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Nickel as a Chemical Modifier for Sensitivity Enhancement and Fast Atomization Processes in Electrothermal Atomic Absorption Spectrometric Determination of Cadmium in Biological and Environmental Samples J. C. FEO, M. A. CASTRO, J. M. LUMBRERAS, B. de CELIS, and A. J. ALLER† Department of Biochemistry and Molecular Biology, Area of Analytical Chemistry, University of León, E-24071 León, Spain
A systematic study of the efficiency of protons, Ni, Pd and Th as chemical modifiers for the determination of cadmium by electrothermal atomic absorption spectrometry (ETAAS) using fast temperature programs was made for platform atomization. A comparison was made in terms of the salt type, absorbance–time profiles and elimination of the sodium chloride interference. The results were adapted to develop a method for the ETAAS determination of cadmium in biological and environmental samples. The highest sensitivity to determine cadmium in biological and environmental samples was obtained using nickel (together with protons) as a chemical modifier. The accuracy of the method was tested by the determination of cadmium in different certified reference materials. The best detection limit and the characteristic mass of Cd were found to be 0.03 ng mL–1 and 0.35 pg, respectively. (Received April 10, 2003; Accepted September 8, 2003)
Introduction Cadmium is one of the most toxic elements for animals and humans. Industrial exposures to cadmium can cause acute and chronic toxicity in humans. Low-level long-term exposure to cadmium seems to be associated with abnormally high hypertension, being related to renal sodium retention or to a direct effect on vascular smooth muscle.1 An additional problem of cadmium toxicity is its accumulative character, because it can be accumulated to an amount of about 30 mg during a lifetime. From an analytical point of view, a problem emerges because the concentrations of this element in the majority of samples are usually around, and even below, the detection limits of the most sensitive techniques. Regarding these low concentrations, the accurate determination of Cd in biological and environmental samples is a very important goal for analytical chemists. Cadmium can be successfully determined by different However, electrothermal atomic absorption techniques.2 spectrometry (ETAAS) is many times the technique of choice for high-sensitivity analysis. Nonetheless, problems exist in the determination of Cd by ETAAS derived from losses by vaporization at temperatures prior to atomization and from spectral and chemical interferences.3 The removal of matrix components during the pyrolysis step is difficult in many cases owing to the high volatility of cadmium. Attempts were made to eliminate the drawbacks originating by the presence of alkaline and alkaline earth halides by various means. Thus, a two step electrothermal atomizer4 and a filter † To whom correspondence should be addressed. E-mail:
[email protected]
furnace5 have been proved to be useful for volatile elements, such as cadmium. Cadmium has been separated by a coprecipitation process with Ni diethyldithiocarbamate,6 and by different preconcentration methods.7–9 Nonetheless, to solve the lack of sensitivity, the method of choice has routinely been the addition of chemical modifiers.10–13 Many chemical modifiers have been used in the determination of cadmium, palladium and magnesium, being the most largely used,14–19 sometimes in combination with other species (ammonium phosphate,16,18,19 nickel19 and sodium azide).20 Ammonium salts20–23 were used for the first time as chemical modifiers to thermally stabilize Cd. Nonetheless, ammonium phosphate causes spectral Other modifiers used were silver and interference.24 lanthanum,25 thiourea,26 polytetrafluoroethylene as a fluorination reagent27 and some acidic compounds (inorganics, such as hydrofluoric acid28,29 and organics, such as ascorbic and oxalic acids).30 Nonetheless, the effects of other similar media (HNO3, HCl, HF, H2SO4, HClO4, acetic acid, hydroxilammonium chloride and ammonium acetate) on the atomization of cadmium have also been studied.31 Of course, a permanent chemical modifier approach using ruthenium gave comparable results.17 Other permanent modifiers based on iridium have also proved to be very suitable for the determination of cadmium in complex matrices.32,33 Pd-base modifiers have usually been the accepted choice. Using the Pd-Mg modifier in combination with ammonium nitrates,14 up to 3000 mg L–1 NaCl did not interfere in the determination of Cd. Developed methods for the determination of cadmium have been applied to many matrices. Some of them are natural water samples,9 animal tissues,34 biological samples,14,35 soils, sediments, coals, ashes, sewage sludges, and plants,31,36 foods,37 ceramicware38 and urine.11,39,40 In this work, a comparison of three chemical modifiers (Ni,
