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Flow injection sorbent extraction with dialkyldithiophosphates as chelating agent for the determination of zinc by flame atomic absorption spectrometry
ANALYTICA
CHIMICA
ACTA ELSEVIER
Analytica Chimica Acta 309 (1995) 39.5-403
Flow injection sorbent extraction with dialkyldithiophosphates as chelating agent for the determination of zinc by flame atomic absorption spectrometry Renli Ma, Willy Van Mol, Freddy Adams Department
of Chemistry,
University ofAnhverp
*
(UIA), B-2610 Wilrijk, Belgium
Received 29 July 1994; revised 28 November 1994; accepted 16 January 1995
Abstract Dialkyldithiophosphates, (RO),P(S)S-, were used as the complexing agent for flow injection on-line sorbent extraction for the determination of zinc by flame atomic absorption spectrometry. The extractability with diethyl-, di-n-propyl-, di-2-propyl-, di-n-butyl-, di-isobutyl-, di-sec.-butyl-, di-n-pentyl- and di-n-hexyldithiophosphates was compared in respect of the effects of pH, alkyl group, masking agent and reagent concentration. Di-sec.-butyldithiophosphate (OS%, m/v) at pH 3 with 0.1 M citrate as a masking agent for iron, was used for the determination of zinc in a number of certified biological and environmental reference materials from the Community Bureau of Reference (BCR). All analytical results were in good
agreement with the certified values. The detection limit (3~) of peak height measurement, using a flow spoiler in the spray chamber, was 0.5 pg 1-r with an enhancement factor of 35 for a 20-s sample loading at 8.7 ml min-‘. Keywords: Waters
Atomic
absorption
spectrometry;
Flow injection;
Dialkyldithiophosphates;
1. Introduction In comparison with other chelating agents containing two sulphur donors, such as alkyl xanthates (alkyldithiocarbonates) and dialkyldithiocarbamates, dialkyldithiophosphates (DDP), (RO),P(S)S-, are unique for use in acid media where they are stable and selective. The half-life of diethyldithiophosphoric acid at room temperature is 4.8 h in 10 M HCl and 250 h in 1 M HCl [l]. Increase in the hydropho-
* Corresponding
author.
0003-2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SsDIOOO3-2670(95)00053-4
Zinc; Biological
materials;
Environmental
samples;
bit feature of the alkyl groups enhances the stability against hydrolysis [2]. The reagent solubility in aqueous media decreases with the hydrophobic character of the alkyl groups and with acid or salt concentration 131. Systematic investigations on the solvent extractability with one reagent for different elements 14-61 or different reagents for one element [6-91 have been reported. The most often used reagents for solvent extraction are diethyl [4,5,10,11], di-n-butyl [5,6,11,12] and di-sec.-butyl [13,14] derivatives. Good extraction efficiency is achieved for the elements that form sulphides of very low water-solubility such as Ag, Cd, Cu, Hg and Pb, while alkali and alkaline earth metals are not extracted [4-61. The
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Chimica Acta 309 (1995) 395-403
number of extractable elements increases with the chain length of the straight-chain alkyl groups [5,6]. The extractability, e.g., for Zn, increases with the chain length of the alkyl groups [6-91 and branching in the alkyl groups [7,8]. Au(III), Cu(II), Fe(III) and Tl(II1) can be reduced by the reagents [4,6,10,15]. The reducing ability of the reagents increases with the size of the alkyl groups [16]. DDP reagents are generally laboratory-made because they are not commercially available for analytical use except diethyldithiophosphate. The synthesis is simple and easy by reacting phosphorus pentasulphide with the corresponding alcohols [17,18], but reported procedures differ [9,13,15,17,19]. The yield and purity are dependent on the quality of the reactants, correct stoichiometric quantities, mixing mode, reaction temperature and reaction time [18]. As the free acids may be somewhat unstable thermodynamically to hydrolysis and oxidation [1,2,5,6,16], they are preferably converted into the more stable salts, e.g., the ammonium salts [2,5,6]. Further purification may be carried out by recrystallisation from benzene [13] or ethyl acetate [6,7,15]. Flow injection (FI) liquid-solid sorbent extraction with a reversed-phase hydrophobic sorbent packed column is a technique combining the advantages of solvent extraction with the column operation in which the consumption of time, sample and reagent is reduced and the reproducibility is improved 120,211. Dithiocarbamate reagents have generally been used as the complexing agent [20-221, however, they are unstable in acidic media and unselective among the heavy metals. It is of interest to clarify the FI sorbent extraction characteristics of DDP reagents. Arnmonium diethyldithiophosphate (DEtDPA) in acid media (pH l-2) has been used in our previous work in FI sorbent extraction systems for the selective determination of Cd, Cu and Pb with flame or graphite furnace atomic absorption spectrometry (FAAS or GFAAS) [22,23]. The importance of Zn as an essential trace element in human and animal health is well known and AAS has been extensively employed for Zn determination. Detection by GFAAS may require a sample dilution with a risk of contamination and the introduction of a matrix modifier with a high blank value. The relatively high concentration level in the samples and the relatively high sensitivity of AAS for
this element suggest FAAS to be the detection technique of choice combined with preconcentration and separation procedures. Solvent extraction of Zn with DDP reagents, especially with the di-n-butyl derivative, has been studied thoroughly [6-9,11,12] and the extract could be analysed by FAAS [ll]. FI liquidliquid extraction with a dithiocarbamate reagent [24] and FI liquid-solid chelating adsorption - ion exchange with Chelex-100 resin or 8-quinolinol-immobilized controlled pore glass [25-271 have been applied for Zn determination by FAAS. Hitherto no determination of Zn with FI sorbent extraction has been reported, hence it is of interest, with this technique, to establish a selective determination of Zn after separation from the major matrix components including not only alkali and alkaline earth elements but also other heavy metals. Using FI on-line analyte preconcentration techniques, the risk of contamination can significantly be decreased in a closed flow system and the blank value can be effectively decreased with on-line purification of the reagents. In the present work, the FI sorbent extraction behaviour of DDP reagents for Zn in respect of the effects of pH, alkyl group, masking agent and reagent concentration is investigated. As a result an analytical system has been developed for the determination of Zn by FAAS and applied to a number of certified reference materials (biological materials and environmental samples) from the Community Bureau of Reference (BCR): cod muscle (CRM 4221, hay powder (CRM 129), single cell protein (CRM 274), mussel tissue (CRM 278), plankton (CRM 4141, calcareous loam soil (CRM 1411, river sediment (CRM 320), sea water (CRM 403) and estuarine water (CRM 505).
