Determination of trace elements in pumpkin seed oils and pumpkin seeds by ICP-AES

Abstract

Inductively coupled plasma atomic emission spectrometry (ICP-AES) was used for the determination of the elements present in pumpkin seed oil and pumpkin seeds. After extraction of the oil from pumpkin seeds a residue was obtained. For the determination of elements three types of samples were used: oil, seeds and the residue (after oil extraction). Three different digestion types were applied: open vessel and closed vessel digestion in a steel bomb as well as microwave digestion in a closed system. For most elements of interest the measured concentration differs depending on the digestion method used. After microwave digestion a good reproducibility for ICP-AES measurements for all elements was found. Determination of Zn by ICP-AES is possible both after open and closed vessel digestion (steel bomb or microwave digestion). No loss of volatile compounds in pumpkin oil during preparation or microwave digestion prior to ICP-AES analysis could be observed. In general, the recoveries for all elements in pumpkin seed oils and seeds were >95%, for S only they were <50%. Differences in the element concentration were found for seed oil obtained from the same seeds but prepared by two different procedures, extracted by Soxhlet and commercially produced after roasting of the seeds. The differences of the measured element concentrations after application of different types of dissolution procedures are discussed. The closed vessel dissolution was found to be the best procedure prior to the ICP-AES determination of metals. The method is evaluated by application of the standard addition method and by recovery experiments. Addition of salt during the oil production procedure causes approx. 10 times the Ca, K, Mg, and Na amounts compared to the Soxhlet extracted oil produced in the laboratory. Higher amounts of Na could be registered in the residues as well. The LODs were <0.1 µg g⁻¹ for Ca, Cd, Mg, Mn, Ti, and Zn, in the range of 0.1 to 0.8 µg g⁻¹ for Co, Cu, Fe, K, Mo, Na, Ni, Pb, and V, and >0.8 µg g⁻¹ for Al, Cr and P in pumpkin oil samples.

---

1. Introduction

The determination of trace elements in edible seed oils and seeds has gained more importance during the last few years because they contain naturally occurring antioxidants and essential elements. The content of metals and their species (chemical forms) in edible seed oils depends on several factors. The metals can be incorporated into the oil from the soil or be introduced during the production process. The presence of trace metals such as Cu, Fe, Mn, Ni, and Zn are known to have different effects on the oxidative stability of edible seed oils. Lead and copper are potentially present in oils caused by environmental contamination.^1–3

Hydrogenation of edible seed oils and fats has been performed using nickel catalysts. The presence of copper and iron can be caused by the processing equipment as well. Because of the metabolic functions of the metals, a development of fast and accurate analytical methods for trace element determination in oils is important from the point of view of both quality control of production and food analysis. The authenticity of products is important from the standpoints of both commercial value and health aspects. Over the years, a high degree of sophistication has evolved for chromatographic methods^4–6 for the analysis of components of oils and fats. At the same time spectroscopic methods^7–10 are emerging as potential tools for a rapid screening of samples for the detection of their adulteration. Inductively coupled plasma atomic emission spectrometry (ICP-AES) and atomic absorption spectrometry (AAS) are the most commonly used techniques for the determination of metals in different samples.^11,12 Fats and oils are particularly difficult to analyze for their trace metal contents. Different digestion methods were applied for oil digestion prior to spectrometric measurements. Many of the used wet or dry digestion methods are not recommended for use in high fat material because of the associated safety hazards.¹³ A direct determination method like dilution with an appropriate organic solvent followed by direct aspiration into an atomic absorption flame or an ICP-excitation unit is sometimes not sensitive enough. In most cases the analytical method exhibits changes in the excitation power for organic media compared to aqueous solutions.¹⁴ Also calibration is more difficult in organic media.^15,16 Optional methods of sample introduction into ICP-AES are the emulsions-formation techniques. Their application does not require destruction of the organic matter or the use of large amounts of organic solvent. Their disadvantage compared to aqueous systems is the difficulty occurring in calibration since aqueous standard solution cannot be used in this case. The stability of emulsions, selection of the surfactant, the proportions of the phases, the preparation procedure, and the nature of the analyte compounds present in the matrix are usually limitations for such a method.^8,17,18

