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Simultaneous determination of aluminium and beryllium at the subnanogram per millilitre level by solid-phase derivative spectrophotometry
ANALmcA
CHIMICA ACT4
ELSEVIER
Analytica Chimica Acta 327 (1996) 73-82
Simultaneous determination of aluminium and beryllium at the subnanogram per millilitre level by solid-phase derivative spectrophotometry Ma Carmen Valencia, Department
Said Boudra, J. Manuel Bosque-Se&a*
of &uaiytical Chemistry, Faculty of Sciences, University of Granada, 18071 Granada, Spain
Received 3 August 1995; revised 2 January 1996; accepted 6 January 1996
Abstract The applicability of derivative solid-phase spectrophotometry is demonstrated for the resolution of mixtures of aluminium and beryllium with closely overlapping absorption profiles. A spectrophotometric method for the simultaneous determination of beryllium and ahrminium based on first-derivative solid-phase spectrophotometry is proposed. Both metal ions were fixed on a dextran-type anion-exchanger gel at pH 4.4, as coloured complexes with Eriochrome Cyanine R in the presence of ethylenediaminetetra-acetic acid. The absorption spectrum of the gel, packed in a 1 mm cell, was recorded directly. The application range is up to 60ng ml-’ for aluminium and up to 4.0ngml-’ for beryllium, and the RSD is 3.5% and 3.8%, respectively. The detection limit is 0.04 ng ml-’ for beryllium and 2.25 ng ml-’ for altinium. The relative error obtained in the analysis of a synthetic aqueous solution is 1.50% for beryllium and 1.75% for aluminium. The method was applied to the determination of both analytes in natural water, orange juice, geological and botanical samples. Keywords: Eriochrome Cyanine R; Aluminium and beryllium determination; Spectrophotometry; Waters; Geological samples; Botanical samples; Fruit juices
1. Introduction Numerous complexes of Be(B) and Al(III) with azo reagents, xanthene and triphenylmethane reagents, Eriochrome Cyanine R (ECR) [ 1,2] and Chrome Azurol S [3,4] have been proposed for the spectrophotometric determination of both the elements individually. Of these reagents, ECR, the sodium salt of sulfohydroxy dimethylfuchsone dicarboxylic acid, has been employed for the determination of
* Corresponding author. SOOO3-2670/96/$15.000 1996 Elsevier Science B.V. All rights reserved PZZSOOO3-2670(96)00059-l
both ions through the corresponding red complexes formed at slightly acidic pH [ 1,2]. The determination of aluminium and beryllium in mixtures is difficult because these two elements mutually interfere in the method owing to a considerable overlap of the spectra. However, spectrophotometry in combination with derivative techniques has been shown to be useful in the analysis of mixtures of analytes when their spectra show overlapping. Hence, it has been applied to the analysis of mixtures of uranium and thorium [5], beryllium and ahnninium [6] or uranium and beryllium [7].
14
MC.
Kzlencia et al./Analytica
Solid-phase spectrophotometry (SPS) [8] is based on the direct absorptiometric measurements of coloured compounds fixed on a solid support. Methods for the determination of individual compounds have been proposed using both derivative techniques and SPS [9,10], but the resolution of mixtures of compounds is only used for the simultaneous determination of sulphonamides [ 1 l] of samarium and europium [12] without preceding complex formation. In this paper we propose a simple, sensitive and rapid method for the simultaneous determination of micro amounts of aluminium and beryllium using ECR with first-derivative SPS. The determination of these analytes in natural water, geological samples, vegetable tissues, and fruit juices was accomplished satisfactorily.
Chimica Acta 327 (1996) 73-82
2.2. Apparatus A Perkin-Elmer Lambda 2 UV/Vis spectrophotometer connected to an IE 486 computer fitted with a Perkin software package (PECSS) was used for all the measurements and treatment of data. Furthermore, an Agitaser Model 2000 rotatingbottle agitator, an URA 2610 desk centrifuge and a Crison Digit-501 pH-meter with a combined glasscalomel electrode were used. The microwave system used for sample preparation was a Moulinex microwave oven Model 430, with 100% power at 665 W and 30 1 capacity, and a microwave acid digestion bomb of 25ml volume designed for pressures of up to 8 x lo6 Pa. Atomic spectroscopic determinations were carried out with a Perkin-Elmer 5000 atomic absorption spectrophotometer equipped with an HGA-500 graphite furnace.
2. Experimental 2.3. Absorbance
measurements
2.1. Reagents All chemicals used were of analytical grade unless stated otherwise, the water used for dilution of reagents and samples was pre-treated by reverse osmosis and all experiments were carried out at room temperature. ECR (Carlo Erba) solution: Prepared by dissolving the necessary amount of the dye in reverse osmosis water and adjusting the final acid concentration to 5x 10F3M with HCl. This solution was freshly prepared every day. Beryllium stock solution (0.1 g 1-l): Prepared from BeS04.4Hz0 (Merck) in 6.04 x lo-’ M HC104 [ 131. Aluminium stock solution (0.1 g 1-l): Prepared from Alz(SO,&. 18HzO (Merck) in 6.04 x 1O-2 M HC104 [ 141. Ethylenediaminetetra-acetic acid solution (EDTA), 0.0165 M: Prepared from C1&11&Na20s.2H20 (Riedel-de-Ha&r), dissolved in reverse osmosis water. Solid ion exchanger. Sephadex DEAE A-25 anionexchanger (Sigma) was used in the chloride form as received from the supplier and without pre-treatment in order to avoid contamination. Buffer solutions. Solutions of the required pH were prepared from 2M sodium acetate solution (Merck) and 2 M acetic acid (Merck).
The absorbance of the reaction product sorbed on the exchanger was measured at the maximum absorption maximum wavelength of the complex A and at 750nm (the latter wavelength is within thtyange where only the exchanger absorbs light) against a 1 mm cell well packed with resin equilibrated with blank solution. The difference has been used to evaluate the concentration [ 151. The first-order derivative spectra, however, obtained from the stored data of zero order spectra, eliminate the necessity for measurements at two wavelengths, as the amplitude of derivative plots of the complex is proportional to the concentration of the analyte in solution. 2.4. Basic procedure An appropriate sample volume containing between 0.5 and 4.0 pg of Be(I1) and between 10 and 40 ug of Al(III) was made up to 1OOOml with water obtained by reversed osmosis and transferred to a 21 polyethylene bottle. Then, 1.5 ml of EDTA of 0.0165 M, 2ml ECR 7.46x 10m3M, 5 ml of buffer solution (pH 4.4), and 0.08Og of the Sephadex DEAE A-25 anion-exchanger were added. The mixture was shaken mechanically for 45min. The coloured
M.C. klencia
exchanger beads were then collected by filtration under suction and, with the aid of a pipette, were packed into a 1 mm cell together with a small portion of the filtrate. The cell was centrifuged for 1 min at 25 g. A blank solution containing all of the reagents was prepared in the same way as described for the sample. The absorption spectra were recorded between 400 and 750nm with a scan speed of 480 nm rnin-’ against the blank and stored in a disk file. The lirstderivative spectra were calculated by the SavitzkyGolay method [ 16,171 using a window of 19 nm. First-derivative measurements were taken as the vertical distance from the first-derivative spectrum at 594.0nm (lD5s4.c) to the baseline for aluminium and from 574.0 nm (1Ds74,0) to the baseline for beryllium. Calibration graphs were constructed in the same way using aluminium and beryllium of known concentrations.