1632 Table 1
ANALYTICAL SCIENCES DECEMBER 2003, VOL. 19 Graphite furnace program
Parameter
Dry/pyrolysis
Atomization
Cleaning
Temperature, ˚C Ramp time, s Hold time, s Purge, position Read on, s
250 12 10 2 —
1400 0 4 0 5
2200 — 3 3 —
Pd and Th) used alone and mixed together with protons for the ETAAS determination of Cd in the presence of sodium chloride was systematically carried out. Fast furnace programs, in which the drying and pyrolysis steps are joined, or even skipped, have proved to be very usefull in many cases because of their increased analytical throughput.41 Occasionally, however, some residual high background signals could remain, but only if the D2 lamp correction system is used. Thus, a fast temperature program was used in this work because it provides faster analysis and it is much easier to use in practice than a conventional program.42 Recently, we have found some advantages using chemical modifiers43 other than palladium, which has been largely considered to be a universal modifier. Thorium was selected because it was proved to be advantageous for those analytes capable to form oxides prior to the atomization stage. Nickel was the first modifier used, and it seems to provide sensitivity values higher than palladium for some situations. The performance of the proposed chemical modifier (Ni + protons) was accomplished by the determination of Cd in different biological and environmental certified reference materials.
Experimental Apparatus The experiments were carried out using a Thermo Jarrel Ash atomic absorption spectrophotometer (SH 11) equipped with a heated graphite atomizer (Model CTF-188) and a Smith–Hieftje background-correction system. The operating parameters used were as follows: wavelength, 228.8 nm; the cadmium hollow cathode lamp, Visimax II, was used under the recommended conditions; slitwidth, 1 nm. Standard uncoated rectangular graphite tubes and standard pyrolytic graphite-coated platforms were used for platform atomization. An Epson 118 recorder was used. Solutions as an aerosol were injected into the graphite atomizer by means of an automatic system (Fastac) and argon (99.995% purity) served as the purge gas. The temperature program used is shown in Table 1. A volume (10 µL) of the solution was injected into the atomizer at room temperature. Three replicate determinations based on the integrated absorbances (PA) was used for each measurement. A Mettler AE 240 semimicro analytical balance (sensitivity ± 0.01 mg) was used for weighing samples. A pH-meter (Crison Model Digit 505) was used to measure the acidity of the aqueous phase. In other experiments, the atomization and cleaning temperatures were suddenly stopped at a value lower than 2200˚C. Then, the platform surface was examined by scanning electron microscopy. An electron microprobe analysis of a graphite platform surface was performed in a JEOL scanning electron microscope (Model JSM-6100), equipped with an energy-dispersive X-ray detecting system (LINK) and operating under the recommended conditions (15 kV acceleration voltage and 5 nA probe current).
Fig. 1 Effect of the concentration of some metal chlorides on the ETAAS atomic absorption of 10 ng mL–1 Cd for platform atomization.
Reagents and samples A cadmium stock solution (1000 µg mL–1 Cd) was prepared by dissolving a suitable amount of Cd oxide of the highest quality (Suprapur, Merck) in a smaller quantity of nitric acid, and then adding an appropriate volume of demineralized water. Test solutions were prepared by appropriate dilution of the stock solution immediately prior to their use. Stock solutions of those elements used as chemical modifiers were prepared from Th(NO3)4, Ni(NO3)2, Pd(NO3)2, and NiCl2 dissolved in demineralized water, and PdCl2 dissolved in a HCl solution prepared by adding 1.0 mL conc. HCl (d = 1.19, and 37.9% w/w) and diluting with water to 50 ml. All chemicals used in this study were of analytical reagent grade and were obtained from Merck. Distilled, deionized water (resistivity 18 MΩ cm) was used for preparing the samples and standards. The certified reference material (CRM), analyzed to determine the accuracy of the proposed procedures, was Community Bureau of Reference (BCR) CRM 505 estuarine water. National Institute of Standards and Technology (NIST) Standard Reference Materials SRM 1577b Bovine Liver, SRM 1567a Wheat Flour and SRM 1568a Rice Flour were digested in triplicate with nitric acid. The digests were obtained by decomposing 0.2 g of bovine liver and 4 g of wheat flour and rice flour, each of a dry powder sample material, with 10.0 mL of 1 + 1 v/v nitric acid. The solutions were evaporated to near dryness at 155˚C and the volumes made up to 25 ml with 0.05 mol L–1 nitric acid. Prior to the determination of cadmium, the digests of bovine liver and wheat flour were diluted until 100 ml with 0.01 mol L–1 nitric acid.