2. Experimental
2.1. Apparatus A Perkin-Elmer Model FIAS 200 flow injection accessory and a Perkin-Elmer Model 3030 atomic absorption spectrometer were used. The FI on-line sorbent extraction system and the FAAS detector have been described in our previous work [22]. A precolumn, the same as the extraction column, for
R. Ma et al./Analytica
397
Chimica Acta 309 (1995) 395-403
on-line reagent purification to control the blank, was used for the analysis of sea water. pH adjustment and measurement were performed with a Schott CG 820 pH meter which had been calibrated with standard buffer solutions of pH 4, 7 and 10 (Merck).
3. Results and discussion
2.2. Reagents
3.1. Sorbent extraction
Suprapur concentrated acids (Merck) were used for the solid sample digestion and pH adjustment. All other chemicals used, including those for the complexing-agent synthesis, were of analytical grade. Methanol was used as eluent for the FI sorbent extraction without further purification. Standard solutions of tested metals were made by dilution from 1000 mg 1-l stock solutions (Merck). Deionized water from a Mini-Q water system (Millipore) was used throughout. DEtDPA was purchased from Aldrich. Other DDP reagents as the ammonium salts (DDPA) including the di-n-propyl (DPrDPA), di-Zpropyl (2-DPrDPA), di-n-butyl (DBuDPA), di-isobutyl (i-DBuDPA), disec.-butyl (2-DBuDPA), di-n-pentyl (DPeDPA) and di-n-hexyl (DHeDPA) derivatives were prepared based on the procedure used previously in our laboratory [13]. Phosphorus pentasulphide (0.25 mol or 55.5 g) with 50-80 ml of benzene in a 500-ml three-necked flask was heated with a water bath at 75-80” C. One mol of a particular alcohol (75-125 ml) was added dropwise while stirring and condensing. After 2-h refluxing with stirring, ammonia gas was bubbled through the hot solution until the gas started to appear from the condenser during which a white precipitate appeared. The solution was slowly cooled to room temperature. The product was filtered off and rinsed twice with 10 ml of benzene, and dried in a vacuum system. The recovery of recrystallisation from benzene, e.g., for 2-DBuDPA, was about 80%. DDPA reagents in acetate, citrate or oxalate medium, contents as required, were adjusted to the required pH values with dilute nitric acid or ammonia solution. 2.3. Digestion
biological and environmental samples, respectively [23]. The residue was dissolved in 50 ml of acidified water.
behauiour of DDP reagents
A series of DDPA homologues as the chelating agent for FI sorbent extraction were compared for extractability of Zn, the analyte of interest, and Fe(III), a typical coexisting heavy metal ion. The optimum reagent should provide the maximum sensitivity for Zn with good selectivity from interfering elements, such as Fe. Effect of pH For all DDPA reagents except 2-DPrDPA, at 0.5% (m/v> in 0.1 M acetate medium, the pH dependence of the sorbent extractability was examined between pH 0 and 12. It appeared that DPeDPA did not dissolve very well at pH 1 while DHeDPA did not dissolve below pH 2. The four butyl and pentyl derivatives produced the same maximum sensitivity as shown in Fig. 1, in which peak height measurements were used. Zn can be quantitatively extracted up to pH 6 with both 2-DBuDPA and DPeDPA
of solid samples PH
Accurately weighed 0.1 g of solid standard reference material was digested as reported earlier for
Fig. 1. pH dependence of the 211 extraction. i-DBuDPA; A, 2-DBuDPA; 0, DPeDPA.
0.
DBuDPA,
398
R. Ma et al. /Analytica
Chimica Acta 309 (1995) 395-403
whereas the lower pH limit for quantitative extraction is 2 with DPeDPA or down to 0 with 2-DBuDPA. With DPeDPA at pH 1 the relatively lower extractability is probably due to the low solubility of the reagent. Zn can also be extracted quantitatively within the pH range l-4 with i-DBuDPA or pH 2-3 with DBuDPA. In the case of peak area measurements the pH ranges with 2-DBuDPA and DPeDPA for quantitative extraction are just up to pH 4. The acid tolerance of the Zn extraction increases in the order of DBuDPA < i-DBuDPA < 2DBuDPA which disagrees with the result in solvent
extraction reported by Handley and Dean [6] where the acid tolerance of the Zn extraction decreases with increased branching of the butyl chain near the dithiophosphoric functional group. Zn can not be extracted with DEtDPA over the whole pH range and can be extracted only partly with DPrDPA around pH 2-3. With DHeDPA a stable signal was obtained between pH 3 and 6 (peak height) or between pH 3 and 4 (peak area). The signal, however, was lower than the maximum signal produced by DBuDPAs and DPeDPA. The reason for this is not clear.
,lo %--Relative peak height .-. .__-l-__.-_“---__-.--.-..---.T (a)
DE1DM
DPrDM Z-DPrDM DBuDM CDBuDMMwDIw
DPoDM
DHoDM
DDPA Reagent
,20 lb Relative peak height --.-1--_.-.-.-.-..“..“-.-
.._....._._.. -_I.-.-.-__- .....” ......._.....................” . .“.._ ....“_“...”. ...”. . - ............... ..._
100 80
“_.--._.“..“...-.-._.---
. . __.-._--.-
-.-._“.-.-l- I._. I_... “..._.
60 40
20 0
DEtDM
DPrDW WPrDM
DBuDM I-DI)UDMIQU~ DDFA
Fig. 2. Effect of alkyl group on the extraction
DPoD#r DHoDM
Reagent
of (a) Zn and (b) Fe. W , without citrate;
0, with citrate.