3. Weatment
75
et al.LAnalytica Chimica Acta 327 (19%) 73-82
of samples
3.1. Natural water sample
to treatment and assay to verify the traceability of the method. 0.1 g of the sample was weighted into a PTFE crucible and 2 ml of concentrated sulphuric acid was added, and the crucible was heated on a sand bath for 20min. 3 ml of concentrated hydrofluoric acid was added drop by drop to avoid foam formation. After complete dissolution of the solid, the mixture was heated until the volume of the solution was reduced by l-2 ml. The solution was finally diluted to 100 ml with reverse osmosis water in a calibrated flask. Alliquots were analysed as described in Section 2.4. 3.4. Botanical
samples
Two reference materials, ecorce de pin and ryegrass, from Comite Inter-Instituts (CII, France) (obtained for inter-laboratory tests) were subjected to digestion. 0.2 g of the ecorce de pin or of the ryegrass sample were weighed into a FTFE vessel. Then, the digestion agent was added (1 ml of concentrated nitric acid) and the vessel was heated in the Moulinex microwave system at 665 W for 30 s [20]. Then the bomb was cooled and an adequate volume (up to 5-1Oml) of water was added. The solution was shaken and transferred into a test tube. The digest was diluted with water so that the concentrations of the elements fell within the working range of the proposed method and Al@) and Be@) were determined, as described in Section 2.4.
The water samples were preserved by the addition of nitric acid (2ml of concentrated nitric acid per litre) and collected in a polyethylene container that was carefully cleaned with nitric acid. Prior to the the water samples were filtered measurements, through 0.45 urn pore size membrane filters (Millipore, Type HA). The sample was stored at 4°C until analysis and the usual general precautions were taken to prevent contamination [ 181.
4. Results and discussion
3.2. Orange juices
4.1. Effect of experimental
40 ml of the sample were centrifuged for 15 min in a test tube at 2000rpm (2Og), and the supernatant was separated and filtered through 0.45 pm pore size membrane filters (Millipore, Qpe HA). The content of the analytes was determined in an aliquot, as described in Section 2.4.
On the Sephadex DEAE A-25 anion-exchanger, ECR forms a ternary complex with beryllium and EDTA (pH 5.0), with an absorption maximum at 580nm [13] and a complex with aluminium (pH 3.8), with an absorption maximum at 590~ [14]. In order to obtain a net absorbance signal for both complexes simultaneously, a study of the influence of the pH was performed. Fig. 1 shows this influence, in which it can be seen that at a pH of 4.4 both complexes exhibit equal absorbance, and therefore, this pH was selected for the simultaneous determina-
3.3. Geological
sample
A reference material (Granite de Beauvoir, MA-N) certified for inter-laboratory tests [19] was subjected
variables
16
MC. Kzlencia et ai./Analytica
Chimica Acta 327 (1996) 73-82
The order of addition used here was: EDTA + ECR + buffer + exchanger.
$ c, g 0.2-
01 3
II)” I
3.5
+
B
A
8 0.6% 2 $0.4-
analyte
I
1
4
4.5
I
5
.
4.2. Spectrophotometric
I
5.5
PH Fig. 1. Influence of pH on colour development. Conditions: lOpgl-’ of aluminium, lOpgl-’ of beryllium, ECR=1.49x lo-’ M, EDTA=8.25 x 10e6 M, 80 mg Sephadex DEAE A-25, sample volume 5OOml. stirring time 15 min. 1 mm optical path length.
tion. The best of the buffer systems examined was acetic acid-sodium acetate. The net absorbance of these complexes increases with ECR concentration, but at the same time the “background noise” also increases. In consequence, we chose a concentration of 1.5 x lop5 M of ECR as the most appropriate value for the simultaneous determination of both complexes using the basic procedure. EDTA forms a ternary complex with beryllium and ECR with enhanced absorbance but its presence decreases the net absorbance of the aluminium-ECR complex. The concentration of EDTA must be chosen as a compromise between obtaining the strongest signal for the beryllium complex (maximum concentration of EDTA) and obtaining a sufficiently sensitive signal for the aluminium complex, due to the very low beryllium to aluminium molar ratio in the natural samples. A volume of 1.5 ml of 0.0165 M EDTA was chosen as the most suitable. The optimum stirring time was 45min for a 1OOOml sample system. Under these conditions, the absorbances remained stable for at least 2.5 h after equilibration. The centrifugation time used was 20 s at 2000 t-pm (20 g). For all measurements, unless stated otherwise, 0.08g of exchanger was used as a compromise between maximum sensitivity and manageability.
measurements
Fig. 2 shows the net absorption spectrum of the aluminium complex, which has an absorption maximum at 594nm (spectrum A), and of the beryllium complex, with an absorption maximum at 574 nm (spectrum B). The absorption spectrum of the mixture of both complexes (Be(II)/Al(III)) molar ratio 0.3) with a maximum at 584nm was located between the absorption maxima of the two components (spectrum C). All measurements were performed against a reagent blank because the reagent also absorbs at these wavelengths. As can be seen from Fig. 2, because of a large overlap between the two spectra, significant diff~culties arise in the spectrophotometric determination of these ions when they are present in the same sample. To overcome this problem, derivative spectrophotometry can be used; this involves the differentiation of zero-order spectrum with respect to wavelength. Fig. 3 shows the first-derivative absorption spectra of individual complexes of Al@) (curve A) and Be@) (curve B) and a mixture of both complexes (curve C) with ECR, in the presence of EDTA. It can be seen that, because of the closeness of the two overlapping spectra of the aluminium and beryllium
Wavelength
(nm)
Fig. 2. Net absorption spectra ECR complexes at pHd.4: (A) Al@)-ECR complex; (B) Be(B)-EDTA-ECR complex; (C) mixture of both complexes. Conditions: 20~1~’ of aluminium, 2 pgl-’ of beryllium, ECR=~.SX~O-~M, EDTA=2.5x lo-‘M, 80mg Sephadex DEAE A-25, sample volume lOOOml, sting time 45 min, 1 mm optical path length.
M.C. tilencia et al./AnalyticaChimicaActa 327 (19%) 73-82
17
Ax were tested, and a width of 19 nm was selected to obtain the best signal-to-noise ratio. The response time is automatically selected by the spectrophotometer in accordance with the optical energy and scanning speed. 4.4. Analytical mination mixtures -0.2
! 520
640 560 Wavelength
700
760
(nm)
Fig. 3. First-derivative absorption spectra of ECR complexes: (A) Al(III)-ECR complex; (B) Be(II)-EDTA-ECR complex; (C) mixture of both complexes. Conditions: 2Opgl-’ of aluminium, of beryllium, ECR=1.5 x 1O-5 M, EDTA=2.5 x 10e5 M, 2 pgl-’ pH=4.4, 80mg Sephadex DEAE A-25, sample volume lOOOml, stirring time 4.5 min. Scan speed 480 nm/min, smoothing 19 points, Ax 19nm.
complexes, these spectra are not sufficiently resolved to give two distinct peaks in the first-derivative spectrum of the mixture of both complexes. For this reason, the zero-crossing method [21] was used to resolve the mixture. The heights hi and h2 in the firstderivative spectrum of the mixture (Fig. 3, curve C) corresponding to the values measured at 574.0 and 594.Omn are proportional to the aluminium and beryllium concentration, respectively. 4.3. Selection of optimum instrumental conditions The instrumental parameters affecting the shape of the derivative spectra are: the scan speed, smoothing and wavelength increment, which must be optimised to obtain a good selectivity and higher sensitivity in the determination. The scan speed does not affect the derivative signal in the interval between 120 and 480 nm/min-‘, but higher speeds reduce the signal. Hence, a fast wavelength scanning speed of 480 nm rnin-’ was selected. For the smoothing operation of the spectra, the Savitzky-Golay method with windows comprising 5-19 points was used. The number of points has no appreciable effect on the first-derivative signal. For the calculation of the first-derivative spectra different
parameters for simultaneous deterof aluminium and beryllium in
The “lack of fit” test (LOF) was used to check the calibration linearity of the method for every analyte. Aluminium was tested over a O-60 ugl-’ range (PLor=43%) and beryllium was tested over O4.0 ug 1-l range (PLoF=29%). With the aim of proving the mutual independence of the analytical signals of Al@) and Be@), that is to confirm that hi and h2 are independent of aluminium and beryllium concentrations, respectively, four calibration graphs were obtained from height (h) measurements for standards containing beryllium (between 0.5 and 4 ug 1-l) or aluminium (between 10 and 40 ug 1-l) both in the presence of different concentrations of the other ion and in its absence. The slope, intercept, standard deviations of the slope and intercept, determination coefficient and standard deviation of the regression (S& obtained for all the calibration graphs (n=5) are summarized in Table 1. For both ions, aluminium and beryllium, the slope of the calibration line in the absence of the other metal ion is not significantly different from the slope of the calibration lines in its presence (P-value > 30% in all cases). In addition, the intercept of every calibration graph is not significantly different from zero (P-value > 27% in all cases). Therefore, it can be concluded that the height of the derivative signal of the mixture at the zero-crossing point of the derivative spectrum of one of two components is a function of the concentration of the other component only. Morever, the values of the coefficient of determination (Z?) indicate good fitting of the experimental values for all the calibration graphs obtained. The standard deviations of the derivative spectrum measured for the blank (lDt,iank), obtained by measuring the height of the derivative signal at the zero-crossing point for aluminium (574.Onm) and beryllium (594.0nm) were 0.0015 A and 0.0010 A, respectively (n=9). The detection limits according to
78
MC.
