glassy carbon working electrodes was used (Istran, Ltd., Bratislava, Slovakia). The effective diameter and length of the electrode were approximately 5 mm and 3 mm, respectively. The electrode surface and void volume were about 25 cm² and 20 µl, respectively. The operation parameters are listed in Table 2.

Table 2 Operation parameters of the flow-through electrochemical analyser

Parameter	Value
Deposition current	−4 mA
Quiescence potential	−1000 mV
Quiescence time	10 s
Terminal potential	100 mV
Regeneration potential	200 mV
Standby potential	200 mV
Stripping current	−50 µA
Sample volume	2 ml
Blank volume	2 ml
Rinsing volume	3 ml
Flow rate	3 ml min^−1

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The hair samples were mineralised in a dry digester system APION (TESSEK Praha, Czech Republic) and a microwave assisted wet digester Milestone, MLS 1200 MEGA.

For Cu removal a minicolumn of inner diameter and length of 5 mm and 20 mm, respectively was used. The column was packed with Iontosorb OXIN 100 chelating sorbent with chemically bound 8-hydroxychinolinol groups on pearl cellulose (IONTOSORB, Usti nad Labem, Czech Republic). The column was rinsed with 50 ml of 1 mol dm⁻³ NaOH, then with water and 1 mol dm⁻³ HCl until the eluted solution became colourless. Finally, the column was rinsed with water until a neutral pH. After use, the column was regenerated with 1 mol dm⁻³ HCl and rinsed with water.

Reagents and solutions

Analytical-reagent grade chemicals were used in all experiments. Deionised and degassed water was used for the preparation of all solutions.

Magnesium nitrate solution: 10 g dm⁻³ Mg(NO₃)₂ in 15% (v/v) HNO₃ (MERCK, No. 5813).

Rhodium matrix modifier, 2.5 mg dm⁻³ Rh, prepared from Rhodium(III)-chloride-Trihydrate (Degussa).

Carrier electrolyte for chronopotentiometry: 0.1 mol dm⁻³ NaCl and 0.002 mol dm⁻³ HCl.

Electrolyte for sample adjustment: 0.1 mol dm⁻³ NH₃ and 0.1 mol dm⁻³ NH₄Cl.

The Ni calibration solutions were prepared from a Ni certified reference material with 1.000 g dm⁻³ Ni (Slovak Metrological Institute, Bratislava, Slovak Republic).

Sample preparation

Hair samples were washed in acetone, rinsed with a detergent solution and pure water.¹⁷ The samples were dried at 60 °C.

Two methods were used for sample digestion and preparation:

Method A. 0.3 g of the sample was digested in the PTFE vessel of microware digester with 5 ml of concentrated nitric acid and 1 ml of hydrogen peroxide. On completing the digestion and cooling down the digested sample was transferred to a 25 ml volumetric flask and its volume was adjusted to 25 ml with water. This sample solution was analysed by GF AAS.

Method B. 0.3 g of the sample was weighted to the quartz vessels of the dry asher APION. The sample was destroyed with a mixture of oxidising gases (oxygen, nitrous oxides and ozone formed during catalytic burning of gaseous ammonia in oxygen) at 400 °C for 14 hours.

GF AAS measurements: the formed ash was leached with 10 ml of 1 mol dm⁻³ HNO₃ and the solution was analysed.

Chronopotentiometric measurements: the ash was leached with 5 ml of 0.1 mol dm⁻³ HCl. This solution was let pass the minicolumn at a flow rate of 1–3 ml min⁻¹. The column was rinsed with 5 ml of 0.1 mol dm⁻³ HCl giving a total volume of 10 ml of the treated sample solution. On adding 2 ml of 1 mol dm⁻³ NH₄OH the resulting solution was analysed.