R. Ma et al. / Analytica Chimica Acta 309 (1995) 395-403
The maximum signals for Fe(III) were pH ca. 1 with DEtDPA, within pH DPrDPA; pH 2-3 with DBuDPAs; around DPeDPA or at pH 3 with DHeDPA. A pH value of 3.0 was selected for all further experiments.
obtained at l-3 with pH 2 with reagents in
Effect of the alkyl group Using 0.5% (m/v) of all DDPA reagents in 0.1 M acetate-ammonium medium with or without 0.1 M citrate at pH 3, the extractability of the reagents for each element was compared. Zn can be extracted quantitatively with DBuDPAs and DPeDPA while it can only partly be extracted with DPrDPAs and DHeDPA and can not be extracted with DEtDPA. Citrate at 0.1 M inhibits the extraction with DPrDPAS but does not influence the extraction with DBuDPAs and DPeDPA (Fig. 2a). The extractability for Zn, also considering the tolerance to acid and masking agent, increases with the number of carbon atoms and the branching of the alkyl chains from two (ethyl-) to five (pentyl-) carbons in the reagents: DEtDPA x DPrDPA < 2-DPrDPA < DBuDPA < i-DBuDPA < 2-DBuDPA < DPeDPA. With DHeDPA, the signal was slightly increased in the presence of citrate. As can be seen from Fig. 2b, the extraction of Fe(III) is greatest with i-DBuDPA. Extraction with all reagents was significantly inhibited by citrate.
399
Effects of masking agent and reagent concentration Several commonly used masking agents for Fe(II1) were examined to eliminate its interference in the determination of Zn. Using 0.1 M of each masking agent with 0.5% (m/v) DBuDPA in acetate medium at pH 3, 400 pg 1-l Fe(III) was masked completely by oxalate or mostly by phosphate, citrate, tartrate and fluoride, but not by thiocyanate and ascorbic acid. The Zn signal was depressed almost completely by oxalate, significantly by phosphate and tartrate and slightly by fluoride and thiocyanate. With DBuDPA, Tiron or particularly Ferron affected the extraction of Zn seriously without masking Fe(II1) very effectively. Because of their relatively high masking efficiency for Fe, the influence of citrate or oxalate content was studied further with different concentrations of 2-DBuDPA or DPeDPA, respectively, at pH 3. The results with 2-DBuDPA and citrate are illustrated in Fig. 3 for peak height measurements. Zn can be extracted quantitatively with 2-DBuDPA or DPeDPA at concentrations higher than 0.05% (m/v) without any masking agent. The minimum reagent concentrations for quantitative extraction become higher as the masking agent contents are increased. Zn determination with 0.5% 2-DBuDPA was not affected by oxalate at a concentration lower than 0.0035 M or citrate lower than 0.25 M. With 0.5% DPeDPA it was not affected by oxalate up to 0.07 M
“‘1 (a)
Fig. 3. Effect of 2-DBuDPA 0.5 M of citrate.
and citrate concentrations
on the extraction
of (a) Zn and (b)Fe. 0, 0 M; 0, 0.05; A, 0.1 M; 0, 0.25 M; + ,
400
R. Ma et al. /Analytica
Chimica Acta 309 (1995) 395-403
or citrate up to 0.5 M. The tolerance of Zn extraction with DPeDPA to masking agents is better than that with 2-DBuDPA. Fe(II1) can be extracted well with between 0.01 and 0.1% 2-DBuDPA or between 0.005 and 0.1% DPeDPA without any masking agent. At higher reagent concentrations the sensitivity decreases, probably because the reagents reduce Fe(II1) to Fe(U) which is less well extracted [6]. This effect becomes more serious with the higher homologue DPeDPA. The Fe extraction is inhibited seriously by the masking agents over the whole reagent concentration range. With 0.5% (m/v) of these two reagents the same masking agent content was required to mask Fe completely, 0.07 M oxalate or 1 M citrate. Consequently, Zn could be quantitatively and selectively extracted with 0.5% 2-DBuDPA and 0.1-0.25 M citrate whereas Fe was mostly masked. Zn also could be quantitatively and selectively extracted using 0.5% DPeDPA with 0.07 M oxalate, where Fe was masked completely. These two reagent solutions were chosen for further study of the determination of Zn. 3.2. Determination of zinc Sample loading rate High sample flow rates can result in low extraction efficiency. The influence of the sample loading rate on the preconcentration efficiency was examined with 0.5% (m/v) DPeDPA. Different sample load-
ing times were set at different rates to load the same sample volume. A slight increase in the Zn sensitivity was observed as the sample loading rate was decreased. From 8.7 ml min-’ to 4.3 ml min-’ of the sample loading rate, the sensitivity increased around 10% while the sampling frequency decreased by ca. 50%. A sample loading rate of 8.7 ml min-’ with a pump speed of 100 rev. min-’ was hence selected. The ratio of the reagent flow rate to the sample flow rate was kept at 1:4. Interferences In the FI sorbent extraction, coexisting extractable elements at high concentrations, e.g., Fe, may interfere in the analyte determination [21-231. For the extraction column used, the extractable metals are generally tolerated up to 1 mg 1-l [22]. The influence of Fe(II1) on the sorbent extraction in terms of the Zn signal and the sample-loaded volume was tested with 0.5% (m/v) 2-DBuDPA and 0.1 M citrate at pH 3. Samples of 5 pg 1-l Zn containing Fe(II1) at different concentrations were loaded for 120 s in order to measure the loaded volume accurately. No interference was caused by 10 mg ll’ Fe. The sample volume taken up was not affected with 100 mg 1-l Fe whereas the peak height was depressed by 17% and the peak area was increased by 13%. This implies that the eluting rate was decreased by the clogging effect from the large amount of extracted complexes, resulting in lower and broader
la; (W 1201001 ‘i;
P 60- cl
z
:n,,,,,oo J
Iron
concentration
Fig. 4. Interference on the Zn determination A, peak height; A, peak area.
-
_
E
60-
&?
: 40-
00.1 (mg/l)
Nickel
!zoncen
from (a) Fe and (b) Ni using 0.5% DPeDPA with 0.07 M oxalate.
