Determination of trace aluminum in biological and water samples by cloud point extraction preconcentration and graphite furnace atomic absorption spectrometry detection

Available online at www.sciencedirect.com

Journal of Hazardous Materials 154 (2008) 1127–1132

Determination of trace aluminum in biological and water samples by cloud point extraction preconcentration and graphite furnace atomic absorption spectrometry detection Hongbo Sang, Pei Liang ∗ , Dan Du Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, PR China Received 4 September 2007; received in revised form 7 November 2007; accepted 7 November 2007 Available online 17 November 2007

Abstract A cloud point extraction (CPE) method for the preconcentration of trace aluminum prior to its determination by graphite furnace atomic absorption spectrometry (GFAAS) has been developed. The CPE method is based on the complex of Al(III) with 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (PMBP), and then entrapped in non-ionic surfactant Triton X-114. PMBP was used not only as chelating reagent in CPE preconcentration, but also as chemical modifier in GFAAS determination. The main factors affecting CPE efficiency, such as pH of sample solution, concentration of PMBP and Triton X-114, equilibration temperature and time, were investigated in detail. An enrichment factor of 37 was obtained for the preconcentration of Al(III) with 10 mL solution. Under the optimal conditions, the detection limit of this method for Al(III) is 0.09 ng mL−1 , and the relative standard deviation is 4.7% at 10 ng mL−1 Al(III) level (n = 7). The proposed method has been applied for determination of trace amount of aluminum in biological and water samples with satisfactory results. © 2007 Elsevier B.V. All rights reserved. Keywords: Cloud point extraction; Preconcentration; Aluminum; Graphite furnace atomic absorption spectrometry

1. Introduction Aluminum is the third most abundant element in the Earth’s crust (8.1% by weight), and is a non-essential element to which humans are frequently exposed [1]. Aluminum is widespread throughout nature, air, water, plants and consequently in all the food because of its wide use. During recent years, much interest has been raised by the toxicity and biological effect of aluminum [2]. Some studies suggest that aluminum may be accumulated in the brain via different routes (drinking waters, food, and medicines) and interfere with the normal activities of nervous system. This metal ion has been considered as a possible cause of renal osteodystrophy, Parkinson disease and Alzheimer’s disease [3–5]. The determination of very low levels of aluminum has become increasingly very important in environmental and clinical chemistry since its negative role in the human life.

∗

Corresponding author. Fax: +86 27 67867961. E-mail address: [email protected] (P. Liang).

0304-3894/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2007.11.018

Graphite furnace atomic absorption spectrometric (GFAAS) method is a much suitable technique for the determination of aluminum because of its high sensitivity, precision, selectivity, and versatility. However, it is generally impossible to determine trace aluminum in biological and environmental samples directly because of interfering species in the surrounding matrix, or the concentration of the analyte being below the detection limit of the instrument. So preconcentration and separation techniques are still necessary. The widely used techniques for the separation and preconcentration of aluminum include liquid–liquid extraction [6,7], ion exchange [8,9], solid-phase extraction [10–12] and single drop microextraction [13], etc. Separation and preconcentration based on cloud point extraction (CPE) are becoming an important and practical application of surfactants in analytical chemistry [14,15]. The technique is based on the property of most non-ionic surfactants in aqueous solutions to form micelles and to separate into a surfactant-rich phase of a small volume and a diluted aqueous phase when heated to a temperature known as the cloud point temperature. The small volume of the surfactant-rich phase obtained with this methodology permits the design of extraction schemes

1128

H. Sang et al. / Journal of Hazardous Materials 154 (2008) 1127–1132

Fig. 1. Structural formulae of PMBP and the complex.

