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Behaviour of the dithiocarbamate complexes of arsenic, antimony, bismuth, mercury, lead, tin and selenium in methanol with a hydride generator
Talanta ELSEVIER
Talanta
43 (1996)
479-486
Behaviour of the dithiocarbamate complexes of arsenic, antimony, bismuth, mercury, lead, tin and selenium in methanc with a hydride generator L. Vuchkova, Faculty Received
qf Chemisrry,
Uniuersiry
Z June 1995; revised
S. Arpadjan” oj Sofia.
30 August
I126 Sqfia,
1995; accepted
Bulgaria 9 October
1995
Abstract
A study was carried out with a continuous hydride generator coupled to an atomic emissionspectrometerwith inductively-coupledplasmato determinewhether hydridesof As, Bi. Pb, Sb, Sn and Se and mercury vapor could be generatedin methanol solutionsof their dithiocarbamatecomplexes.It was found that (with the exception of Pb) hydride generationwith sufficient efficiency for simultaneousmulti-elementdetermination is achievedusing 0.25’%1 NaBH4-0.6 mol 1~’ HCl asreaction medium.The detection limit was found to be 0.2 ng ml - ’ for As, 30 ng ml-- ’ for Bi, 0.03 ng ml ’ for Se, Sb and Sn. Kevwords: Inductively-coupled plasma atomic emission _ _ _ mate complexes in methanol; Solid phase extraction
spectrometry;
1. Introduction The generation of volatile covalent hydrides and mercury vapor has become a widely-used technique in atomic spectrometry for analysis of As, Sb, Se, Sn, Ge, Te, Bi, Hg and Pb in various matrices. For the determination of extremely low element concentrations and for the elimination of matrix interferences a preliminary preconcentration is necessary. Liquid-liquid extraction preconcentration methods have been described in combination with subsequent hydride generation * Corresponding 0039-9140/96/Sl5.00 SSDI
author. ‘0 1996 Elsevier
0039-9140(95)01780-1
Science
B.V.
All rights
reserved
Continuous
hydride
generator;
Dithiocarba-
(HG) directly in non-aqueous media without any mineralisation or re-extracting procedures [ 1- 111. However, the solvent extraction method offers limited enrichment factors and is unsuitable for automation of the analysis and for preservation and transportation of pretreated samples. This is the reason for the recent enhanced interest in preconcentration methods using column solid phase extraction. An effective preconcentration technique allowing higher concentration factors than with liquid extraction was developed using a water-insoluble ligand, such as ammonium hexahydroazepine- 1-dithiocarboxylate (ammonium hexamethylenedithiocarbamate) (HMDC), physically immobilized on polyurethane foam, and dis-
480
L. Vuchkoua,
S. Arpadjan
posable syringes as microcolumns [12]. In a preliminary study optimum conditions were established for quantitative solid phase extraction of As, Bi, Hg, Sb, Se, Sn and Pb. A complete elution was achieved by total dissolution of the sorbed analyte dithiocarbamate complexes in organic solvents [12,13]. The application of a water-miscible solvent such as methanol simplifies the analytical procedure due to the possibility of using the conventional hydride generator for aqueous solutions without any changes. However, no data are available on whether elemental hydrides and mercury vapor could be generated from methanol solutions of chelate complexes of the analytes and then followed by inductively-coupled plasma atomic emission spectrometry (ICP-AES) measurements. The aims of the present work are: (i) to investigate the hydride generation of As, Sb, Sn, Se, Bi and Pb when present as dithiocarbamate complexes in methanol solutions; (ii) to investigate the behaviour of Hg under the same conditions; and (iii) to optimize the working conditions for combination of the HG in methanol solutions of the analyte dithiocarbamate complexes with an inductively-coupled plasma optimized for simultaneous determination of the above elements by atomic emission spectrometry. 2. Experimental 2.1. Reagents and apparatus
All reagents used were of analytical-reagent grade. Redistilled water was used throughout. Stock solutions of 0.25% and 0.5% NaBH, (Merck) in 0.4% NaOH were prepared daily and stored in a polyethylene bottle before use. The multielement standard solutions for all the studied elements were prepared from Titrisol (Merck, Germany). HMDC (Merck) was used as received. The solvent methanol (Merck) was additionally purified by distillation. The ICP-AES measurements were performed with a simultaneous ICP spectrometer Spectroflame combined with a continuous hydride generator 341-ARL. The ICP and HG operating conditions are given in Table 1.