&g/l)
loo0
0, sample loaded volume;
R. Ma et al. /Analytica Chimica Acta 309 (1995) 395-403
peak signals. With 500 mg 1-l Fe the column was clogged too seriously to continue extraction and elution. Using a higher citrate content (0.25 M) the tolerance to Fe could be substantially increased. The interference of Fe(II1) on the Zn determination was also studied using 0.5% (m/v) DPeDPA and 0.07 M oxalate at pH 3 (Fig. 4a). No significant influence on the sample volume taken up occurred with 500 mg 1-l Fe while the peak height was depressed by 32% and the peak area was increased by 32%. It suggests again that the eluting rate could be decreased although the sample-loading rate was normal. With 1000 mg 1-l Fe, the sample volume and peak height or area were all decreased in terms of sample volume ( - 25%), peak area ( - 55%), peak height (-SO%), because of the decrease in both sample-loading and eluting rates and probably also because of the inhibition of the complexation and extraction of Zn. Ni was also tested as a possible contaminant. In the case of 100 mg 1-l Ni with a normal sample volume taken up, the peak height and area were decreased by ca. 10% which might just be caused by inhibition of the complexation and extraction of Zn by competing for the complexing agent and the binding sites. With 1000 mg 1-l Ni, the sample volume ( - 20%), peak area ( - 60%) and peak height ( - 80%) were all decreased (Fig. 4b). The Zn signal peaks with 500-1000 mg 1-r Fe or 1000 mg 1-l Ni showed considerable tailing. Blank One problem of the Zn determination involving a preconcentration step is the high blank value. With the FI technique the blank signal from the reagent solution can be controlled by using a precolumn for on-line purification. The Zn blank at different preconcentration times was studied with or without a precolumn for 0.5% (m/v) 2-DBuDPA with 0.1 M citrate at pH 3 (Fig. 5). The blank values were first stable within the preconcentration time of 20 s without a precolumn or 30 s with a precolumn. Then the blank values increased almost linearly with time, but more slowly when using the precolumn. No obvious difference of the blank values was observed with or without a precolumn for preconcentration times less than 20 s in which the blank might originate mainly from the eluent and the spectral blank of the deu-
401
5‘ LmO.O6 .a, I 0.05 Y : a 0.04 V Y [r: 0.03 0
m0.02
0.01
Fig. 5. Blank of Zn at different sample-loading a precolumn; A, with a precolumn.
times. A, without
terium arc background correction for the organic reagent and solvent. For Zn determination in sea water, however, a much longer preconcentration time is required. In that case blank control with a precolumn is definitely necessary. Analytical performance With a 20-s sample loading at 8.7 ml min-’ for peak height measurement using a flow spoiler in the spray chamber, the enhancement factor in comparison with conventional FAAS without background correction is 35, which is the same as that of Cu and Cd with DEtDPA [22]. The detection limit (3~1 is 0.5 pg 1-l ; the calibration graph is linear up to 50 pg 1-l (r = 0.9999, n = 9). The reproducibility (relative standard deviation) with deuterium background correction is generally between 1 and 2% and not worse than 3% (n = 5) within the linear working range. Analysis of reference materials The capability and accuracy of the developed system was tested by analysing a number of BCR certified biological and environmental reference materials. In the biological materials and water samples, there are no high concentrations of extractable interfering elements, for which the analyte has to be preconcentrated. For the digested environmental samples, however, the Fe contents are 5-10 mg 1-l
R. Ma et al. /Analytica Chimica Acta 309 (1995) 395-403
402 Table 1 Determination of Zn in reference and 0.1 M citrate Reference material
Cod muscle Hay powder Single cell protein Mussel tissue Plankton Calcareous loam soil River sediment Estuarine water a*b Sea water a a Concentration b Ref. [28].
4. Conclusions materials with 0.5% 2-DBuDPA
Certified concentration
Measured concentration
(pgg-l)
(pgg-‘1
19.6jzO.S 32.1 f 1.7 42.7 f 1.0 76*2 112&-3 s1.3*3.7 142*3 172+11 25.7k2.9
19.9*0.1 31.4f0.9 42.4 f 0.3 75+2 114+ 1 81.9rt4.6 142rt5 163*6 27.4 + 1.2
in nmol kg-‘.
in the made-up solutions for analysis with 20-s sample loading. An effective matrix separation is necessarily provided for these samples by using a masking agent. Considering the presence of dissolved organic matter as potential competing complexing agents in the water samples, especially in the estuarine water [28], a relatively low content of the masking agent is advisable. 2-DBuDPA at 0.5% (m/v> with 0.1 M citrate at pH 3 was used for the analysis. The analytical results of all reference materials are listed in Table 1 and are in good agreement with the certified values (within 95% confidence intervals). The measured concentration is the mean and standard deviation of the determinations on three independent digest solutions for the solid samples or five determinations for the water samples. The analysis of biological materials using DPeDPA was also carried out. The result was too low for direct analysis of the cod muscle sample whose acidity was very high without being heated to dryness during the digestion. The final pH of its mixture with the reagent solution for the extraction appeared to be lower than 1. After adjusting the sample acidity with ammonia to ensure the final pH of the mixture solution above 2, satisfactory results were achieved. An important disadvantage of the Zn extraction with DPeDPA is the lower tolerance to the sample acidity, although higher masking-agent contents can be used.
In FI sorbent extraction with a DDP as chelating agent, the extractability to Zn increases with the chain length and branching of the alkyl substituent groups in the reagents and the tolerance to masking agents increases with the reagent concentration. An analytical method has been presented for Zn determination based on the combination of FI sorbent extraction preconcentration as a DDP complex with FAAS detection, which would be a good alternative to the determination by GFAAS after sample dilution. 2-DBuDPA at 0.5% (m/v) with 0.1-0.2 M citrate at pH 3 is the optimum reagent solution considering the sensitivity, selectivity and acid tolerance especially for those acid digest samples. No pH adjustment is necessary prior to the extraction which is normally required for the chelating adsorption-ion exchange procedures. The method has been successfully applied to a number of BCR certified biological and environmental reference materials. Applications to high purity ferrous metals and chemicals are predictable. In an attempt to establish the analytical systems for different heavy metals by using DDP reagents to FI sorbent extraction, the systematic investigations of more elements such as Co, Mn and Ni are in progress.
Acknowledgements The authors are grateful to M. Ceulemans and B. de la Calle Guntinas for their help in the preparation of this paper.
References [l] H. Bode and W. Amswald, Z. Anal. Chem., 185 (1962) 99. [2] G. Cote and D. Bauer, Anal. Chem., 56 (1984) 2153. [3] R.H. Zucal, J.A. Dean and T.H. Handley, Anal. Chem., 35 (1963) 988. [4] H. Bode and W. Arnswald, Z. Anal. Chem., 185 (1962) 179. [S] A.K. De, S.M. Khopkar and R.A. Chalmers, Solvent Extraction of Metals, Van Nostrand Reinhold, London, 1970, p. 169. [6] T.H. Handley and J.A. Dean, Anal. Chem., 34 (1962) 1312. [7] T.H. Handley, R.H. Zucal and J.A. Dean, Anal. Chem., 35 (1963) 1163.