Procedures based on trace element extraction by different extractants are often time consuming and prone to losses and contamination.¹⁹ An alternative methodology is microwave-enhanced dissolution chemistry. It enables fast, efficient and reproducible sample preparation. Sample solutions are heated so effectively that the reaction time can be reduced drastically, often from days to minutes. Furthermore the level of the reaction and process control offered by microwave heating is better than by any other heating method.^8,20

The aim of this work was the determination of elements in pumpkin seed oil and seeds by ICP-AES. Furthermore differences in element concentrations between oil extracted by Soxhlet (self extracted) and commercially pressed pumpkin seed oil from roasted seeds should be registered. Self extracted oil was gained by Soxhlet extraction without salt addition during the procedure. The resulting product was used for comparison to commercially pressed pumpkin oil.

After the oil is extracted from pumpkin seeds a residue is obtained. The concentrations of the elements in oil, seeds and residue were determined, applying three different digestion types: open vessel and closed vessel digestion in a steel bomb as well as microwave digestion in a closed system.

2. Experimental

2.1. Chemicals and glassware

Nitric acid (p.A.), hydrogen peroxide (30% H₂O, p.A.) and single element standards p.A. (Merck, Darmstadt, Germany) were used for the experimental work. Aqueous standard stock solutions were used after appropriate dilution. Oil and seed samples were commercially available. All glassware was cleaned with nitric acid prior to use.

2.2. Apparatus

For the element detection and determination an ARL 3580 ICP spectrometer (ARL, Ecublens, Switzerland) equipped with an HF-generator (Henry, 27.12 MHz) and RF power supply (1200 W) was used. The measurements were carried out using the sequential Paschen–Runge part of the spectrometer equipped with a grating (1080 lines mm⁻¹), a Fassel type torch and a computer (DEC 316 sx). The gas flows used were/L min⁻¹: outer gas/12, intermediate/0.8, aerosol carrier/1. Observation height was 15 mm above coil. All measurements were performed with a Babington-type nebulizer (ARL MDSN). For microwave digestion of the samples a high performance microwave digestion unit MLS-1200 MEGA equipped with an EM-30 unit (Milestone GmbH) was applied.

2.3. Sample preparation for ICP-AES

Samples of oils, seeds and residues were weighed and subsequently digested by three different procedures: open vessel wet digestion, closed vessel wet digestion under pressure in a steel bomb or using the microwave digestion unit. After digestion with a mixture of nitric acid and hydrogen peroxide clear solutions were obtained and the analytes were determined by ICP-AES.

Prior to the analysis of the elements by ICP-AES the oil was extracted from the ground seeds using a Soxhlet apparatus (petroleum ether, bp 60–90 °C, 24 h). After extraction of the oil from the seeds a residue was left. For the determination of the elements three types of samples were used: oil, seeds and residue.

For this purpose 1 g each of the oils, seeds and residues were weighed for the open vessel digestion. Open vessel digestions of the samples were performed in glass vessels after addition of 5 mL of HNO₃ conc. and 15 drops of H₂O₂ at approx. 120 °C and heating of the samples for approximately 12 h. During the digestion procedure the same amount of acid and hydrogen peroxide was added to the samples again twice. After digestion all the samples were transferred into 10 or 20 mL volumetric flasks and filled to volume. For each series of digestions a reagent blank was prepared.

For the closed vessel digestion of the samples steel bombs equipped with a PTFE inlet were used. To 0.25 g oil, seeds or residue 5 mL of HNO₃ conc. and 15 drops of H₂O₂ were added and the sample was heated to 100 °C for approx. 12 h. After digestion all the samples were transferred into 10 mL volumetric flasks and diluted to volume with 1 M HNO₃ for measurement.