/ Talanta
43 (1996)
479-486
The HMDC-immobilized polyurethane foam and the sorbent columns for solid phase extraction were prepared as described earlier [12]. 2.2. Preparation
of test solutions
To 20.00 ml of a standard solution containing 50 ng ml ~ ’ As(III), Sb(III), Bi(III), Se(IV); 60 ng ml ~ ’ Sn( IV); 20 ng ml - ’ Hg(II) and 100 ng ml - ’ Pb 5 ml acetic buffer at pH 4.66 was added. This solution was pumped through a sorbent column filled with HMDC-immobilized polyurethane foam using the conditions reported previously [12]. Then the organic chelating ligand, HMDC, together with the analyte-dithiocarbamate complexes formed were completely dissolved in 9.0 ml methanol or in 9.0 ml aqueous methanol solution with 66% methanol (6.0 ml CH,OH + 3.0 ml H,O). 2.3. Recommended procedure
In 150 ml of a water sample 3 g KI is dissolved and heated gently at 60°C for 1 h to ensure that the analytes are present in their lower oxidation states. The pH is adjusted to 4.5 + 0.5 with acTable ICP
1
operating parametersand
Incident power (kW) Argon flow rates (I min-‘): coolant carrier sheath Observation height (mm) Pre-integration time (s) Integration time (s) Wavelengths (nm): As 1 Bi I Hg 1 Pb II Sb 1 Se 1 Sn II Reductant Reductant flow rate (ml min-‘) Acid flow rate (ml min-‘) Sample flow rate (ml min ‘)
HG
conditions 1.25 17 0.9 0.85 15 30 5 189.04 230.0 184.95 220.35 206.8 196.09 189.98 NaBH, solution 0.4% NaOH 2.4 1.2 6.0
in
L. Vuchkooa,
S. Arpadjan
1 Talanta
43 (1996)
481
479-486
0.25% NaBH4 - 0.6M HCI
As
Ha :
Fig. 1. Analyte line intensity generation: + L in presence
sb
Pb
tza 66(HcH3OH-L %%% CEJOH-L
LF2*tHcL
relative to aqueous reference solutions using 0.25% of HMDC; - L in absence of HMDC. 0.25%
sn
SO
NaBH4
NaBH,-0.6
M HCI as reaction
medium
for hydride
- l.ZM HCI
R * 1
1 n t 0 n I I t Y
m Fig. 2. As Fig.
a m
66HCH30H+L C!H3OH+L
I but with
0.25%
etate buffer or if necessary with some drops of dilute ammonia or HCl. The sample is pumped through the sorbent column at a flow rate of 2 ml min’. Then 5.0 ml CH,OH is placed in a dry quartz beaker and passed six times through the sorbent with the aid of the syringe plunger. This methanol solution of the analyte-dithiocarbamate complexes is connected to the sample channel of the hydride generator. The calibration solutions were prepared from aqueous multielement standard solutions by appropriate dilution with a methanol solution of HMDC (10 g l- I).
NaBH,-1.2
66%CH3OH-L CHjOH-L
M HCI
as reaction
medium.
3. Results and discussion
The influence of methanol and of the chelateforming ligand HMDC on the yield of hydride was investigated for the following reaction systems: A, 0.25% NaBH,-0.6 M HCl; B, 0.25% NaBH,- 1.2 M HCl; C, 0.25% NaBH,-0.6 M HNO,; D, 0.25% NaBH,- 1.2 MHNO,; E, 0.25% NaBH,-2 M HNO,; F, 0.5% NaBH,-2 M HNO,. The intensity of the emission signal of the analytes when present as dithiocarbamate complexes
L. Vuchkooa,
482
S. Arpadjan
: Talantu
43 (1996)
479-486
0.25%NaBH4-0.6MHN03 2.50 R e I I n t e n s 1 t
2.00
1.50
1.00
050
Y 0.W
As
Fig. 3. As Fig.
Bi
Hg
m
66HCHJOH+L
m
CH30H+L
1 but with
0.25%
-Pb
sb m m
NaBH,-0.6
Se
Sn
66%CE3OH-L CHJOH-L
M HNO,
as reaction
medium.
0.25?hNaBH4-1.2MHN03 350, R * I
I s.oof
J Pb
m
Fig. 4. As Fig.