Results and discussion

Digestion of hair samples

Two types of sample digestion were tested, a wet digestion and a dry ashing. Statistical tests have shown no significant differences in results obtained by these techniques. The wet digestion in a microwave assisted digester is faster (few minutes only) than the dry ashing procedure demanding 12–14 hours. On the other hand, the dry asher produces a leached ash sample in a 1 mol dm⁻³ HNO₃, which is less corrosive to the AAS instrument than the mineralizate from wet digestion consisting of excess concentrated nitric acid, hydrogen peroxide and nitrous gases. Moreover, the product of wet digestion was not compatible with the chronopotentiometric method, which demanded a complete removal of nitric acid from the digested sample by evaporation of the digested sample to dryness. The dry digestion produced an ash which could directly been leached with the electrolyte solution for the chronopotentiometric measurement of Ni. Hence, this digestion technique was used in further experiments.

AAS measurements

The need of a matrix modifier for GF AAS determination was investigated first. A digested hair sample in 1 mol dm⁻³ HNO₃, then with addition of Mg(NO₃)₂ modifier recommended by the producer of AAS instrument, and finally a Rhodium modifier¹⁸ as a representative of noble metal modifiers were tested.

The pyrolysis temperatures are influenced in a similar way for all three cases (Fig. 1). The highest sensitivity for the 1 mol dm⁻³ HNO₃ medium is achieved at temperatures of 2300–2400 °C. In the case of the Mg(NO₃)₂ modifier (10 µl) this temperature was about 200 °C lower and the Rh modifier (10 µl) shifted the atomisation temperature to unacceptably high values. The background signal was significantly influenced by the matrix modifiers (Fig. 2). Without a modifier, the background signal was at a level of 0.02–0.04 absorption units virtually independent on pyrolysis and atomisation temperatures. With the Rh modifier, the background signal decreases with pyrolysis temperatures and is virtually independent on the atomisation temperatures. The Mg(NO₃)₂ modifier, however, gives rise to a steadily increasing background signal both when increasing the pyrolysis and atomisation temperatures. In fact, this modifier stabilises the sample matrix and is therefore less suitable for this application. These observations confirmed some earlier conclusions¹⁹ that there is virtually no need to use a matrix modifier in Ni determination in digester hair samples. Hence, no matrix modifier, pyrolysis and atomisation temperatures of 700 °C and 2300 °C (Fig. 1), respectively, have been used in further measurements.

Fig. 1 Influence of the pyrolysis and atomisation temperatures on the net signal of Ni.

Fig. 2 Influence of the pyrolysis and atomisation temperatures on the background signal.

The limit of detection obtained by the IUPAC recommended method²⁰ was found to be 0.02 mg dm⁻³.

Stripping chronopotentiometry

The electrochemical determination of Ni is based on the deposition of Ni ions from ammonia buffer solutions on the surface of porous carbon electrode followed by stripping of the deposit into a slightly acidic NaCl solution by constant current:

Ni²⁺ = Ni⁰ − 2e⁻ (1)

The deposition can be done either by means of a constant current or constant potential. The former procedure ensured better reproducibility and higher electrochemical yields, so it was used in further experiments.

The measurement is influenced by some species, especially those forming stable complexes with nickel ions and metal ions giving stripping signals near to that of Ni such as copper. Due to the applied digestion method, no complex forming species can be expected in the ashed samples. Cadmium and copper could also give signals near that of nickel, but the cadmium content was negligible in hair samples. However, hair samples inherently contain significant amounts of copper, which adversely influences the accuracy of nickel measurement owing to some overlapping of their stripping peaks. To minimise this interference the carrier electrolyte, Ni is being stripped to, was optimised to achieve the best resolution of the Ni and Cu stripping peaks. The best electrolyte found was that containing 0.1 mol dm⁻³ NaCl, 0.002 mol dm⁻³ HCl. Unfortunately, the adverse influence of copper still remained significant (Table 3). Copper concentrations in the analysed sample solutions depress the Ni signal in 10% already at a 1 : 1 ratio of Cu : Ni, at a ratio of 2 : 1 the Ni signal decreases to about 18% of the original value.