For the microwave digestion procedure oil (0.5 g), seeds (0.5 g) and residue (0.5 g) were weighed into the digestion vessels. The digestions were performed by adding 4 mL of HNO₃ conc. and 2 mL H₂O₂ (30% v/v) to the sample. The microwave program applied is listed in Table 2 (see later). A rotating turntable was used to insure homogeneous distribution of the microwave radiation in the oven. Temperature control required a temperature sensor in one vessel during the entire decomposition. The oil, seeds and residues were digested according to the following optimized program (Power in W/time in min): 250/2, 0/1, 250/2, 600/1, 400/5, ventilation 3.0 min. The internal temperature was limited to 240 °C during the last step and ventilation. After cooling all the digests were transferred into 10 mL volumetric flasks and diluted to volume with HNO₃ (1% v/v). Reagent blanks were prepared similarly to the samples.

The element content of commercially produced pumpkin seed oil (“roasted seeds”), self extracted oil (Soxhlet apparatus) and salad oil products available at local markets were used for comparison. In the laboratory the different oils were extracted by a Soxhlet apparatus from ground seeds without addition of salt.

2.4. Calibration procedure

The system was calibrated using aqueous mixed standards. All calibration curves were based on five different concentrations, including the blank. Standard solutions were prepared by diluting a 1000 mg L⁻¹ multielement solution (ICP Multielement Standard IV, Merck, Darmstadt, FRG). The concentration ranges for the elements were: 5 to100 mg L⁻¹ of Al, Cd, Co, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, S, Ti, V, and Zn, and 500 to 1000 mg L⁻¹ of Ca, K, Na, and P. Calibration ranges were modified according to the expected concentration ranges of the elements of interest.

2.5. Spiking procedures

Standard additions were performed by addition of specific volumes of aqueous standard solutions to the set of the samples prepared. Sample spikes were prepared in triplicate (a, b and c). The oils were weighed, approx. 0.5 g, into digestion flasks for the microwave digestion procedure and, approx. 0.25 g, for the steel bomb. Standards were added (1 mL of standard c = 100 mg L⁻¹) and the samples were digested with a mixture of nitric acid and hydrogen peroxide in the microwave oven or a steel bomb. After digestion the clear digests obtained were transferred into 10 mL flasks and measured by ICP-AES. To pumpkin oil (p.oil 1) and pumpkin seeds (sample LSI4) 10 mg L⁻¹ of Cd, Cu, Mo, Na, Ni, and Ti were added; to pumpkin oil (sample LOI2) and pumpkin seeds (sample LSI5) 10 mg L⁻¹ of Al, Ca, Co, Fe, P, and Zn, were added; to pumpkin oil (sample LOI1) and pumpkin seeds (sample LSI5) 10 mg L⁻¹ of Cr, K, Mg, Mn, Pb, S, and V were added. After digestion the clear digests were transferred into 10 mL flasks and measured by ICP-AES. Furthermore the residues were weighed (approx. 0.5 g) into digestion flasks for the microwave digestion procedure and standards were added (1 mL of standard c = 100 mg L⁻¹). The samples were digested with a mixture of nitric acid and hydrogen peroxide, transferred into 10 mL flasks and measured by ICP-AES.

2.6. The limits of detection (LOD)

The limits of detection (LOD) were calculated according to Boumans¹⁵ using 3σ. LODs were determined in pure element standards and in the pumpkin oil samples. The limits of detection were determined by measuring an appropriate reagent blank solution ten times and a standard solution three times. The detection limit was calculated as the concentration equivalent to three times the standard deviation of the signal of the oil blank solution. All standard deviations are based on measurements in triplicate.

2.7. ICP-AES determination

Prior to the sequential analysis line selection was performed for every element (Table 1).

Table 1 Limits of detection and line selection for determination of elements in pumpkin oil

Element	LOD/µg g^−1 pumpkin oil	Wavelength/nm
Al	0.920	396.1
Ca	0.041	393.3
Cd	0.044	226.5
Co	0.356	239.8
Cr	0.880	267.7
Cu	0.293	327.3
Fe	0.320	259.9
K	0.248	766.5
Mg	0.030	279.5
Mn	0.032	257.6
Mo	0.161	202.0
Na	0.139	589.6
Ni	0.164	341.4
P	1.101	178.3
Pb	0.166	220.3
S	—	181.0
Ti	0.042	337.2
V	0.114	310.2
Zn	0.051	206.2

---

The background correction position was selected as +0.14 nm. The integration time used was 5 s. For the determination of K, Na, P, and S sequential spectral lines were used, while for the other elements a simultaneous program was applied.