that are simple, cheap, highly efficient, speedy and lower toxicity to the environment than those extractions that use organic solvents. CPE has been used to separate and preconcentrate organic compounds as a step prior to their determination by liquid chromatography [16] and capillary electrophoresis [17]. The phase separation phenomenon has also been used for the extraction and preconcentration of metal ions after the formation of sparingly water-soluble complexes [18,19]. CPE as a preconcentration step in conjunction with detection by spectrophotometry, spectrofluorometry, FAAS, ICP-AES and HPLC for the determination of various metal ions has been widely studied [20–28]. CPE coupled with spectrophotometry and spectrofluorometry for the determination of aluminum has also been reported [29–32]. CPE coupled with GFAAS could obtain lower detection limit compared with other configurations [19]. In this paper, a CPE method based on the complex of Al(III) with 1-phenyl-3-methyl4-benzoyl-5-pyrazolone (PMBP, its structure is shown in Fig. 1) and using Triton X-114 as surfactant was proposed for separation and preconcentration of aluminum prior to its determination by GFAAS. The main factors affecting CPE efficiency were investigated. The proposed method has been applied for determination of trace amount of aluminum in biological and water samples with satisfactory results. 2. Experimental 2.1. Instrumentation A TBS-990 atomic absorption spectrophotometer (Beijing Purkinge General Instrument Co. Ltd., Beijing, PR China) with a deuterium background correction and a GF990 graphite furnace atomizer system was used. An aluminum hollow-cathode Table 1 Operating parameters for GFAAS Parameters Lamp current (mA) Wavelength (nm) Slit (nm) Ar flow rate (mL min−1 ) Sample volume (␮L)

6.0 309.3 0.4 200 (stopped during atomizing) 20

Temperature program Drying Ashing Atomizing Cleaning

100 ◦ C (ramp 20 s, hold 10 s) 1300 ◦ C (ramp 10 s, hold 20 s) 2200 ◦ C (ramp 0 s, hold 3 s) 2400 ◦ C (ramp 1 s, hold 3 s)

lamp was used as radiation source at 309.3 nm. The optimum operating parameters for GFAAS are given in Table 1. The pH values were measured with a Mettler Toledo 320-S pH meter (Mettler Toledo Instruments Co. Ltd., Shanghai, PR China) supplied with a combined electrode. A thermostated bath maintained at the desired temperatures was used for the cloud point experiments. An 80-2 centrifuge (Changzhou Guohua Electric Appliance Co. Ltd., PR China) was used to accelerate the phase separation. 2.2. Standard solution and reagents The non-ionic surfactant Triton X-114 was obtained from Sigma (St. Louis, MO, USA) and was used without further purification. Stock standard solution of Al(III) at a concentration of 1000 ␮g mL−1 was obtained from the National Institute of Standards (Beijing, PR China). Working standard solutions were obtained by appropriate dilution of the stock standard solutions. A 1.0 × 10−2 mol L−1 solution of PMBP was prepared by dissolving appropriate amounts of this reagent in absolute ethanol from the commercially available product (Shanghai ZhenXing First Chemical Factory, Shanghai, PR China). The following buffers were used to control the pH of the solutions: sodium acetate–acetic acid (pH 3–6), ammonium acetate–ammonia (pH 6–8) and ammonium chloride–ammonia (pH 8–10). All other reagents were of analytical reagent grade or better. Double distilled water was used throughout the entire study. The pipettes and vessels used for trace analysis were kept in 10% nitric acid for at least 24 h and subsequently washed four times with double distilled water. 2.3. Procedures For CPE, aliquots of 10 mL of a solution containing the analyte, Triton X-114 and PMBP buffered at a suitable pH were kept in the thermostatic bath maintained at 40 ◦ C for 20 min, and the surfactant-rich phase can settle through the aqueous phase. The phase separation could be accelerated by centrifuging for 5 min at 3000 rpm. After cooling in an ice bath, the surfactant-rich phase became viscous and was retained at the bottom of the tube. The aqueous phases can readily be discarded simply by inverting the tubes. To decrease the viscosity of the extract and allow its pipetting, 200 ␮L of 0.1 mol L−1 HNO3 was added to the surfactant-rich phase. 20 ␮L of the diluted extract was introduced into the GFAAS by manual injection. Calibration was performed against aqueous standards submitted to the same CPE procedure. A blank submitted to the same

H. Sang et al. / Journal of Hazardous Materials 154 (2008) 1127–1132

Fig. 2. Ashing curves (a) and atomization curves (b) for Al(III) with and without CPE procedure. Al(III): 10 ng mL−1 ; CPE conditions: 2 × 10−4 mol L−1 PMBP, 1.0 g L−1 Triton X-114, pH 7.0.

procedure was measured parallel to the samples and calibration solutions.