Results and Discussion Behavior of the chemical modifiers on the atomization of cadmium The effect of three metals (Na, Ni and Pd, as chlorides) on the electrothermal atomization of Cd was evaluated for platform atomization using the fast temperature program given in Table 1. The obtained results (Fig. 1) support the idea that the integrated absorbances decrease with the chloride concentration, but only in a systematic way for NaCl. However, differences from the effect of nickel and palladium chlorides were also
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Fig. 2 Effect of the concentration of some metal nitrates on the ETAAS atomic absorption of 10 ng mL–1 Cd for platform atomization.
Fig. 3 Effect of the nitrate concentration (as sodium nitrate and nitric acid) on the ETAAS atomic absorption of 10 ng mL–1 Cd for platform atomization.
established. Nickel chloride showed a better behavior than palladium chloride because higher integrated absorbances were obtained. This suggests that a beneficial effect seems to be derived from the presence of nickel. For that reason, the effect of the metals, but added as nitrate, was also tested. For a comparison, we also included a different metal (Th, as nitrate), which has proved to show good results as a chemical modifier for other analytes.43 The results derived from such situations (Fig. 2) are shown to be different from those found for the metal chlorides. Comparing the results from Figs. 1 and 2, it is also possible to derive that the nitrate anion takes a role in the atomization mechanism of Cd, probably favoring a more oxidative environment. In this respect, we tried to prove this statement by following a study controlling the concentration of the nitrate anion (Fig. 3). Figure 3 shows important differences in the behavior of the nitrate anion, depending on the cation present (sodium or proton). No beneficial effects were derived from the presence of nitric acid, which produces a decrease in the integrated absorbances of Cd. The opposite is true for sodium nitrate. The effect of protons on the atomization of Cd was also tested for both hydrochloric acid and nitric acid (Fig. 4). The presence of either low or high amounts of nitric acid does not favor the atomization of Cd. Hydrochloric acid at concentrations below 2000 mg L–1 does not produce a strong effect on the atomization signal of Cd. Peak profile characteristics The typical atomic absorption profiles of Cd in aqueous solutions without and with Na, Ni, Pd, Th and protons were comparatively studied (Fig. 5). Different atomization signal shapes were obtained for Cd, depending on the concomitant present. In general, Cd atomic absorption signals appear at later times if Pd is present. However, a strong increase in the integrated absorbance for Cd was achieved in the presence of Ni (as nitrate). The presence of Ni produces only one Cd absorbance–time peak, while the presence of Pd, Na and H (as nitrates) generates a double-peak profile for platform atomization, suggesting a two-precursor mechanism for the atomization of Cd. For these cases, broader and time-delayed Cd atomic absorption peaks appear. It seems to be evident that the largest sensitivity is achieved when Ni (as nitrate) is present. The mechanism of that behavior
Fig. 4 Effect of the concentration of protons (as hydrochloric and nitric acid) on the ETAAS atomic absorption of 10 ng mL–1 Cd for platform atomization.
can be explained as being due to the formation of a thermally stable compound between analyte (Cd metal) and the modifier (Ni metal) during the earlier thermal treatment process. Obviously, the increased Cd atomic absorption signal obtained, if Ni (as nitrate) is present, as suggested from the corresponding peak profile (Fig. 5), could be explained as a result of a large affinity for nickel and a highly efficient atomization process in the presence of Ni. The main stabilization mechanism from Th seems to be related to the formation of some metal oxide. In this case, no beneficial effect was derived for the atomization of Cd (Fig. 2) owing to the fact that the amount of Cd oxide is minimal on the graphite surface. Efficiency of chemical modifiers to overcome sodium chloride interference The behavior of different compounds to overcome the negative effect derived from the presence of sodium chloride was proved by adding separately nitric acid, nickel nitrate, palladium nitrate and thorium nitrate to the cadmium solutions.