Table 1 Statistical analysis of the determination phase spectrophotometry. Element determined
Valencia et al./Analytica
of almninium
(lo-40
Other metal present Element
Chimica Acta 327 (1996) 73-82
~g 1-l) and beryllium
Slope ((SD)x 10’)
(0.54
pg 1-I) in the mixtures
Intercept ((SD)x 103)
Concentration
by first-derivative
solid-
Determination Coefficient
Standard deviation
(R’)
((SR,J x ld)
(pg 1-Q ‘WII)
Be(B)
0.5 1.5 2.5
0.20 0.19 0.18 0.18
(0.01) (0.01) (0.01) (0.01)
-6.4 -3.2 -2.6 -2.9
(4.26) (2.50) (2.36) (2.25)
0.988 0.990 0.992 0.992
0.35 0.32 0.30 0.29
Be(K)
WII)
20 30 40
6.76 6.36 6.39 6.21
(0.43) (0.09) (0.08) (0.16)
-7.3 0.5 1.9 0.5
(10.6) (2.1) (2.2) (2.8)
0.988 0.992 1.000 0.998
1.23 0.31 0.24 0.40
Table 2 Effect of foreign ions on the determination Foreign ion
clPod3SiOg’N03so,2Ca(IU Mg(II) Mn(Q
Tolerance
of 20 pg 1-l of Al@) level (pg 1-l)
‘ww
Be@)
11000 9100 6000 5400 1000 3100 3700 3100
9000 5100 9800 6600 1000 3600 3500 600
IUPAC based on three times the standard deviation of the blank were O.O4pgl-’ for beryllium and 2.25 pg 1-t for aluminium and the quantitation limits (10 times the stand. dev.) were 0.15 ug I-’ for beryllium and 7.5 pgl-’ for altinium. The relative standard deviations (RSD, ~10) were 3.8% for 2 pg 1-l of beryllium and 3.5% for 20 pgl-’ of aluminium. 4.5. Effects of foreign
ions
The effect of foreign ions on the determination of aluminium and beryllium using 2 pg 1-l of beryllium and 2Opg 1-l of aluminium, was studied following the basic procedure. Tolerance is defined as the amount of foreign ions which produces an error of f 5% in the determination of the analyte. The results are summarized in Table 2.
and 2 c(g 1-l of Be(I1) Tolerance
Foreign ion
Pb(II) Cu(I1) Cd(I1) Fe(III) Zn(I1) WI) Ni(I1) Co(I1)
level (pg 1-l)
Al(II1)
Be@)
600 500 200 190 180 80 75 65
1000 700 220 350 190 12 85 75
4.6. Recovery of aluminium synthetic samples
and
beryllium
in
The proposed method was applied to the analysis of several synthetic mixtures of aluminium and beryllium in different molar ratios with the intention of checking its accuracy in particular conditions. Table 3 summarizes the results obtained for each molar ratio. The percentage of recovery is between 94% and 116% for beryllium and 94% and 103% for alum&urn showing that the accuracy is acceptable in all instances.
4.7. Analytical
applications
To test the applicability of the proposed method, it was used to simultaneously determine beryllium and
M.C. Valencia et al./Analytica Chimica Acta 327 (1996) 73-82
79
Table 3 Recovery study for almuinium and beryllium in synthetic mixture samples Be(II)/Al(III) molar ratio
0.037 0.100 0.150 0.200 0.300 0.300 0.450 0.608 0.600 1.200
Be(n)
Al(W
Added
Found
Recovery
Added
Found
Recovery
(P8 1-P
(cl8 1-P
(46)
(P8 1-Q
(P8 10
(S)
0.5 1.0 1.0 2.0 2.0 3.0 3.0 2.0 4.0 4.0
0.58 1.06 1.01 1.99 1.90 2.96 2.93 1.88 3.95 3.91
116 106 101 100 95 99 98 94 99 98
40 30 20 30 20 30 20 10 20 10
40.4 28.2 19.1 31.0 19.2 29.3 20.3 9.7 19.2 9.9
101 94 96 103 96 98 102 97 96 99
aluminium in different samples: solution, natural water, granite, grass and orange juice.
a synthetic aqueous Bcorce de pin, rye-
4.7.1. Determination of aluminium and beryllium in a synthetic aqueous solution With the aim of checking the accuracy of the method, synthetic aqueous solution was prepared containing aluminium and beryllium and the usual ions in natural water: NOs(9000 pgl-‘), ClSi032(8000 ug l-l), POa3(3000 ug 1-k (3000 ug l-l), soa2(1000 ug 1-k Ca(n) (3000 Pg 0, (3000 cLg1-‘), Mn(Q Mg(W (300 pg l-l), Cd@) (100 ug l-l), Cu(II) (100 pg l-l), Zn(II) (100 ug l-l), Pb@) (100 ug l-l), Fe@) (100 pg l-l), Ni(II) (50 ug l-l), Co@) (50 ug l-l), Al(III) (40 ugl-‘), U(VI) (10 ugl-‘) and Be(I1) (2ugl_‘). For all measurements, 1000 ml synthetic water was used and the samples were analysed as described in Section 2.4. The average value (5 determinations) and relative standard deviation (RSD%) for aluminium and beryllium concentrations calculated from the calibration graphs were 40.7 pgl-’ (2.58%) and 1.97 ugl-’ (2.68%), respectively, with a relative error of 1.75% for aluminium and 1.50% for beryllium. In addition, recoveries of 100-106% for aluminium and 95-102% for beryllium indicate the good accuracy of the method even in the presence of foreign ions.
4.7.2. Determination of aluminium and beryllium in natural water The proposed method was applied to the determination of aluminium and beryllium in water samples by the standard additions method. The constant error was corrected according to Cardone blank [22]. The results for aluminium and beryllium obtained by the proposed method and those from Atomic Absorption Spectrometry with Graphite Furnace (AASGF) for aluminium, are presented in Table 4. The statistical comparison by F-test and t-test showed no significant difference between the average value derived from the SPS method and from AASGF, and the precision of both methods is the same (the P-values are 33% and 13%, respectively). Because an appropriate reference method to compare the beryllium content obtained was not available, a validation study was carried out following the statistical protocol proposed by Cuadros Rodriguez et al. [23]. In this procedure and when a certain prerequisites are fulfilled, the accuracy of the results is tested by comparing the analyte contents obtained from the standard-additions calibration (AC) and the standard calibration (SC), using a t-test for the comparison of the two means. The application of the accuracy test in the present determination of beryllium indicated that ted = 0.455 < ttit = 2.145 (a = 0.05, Df = 14), that is to say there is no significant difference between the means obtained from the AC and SC graphs, and therefore it may be concluded that the method is accurate.