1129

Fig. 3. Effect of pH on the extraction recovery. 10 ng mL−1 Al(III), 2 × 10−4 mol L−1 PMBP and 1.0 g L−1 Triton X-114.

fact that the chelate formation of Al(III) with PMBP prohibits the formation of Al carbide, and alters the atomization mechanism of aluminum [33]. From the ashing and atomization curves, the optimal ashing and atomization temperatures are 1300 ◦ C and 2200 ◦ C for Al(III), respectively.

2.4. Preparation of samples Portions (0.5000 g) of standard reference material of human hair (GBW09101, PR China) were transferred into PTFE beakers, 10 mL of concentrated HNO3 and 3 mL of H2 O2 were added, heated until the solution become transparent, continuously heated to near dryness. The residue was dissolved in 0.1 mol L−1 HNO3 and made up to 50 mL with distilled water. For preparation of human urine, 1.0 mL urine was transferred into 50 mL volumetric and diluted to the mark with distilled water. Lake water sample was collected from East Lake (Wuhan, PR China), and tap water sample was freshly collected from a laboratory, after allowing the water to flow for 5 min. All the water samples were filtered through a 0.45 ␮m pore size membrane filter to remove suspended particulate matter and were stored at 4 ◦ C in the dark. 3. Results and discussion 3.1. Ashing and atomization curves Ashing and atomization curves were established using 10 ng mL−1 Al(III) solutions submitted to CPE procedure and diluted with 10 mL of 0.1 mol L−1 HNO3 . 20 ␮L of the diluted extract was used for GFAAS analysis. The ashing and atomization curves of Al(III) without CPE procedure were also studied with 10 ng mL−1 Al(III) in 0.1 mol L−1 HNO3 . Fig. 2 shows the ashing and atomization curves for Al(III) with and without CPE procedure. As can be seen, with CPE procedure, the ashing temperature could be increased by 500 ◦ C over the Al(III) solution without CPE procedure, and the aluminum signal was enhanced by two times. There was no difference in the shape of the atomization curve for aluminum with and without CPE procedure, only the values of absorbance were different. This is due to the

3.2. Effect of pH on CPE The extraction of metal ion by CPE method involves the formation of a metal-chelate with sufficient hydrophobic to be extracted into the small volume of surfactant-rich phase, thus obtaining the desired preconcentration. The pH plays a unique role on metal-chelate formation and subsequent extraction, and is proved to be a main parameter for CPE method. It has been reported that the optimal pH range of chelate formation between Al(III) and PMBP in water solution was pH 5–10 [34]. Fig. 3 shows the effect of pH on the extraction recovery of Al(III), which were calculated based on the amounts of analyte in the starting samples and in the surfactantrich solution after extraction. It can be seen that quantitative extraction (>95%) was obtained for Al(III) in the pH range of 6.0–9.0. Hence, a pH of 7.0 was chosen for the subsequent work. 3.3. Effect of PMBP concentration 10 mL of a solution containing 0.1 ␮g of Al(III) in 1.0 g L−1 Triton X-114 and at a medium buffer of pH 7.0 containing varTable 2 Tolerance limits of coexisting ions Coexisting ions

Foreign ion to analyte ratio

K+ ,

5000 1000 100 50 5000

Na+

Ca2+ , Mg2+ , Ba2+ Cu2+ , Mn2+ , Zn2+ , Cd2+ , Ni2+ , Pb2+ Cr3+ , Fe3+ SO4 2− , Cl− , NO3 −

Al(III): 10 ng mL−1 ; CPE conditions: 2 × 10−4 mol L−1 PMBP, 1.0 g L−1 Triton X-114, pH 7.0.

1130

H. Sang et al. / Journal of Hazardous Materials 154 (2008) 1127–1132

Table 3 Comparison of the published methods with the proposed method in this work Separation method

Detection method

Enrichment factor

Detection limit (␮g L−1 )

Reference

LLE SPE SPE CPE CPE CPE CPE CPE

ETAAS Spectrophotometry GFAAS Spectrophotometry ICP-AES Spectrofluorimetry Spectrophotometry GFAAS