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Chimica Acta 309 (1995) 395-403
[8] V.F. Toropova, A.R. Garifzyanov and I.E. Panfilova, Talanta, 34 (1987) 211. (91 Y. Sasaki, S. Tagashira, Y. Murakami, K. Okada and K. Hayashi, Bunseki Kagaku, 38 (1989) 707. [lo] K. Hayashi, Y. Sasaki, S. Tagashira, S. Inomata, J. Rikitake, Y. Hisatomi and M. Suzuki, Bunseki Kagaku, 33 (1984) 247. [ll] T.V. Rodionova and V.M. Ivanov, Zh. Anal. Khim., 41 (1986) 2181. [12] S. Wingefors and J. Rydberg, Acta Chem. Stand., A34 (1980) 313. [13] D. Chakraborti, W. De Jonghe and F. Adams, Anal. Chim. Acta, 120 (1980) 121. [14] D. Chakraborti, K.J. Irgolic and F. Adams, J. Assoc. Off. Anal. Chem., 67 (1984) 277. [15] W. Rudzinski and Q. Fernando, Anal. Chem., 50 (1978) 472. [16] R.F. Makens, H.H. Vaughan and R.R. Chelberg, Anal. Chem., 27 (1955) 1062. [17] T.W. Mastin, G.R. Norman and E.A. Weilmuenster, J. Am. Chem. Sot., 67 (1945) 1662. [18] R. Zimmerling, T. Dimmig, G. Jlger and D. StrBhI, Z. Chem., 30 (1990) 372.
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CHIMICA
ACTA ELSEVIER
Analytica Chimica Acta 309 (1995) 39.5-403
Flow injection sorbent extraction with dialkyldithiophosphates as chelating agent for the determination of zinc by flame atomic absorption spectrometry Renli Ma, Willy Van Mol, Freddy Adams Department
of Chemistry,
University ofAnhverp
*
(UIA), B-2610 Wilrijk, Belgium
Received 29 July 1994; revised 28 November 1994; accepted 16 January 1995
Abstract Dialkyldithiophosphates, (RO),P(S)S-, were used as the complexing agent for flow injection on-line sorbent extraction for the determination of zinc by flame atomic absorption spectrometry. The extractability with diethyl-, di-n-propyl-, di-2-propyl-, di-n-butyl-, di-isobutyl-, di-sec.-butyl-, di-n-pentyl- and di-n-hexyldithiophosphates was compared in respect of the effects of pH, alkyl group, masking agent and reagent concentration. Di-sec.-butyldithiophosphate (OS%, m/v) at pH 3 with 0.1 M citrate as a masking agent for iron, was used for the determination of zinc in a number of certified biological and environmental reference materials from the Community Bureau of Reference (BCR). All analytical results were in good
agreement with the certified values. The detection limit (3~) of peak height measurement, using a flow spoiler in the spray chamber, was 0.5 pg 1-r with an enhancement factor of 35 for a 20-s sample loading at 8.7 ml min-‘. Keywords: Waters
Atomic
absorption
spectrometry;
Flow injection;
Dialkyldithiophosphates;
1. Introduction In comparison with other chelating agents containing two sulphur donors, such as alkyl xanthates (alkyldithiocarbonates) and dialkyldithiocarbamates, dialkyldithiophosphates (DDP), (RO),P(S)S-, are unique for use in acid media where they are stable and selective. The half-life of diethyldithiophosphoric acid at room temperature is 4.8 h in 10 M HCl and 250 h in 1 M HCl [l]. Increase in the hydropho-
* Corresponding
author.
0003-2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SsDIOOO3-2670(95)00053-4
Zinc; Biological
materials;
Environmental
samples;
bit feature of the alkyl groups enhances the stability against hydrolysis [2]. The reagent solubility in aqueous media decreases with the hydrophobic character of the alkyl groups and with acid or salt concentration 131. Systematic investigations on the solvent extractability with one reagent for different elements 14-61 or different reagents for one element [6-91 have been reported. The most often used reagents for solvent extraction are diethyl [4,5,10,11], di-n-butyl [5,6,11,12] and di-sec.-butyl [13,14] derivatives. Good extraction efficiency is achieved for the elements that form sulphides of very low water-solubility such as Ag, Cd, Cu, Hg and Pb, while alkali and alkaline earth metals are not extracted [4-61. The
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number of extractable elements increases with the chain length of the straight-chain alkyl groups [5,6]. The extractability, e.g., for Zn, increases with the chain length of the alkyl groups [6-91 and branching in the alkyl groups [7,8]. Au(III), Cu(II), Fe(III) and Tl(II1) can be reduced by the reagents [4,6,10,15]. The reducing ability of the reagents increases with the size of the alkyl groups [16]. DDP reagents are generally laboratory-made because they are not commercially available for analytical use except diethyldithiophosphate. The synthesis is simple and easy by reacting phosphorus pentasulphide with the corresponding alcohols [17,18], but reported procedures differ [9,13,15,17,19]. The yield and purity are dependent on the quality of the reactants, correct stoichiometric quantities, mixing mode, reaction temperature and reaction time [18]. As the free acids may be somewhat unstable thermodynamically to hydrolysis and oxidation [1,2,5,6,16], they are preferably converted into the more stable salts, e.g., the ammonium salts [2,5,6]. Further purification may be carried out by recrystallisation from benzene [13] or ethyl acetate [6,7,15]. Flow injection (FI) liquid-solid sorbent extraction with a reversed-phase hydrophobic sorbent packed column is a technique combining the advantages of solvent extraction with the column operation in which the consumption of time, sample and reagent is reduced and the reproducibility is improved 120,211. Dithiocarbamate reagents have generally been used as the complexing agent [20-221, however, they are unstable in acidic media and unselective among the heavy metals. It is of interest to clarify the FI sorbent extraction characteristics of DDP reagents. Arnmonium diethyldithiophosphate (DEtDPA) in acid media (pH l-2) has been used in our previous work in FI sorbent extraction systems for the selective determination of Cd, Cu and Pb with flame or graphite furnace atomic absorption spectrometry (FAAS or GFAAS) [22,23]. The importance of Zn as an essential trace element in human and animal health is well known and AAS has been extensively employed for Zn determination. Detection by GFAAS may require a sample dilution with a risk of contamination and the introduction of a matrix modifier with a high blank value. The relatively high concentration level in the samples and the relatively high sensitivity of AAS for
this element suggest FAAS to be the detection technique of choice combined with preconcentration and separation procedures. Solvent extraction of Zn with DDP reagents, especially with the di-n-butyl derivative, has been studied thoroughly [6-9,11,12] and the extract could be analysed by FAAS [ll]. FI liquidliquid extraction with a dithiocarbamate reagent [24] and FI liquid-solid chelating adsorption - ion exchange with Chelex-100 resin or 8-quinolinol-immobilized controlled pore glass [25-271 have been applied for Zn determination by FAAS. Hitherto no determination of Zn with FI sorbent extraction has been reported, hence it is of interest, with this technique, to establish a selective determination of Zn after separation from the major matrix components including not only alkali and alkaline earth elements but also other heavy metals. Using FI on-line analyte preconcentration techniques, the risk of contamination can significantly be decreased in a closed flow system and the blank value can be effectively decreased with on-line purification of the reagents. In the present work, the FI sorbent extraction behaviour of DDP reagents for Zn in respect of the effects of pH, alkyl group, masking agent and reagent concentration is investigated. As a result an analytical system has been developed for the determination of Zn by FAAS and applied to a number of certified reference materials (biological materials and environmental samples) from the Community Bureau of Reference (BCR): cod muscle (CRM 4221, hay powder (CRM 129), single cell protein (CRM 274), mussel tissue (CRM 278), plankton (CRM 4141, calcareous loam soil (CRM 1411, river sediment (CRM 320), sea water (CRM 403) and estuarine water (CRM 505).