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ANALYTICAL SCIENCES DECEMBER 2003, VOL. 19 Table 2
Accuracy of the procedure (n = 3)
NIST Sample
Cd certified/ µg g–1a
Cd found/ µg g–1a
Seawater CRM 505 Bovine liver SRM 1577b Wheat flour SRM 1567a Rice flour SRM 1568a
0.09 ± 0.05 0.500 ± 0.003 0.026 ± 0.002 0.022 ± 0.002
0.089 ± 0.025 0.497 ± 0.003 0.027 ± 0.002 0.021 ± 0.002
The error is three standard deviations. a. ng mL–1 for seawater.
Fig. 5 Atomic absorption profiles for platform atomization of Cd and Cd together with different compounds.
Fig. 6 Effect of the concentration of protons (as nitric acid) on the ETAAS Cd atomic absorption for platform atomization.
None of the metal modifiers were able to counteract the negative effect due to the presence of sodium chloride. The only compound showing a positive effect was nitric acid, which
completely overcome the negative effect owing to the presence of sodium chloride (Fig. 6). In this case, no negative atomic absorption signals of Cd were obtained if Cd together with NaCl was introduced into the tube with a nitric acid solution. In spite of the overcoming shown by the presence of nitric acid, no significative improvement in the Cd atomic absorption signals was derived. In order to increase the sensitivity, a metal chemical modifier was used in combination with protons. Thus, nickel, palladium and thorium (as nitrates) were tested as chemical modifiers together with protons (as nitric acid). In the presence of protons, only nickel provides the most sensitive results (Fig. 6). It is interesting to note that with the fast temperature program used (Table 1), the mixture constituted by Pd together with protons was not effective in any case to overcome the drawback derived by the presence of NaCl. Consequently, the Cd atomic absorption peak profiles always show negative peaks for all of these situations. This lack of effectiveness of protons in eliminating NaCl interference can be explained as being due to the high affinity of Pd for chlorine.44 The above is particularly shown in Fig. 7, where the coexistence of sodium, chlorine and palladium on the graphite surface is evident. On the contrary, the atomization of Cd solutions also containing NaCl together with protons and Pd only provides unaltered positive Cd atomic absorption signals if high pyrolysis temperatures (above 1000˚C) are used. Figures of merit and applications As a result of the above experiments, a mixture of Ni + protons (as nitric acid) was chosen as the chemical modifier for the determination of Cd in a NaCl matrix. Calibration graphs were constructed from aqueous Cd standards treated with the modifier. The optimum linear range was established over which the measured and predicted absorbances differed by less than 5%. The sensitivity (0.04 ng–1 mL) represents the slope of the analytical calibration graph of absorbance versus concentration (in ng mL–1) obtained by a linear regression. The detection limit is defined as three-times the standard deviation of nine consecutive measurements of blank solutions using an integrated absorbance mode, and the characteristic mass is expressed as the mass of a sample corresponding to an integrated absorbance of 0.0044. The detection limit (0.03 ng mL–1) and characteristic mass (0.35 pg) of the analyte were determined in the presence of a modifier for the sensitivity of the proposed method. The characteristic mass derived in this method represents a good value which is similar to the best values (0.4 – 0.8 pg) found in the literature.5,18,38 Something similar happens with the detection limit found in this work, which is similar to those reported in other studies using a similar technique.18,40 We have found only one study providing a slightly lower detection limit (0.007 µg L–1)29 than the presented at herein, although it was derived by using an a priori calculation,45 different from the way recommended by IUPAC.
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Fig. 7 Secondary electron (SE) image and X-rays maps from the same area for carbon, oxygen, sodium, chlorine sand palladium. The EDS X-ray spectrum was derived from a graphite platform used to heat a Cd solution (10 ng mL–1) together with sodium chloride (4000 mg L–1), Pd nitrate (4000 mg L–1) and protons (4000 mg L–1) (as nitric acid) according to the temperature program, but up to a temperature value of the platform surface of 900˚C (magnification, 10000X).
The repeatability (n = 9) varies from 2.30 until 4.25% for cadmium concentrations between 0.5 and 12.5 ng mL–1, but the best relative standard deviation (2.30%) was obtained for a Cd concentration of 1 ng mL–1. The accuracy of this procedure was tested by replicate determinations of cadmium in different certified reference materials. The measured cadmium concentrations agree with the certified values (Table 2). The absorbance blank level of Cd was of 0.001.
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