3 20 150 50 200 10 20 37

0.3 0.3 0.02 3.0 0.25 0.79 0.52 0.09

[7] [10] [11] [29] [30] [31] [32] This work

ious amounts of PMBP were subjected to the CPE process. The extraction recovery increased up to a PMBP concentration of 1.5 × 10−4 mol L−1 and reaches near 100%. A PMBP concentration of 2.0 × 10−4 mol L−1 was chosen to account for other extractable species that might potentially interfere with the assaying of Al(III). 3.4. Effect of Triton X-114 concentration A successful CPE would be that maximizes the extraction efficiency through minimizing the phase volume ratio, thus maximizing its enrichment factor. The variation in extraction efficiency of aluminum within the Triton X-114 range of 0.1–2.0 g L−1 was examined. Quantitative extraction was observed when Triton X-114 concentration above 0.9 g L−1 . So a concentration of 1.0 g L−1 was chosen as the optimum surfactant concentration in order to achieve the highest possible extraction efficiency. 3.5. Effects of equilibration temperature and time It was desirable to employ the shortest equilibration time and the lowest possible equilibration temperature, as a compromise between completion of extraction and efficient separation of phases. The dependence of extraction efficiency upon equilibration temperature and time was studied with a range of 25–60 ◦ C and 5–30 min, respectively. The results showed that an equilibration temperature of 40 ◦ C and a time of 20 min were adequate to achieve quantitative extraction. Table 4 Determination of Al(III) (ng mL−1 ) in biological and water samples (n = 5) Samples

Added

Founda

Human urine

0 10 20

21.5 ± 0.9 31.8 ± 1.5 41.1 ± 2.2

– 103 98

Tap water

0 10 20

6.4 ± 0.3 16.0 ± 0.7 26.2 ± 1.2

– 96 99

Lake water

0 10 20

11.1 ± 0.6 20.6 ± 0.8 30.3 ± 1.5

– 95 96

a

Mean of five determinations.

3.6. Interferences In the view of the high selectivity provided by GFAAS, the interferences studied were those related to the preconcentration step. Cations that may react with PMBP and extracted to the micelle phase were studied. For the study, 10 mL of solution containing 10 ng mL−1 Al(III) and interfere ion in different interferent-to-analyte ratios was subjected to the extraction procedure. The tolerance limits of the coexisting ions, defined as the largest amount making the recovery of Al(III) changed less than 5%, are given in Table 2. It can be seen that the major cations and anions in water samples have no obvious influence on CPE of Al(III) under the selected conditions. 3.7. Characteristics of the method Under the optimal experimental conditions, the calibration curve for Al(III) is linear up to 90 ng mL−1 with a correlation coefficient (r) of 0.9981. The relative standard deviation (R.S.D.) for seven samples of 10 ng mL−1 of Al(III) subjected to the complete procedure is 4.7%. The limit of detection (LOD) of this method, calculated as three times the standard deviation of the blank signals, is 0.09 ng mL−1 . The enrichment factor, calculated as the ratio of absorbance of preconcentration sample to that obtained without preconcentration, is 37. Table 3 compares the characteristic data of the present method with those reported in literatures. Generally, the enrichment factor obtained by the present method is comparable to those reported methods, and the detection limit is better than them. The higher enrichment factor reported in Refs. [11,30] was obtained by using 750 mL and 50 mL sample solution. The enrichment factor and LOD of our work could be improved by using larger volume of sample solution.

Recovery (%)

3.8. Analysis of biological and water samples To validate the proposed method, the contents of aluminum in certified reference material of human hair (GBW09101, PR China) were determined by the method. The determined value (13.5 ± 1.5 ␮g g−1 , n = 5) was not significantly different from the certified value (13.3 ± 2.3 ␮g g−1 ). The proposed method has been applied for the determination of aluminum in human urine, tap and lake water samples collected in Wuhan, PR China. In addition, the recovery exper-