80 Table 4 Determination Sample
MC. Wencia
of aluminium
and beryllium
et al./Analytica
Chimicn Acta 327 (1996) 73-82
in different samples
Al(U)
Be(R)
(ml) Found (ug) Natural water 200.0 600.0 Grange juice 2.0 2.5
2.66 9.10
Content” (ugm I-‘)(RSD%)
0.013 (22.4) 0.015 (5.1)
26.90 33.53
13.5b (0.5) 13.4 (0.2)
31.37 33.70 34.83 34.35 33.03
12.5 (3.5) 13.5 (0.9) 13.9 (2.5) 13.7b (3.5) 13.2b (5.5)
Content by GFAAS” (ngm ll’)(RSD%)
0.014 (2.1)
14.0 (8.1)
Added
Found
ug
ug
Content” ugl-‘(RSD%)
-
0.102 0.34
0.51 (4.1) 0.57 (2.9)
-
-
-
1.00 2.00 3.00 1.05 2.10
0.94 2.01 3.21 1.05 2.10
400c 800” 1200” 420bsc 840b.”
(2.1) (0.6) (2.5) (2.3) (0.5)
“Each value is an average of three determinations. bEach value is an average of four determinations. ‘Hypothetical content according to sample dilution.
4.7.3. Determination of aluminium and beryllium in orange juice Different aliquots of a commercially available orange juice were analysed for the simultaneous determination of aluminium and beryllium using the basic procedure. Because no significant difference between the slopes of the standard calibration graph and the standard additions graphs was observed, the standard calibration graph was used in all cases. Table 4 shows the results obtained for each sample volume analysed. The average value (four determinations) of the beryllium concentration was lower than the IUPAC detection limit in all cases tested. To check the possibility of simultaneous determination of aluminium and beryllium in this sample, a recovery study for beryllium was carried out in which two different amounts of beryllium were added to 25Oml samples of orange juice prior to their centrifugation, and the aliquots were analysed as described in Section 2.4. The results, presented in Table 4, show that given values found for beryllium produce a recovery of lOO-lOl%, and that with respect to aluminium, the comparison with the results from AASGF showed no significant difference (Pvalue of 34%), we then have clear indications of the suitability of the proposed method for the samples. Furthermore, the precision of SPS method is similar than that obtained from AASGF.
4.7.4. Determination of aluminium and beryllium in geological samples The Basic Procedure (Section 2.4) was applied to aliquots of the solution obtained from a granite sample after the treatment previously described. The results obtained using the standard calibration graph are presented in Table 5. The statistical comparison by t-test showed no significant difference between the aluminium and beryllium values obtained from the proposed method and the certified values (the Pvalues are 9% and 33%, respectively). 4.7.5. Determination of aluminium and beryllium in botanical samples Aliquots of the two different reference materials were analysed using the Basic Procedure. The statistical comparison of the slopes of the standard calibration and the standard additions graphs, in both materials, showed no significant difference between them. In all the cases tested, the concentration of beryllium was lower than the IUPAC detection limit. To demonstrate the possibility of simultaneous determination of aluminium and beryllium, samples spiked with different amounts of beryllium were analysed. Table 5 summarizes the results obtained, showing at the same time that the recoveries for beryllium are acceptable: the comparison of the aluminium values found by the proposed method with
M.C. Wencia Table 5 Determination Sample
of aluminimn
and beryllium
et al./Andytica
in Geological
Chimica Acta 327 (19%) 73-82
and Botanical
AKIH)
81
samples Be(n)
(ml)
Granite de Beauvoirb Bcorce de pin’ 0.5 0.7
Ryegrass” 0.3 0.5 0.5
Found
Content”
Certified”
Added
Found
Content
Certified
(I.Lg)
(olg g-‘)(RSD%))
((pg g-‘)(RSD%))
(pg 1-l)
(pg 1-l)
(Olgg-‘)(RSD%))
((pg g-‘)(RSD%))
61.64*
17.98(1.8)
17.62
0.19*
293(3.7)
298(40.9)
25.67 31.32 31.30c 33.37 34.89
2445(1.7) 2131(3.1) 213OG.8) 2270(1.1) 2373(2.1)
2450(16.7)
9.92 17.04 17.18*
1637(1.3) 1687(10.6) 17OH8.3)
1608(7.5)
-
0.64 1.00 2.00
0.63” 1.00 1.98
48’(1.6) 68’(6.8) 135’(2.2)
-
0.45
0.45*
45’(2.4)
’ The content dete.rmin~ were Al203 in percentage, for Granite de Beauvoir. b Granite de Beauvoir, MA-N. Rock reference material: data are average values of 18 laboratories. ’ Reference Material from Conrite Inter-Instituts (CII, France). Data are average values of: &corce de pin, 11 and ryegrass, Each value is an average of three determinations, excepting * five and ’ four determinations. f Hypothetical content according to sample dilution.
the certified value 234% to perform analytes in
values show no significant difference (Pin all cases), indicating that it is possible the simultaneous determination of both these types of sample.
4.8. Conclusion An original, simple, sensitive and inexpensive method for the simultaneous determination of both aluminium and beryllium at the subnanogram per millilitre level by SPDS has been developed. It provides a high sensitivity with detection limits of 0.04 and 2.25 pgl-’ for beryllium and aluminium, respectively. The suitability of the proposed procedure has been demonstrated for the successful determination of both analytes in different types of samples with a wide concentration range.
References [l] U.T. Hill, Anal. Chem., 28 (1958) 521. 121 U.T. Hill, Anal. Chem., 31 (1959) 42. [3] K. Hayashi, Y. Sasaki, S. Tagashira and E. Kosaka, Chem., 58 (1986) 1444.
Anal.
10 laboratories.
[41 Z. Marczenko and M. Jarosz, Analyst, 107 (1982) 1431. L.K. Ravindranath and S. [51 A. Ramesh, J. Krishnamacharyulu, Brahmaji Rao, Analyst, 117 (1992) 1037. %I N. Kumar Agnihotri, H. Bhajan Singh, R. Lal Sharma and V Kumar Singh, Talanta, 40 (1993) 415. [71 N. Kumar Agnihotri, V. Kumar Singh and H. Bhajan Singh, Analyst, 120 (1995) 1809. L.F. Capit&-Vallvey, M.C. Valencia, J.M. VI F. Capit& Basque-Sendra, F. Molina and I. de Orbe, Analusis, 19 (1991) 177. [91 H. I&ii, Fresenius’ Z. Anal. Chem., 319 (1984) 23. WY M.I. Shtokalo, E.E. Kostenko and I.Z. Zhuk, J. Anal. Chem., 47 (1992) 1335. L.F. Capitan-Vallvey, I. de Orb-e, M.C. Valencia and J.J. [ill Berzas-Nevado, Anal. Chim. Acta, 282 (1993) 75. WI H. Ishii and K. Satoh, Fresenius’ Z. Anal. Chem., 312 (1982) 114. I131 MC. Valencia, S. Boudra and J. M Bosque-Sendra, Analyst., 118 (1993) 1333. M.C. Valencia and S. Boudra, H41 J. M Basque-Sendra, AnalLea., 8 (1994) 1579. M.C. Valencia and L.F. Capit&-Vallvey, u51 F. Capi& Miiochim. Acta, III (1984) 303. W51 A. Savitxky and M.J.E. Golay, AnaLChem., 36 (1964) 1627. 44 Cl71 J. Steiner, Y. Termonia and J. Deltour, Anal.Chem., (1972) 1906. WI American Public Health Asociation, American Water Works Association and Water Pollution Control Federation. Standard Methods for the Examination of Water and Wastewater, 15th Ed., A.P.H.A., Washington DC., 1981.