3. Results and discussion

Table 2 shows the results obtained for all three digestion methods for Al, Fe and Zn only. For the other elements investigated the results varied drastically depending on the digestion method used. Determination of Zn in pumpkin oils, seeds and residues by ICP-AES is possible both after open and closed vessel digestion (steel bomb or microwave digestion). The differences of the results for Al and Fe obtained by using open and closed vessel methods varied significantly according to a t-test. Lower results up to approx. 50% were obtained for other elements after open vessel digestion compared to the closed vessel digestion (steel bomb and microwave) methods. The differences between the results for both closed vessel digestions are not statistically significant for most of the elements. This indicates that no loss by volatilization occurred in both cases. Comparison of the two closed vessel methods of sample preparation, microwave digestion vs. steel bomb, results in the conclusion that microwave digestion is less time consuming. Further, more samples can be digested at the same time. After the microwave digestion good reproducibility for ICP-AES measurements for all elements was observed (approx. 5% RSD), indicating that the microwave digestion of the samples was complete. For testing of the losses of elements occurring during the laboratory procedure of oil production (evaporating of the solvent and complete removal of the solvent using a vacuum pump) commercially produced pumpkin oil was also treated in the same way as the oil extracted in the laboratory. The microwave digestion of commercially produced pumpkin oils was performed by adding nitric acid and hydrogen peroxide to the samples followed by ICP-AES determination of metals. No loss of the elements during preparation or microwave digestion prior to the ICP-AES analysis could be noticed. The procedure is not limited to non-volatile metals as can be seen from Table 3. According to the results it could be concluded that microwave digestion is an appropriate technique for digestion of edible oil samples.

Table 2 Determination of Al, Fe and Zn in pumpkin oil (sample 1) and seeds (sample 1) after microwave digestion (mW), open vessel digestion (op) and digestion in steel bomb (sb). Results in µg g⁻¹

Pumpkin oil	Al	Fe	Zn	Pumpkin seeds	Al	Fe	Zn
Microwave digestion	31.9	67.83	12.21	Microwave digestion	69.86	139.13	59.64
Steel bomb	32.03	60.16	11.64	Steel bomb	72.66	151.08	53.15
Open vessel	24.76	47.19	12.74	Open vessel	59.87	93.53	54.23

---

Table 3 Pumpkin oil A, B and C^a

	Ca		Cd		Cr		Cu		Fe		Mg		Mo		Na		Ti		V		Zn		Al, Co, Ni, Mn, P, K, Pb
a A—commercially produced pumpkin oil. B—commercially produced pumpkin oil was diluted with petroleum ether and the solvent was evaporated on a rotary evaporator at 45 °C. C—commercially produced pumpkin oil was filled in a round bottom flask, connected to a vacuum pump at room temperature for 30 min. Results are given in µg g^−1. b The LOD were 0.1 µg g^−1 for Co, K, Mn, Ni, Pb and >0.8 µg g^−1 for Al and P.
A a		1.46		1.74		6.63		9.76		15.45		46.04		0.84		34.41		3.87		24.65		3.22		<DL^b
A b		6.13		1.72		4.93		9.91		14.4		41.12		0.81		33.35		3.84		22.26		2.88		<DL^b
B a		7.85		1.76		8.6		14.77		17.75		48.62		0.91		37.82		3.91		26.51		3.55		<DL^b
B b		5.43		1.71		5.05		10.59		15.88		40.46		0.79		36.42		3.83		24.86		3.25		<DL^b
C a		7.28		1.76		8.46		14.59		17.7		48.86		0.9		36.61		3.91		26.59		3.49		<DL^b
C b		6.4		1.72		7.44		12.83		15.56		42.97		0.82		32.2		3.84		23.38		3.07		<DL^b
Mean		5.8		1.7		6.8		12.1		16.1		44.7		0.8		35.1		3.9		24.7		3.2
SD		2.30		0.02		1.61		2.30		1.34		3.70		0.05		2.20		0.04		1.71		0.25
%RSD		39.4		1.1		23.5		19.0		8.3		8.3		5.9		6.2		1.0		6.9		7.7