2. Experimental
2.1. Apparatus A Perkin-Elmer Model FIAS 200 flow injection accessory and a Perkin-Elmer Model 3030 atomic absorption spectrometer were used. The FI on-line sorbent extraction system and the FAAS detector have been described in our previous work [22]. A precolumn, the same as the extraction column, for
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on-line reagent purification to control the blank, was used for the analysis of sea water. pH adjustment and measurement were performed with a Schott CG 820 pH meter which had been calibrated with standard buffer solutions of pH 4, 7 and 10 (Merck).
3. Results and discussion
2.2. Reagents
3.1. Sorbent extraction
Suprapur concentrated acids (Merck) were used for the solid sample digestion and pH adjustment. All other chemicals used, including those for the complexing-agent synthesis, were of analytical grade. Methanol was used as eluent for the FI sorbent extraction without further purification. Standard solutions of tested metals were made by dilution from 1000 mg 1-l stock solutions (Merck). Deionized water from a Mini-Q water system (Millipore) was used throughout. DEtDPA was purchased from Aldrich. Other DDP reagents as the ammonium salts (DDPA) including the di-n-propyl (DPrDPA), di-Zpropyl (2-DPrDPA), di-n-butyl (DBuDPA), di-isobutyl (i-DBuDPA), disec.-butyl (2-DBuDPA), di-n-pentyl (DPeDPA) and di-n-hexyl (DHeDPA) derivatives were prepared based on the procedure used previously in our laboratory [13]. Phosphorus pentasulphide (0.25 mol or 55.5 g) with 50-80 ml of benzene in a 500-ml three-necked flask was heated with a water bath at 75-80” C. One mol of a particular alcohol (75-125 ml) was added dropwise while stirring and condensing. After 2-h refluxing with stirring, ammonia gas was bubbled through the hot solution until the gas started to appear from the condenser during which a white precipitate appeared. The solution was slowly cooled to room temperature. The product was filtered off and rinsed twice with 10 ml of benzene, and dried in a vacuum system. The recovery of recrystallisation from benzene, e.g., for 2-DBuDPA, was about 80%. DDPA reagents in acetate, citrate or oxalate medium, contents as required, were adjusted to the required pH values with dilute nitric acid or ammonia solution. 2.3. Digestion
biological and environmental samples, respectively [23]. The residue was dissolved in 50 ml of acidified water.
behauiour of DDP reagents
A series of DDPA homologues as the chelating agent for FI sorbent extraction were compared for extractability of Zn, the analyte of interest, and Fe(III), a typical coexisting heavy metal ion. The optimum reagent should provide the maximum sensitivity for Zn with good selectivity from interfering elements, such as Fe. Effect of pH For all DDPA reagents except 2-DPrDPA, at 0.5% (m/v> in 0.1 M acetate medium, the pH dependence of the sorbent extractability was examined between pH 0 and 12. It appeared that DPeDPA did not dissolve very well at pH 1 while DHeDPA did not dissolve below pH 2. The four butyl and pentyl derivatives produced the same maximum sensitivity as shown in Fig. 1, in which peak height measurements were used. Zn can be quantitatively extracted up to pH 6 with both 2-DBuDPA and DPeDPA
of solid samples PH
Accurately weighed 0.1 g of solid standard reference material was digested as reported earlier for
Fig. 1. pH dependence of the 211 extraction. i-DBuDPA; A, 2-DBuDPA; 0, DPeDPA.
0.
DBuDPA,
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whereas the lower pH limit for quantitative extraction is 2 with DPeDPA or down to 0 with 2-DBuDPA. With DPeDPA at pH 1 the relatively lower extractability is probably due to the low solubility of the reagent. Zn can also be extracted quantitatively within the pH range l-4 with i-DBuDPA or pH 2-3 with DBuDPA. In the case of peak area measurements the pH ranges with 2-DBuDPA and DPeDPA for quantitative extraction are just up to pH 4. The acid tolerance of the Zn extraction increases in the order of DBuDPA < i-DBuDPA < 2DBuDPA which disagrees with the result in solvent
extraction reported by Handley and Dean [6] where the acid tolerance of the Zn extraction decreases with increased branching of the butyl chain near the dithiophosphoric functional group. Zn can not be extracted with DEtDPA over the whole pH range and can be extracted only partly with DPrDPA around pH 2-3. With DHeDPA a stable signal was obtained between pH 3 and 6 (peak height) or between pH 3 and 4 (peak area). The signal, however, was lower than the maximum signal produced by DBuDPAs and DPeDPA. The reason for this is not clear.
,lo %--Relative peak height .-. .__-l-__.-_“---__-.--.-..---.T (a)
DE1DM
DPrDM Z-DPrDM DBuDM CDBuDMMwDIw
DPoDM
DHoDM
DDPA Reagent
,20 lb Relative peak height --.-1--_.-.-.-.-..“..“-.-
.._....._._.. -_I.-.-.-__- .....” ......._.....................” . .“.._ ....“_“...”. ...”. . - ............... ..._
100 80
“_.--._.“..“...-.-._.---
. . __.-._--.-
-.-._“.-.-l- I._. I_... “..._.