82 [19] [20] [21]
MC. Valencia et al./Analytica K. Govindaraju and I. Roelandts, Geostandards Newslett., 17 (1993) 227. M. Lachica, Analusis, 18 (1990) 331. T.C. O’Haver and G.L. Green, Anal. Chem., 48 (1976) 312.
Chimica Acta 327 (19%) 73-82 [22] [23]
M.J. Cardone, Anal. Chem., 58 (1986) 438. L. Cuadros Rodriguez, A.M. Garcia Campaiia, C. Jimenez Linares, F. AIt% Barrero and M. Rom&n Ceba, J. Assoc. Off. Anal. Chem., 78 (1995) 795.
CHIMICA ACT4
ELSEVIER
Analytica Chimica Acta 327 (1996) 73-82
Simultaneous determination of aluminium and beryllium at the subnanogram per millilitre level by solid-phase derivative spectrophotometry Ma Carmen Valencia, Department
Said Boudra, J. Manuel Bosque-Se&a*
of &uaiytical Chemistry, Faculty of Sciences, University of Granada, 18071 Granada, Spain
Received 3 August 1995; revised 2 January 1996; accepted 6 January 1996
Abstract The applicability of derivative solid-phase spectrophotometry is demonstrated for the resolution of mixtures of aluminium and beryllium with closely overlapping absorption profiles. A spectrophotometric method for the simultaneous determination of beryllium and ahrminium based on first-derivative solid-phase spectrophotometry is proposed. Both metal ions were fixed on a dextran-type anion-exchanger gel at pH 4.4, as coloured complexes with Eriochrome Cyanine R in the presence of ethylenediaminetetra-acetic acid. The absorption spectrum of the gel, packed in a 1 mm cell, was recorded directly. The application range is up to 60ng ml-’ for aluminium and up to 4.0ngml-’ for beryllium, and the RSD is 3.5% and 3.8%, respectively. The detection limit is 0.04 ng ml-’ for beryllium and 2.25 ng ml-’ for altinium. The relative error obtained in the analysis of a synthetic aqueous solution is 1.50% for beryllium and 1.75% for aluminium. The method was applied to the determination of both analytes in natural water, orange juice, geological and botanical samples. Keywords: Eriochrome Cyanine R; Aluminium and beryllium determination; Spectrophotometry; Waters; Geological samples; Botanical samples; Fruit juices
1. Introduction Numerous complexes of Be(B) and Al(III) with azo reagents, xanthene and triphenylmethane reagents, Eriochrome Cyanine R (ECR) [ 1,2] and Chrome Azurol S [3,4] have been proposed for the spectrophotometric determination of both the elements individually. Of these reagents, ECR, the sodium salt of sulfohydroxy dimethylfuchsone dicarboxylic acid, has been employed for the determination of
* Corresponding author. SOOO3-2670/96/$15.000 1996 Elsevier Science B.V. All rights reserved PZZSOOO3-2670(96)00059-l
both ions through the corresponding red complexes formed at slightly acidic pH [ 1,2]. The determination of aluminium and beryllium in mixtures is difficult because these two elements mutually interfere in the method owing to a considerable overlap of the spectra. However, spectrophotometry in combination with derivative techniques has been shown to be useful in the analysis of mixtures of analytes when their spectra show overlapping. Hence, it has been applied to the analysis of mixtures of uranium and thorium [5], beryllium and ahnninium [6] or uranium and beryllium [7].
14
MC.
Kzlencia et al./Analytica
Solid-phase spectrophotometry (SPS) [8] is based on the direct absorptiometric measurements of coloured compounds fixed on a solid support. Methods for the determination of individual compounds have been proposed using both derivative techniques and SPS [9,10], but the resolution of mixtures of compounds is only used for the simultaneous determination of sulphonamides [ 1 l] of samarium and europium [12] without preceding complex formation. In this paper we propose a simple, sensitive and rapid method for the simultaneous determination of micro amounts of aluminium and beryllium using ECR with first-derivative SPS. The determination of these analytes in natural water, geological samples, vegetable tissues, and fruit juices was accomplished satisfactorily.
Chimica Acta 327 (1996) 73-82
2.2. Apparatus A Perkin-Elmer Lambda 2 UV/Vis spectrophotometer connected to an IE 486 computer fitted with a Perkin software package (PECSS) was used for all the measurements and treatment of data. Furthermore, an Agitaser Model 2000 rotatingbottle agitator, an URA 2610 desk centrifuge and a Crison Digit-501 pH-meter with a combined glasscalomel electrode were used. The microwave system used for sample preparation was a Moulinex microwave oven Model 430, with 100% power at 665 W and 30 1 capacity, and a microwave acid digestion bomb of 25ml volume designed for pressures of up to 8 x lo6 Pa. Atomic spectroscopic determinations were carried out with a Perkin-Elmer 5000 atomic absorption spectrophotometer equipped with an HGA-500 graphite furnace.
2. Experimental 2.3. Absorbance
measurements
2.1. Reagents All chemicals used were of analytical grade unless stated otherwise, the water used for dilution of reagents and samples was pre-treated by reverse osmosis and all experiments were carried out at room temperature. ECR (Carlo Erba) solution: Prepared by dissolving the necessary amount of the dye in reverse osmosis water and adjusting the final acid concentration to 5x 10F3M with HCl. This solution was freshly prepared every day. Beryllium stock solution (0.1 g 1-l): Prepared from BeS04.4Hz0 (Merck) in 6.04 x lo-’ M HC104 [ 131. Aluminium stock solution (0.1 g 1-l): Prepared from Alz(SO,&. 18HzO (Merck) in 6.04 x 1O-2 M HC104 [ 141. Ethylenediaminetetra-acetic acid solution (EDTA), 0.0165 M: Prepared from C1&11&Na20s.2H20 (Riedel-de-Ha&r), dissolved in reverse osmosis water. Solid ion exchanger. Sephadex DEAE A-25 anionexchanger (Sigma) was used in the chloride form as received from the supplier and without pre-treatment in order to avoid contamination. Buffer solutions. Solutions of the required pH were prepared from 2M sodium acetate solution (Merck) and 2 M acetic acid (Merck).
The absorbance of the reaction product sorbed on the exchanger was measured at the maximum absorption maximum wavelength of the complex A and at 750nm (the latter wavelength is within thtyange where only the exchanger absorbs light) against a 1 mm cell well packed with resin equilibrated with blank solution. The difference has been used to evaluate the concentration [ 151. The first-order derivative spectra, however, obtained from the stored data of zero order spectra, eliminate the necessity for measurements at two wavelengths, as the amplitude of derivative plots of the complex is proportional to the concentration of the analyte in solution. 2.4. Basic procedure An appropriate sample volume containing between 0.5 and 4.0 pg of Be(I1) and between 10 and 40 ug of Al(III) was made up to 1OOOml with water obtained by reversed osmosis and transferred to a 21 polyethylene bottle. Then, 1.5 ml of EDTA of 0.0165 M, 2ml ECR 7.46x 10m3M, 5 ml of buffer solution (pH 4.4), and 0.08Og of the Sephadex DEAE A-25 anion-exchanger were added. The mixture was shaken mechanically for 45min. The coloured
M.C. klencia
exchanger beads were then collected by filtration under suction and, with the aid of a pipette, were packed into a 1 mm cell together with a small portion of the filtrate. The cell was centrifuged for 1 min at 25 g. A blank solution containing all of the reagents was prepared in the same way as described for the sample. The absorption spectra were recorded between 400 and 750nm with a scan speed of 480 nm rnin-’ against the blank and stored in a disk file. The lirstderivative spectra were calculated by the SavitzkyGolay method [ 16,171 using a window of 19 nm. First-derivative measurements were taken as the vertical distance from the first-derivative spectrum at 594.0nm (lD5s4.c) to the baseline for aluminium and from 574.0 nm (1Ds74,0) to the baseline for beryllium. Calibration graphs were constructed in the same way using aluminium and beryllium of known concentrations.