Table 3 Nickel found at various copper contents^a

Cu added/µg dm^−3	Ni found/µg dm^−3
a Ni added 50 µg dm^−3.
10	51.7
20	52.2
25	48.8
50	45.5
80	31.8
100	17.5
200	4.3
1000	<0.1

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Hence, the interfering copper ions had to be removed from the hair sample solutions to gain accurate results. A chelating sorbent Iontosorb Oxin 100 with chemically bound 8-hydroxychinolinol groups on pearl cellulose was tested for this purpose. The sorption of metal ions on such sorbents depends significantly on pH²¹ which could be used for their separation. It was found that in 0.1 mol dm⁻³ HCl the sorption of Ni is negligible whereas Cu ions are completely trapped in the sorbent. The procedure was then modified so that the ash obtained from the digestion was eluted with 0.1 mol dm⁻³ HCl, the resulting solution was let to pass the minicolumn and the column was rinsed with 0.1 mol dm⁻³ HCl to obtain 10 ml of eluate. Finally, ammonia solution was added to the eluate to obtain an optimum ammonia buffer solution and the resulting solution was analysed (Fig. 3).

Fig. 3 Chronopotentiometric signals of (a) Ni standard (100 µg dm⁻³), (b) hair sample, and (c) the same hair sample after removal of Cu by means of chelating sorbent Iontosorb Oxin 100. Ni found 0.438 µg g⁻¹ (Ni found by GF AAS 0.47 µg g⁻¹). The integration boarders set on the stripping peak of the hair sample.

Figures of merit

Analytical figures of merit of both methods are listed in Table 4. The values of trueness and precision were obtained from 10 analyses of a hair sample with not detected native nickel content and spiked with a known amount of Ni. The chronopotentiometric method possesses a broader linear concentration range and a lower detection limit. However, the sample preparation for AAS measurement is simpler and faster than that for chronopotentiometry, namely there is no need to remove copper from the sample solution.

Table 4 Analytical figures of merit

Parameter	GF AAS	Chronopotentiometry
a Obtained by means of a hair sample spiked with Ni. Concentration of Ni in the measured solution was 15 µg dm^−3.
Limit of detection/mg kg^−1	0.15	0.032
Limit of quantification/mg kg^−1	0.44	0.10
Linear range/mg kg^−1	0.44–3.3	0.10–20
Trueness^a (%)	4.3	5.8
Precision^a (%)	4.1	6.1

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Real samples

Hair samples obtained from various regions of Slovakia (urban, industrial, countryside) were analysed by the elaborated procedures (Table 5). Though not a perfect match but except for one sample acceptable agreement was observed between the two methods.

Table 5 Determination of nickel in hair samples

Sample	Ni found/mg kg^−1
	Chronopotentiometry	GF AAS
1141	0.434 ± 0.040	<0.44
7740	0.939 ± 0.031	1.003 ± 0.020
1132	0.352 ± 0.004	<0.44
7756	1.78 ± 0.13	1.500 ± 0.030
13 800	0.403 ± 0.030	0.542 ± 0.011
13 797	1.56 ± 0.12	1.650 ± 0.034
13 803	0.633 ± 0.047	0.614 ± 0.012
8539	<0.10	<0.44

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Conclusions

Graphite furnace AAS and stripping chronopotentiometry have proved to be useful for Ni determination in hair samples. The spectroscopic method is simpler and faster but has a worse detection limit and narrower linear range than the electrochemical one. Notwithstanding this, owing to the widespread acceptance of AAS remains the less known chronopotentiometric method more an alternative or control method for such applications. Its use would be promoted if nickel concentrations lower than the detection limit for AAS should preferably be quantified.

Acknowledgements

The authors appreciate the financial support of the Slovak Grant Agency of Science VEGA (Project No.1/0500/08) and the Slovak Research and Development Agency APVV (Project No. 0057-06).

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