60 40
20 0
DEtDM
DPrDW WPrDM
DBuDM I-DI)UDMIQU~ DDFA
Fig. 2. Effect of alkyl group on the extraction
DPoD#r DHoDM
Reagent
of (a) Zn and (b) Fe. W , without citrate;
0, with citrate.
R. Ma et al. / Analytica Chimica Acta 309 (1995) 395-403
The maximum signals for Fe(III) were pH ca. 1 with DEtDPA, within pH DPrDPA; pH 2-3 with DBuDPAs; around DPeDPA or at pH 3 with DHeDPA. A pH value of 3.0 was selected for all further experiments.
obtained at l-3 with pH 2 with reagents in
Effect of the alkyl group Using 0.5% (m/v) of all DDPA reagents in 0.1 M acetate-ammonium medium with or without 0.1 M citrate at pH 3, the extractability of the reagents for each element was compared. Zn can be extracted quantitatively with DBuDPAs and DPeDPA while it can only partly be extracted with DPrDPAs and DHeDPA and can not be extracted with DEtDPA. Citrate at 0.1 M inhibits the extraction with DPrDPAS but does not influence the extraction with DBuDPAs and DPeDPA (Fig. 2a). The extractability for Zn, also considering the tolerance to acid and masking agent, increases with the number of carbon atoms and the branching of the alkyl chains from two (ethyl-) to five (pentyl-) carbons in the reagents: DEtDPA x DPrDPA < 2-DPrDPA < DBuDPA < i-DBuDPA < 2-DBuDPA < DPeDPA. With DHeDPA, the signal was slightly increased in the presence of citrate. As can be seen from Fig. 2b, the extraction of Fe(III) is greatest with i-DBuDPA. Extraction with all reagents was significantly inhibited by citrate.
399
Effects of masking agent and reagent concentration Several commonly used masking agents for Fe(II1) were examined to eliminate its interference in the determination of Zn. Using 0.1 M of each masking agent with 0.5% (m/v) DBuDPA in acetate medium at pH 3, 400 pg 1-l Fe(III) was masked completely by oxalate or mostly by phosphate, citrate, tartrate and fluoride, but not by thiocyanate and ascorbic acid. The Zn signal was depressed almost completely by oxalate, significantly by phosphate and tartrate and slightly by fluoride and thiocyanate. With DBuDPA, Tiron or particularly Ferron affected the extraction of Zn seriously without masking Fe(II1) very effectively. Because of their relatively high masking efficiency for Fe, the influence of citrate or oxalate content was studied further with different concentrations of 2-DBuDPA or DPeDPA, respectively, at pH 3. The results with 2-DBuDPA and citrate are illustrated in Fig. 3 for peak height measurements. Zn can be extracted quantitatively with 2-DBuDPA or DPeDPA at concentrations higher than 0.05% (m/v) without any masking agent. The minimum reagent concentrations for quantitative extraction become higher as the masking agent contents are increased. Zn determination with 0.5% 2-DBuDPA was not affected by oxalate at a concentration lower than 0.0035 M or citrate lower than 0.25 M. With 0.5% DPeDPA it was not affected by oxalate up to 0.07 M
“‘1 (a)
Fig. 3. Effect of 2-DBuDPA 0.5 M of citrate.
and citrate concentrations
on the extraction
of (a) Zn and (b)Fe. 0, 0 M; 0, 0.05; A, 0.1 M; 0, 0.25 M; + ,
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or citrate up to 0.5 M. The tolerance of Zn extraction with DPeDPA to masking agents is better than that with 2-DBuDPA. Fe(II1) can be extracted well with between 0.01 and 0.1% 2-DBuDPA or between 0.005 and 0.1% DPeDPA without any masking agent. At higher reagent concentrations the sensitivity decreases, probably because the reagents reduce Fe(II1) to Fe(U) which is less well extracted [6]. This effect becomes more serious with the higher homologue DPeDPA. The Fe extraction is inhibited seriously by the masking agents over the whole reagent concentration range. With 0.5% (m/v) of these two reagents the same masking agent content was required to mask Fe completely, 0.07 M oxalate or 1 M citrate. Consequently, Zn could be quantitatively and selectively extracted with 0.5% 2-DBuDPA and 0.1-0.25 M citrate whereas Fe was mostly masked. Zn also could be quantitatively and selectively extracted using 0.5% DPeDPA with 0.07 M oxalate, where Fe was masked completely. These two reagent solutions were chosen for further study of the determination of Zn. 3.2. Determination of zinc Sample loading rate High sample flow rates can result in low extraction efficiency. The influence of the sample loading rate on the preconcentration efficiency was examined with 0.5% (m/v) DPeDPA. Different sample load-
ing times were set at different rates to load the same sample volume. A slight increase in the Zn sensitivity was observed as the sample loading rate was decreased. From 8.7 ml min-’ to 4.3 ml min-’ of the sample loading rate, the sensitivity increased around 10% while the sampling frequency decreased by ca. 50%. A sample loading rate of 8.7 ml min-’ with a pump speed of 100 rev. min-’ was hence selected. The ratio of the reagent flow rate to the sample flow rate was kept at 1:4. Interferences In the FI sorbent extraction, coexisting extractable elements at high concentrations, e.g., Fe, may interfere in the analyte determination [21-231. For the extraction column used, the extractable metals are generally tolerated up to 1 mg 1-l [22]. The influence of Fe(II1) on the sorbent extraction in terms of the Zn signal and the sample-loaded volume was tested with 0.5% (m/v) 2-DBuDPA and 0.1 M citrate at pH 3. Samples of 5 pg 1-l Zn containing Fe(II1) at different concentrations were loaded for 120 s in order to measure the loaded volume accurately. No interference was caused by 10 mg ll’ Fe. The sample volume taken up was not affected with 100 mg 1-l Fe whereas the peak height was depressed by 17% and the peak area was increased by 13%. This implies that the eluting rate was decreased by the clogging effect from the large amount of extracted complexes, resulting in lower and broader
la; (W 1201001 ‘i;
P 60- cl
z
:n,,,,,oo J
Iron
concentration
Fig. 4. Interference on the Zn determination A, peak height; A, peak area.
-
_
E
60-
&?
: 40-
00.1 (mg/l)
Nickel
!zoncen
from (a) Fe and (b) Ni using 0.5% DPeDPA with 0.07 M oxalate.