3. Weatment
75
et al.LAnalytica Chimica Acta 327 (19%) 73-82
of samples
3.1. Natural water sample
to treatment and assay to verify the traceability of the method. 0.1 g of the sample was weighted into a PTFE crucible and 2 ml of concentrated sulphuric acid was added, and the crucible was heated on a sand bath for 20min. 3 ml of concentrated hydrofluoric acid was added drop by drop to avoid foam formation. After complete dissolution of the solid, the mixture was heated until the volume of the solution was reduced by l-2 ml. The solution was finally diluted to 100 ml with reverse osmosis water in a calibrated flask. Alliquots were analysed as described in Section 2.4. 3.4. Botanical
samples
Two reference materials, ecorce de pin and ryegrass, from Comite Inter-Instituts (CII, France) (obtained for inter-laboratory tests) were subjected to digestion. 0.2 g of the ecorce de pin or of the ryegrass sample were weighed into a FTFE vessel. Then, the digestion agent was added (1 ml of concentrated nitric acid) and the vessel was heated in the Moulinex microwave system at 665 W for 30 s [20]. Then the bomb was cooled and an adequate volume (up to 5-1Oml) of water was added. The solution was shaken and transferred into a test tube. The digest was diluted with water so that the concentrations of the elements fell within the working range of the proposed method and Al@) and Be@) were determined, as described in Section 2.4.
The water samples were preserved by the addition of nitric acid (2ml of concentrated nitric acid per litre) and collected in a polyethylene container that was carefully cleaned with nitric acid. Prior to the the water samples were filtered measurements, through 0.45 urn pore size membrane filters (Millipore, Type HA). The sample was stored at 4°C until analysis and the usual general precautions were taken to prevent contamination [ 181.
4. Results and discussion
3.2. Orange juices
4.1. Effect of experimental
40 ml of the sample were centrifuged for 15 min in a test tube at 2000rpm (2Og), and the supernatant was separated and filtered through 0.45 pm pore size membrane filters (Millipore, Qpe HA). The content of the analytes was determined in an aliquot, as described in Section 2.4.
On the Sephadex DEAE A-25 anion-exchanger, ECR forms a ternary complex with beryllium and EDTA (pH 5.0), with an absorption maximum at 580nm [13] and a complex with aluminium (pH 3.8), with an absorption maximum at 590~ [14]. In order to obtain a net absorbance signal for both complexes simultaneously, a study of the influence of the pH was performed. Fig. 1 shows this influence, in which it can be seen that at a pH of 4.4 both complexes exhibit equal absorbance, and therefore, this pH was selected for the simultaneous determina-
3.3. Geological
sample
A reference material (Granite de Beauvoir, MA-N) certified for inter-laboratory tests [19] was subjected
variables
16
MC. Kzlencia et ai./Analytica
Chimica Acta 327 (1996) 73-82
The order of addition used here was: EDTA + ECR + buffer + exchanger.
$ c, g 0.2-
01 3
II)” I
3.5
+
B
A
8 0.6% 2 $0.4-
analyte
I
1
4
4.5
I
5
.
4.2. Spectrophotometric
I
5.5
PH Fig. 1. Influence of pH on colour development. Conditions: lOpgl-’ of aluminium, lOpgl-’ of beryllium, ECR=1.49x lo-’ M, EDTA=8.25 x 10e6 M, 80 mg Sephadex DEAE A-25, sample volume 5OOml. stirring time 15 min. 1 mm optical path length.
tion. The best of the buffer systems examined was acetic acid-sodium acetate. The net absorbance of these complexes increases with ECR concentration, but at the same time the “background noise” also increases. In consequence, we chose a concentration of 1.5 x lop5 M of ECR as the most appropriate value for the simultaneous determination of both complexes using the basic procedure. EDTA forms a ternary complex with beryllium and ECR with enhanced absorbance but its presence decreases the net absorbance of the aluminium-ECR complex. The concentration of EDTA must be chosen as a compromise between obtaining the strongest signal for the beryllium complex (maximum concentration of EDTA) and obtaining a sufficiently sensitive signal for the aluminium complex, due to the very low beryllium to aluminium molar ratio in the natural samples. A volume of 1.5 ml of 0.0165 M EDTA was chosen as the most suitable. The optimum stirring time was 45min for a 1OOOml sample system. Under these conditions, the absorbances remained stable for at least 2.5 h after equilibration. The centrifugation time used was 20 s at 2000 t-pm (20 g). For all measurements, unless stated otherwise, 0.08g of exchanger was used as a compromise between maximum sensitivity and manageability.
measurements
Fig. 2 shows the net absorption spectrum of the aluminium complex, which has an absorption maximum at 594nm (spectrum A), and of the beryllium complex, with an absorption maximum at 574 nm (spectrum B). The absorption spectrum of the mixture of both complexes (Be(II)/Al(III)) molar ratio 0.3) with a maximum at 584nm was located between the absorption maxima of the two components (spectrum C). All measurements were performed against a reagent blank because the reagent also absorbs at these wavelengths. As can be seen from Fig. 2, because of a large overlap between the two spectra, significant diff~culties arise in the spectrophotometric determination of these ions when they are present in the same sample. To overcome this problem, derivative spectrophotometry can be used; this involves the differentiation of zero-order spectrum with respect to wavelength. Fig. 3 shows the first-derivative absorption spectra of individual complexes of Al@) (curve A) and Be@) (curve B) and a mixture of both complexes (curve C) with ECR, in the presence of EDTA. It can be seen that, because of the closeness of the two overlapping spectra of the aluminium and beryllium
Wavelength
(nm)
Fig. 2. Net absorption spectra ECR complexes at pHd.4: (A) Al@)-ECR complex; (B) Be(B)-EDTA-ECR complex; (C) mixture of both complexes. Conditions: 20~1~’ of aluminium, 2 pgl-’ of beryllium, ECR=~.SX~O-~M, EDTA=2.5x lo-‘M, 80mg Sephadex DEAE A-25, sample volume lOOOml, sting time 45 min, 1 mm optical path length.
M.C. tilencia et al./AnalyticaChimicaActa 327 (19%) 73-82
17
Ax were tested, and a width of 19 nm was selected to obtain the best signal-to-noise ratio. The response time is automatically selected by the spectrophotometer in accordance with the optical energy and scanning speed. 4.4. Analytical mination mixtures -0.2
! 520
640 560 Wavelength
700
760
(nm)
Fig. 3. First-derivative absorption spectra of ECR complexes: (A) Al(III)-ECR complex; (B) Be(II)-EDTA-ECR complex; (C) mixture of both complexes. Conditions: 2Opgl-’ of aluminium, of beryllium, ECR=1.5 x 1O-5 M, EDTA=2.5 x 10e5 M, 2 pgl-’ pH=4.4, 80mg Sephadex DEAE A-25, sample volume lOOOml, stirring time 4.5 min. Scan speed 480 nm/min, smoothing 19 points, Ax 19nm.