---

In Table 4 the results obtained by the standard addition method and the recovery experiments for pumpkin oil (sample 2) are shown. The expected concentration of the elements was 10 mg L⁻¹ (200 µg g⁻¹). In general, recoveries for all elements in pumpkin oils were >95%. For Na and S they were <50% or 70%, respectively.

Table 4 Results for spike and recovery experiments for pumpkin oil (sample 2) after microwave digestion. Results in µg L⁻¹

	Al	Ca	Cd	Co	Cr	Cu	Fe	K	Mg	Mn	Mo	Na	Ni	P	Pb	S	Ti	V	Zn
p.oil 2a	9.3	10.0	9.6	9.2	9.5	9.3	10.0	10.6	10.6	10.0	9,7	3.4	9.2	9.2	10.2	6.6	13.1	9.8	9.2
p.oil 2b	11.5	11.7	11.4	12.0	11.2	12.8	11.5	12.4	12.8	11.7	11.1	4.8	10.4	9.8	10.0	8.7	16.4	11.6	12.6
p.oil 2c	10.3	10.6	10.3	10.0	10.1	10.4	10.5	9.9	11.7	10.6	10.1	4.8	9.6	13.0	9.1	7.2	15.1	10.6	13.0
Mean	10.4	10.8	10.4	10.4	10.2	10.8	10.7	11.0	11.7	10.8	10.3	4.4	9.7	10.7	9.8	7.5	14.9	10.7	11.6
SD	1.1	0.9	0.9	1.4	0.9	1.8	0.8	1.3	1.1	0.9	0.7	0.8	0.6	2.0	0.6	1.1	1.7	0.9	2.1

---

Results for spike experiments in pumpkin oil (sample 1) and pumpkin seeds (sample 1) are given in Table 5. The expected concentration of elements was 10 mg L⁻¹ (200 µg g⁻¹). Recoveries for all elements in pumpkin oil and pumpkin seeds were approx. 10 mg L⁻¹, or >95%, except for S they were <50%. The results clearly demonstrate that this type of digestions and ICP-measurements are suitable for all elements except S.

Table 5 Results for spike experiments with pumpkin oil (sample 1) and pumpkin seeds (sample 1) after digestion in a steel bomb. Results in µg L⁻¹

	Al	Ca	Cd	Co	Cr	Cu	Fe	K	Mg	Mn	Mo	Na	Ni	P	Pb	S	Ti	V	Zn
p. oil 1	1.6	0.7	10.6	2.9	1.7	9.2	3.5	2.1	0.9	0.3	9.3	11.1	11.0	5.9	0.2	0.0	13.9	0.0	0.7
p. oil 2	10.9	10.9	0.5	13.9	1.6	0.0	9.5	1.3	0.8	0.2	0.1	1.0	0.0	13.7	0.1	0.0	0.2	0.0	9.4
p. oil 3	1.0	0.7	0.6	3.0	10.5	0.0	3.4	11.8	11.4	11.7	0.0	1.2	0.0	5.6	10.2	0.0	0.2	10.0	0.6
p.seed 4	3.1	27.3	10.1	3.2	0.3	10.3	5.9	467.6	84.8	2.9	10.1	9.3	9.1	493.3	0.0	0.0	15.8	1.3	3.3
p.seed 5	11.3	31.8	1.7	14.6	0.3	0.8	15.9	500.6	95.8	2.1	0.2	1.9	0.0	514.6	0.0	0.0	3.8	1.3	15.2
p.seed 6	3.0	28.0	1.7	3.1	11.2	0.8	5.1	497.6	100.1	13.3	0.4	1.9	0.0	482.5	9.8	5.6	3.8	9.1	2.9