H. Sang et al. / Journal of Hazardous Materials 154 (2008) 1127–1132

iments of different amounts of aluminum were carried out, and the results were shown in Table 4. The results indicated that the recoveries were reasonable for trace analysis, in a range of 95–103%. 4. Conclusions In this work, the use of micelle systems as a separation and preconcentration for aluminum offers several advantages including low cost, safety, preconcentration aluminum with high recoveries and very good extraction efficiency. The surfactantrich phase can be easily introduced into the graphite furnace after dilution with 0.1 mol L−1 HNO3 and directly determined by GFAAS. The proposed method can be applied to the determination of trace amount of aluminum in various real samples. Acknowledgement Financial support from National Nature Science Foundation of China (Grant no. 20705010) is gratefully acknowledged. References [1] J. Tria, E.C.V. Butler, P.R. Haddad, A.R. Bowie, Determination of aluminium in natural water samples, Anal. Chim. Acta 588 (2007) 153– 165. [2] R.A. Yokel, M.S. Golub (Eds.), Research Issues in Aluminum Toxicity, Taylor & Francis, Bristol, PA, 1997. [3] G.L. Klein, Aluminum: new recognition of an old problem, Curr. Opin. Pharmacol. 5 (2005) 637–640. [4] M.L. Hegde, P. Shanmugavelu, B. Vengamma, T.S.S. Rao, R.B. Menon, R.V. Rao, K.S.J. Rao, Serum trace element levels and the complexity of inter-element relations in patients with Parkinson’s disease, J. Trace Elem. Med. Biol. 18 (2004) 163–171. [5] A. Shokrollahi, M. Ghaedi, M.S. Niband, H.R. Rajabi, Selective and sensitive spectrophotometric method for determination of sub-micromolar amounts of aluminium ion, J. Hazard. Mater. 151 (2008) 642– 648. [6] M. Buratti, C. Valla, O. Pellegrino, F.M. Rubino, A. Colombi, Aluminum determination in biological fluids and dialysis concentrates via chelation with 8-hydroxyquinoline and solvent extraction/fluorimetry, Anal. Biochem. 353 (2006) 63–68. [7] J. Komarek, R. Cervenka, T. Ruzicka, V. Kuban, ET-AAS determination of aluminium in dialysis concentrates after continuous flow solvent extraction, J. Pharm. Biomed. Anal. 45 (2007) 504–509. [8] S. Kneqevic, R. Milacic, M. Veber, ETAAS determination of aluminum and copper in dialysis concentrates after microcolumn chelating ion-exchange preconcentration, Fresen. J. Anal. Chem. 362 (1998) 162–166. [9] S.B. Erdemoglu, K. Pyrzyniska, S. Gucer, Speciation of aluminum in tea infusion by ion-exchange resins and flame AAS detection, Anal. Chim. Acta 411 (2000) 81–89. [10] M.B. Luo, S.P. Bi, Solid phase extraction-spectrophotometric determination of dissolved aluminum in soil extracts and ground waters, J. Inorg. Biochem. 97 (2003) 173–178. [11] I. Narin, M. Tuzen, M. Soylak, Aluminium determination in environmental samples by graphite furnace atomic absorption spectrometry after solid phase extraction on Amberlite XAD-1180/pyrocatechol violet chelating resin, Talanta 63 (2004) 411–418. [12] O.Y. Nadzhafova, O.A. Zaporozhets, I.V. Rachinska, L.L. Fedorenko, N. Yusupov, Silica gel modified with lumogallion for aluminum determination by spectroscopic methods, Talanta 67 (2005) 767–772.