complexes, these spectra are not sufficiently resolved to give two distinct peaks in the first-derivative spectrum of the mixture of both complexes. For this reason, the zero-crossing method [21] was used to resolve the mixture. The heights hi and h2 in the firstderivative spectrum of the mixture (Fig. 3, curve C) corresponding to the values measured at 574.0 and 594.Omn are proportional to the aluminium and beryllium concentration, respectively. 4.3. Selection of optimum instrumental conditions The instrumental parameters affecting the shape of the derivative spectra are: the scan speed, smoothing and wavelength increment, which must be optimised to obtain a good selectivity and higher sensitivity in the determination. The scan speed does not affect the derivative signal in the interval between 120 and 480 nm/min-‘, but higher speeds reduce the signal. Hence, a fast wavelength scanning speed of 480 nm rnin-’ was selected. For the smoothing operation of the spectra, the Savitzky-Golay method with windows comprising 5-19 points was used. The number of points has no appreciable effect on the first-derivative signal. For the calculation of the first-derivative spectra different
parameters for simultaneous deterof aluminium and beryllium in
The “lack of fit” test (LOF) was used to check the calibration linearity of the method for every analyte. Aluminium was tested over a O-60 ugl-’ range (PLor=43%) and beryllium was tested over O4.0 ug 1-l range (PLoF=29%). With the aim of proving the mutual independence of the analytical signals of Al@) and Be@), that is to confirm that hi and h2 are independent of aluminium and beryllium concentrations, respectively, four calibration graphs were obtained from height (h) measurements for standards containing beryllium (between 0.5 and 4 ug 1-l) or aluminium (between 10 and 40 ug 1-l) both in the presence of different concentrations of the other ion and in its absence. The slope, intercept, standard deviations of the slope and intercept, determination coefficient and standard deviation of the regression (S& obtained for all the calibration graphs (n=5) are summarized in Table 1. For both ions, aluminium and beryllium, the slope of the calibration line in the absence of the other metal ion is not significantly different from the slope of the calibration lines in its presence (P-value > 30% in all cases). In addition, the intercept of every calibration graph is not significantly different from zero (P-value > 27% in all cases). Therefore, it can be concluded that the height of the derivative signal of the mixture at the zero-crossing point of the derivative spectrum of one of two components is a function of the concentration of the other component only. Morever, the values of the coefficient of determination (Z?) indicate good fitting of the experimental values for all the calibration graphs obtained. The standard deviations of the derivative spectrum measured for the blank (lDt,iank), obtained by measuring the height of the derivative signal at the zero-crossing point for aluminium (574.Onm) and beryllium (594.0nm) were 0.0015 A and 0.0010 A, respectively (n=9). The detection limits according to
78
MC.
Table 1 Statistical analysis of the determination phase spectrophotometry. Element determined
Valencia et al./Analytica
of almninium
(lo-40
Other metal present Element
Chimica Acta 327 (1996) 73-82
~g 1-l) and beryllium
Slope ((SD)x 10’)
(0.54
pg 1-I) in the mixtures
Intercept ((SD)x 103)
Concentration
by first-derivative
solid-
Determination Coefficient
Standard deviation
(R’)
((SR,J x ld)
(pg 1-Q ‘WII)
Be(B)
0.5 1.5 2.5
0.20 0.19 0.18 0.18
(0.01) (0.01) (0.01) (0.01)
-6.4 -3.2 -2.6 -2.9
(4.26) (2.50) (2.36) (2.25)
0.988 0.990 0.992 0.992
0.35 0.32 0.30 0.29
Be(K)
WII)
20 30 40
6.76 6.36 6.39 6.21
(0.43) (0.09) (0.08) (0.16)
-7.3 0.5 1.9 0.5
(10.6) (2.1) (2.2) (2.8)
0.988 0.992 1.000 0.998
1.23 0.31 0.24 0.40
Table 2 Effect of foreign ions on the determination Foreign ion
clPod3SiOg’N03so,2Ca(IU Mg(II) Mn(Q
Tolerance
of 20 pg 1-l of Al@) level (pg 1-l)
‘ww
Be@)
11000 9100 6000 5400 1000 3100 3700 3100
9000 5100 9800 6600 1000 3600 3500 600
IUPAC based on three times the standard deviation of the blank were O.O4pgl-’ for beryllium and 2.25 pg 1-t for aluminium and the quantitation limits (10 times the stand. dev.) were 0.15 ug I-’ for beryllium and 7.5 pgl-’ for altinium. The relative standard deviations (RSD, ~10) were 3.8% for 2 pg 1-l of beryllium and 3.5% for 20 pgl-’ of aluminium. 4.5. Effects of foreign
ions
The effect of foreign ions on the determination of aluminium and beryllium using 2 pg 1-l of beryllium and 2Opg 1-l of aluminium, was studied following the basic procedure. Tolerance is defined as the amount of foreign ions which produces an error of f 5% in the determination of the analyte. The results are summarized in Table 2.
and 2 c(g 1-l of Be(I1) Tolerance
Foreign ion
Pb(II) Cu(I1) Cd(I1) Fe(III) Zn(I1) WI) Ni(I1) Co(I1)
level (pg 1-l)
Al(II1)
Be@)
600 500 200 190 180 80 75 65
1000 700 220 350 190 12 85 75
4.6. Recovery of aluminium synthetic samples
and
beryllium
in
The proposed method was applied to the analysis of several synthetic mixtures of aluminium and beryllium in different molar ratios with the intention of checking its accuracy in particular conditions. Table 3 summarizes the results obtained for each molar ratio. The percentage of recovery is between 94% and 116% for beryllium and 94% and 103% for alum&urn showing that the accuracy is acceptable in all instances.
4.7. Analytical
applications
To test the applicability of the proposed method, it was used to simultaneously determine beryllium and
M.C. Valencia et al./Analytica Chimica Acta 327 (1996) 73-82
79
Table 3 Recovery study for almuinium and beryllium in synthetic mixture samples Be(II)/Al(III) molar ratio
0.037 0.100 0.150 0.200 0.300 0.300 0.450 0.608 0.600 1.200
Be(n)
Al(W
Added
Found
Recovery
Added
Found
Recovery
(P8 1-P
(cl8 1-P
(46)
(P8 1-Q
(P8 10
(S)
0.5 1.0 1.0 2.0 2.0 3.0 3.0 2.0 4.0 4.0
0.58 1.06 1.01 1.99 1.90 2.96 2.93 1.88 3.95 3.91
116 106 101 100 95 99 98 94 99 98
40 30 20 30 20 30 20 10 20 10
40.4 28.2 19.1 31.0 19.2 29.3 20.3 9.7 19.2 9.9
101 94 96 103 96 98 102 97 96 99
aluminium in different samples: solution, natural water, granite, grass and orange juice.
a synthetic aqueous Bcorce de pin, rye-
4.7.1. Determination of aluminium and beryllium in a synthetic aqueous solution With the aim of checking the accuracy of the method, synthetic aqueous solution was prepared containing aluminium and beryllium and the usual ions in natural water: NOs(9000 pgl-‘), ClSi032(8000 ug l-l), POa3(3000 ug 1-k (3000 ug l-l), soa2(1000 ug 1-k Ca(n) (3000 Pg 0, (3000 cLg1-‘), Mn(Q Mg(W (300 pg l-l), Cd@) (100 ug l-l), Cu(II) (100 pg l-l), Zn(II) (100 ug l-l), Pb@) (100 ug l-l), Fe@) (100 pg l-l), Ni(II) (50 ug l-l), Co@) (50 ug l-l), Al(III) (40 ugl-‘), U(VI) (10 ugl-‘) and Be(I1) (2ugl_‘). For all measurements, 1000 ml synthetic water was used and the samples were analysed as described in Section 2.4. The average value (5 determinations) and relative standard deviation (RSD%) for aluminium and beryllium concentrations calculated from the calibration graphs were 40.7 pgl-’ (2.58%) and 1.97 ugl-’ (2.68%), respectively, with a relative error of 1.75% for aluminium and 1.50% for beryllium. In addition, recoveries of 100-106% for aluminium and 95-102% for beryllium indicate the good accuracy of the method even in the presence of foreign ions.
4.7.2. Determination of aluminium and beryllium in natural water The proposed method was applied to the determination of aluminium and beryllium in water samples by the standard additions method. The constant error was corrected according to Cardone blank [22]. The results for aluminium and beryllium obtained by the proposed method and those from Atomic Absorption Spectrometry with Graphite Furnace (AASGF) for aluminium, are presented in Table 4. The statistical comparison by F-test and t-test showed no significant difference between the average value derived from the SPS method and from AASGF, and the precision of both methods is the same (the P-values are 33% and 13%, respectively). Because an appropriate reference method to compare the beryllium content obtained was not available, a validation study was carried out following the statistical protocol proposed by Cuadros Rodriguez et al. [23]. In this procedure and when a certain prerequisites are fulfilled, the accuracy of the results is tested by comparing the analyte contents obtained from the standard-additions calibration (AC) and the standard calibration (SC), using a t-test for the comparison of the two means. The application of the accuracy test in the present determination of beryllium indicated that ted = 0.455 < ttit = 2.145 (a = 0.05, Df = 14), that is to say there is no significant difference between the means obtained from the AC and SC graphs, and therefore it may be concluded that the method is accurate.