---

The limits of detection obtained for the determination of the elements in pumpkin oil are given in Table 1. LODs were determined also in pumpkin seeds and pumpkin oil residue samples. The limits of detection for the elements in those samples were the same as for the elements in pumpkin oil (Table 1). Limits of detection <0.1 µg g⁻¹ were obtained for Ca, Cd, Mg, Mn, Ti, and Zn. For Co, Cu, Fe, K, Mo, Na, Ni, Pb, and V they were in the range from 0.1 to 0.8 µg g⁻¹, and for Al, Cr, and P >0.8 µg g⁻¹.

Further improvements of the method's performance, especially for the S determination are limited by the sample amount and the reagent volumes, necessary for the digestion procedure. Smaller volumes of nitric acid and hydrogen peroxide did not lead to complete digestion. Higher volumes of reagents can even cause excessive foaming and sample loss by the vapour removal system.

The clear sample digests obtained by the method described can be used not only for ICP-AES but also for ICP-MS, graphite furnace or flame atomic absorption spectrometry. Compared to direct analysis of samples after dilution with an organic solvent, the digestion method applied for this study allows a simultaneous determination of metals and avoids limitations due to the introduction of a carbonaceous matrix or contamination caused by the organic solvents used. Figs. 1 and 2 show results for the determination of elements in oil, seeds and residue after oil extraction and subsequent digestion using the microwave unit.

Fig. 1 Determination of Al, Co, Cr, Cu, Fe, Mn, Na, Zn in pumpkin oil, seeds and residue after oil extraction and digestion in the microwave unit.

Fig. 2 Determination of Ca, K, Mg, P in pumpkin oil, seeds and residue after oil extraction and digestion in the microwave unit.

For all residues investigated the concentrations of the elements of interest are higher compared to the ones measured for seeds and oils.

In all samples investigated the concentration of the toxic elements (Cd, Pb) was rather low; approx. 10 µg L⁻¹ or less for Cd and Pb and approx. 6 µg L⁻¹ or less for Mo, Ni and Ti.

Salad pumpkin oil is commercially produced from roasted seeds (PK is the so called “Presskuchen” i.e. molding cake). This type of oil was used also for comparison to the self extracted oil (“Soxhlet” apparatus). Higher concentrations of Ca, K, Mg, Na, and P in PK-oil samples were determined caused by addition of salt (NaCl) to the roasted seeds before pressing of the oil. Salt is added to obtain a higher amount of oil from the seeds. Self extracted oil contained approx. 10 times less Ca, K, Mg, Na, and P than the commercially produced pumpkin oil from “roasted seeds”. The amounts of Ca, K, Mg, Na, and P depend on the procedure of the oil production. For roasted seeds: 70 to 90 µg Ca g⁻¹, 110 to 300 µg K g⁻¹, 150 to 250 µg Mg g⁻¹, 190 to 440 µg Na g⁻¹, and 450 to 950 µg P g⁻¹ were measured. Furthermore the results of the element determinations in the different residue samples were compared to those obtained in oils. No significant differences of the element concentrations were found for the residues after oil extraction by a Soxhlet apparatus and the residues after pressing of the oil. With one exception: for Na a difference of 23.78 to 31.04 µg Na g⁻¹ for residues of ground seeds to approx. 10000 µg Na g⁻¹ for the residues after pressing of the oil was found. As expected adding salt to seeds before pressing of the oil causes an increase of the Na amount of the oil. Self extracted oil was gained from ground seeds using a Soxhlet apparatus (without adding salt). Adding salt in commercially produced pumpkin seed oil causes high amounts of Ca, K, Mg, Na, and especially P.