1131

[13] L.B. Xia, B. Hu, Z.C. Jiang, Y.L. Wu, L. Li, R. Chen, 8-Hydroxyquinolinechloroform single drop microextraction and electrothermal vaporization ICP-MS for the fractionation of aluminium in natural waters and drinks, J. Anal. Atom. Spectrom. 20 (2005) 441–446. [14] A. Sanz-Medel, M.R. Fernandez de la Campa, E.B. Gonzalez, M.L. Fernandez-Sanchez, Organised surfactant assemblies in analytical atomic spectrometry, Spectrochim. Acta 54B (1999) 251–287. [15] F.H. Quina, W.L. Hinze, Surfactant-mediated cloud point extraction: an environmentally benign alternative separation approach, Ind. Eng. Chem. Res. 38 (1999) 4150–4168. [16] A.E. Fernandez, Z.S. Ferrera, J.J.S. Rodriguez, Application of cloud-point methodology to the determination of polychlorinated dibenzofurans in sea water by high-performance liquid chromatography, Analyst 124 (1999) 487–491. [17] R. Carabias-Martinez, E. Rodriguez-Gonzalo, J. Dominguez-Alvarez, J. Hernandez-Mendez, Cloud point extraction as a preconcentration step prior to capillary electrophoresis, Anal. Chem. 71 (1999) 2468–2474. [18] C.D. Stalikas, Micelle-mediated extraction as a tool for separation and preconcentration in metal analysis, Trends Anal. Chem. 21 (2002) 343– 355. [19] M.A. Bezerra, M.A.Z. Arruda, S.L.C. Ferreira, Cloud point extraction as a procedure of separation and pre-concentration for metal determination using spectroanalytical techniques: a review, Appl. Spectrosc. Rev. 40 (2005) 269–299. [20] M.F. Silva, L. Fernandez, R. Olsina, D. Stacchiola, Cloud point extraction, preconcentration and spectrophotometric determination of erbium(III)2-(3,5-dichloro-2-pyridylazo)-5-dimethylaminophenol, Anal. Chim. Acta 342 (1997) 229–238. [21] S. Igarashi, K. Endo, Room temperature phosphorescence of palladium(II)–coproporphyrin III complex in a precipitated nonionic surfactant micelle phase: determination of traces of palladium(II), Anal. Chim. Acta 320 (1996) 133–138. [22] J. Li, P. Liang, T.Q. Shi, H.B. Lu, Cloud point extraction preconcentration and inductively coupled plasma optical emission spectrometry determination of trace chromium and copper in water samples, Atom. Spectrosc. 24 (2003) 169–172. [23] S.A. Kulichenko, V.O. Doroschuk, S.O. Lelyushok, The cloud point extraction of copper(II) with monocarboxylic acids into non-ionic surfactant phase, Talanta 59 (2003) 767–773. [24] J.L. Manzoori, A.B. Tabrizi, Cloud point preconcentration and flame atomic absorption spectrometric determination of Cd and Pb in human hair, Anal. Chim. Acta 470 (2002) 215–221. [25] X.H. Zhu, X.S. Zhu, B.S. Wang, Cloud point extraction for speciation analysis of inorganic tin in water samples by graphite furnace atomic absorption spectrometry, J. Anal. Atom. Spectrom. 21 (2006) 69–73. [26] Z.M. Sun, P. Liang, Q. Ding, J. Cao, Determinationof trace nickel in water samples by cloud point extraction preconcentration coupled with graphite furnace atomic absorption spectrometry, J. Hazard. Mater. 137 (2006) 943–946. [27] A.N. Tang, D.Q. Jiang, Y. Jiang, S.W. Wang, X.P. Yan, Cloud point extraction for high-performance liquid chromatographic speciation of Cr(III) and Cr(VI) in aqueous solutions, J. Chromatogr. A 1036 (2004) 183–188. [28] M. Ghaedi, A. Shokrollahi, F. Ahmadi, H.R. Rajabi, M. Soylak, Cloud point extraction for the determination of copper, nickel and cobalt ions in environmental samples by flame atomic absorption spectrometry, J. Hazard. Mater. 150 (2008) 533–540. [29] L. Sombra, M. Luconi, M.F. Silva, R.A. Olsina, L. Fernandez, Spectrophotometric determination of trace aluminium content in parenteral solutions by combined cloud point preconcentration-flow injection analysis, Analyst 126 (2001) 1172–1176. [30] L.L. Sombra, M.O. Luconi, L.P. Fernandez, R.A. Olsina, M.F. Silva, L.D. Martinez, Assessment of trace aluminium content in parenteral solutions by combined cloud point preconcentration-flow injection inductively coupled plasma optical emission spectrometry, J. Pharm. Biomed. Anal. 30 (2003) 1451–1458. [31] A.B. Tabrizi, Cloud point extraction and spectrofluorimetric determination of aluminium and zinc in foodstuffs and water samples, Food Chem. 100 (2007) 1698–1703.

1132

H. Sang et al. / Journal of Hazardous Materials 154 (2008) 1127–1132

[32] M. Bahram, T. Madrakian, E. Bozorgzadeh, A. Afkhami, Micelle-mediated extraction for simultaneous spectrophotometric determination of aluminum and beryllium using mean centering of ratio spectra, Talanta 72 (2007) 408–414. [33] F.Y. Wang, B. Hu, Z.C. Jiang, Y.L. Wu, Study on the chemical modification of acetylacetone in graphite furnace atomic absorption spec-

trometry determination of trace aluminum, Anal. Lett. 35 (2002) 2593– 2602. [34] E.W. Berg, J.T. Truemper, Vapor pressure–temperature data for various metal ␤-diketone chelates, Anal. Chim. Acta 32 (1965) 245– 252.