80 Table 4 Determination Sample
MC. Wencia
of aluminium
and beryllium
et al./Analytica
Chimicn Acta 327 (1996) 73-82
in different samples
Al(U)
Be(R)
(ml) Found (ug) Natural water 200.0 600.0 Grange juice 2.0 2.5
2.66 9.10
Content” (ugm I-‘)(RSD%)
0.013 (22.4) 0.015 (5.1)
26.90 33.53
13.5b (0.5) 13.4 (0.2)
31.37 33.70 34.83 34.35 33.03
12.5 (3.5) 13.5 (0.9) 13.9 (2.5) 13.7b (3.5) 13.2b (5.5)
Content by GFAAS” (ngm ll’)(RSD%)
0.014 (2.1)
14.0 (8.1)
Added
Found
ug
ug
Content” ugl-‘(RSD%)
-
0.102 0.34
0.51 (4.1) 0.57 (2.9)
-
-
-
1.00 2.00 3.00 1.05 2.10
0.94 2.01 3.21 1.05 2.10
400c 800” 1200” 420bsc 840b.”
(2.1) (0.6) (2.5) (2.3) (0.5)
“Each value is an average of three determinations. bEach value is an average of four determinations. ‘Hypothetical content according to sample dilution.
4.7.3. Determination of aluminium and beryllium in orange juice Different aliquots of a commercially available orange juice were analysed for the simultaneous determination of aluminium and beryllium using the basic procedure. Because no significant difference between the slopes of the standard calibration graph and the standard additions graphs was observed, the standard calibration graph was used in all cases. Table 4 shows the results obtained for each sample volume analysed. The average value (four determinations) of the beryllium concentration was lower than the IUPAC detection limit in all cases tested. To check the possibility of simultaneous determination of aluminium and beryllium in this sample, a recovery study for beryllium was carried out in which two different amounts of beryllium were added to 25Oml samples of orange juice prior to their centrifugation, and the aliquots were analysed as described in Section 2.4. The results, presented in Table 4, show that given values found for beryllium produce a recovery of lOO-lOl%, and that with respect to aluminium, the comparison with the results from AASGF showed no significant difference (Pvalue of 34%), we then have clear indications of the suitability of the proposed method for the samples. Furthermore, the precision of SPS method is similar than that obtained from AASGF.
4.7.4. Determination of aluminium and beryllium in geological samples The Basic Procedure (Section 2.4) was applied to aliquots of the solution obtained from a granite sample after the treatment previously described. The results obtained using the standard calibration graph are presented in Table 5. The statistical comparison by t-test showed no significant difference between the aluminium and beryllium values obtained from the proposed method and the certified values (the Pvalues are 9% and 33%, respectively). 4.7.5. Determination of aluminium and beryllium in botanical samples Aliquots of the two different reference materials were analysed using the Basic Procedure. The statistical comparison of the slopes of the standard calibration and the standard additions graphs, in both materials, showed no significant difference between them. In all the cases tested, the concentration of beryllium was lower than the IUPAC detection limit. To demonstrate the possibility of simultaneous determination of aluminium and beryllium, samples spiked with different amounts of beryllium were analysed. Table 5 summarizes the results obtained, showing at the same time that the recoveries for beryllium are acceptable: the comparison of the aluminium values found by the proposed method with
M.C. Wencia Table 5 Determination Sample
of aluminimn
and beryllium
et al./Andytica
in Geological
Chimica Acta 327 (19%) 73-82
and Botanical
AKIH)
81
samples Be(n)
(ml)
Granite de Beauvoirb Bcorce de pin’ 0.5 0.7
Ryegrass” 0.3 0.5 0.5
Found
Content”
Certified”
Added
Found
Content
Certified
(I.Lg)
(olg g-‘)(RSD%))
((pg g-‘)(RSD%))
(pg 1-l)
(pg 1-l)
(Olgg-‘)(RSD%))
((pg g-‘)(RSD%))
61.64*
17.98(1.8)
17.62
0.19*
293(3.7)
298(40.9)
25.67 31.32 31.30c 33.37 34.89
2445(1.7) 2131(3.1) 213OG.8) 2270(1.1) 2373(2.1)
2450(16.7)
9.92 17.04 17.18*
1637(1.3) 1687(10.6) 17OH8.3)
1608(7.5)
-
0.64 1.00 2.00
0.63” 1.00 1.98
48’(1.6) 68’(6.8) 135’(2.2)
-
0.45
0.45*
45’(2.4)
’ The content dete.rmin~ were Al203 in percentage, for Granite de Beauvoir. b Granite de Beauvoir, MA-N. Rock reference material: data are average values of 18 laboratories. ’ Reference Material from Conrite Inter-Instituts (CII, France). Data are average values of: &corce de pin, 11 and ryegrass, Each value is an average of three determinations, excepting * five and ’ four determinations. f Hypothetical content according to sample dilution.
the certified value 234% to perform analytes in
values show no significant difference (Pin all cases), indicating that it is possible the simultaneous determination of both these types of sample.
4.8. Conclusion An original, simple, sensitive and inexpensive method for the simultaneous determination of both aluminium and beryllium at the subnanogram per millilitre level by SPDS has been developed. It provides a high sensitivity with detection limits of 0.04 and 2.25 pgl-’ for beryllium and aluminium, respectively. The suitability of the proposed procedure has been demonstrated for the successful determination of both analytes in different types of samples with a wide concentration range.
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[41 Z. Marczenko and M. Jarosz, Analyst, 107 (1982) 1431. L.K. Ravindranath and S. [51 A. Ramesh, J. Krishnamacharyulu, Brahmaji Rao, Analyst, 117 (1992) 1037. %I N. Kumar Agnihotri, H. Bhajan Singh, R. Lal Sharma and V Kumar Singh, Talanta, 40 (1993) 415. [71 N. Kumar Agnihotri, V. Kumar Singh and H. Bhajan Singh, Analyst, 120 (1995) 1809. L.F. Capit&-Vallvey, M.C. Valencia, J.M. VI F. Capit& Basque-Sendra, F. Molina and I. de Orbe, Analusis, 19 (1991) 177. [91 H. I&ii, Fresenius’ Z. Anal. Chem., 319 (1984) 23. WY M.I. Shtokalo, E.E. Kostenko and I.Z. Zhuk, J. Anal. Chem., 47 (1992) 1335. L.F. Capitan-Vallvey, I. de Orb-e, M.C. Valencia and J.J. [ill Berzas-Nevado, Anal. Chim. Acta, 282 (1993) 75. WI H. Ishii and K. Satoh, Fresenius’ Z. Anal. Chem., 312 (1982) 114. I131 MC. Valencia, S. Boudra and J. M Bosque-Sendra, Analyst., 118 (1993) 1333. M.C. Valencia and S. Boudra, H41 J. M Basque-Sendra, AnalLea., 8 (1994) 1579. M.C. Valencia and L.F. Capit&-Vallvey, u51 F. Capi& Miiochim. Acta, III (1984) 303. W51 A. Savitxky and M.J.E. Golay, AnaLChem., 36 (1964) 1627. 44 Cl71 J. Steiner, Y. Termonia and J. Deltour, Anal.Chem., (1972) 1906. WI American Public Health Asociation, American Water Works Association and Water Pollution Control Federation. Standard Methods for the Examination of Water and Wastewater, 15th Ed., A.P.H.A., Washington DC., 1981.
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