4. Conclusion

For most elements of interest the concentrations in pumpkin oils, seeds and residues differ considerably depending on the digestion method applied. Determination of Zn by ICP-AES is possible both after open and closed vessel digestion (steel bomb or microwave digestion). The microwave digestion is less time consuming and more samples can be digested at the same time compared to open vessel and steel bomb digestion. After microwave digestion a good reproducibility for ICP-AES measurements of all elements was found. There is no limitation for the determination of non-volatile metals that cannot be lost normally by open vessel digestion. An addition of salt during oil production causes approx. 10 times higher amounts of Ca, K, Mg, and Na than found for self extracted oil. Also in seeds and residues higher amounts of Na could be registered. No loss of volatile elements in pumpkin oil could be observed using microwave digestion prior to the ICP-AES analysis of the elements. In general, recoveries for all elements in pumpkin oils and seeds were >95%, only S could not be detected. The LODs of the elements in pumpkin oils were <0.1 µg g⁻¹ for Ca, Cd, Mg, Mn, Ti, and Zn, in the range from 0.1 to 0.8 µg g⁻¹ for Co, Cu, Fe, K, Mo, Na, Ni, Pb, and V, and higher than 0.8 µg g⁻¹ for Al, Cr, and P in pumpkin oil samples.

Acknowledgements

This work was supported by OEAD (Austrian Academic Exchange Service) scholarships (OEAD research scholarship and Ernst Mach scholarship) in the course of a co-operation between Croatia and Austria (Croatian Ministry of Science and Technology and Austrian Culture Institute).

References

1. L. Allen, P. H. Siitonen and H. C. Thompson, J. Am. Oil Chem. Soc., 1998, 75, 477–481.
2. A. Banks, E. Eddie and J. G. Smith, Nature, 1961, 190, 908–912.
3. A. J. DeJonge, W. E. Coenen and C. Okkerse, Nature, 1965, 206, 573–577.
4. A. Mandl, G. Reich and W. Lindner, Eur. Food Res. Technol., 1999, 209, 400–406.
5. P. L. Buldini, D. Ferri and J. L. Sharma, J. Chromatogr., 1997, 798, 549–555.
6. G. P. Blanch, M. D. Caja, M. L. R. del Castillo and M. Herraiz, J. Agr. Food Chem., 1998, 46, 3153–3157.
7. I. Karadjova, G. Zachariadis, G. Boskou and J. Stratis, J. Anal. At. Spectrom., 1998, 13, 201–204.
8. M. Maurillo, Z. Benzo, E. Marcano, C. Gomez, A. Garaboto and C. Marin, J. Anal. At. Spectrom., 1999, 14, 815–820.
9. R. Calapaj, S. Chiricosta, G. Saija and E. Bruno, At. Spectrosc., 1988, 9, 107–109.
10. F. J. Slikkerveer, A. A. Braad and P. W. Hendrikse, At. Spectrosc., 1980, 1, 30–35.
11. K. Pomazal, C. Prohaska, I. Steffan, G. Reich and J. F. K. Huber, Analyst, 1999, 124, 657–663.
12. C. Prohaska, K. Pomazal, I. Steffan and A. Törvényi, J. Anal. At. Spectrom., 2000, 15, 97–99.
13. C. Feldman, Anal. Chem., 1974, 46, 1609–1613.
14. E. Tserovsky and S. J. Arpadjan, J. Anal. At. Spectrom., 1991, 6, 487–491.
15. P. W. J. M. Boumans, Basic Concepts and Characteristics of ICP-AES, Inductively Coupled Plasma Emission Spectroscopy. Part I, Methodology, Instrumentation, and Performance, ed. P. W. J. M. Boumans, Wiley, New York, 1987, ch. 4.
16. J. L. Fabc and M. L. Ruschak, Anal. Chem., 1985, 57, 1853–1856.
17. I. M. Goncalves, M. Murillo and A. M. Gonzale, Talanta, 1998, 47, 1033–1042.
18. M. Guardia and M. T. Vidal, Talanta, 1984, 31, 799–803.
19. W. J. Price, J. T. H. Roos and A. F. Clay, Analyst, 1970, 95, 760–762.
20. R. C. Richter, D. Link and H. M. Kingston, Anal. Chem., 2001, 1, 30–37.

---