&g/l)
loo0
0, sample loaded volume;
R. Ma et al. /Analytica Chimica Acta 309 (1995) 395-403
peak signals. With 500 mg 1-l Fe the column was clogged too seriously to continue extraction and elution. Using a higher citrate content (0.25 M) the tolerance to Fe could be substantially increased. The interference of Fe(II1) on the Zn determination was also studied using 0.5% (m/v) DPeDPA and 0.07 M oxalate at pH 3 (Fig. 4a). No significant influence on the sample volume taken up occurred with 500 mg 1-l Fe while the peak height was depressed by 32% and the peak area was increased by 32%. It suggests again that the eluting rate could be decreased although the sample-loading rate was normal. With 1000 mg 1-l Fe, the sample volume and peak height or area were all decreased in terms of sample volume ( - 25%), peak area ( - 55%), peak height (-SO%), because of the decrease in both sample-loading and eluting rates and probably also because of the inhibition of the complexation and extraction of Zn. Ni was also tested as a possible contaminant. In the case of 100 mg 1-l Ni with a normal sample volume taken up, the peak height and area were decreased by ca. 10% which might just be caused by inhibition of the complexation and extraction of Zn by competing for the complexing agent and the binding sites. With 1000 mg 1-l Ni, the sample volume ( - 20%), peak area ( - 60%) and peak height ( - 80%) were all decreased (Fig. 4b). The Zn signal peaks with 500-1000 mg 1-r Fe or 1000 mg 1-l Ni showed considerable tailing. Blank One problem of the Zn determination involving a preconcentration step is the high blank value. With the FI technique the blank signal from the reagent solution can be controlled by using a precolumn for on-line purification. The Zn blank at different preconcentration times was studied with or without a precolumn for 0.5% (m/v) 2-DBuDPA with 0.1 M citrate at pH 3 (Fig. 5). The blank values were first stable within the preconcentration time of 20 s without a precolumn or 30 s with a precolumn. Then the blank values increased almost linearly with time, but more slowly when using the precolumn. No obvious difference of the blank values was observed with or without a precolumn for preconcentration times less than 20 s in which the blank might originate mainly from the eluent and the spectral blank of the deu-
401
5‘ LmO.O6 .a, I 0.05 Y : a 0.04 V Y [r: 0.03 0
m0.02
0.01
Fig. 5. Blank of Zn at different sample-loading a precolumn; A, with a precolumn.
times. A, without
terium arc background correction for the organic reagent and solvent. For Zn determination in sea water, however, a much longer preconcentration time is required. In that case blank control with a precolumn is definitely necessary. Analytical performance With a 20-s sample loading at 8.7 ml min-’ for peak height measurement using a flow spoiler in the spray chamber, the enhancement factor in comparison with conventional FAAS without background correction is 35, which is the same as that of Cu and Cd with DEtDPA [22]. The detection limit (3~1 is 0.5 pg 1-l ; the calibration graph is linear up to 50 pg 1-l (r = 0.9999, n = 9). The reproducibility (relative standard deviation) with deuterium background correction is generally between 1 and 2% and not worse than 3% (n = 5) within the linear working range. Analysis of reference materials The capability and accuracy of the developed system was tested by analysing a number of BCR certified biological and environmental reference materials. In the biological materials and water samples, there are no high concentrations of extractable interfering elements, for which the analyte has to be preconcentrated. For the digested environmental samples, however, the Fe contents are 5-10 mg 1-l
R. Ma et al. /Analytica Chimica Acta 309 (1995) 395-403
402 Table 1 Determination of Zn in reference and 0.1 M citrate Reference material
Cod muscle Hay powder Single cell protein Mussel tissue Plankton Calcareous loam soil River sediment Estuarine water a*b Sea water a a Concentration b Ref. [28].
4. Conclusions materials with 0.5% 2-DBuDPA
Certified concentration
Measured concentration
(pgg-l)
(pgg-‘1
19.6jzO.S 32.1 f 1.7 42.7 f 1.0 76*2 112&-3 s1.3*3.7 142*3 172+11 25.7k2.9
19.9*0.1 31.4f0.9 42.4 f 0.3 75+2 114+ 1 81.9rt4.6 142rt5 163*6 27.4 + 1.2
in nmol kg-‘.
in the made-up solutions for analysis with 20-s sample loading. An effective matrix separation is necessarily provided for these samples by using a masking agent. Considering the presence of dissolved organic matter as potential competing complexing agents in the water samples, especially in the estuarine water [28], a relatively low content of the masking agent is advisable. 2-DBuDPA at 0.5% (m/v> with 0.1 M citrate at pH 3 was used for the analysis. The analytical results of all reference materials are listed in Table 1 and are in good agreement with the certified values (within 95% confidence intervals). The measured concentration is the mean and standard deviation of the determinations on three independent digest solutions for the solid samples or five determinations for the water samples. The analysis of biological materials using DPeDPA was also carried out. The result was too low for direct analysis of the cod muscle sample whose acidity was very high without being heated to dryness during the digestion. The final pH of its mixture with the reagent solution for the extraction appeared to be lower than 1. After adjusting the sample acidity with ammonia to ensure the final pH of the mixture solution above 2, satisfactory results were achieved. An important disadvantage of the Zn extraction with DPeDPA is the lower tolerance to the sample acidity, although higher masking-agent contents can be used.
In FI sorbent extraction with a DDP as chelating agent, the extractability to Zn increases with the chain length and branching of the alkyl substituent groups in the reagents and the tolerance to masking agents increases with the reagent concentration. An analytical method has been presented for Zn determination based on the combination of FI sorbent extraction preconcentration as a DDP complex with FAAS detection, which would be a good alternative to the determination by GFAAS after sample dilution. 2-DBuDPA at 0.5% (m/v) with 0.1-0.2 M citrate at pH 3 is the optimum reagent solution considering the sensitivity, selectivity and acid tolerance especially for those acid digest samples. No pH adjustment is necessary prior to the extraction which is normally required for the chelating adsorption-ion exchange procedures. The method has been successfully applied to a number of BCR certified biological and environmental reference materials. Applications to high purity ferrous metals and chemicals are predictable. In an attempt to establish the analytical systems for different heavy metals by using DDP reagents to FI sorbent extraction, the systematic investigations of more elements such as Co, Mn and Ni are in progress.
Acknowledgements The authors are grateful to M. Ceulemans and B. de la Calle Guntinas for their help in the preparation of this paper.
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