66% CH3OH CHjOHtL
I but with
+ L
0.25’1/0 NaBH,-1.2
in methanol (Z,) was compared with that obtained for aqueous solutions (I,) at the same experimental conditions. The results obtained for the relative intensity, Z, ( = IJZ,), are shown in Figs. l-6. The chelate-forming ligand HMDC affects the process of hydride generation. The HG reaction was realised with higher efficiency in methanol solutions without ligand than in those with ligand. The influence of HMDC depends on the stability of the dithiocarbamate complexes of the analytes in methanol and on the rate of hydride formation or reduction to the elemental state (Hg). In the
Sb m m
Se
Sn
66YCH3OH-L CIDOH-L
M HNO,
as reaction
medium
presence of methanol and HMDC Pb cannot form a volatile covalent hydride at all. The ligand HMDC strongly suppresses the formation of bismuthine in all the investigated reaction systems (Figs. l-6) due to the high stability of the Bidithiocarbamate complex and the relatively slow rate of bismuthine formation. The determination of Bi in the presence of HMDC is possible only in 0.25% NaBH,-0.6 M HCl with twice as poor sensitivity (Z, = 0.5). Arsenic forms a volatile hydride independent of the presence of ligand and in the applied reaction
L. Vuchkma,
S. Arpadjan 8.25%
16.00,R e 1 1 n t 6 n
14.00
: Talanta
43 (1996)
479-486
483
NaBH4 - 2IU HNO3
.. I
lZ.al 10.00 8.00~ 6.00.
sb
Pb m 0
Fig. 5. As Fig.
66%CH3OH+L C!ZUOH+L
1 but with
0.25%
m m
NaBH,-2
se
66% CH3OH CHJOH-L
M HNO,
Sn
-L
as reaction
medium.
0.5% NaJSH4 - ZM HNO3 2.00, R e 1 I n t * n I i t
1.50
1.00
0.50
Y 0.00
As
B1
m
Fig. 6. As Fig.
Se
sll 66HCH?JoH+L CHjOHtL
1 but with
0.5% NaBH,-2
system in pure CH,OH as well as in 66% CH,OH (Fig. 1). Selenium forms a volatile hydride in methanol solutions of HMDC with all the studied reduction systems. The efficiency of H,Se formation is higher in HCl medium (I, z 1.9-2.4, Figs. 1 and 2) than in HNO, medium (Z, z 0.85-1.15, Figs. 3, 4 and 6) except for the system 0.25% NaBH,-2 M HNO, where 1, = 4.3 (Fig. 5). In the case of antimony the efficiency of stibine formation is higher (I, z 2) in hydrochloric acidcontaining reaction systems (Figs. 1, 2) than in nitric acid medium (Z, < 1, Figs. 3, 4 and 6).
El tiii%
Sn
66WCH3OH-L CIDOH-L
M HNO,
as reaction
medium.
A strong depression of SnH, formation was observed in the presence of HMDC in the methanol solution for all reaction systems with HNO,. Mercury forms stable dithiocarbamate complexes. Nevertheless, the efficiency of mercury vapor formation is high in the reaction systems containing HCl (I, z 4.5) and HNO, (I, z 1.13). This is probably due to the high reduction rate of mercury to the elemental state. The higher intensity signals for As, Hg, Sb, Se and Sn in methanol in the absence of the chelate-
484
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S. Arpadjan
/ Talanta
43 (1996)
479-486
Table 2 Detection limits (3 0) for HG with ICP-AES for methanol solutions of dithiocarbamate Reaction medium
0.25% NaBH,-0.6 M HCl 0.25% NaBH,-1.2 M HCI 0.25% NaBH,-0.6 M HNO, 0.25% NaBH,- 1.2 M HNO, 0.25% NaBH,-2 M HNO, 0.5% NaBH,-2 M HNO,
complexes of the analytes
Detection limit (ng ml-‘) As
Bi
Hg
Sb
Se
Sn
0.2 0.5 0.8 0.7 0.4
30 -
0.03 0.1 0.2 0.1 0.2 0.2
3
3.3
1.8 3
9 7 9 8.5
2.1 4 -
-
-
forming ligand HMDC (Figs. 1 and 2) than for their aqueous reference solutions could be explained by a lowering of the surface tension, promoting a higher rate of hydride or cold vapor liberation from the liquid phase. This is presumably the explanation for the higher efficiency in pure methanol than in 66% methanol. An exception is observed for bismuth and lead. The generation of bismuthine in HCl medium does not depend (I, z 1) on the replacement of water by CH,OH. 3.1. Choice of working conditions
A higher sensitivity and a more stable plasma were obtained for methanol solutions of HMDC in comparison to aqueous methanol solutions (66% CH,OH). Table 3 Comparison of the detection limits (3 c) of HG-ICP in aqueous solutions and in methanol solutions of the analyte dithiocarbamate complexes after preconcentration by column solid phase extraction (recommended procedure) Element
HG-ICP in aqueous solutions (ng ml-‘)
Recommended procedure (ng ml-‘)
As Bi Hg Sb Se Sn
0.2 0.35 0.02 0.5 0.6 0.3
0.008 1 0.001 0.1 0.1 0.1
The optimum conditions for single element determination can be seen from the calculated limits of detection presented in Table 2. For simultaneous multi-element analysis, elution by total dissolution of the analyte-dithiocarbamate complexes in pure methanol and HG using the reaction system 0.25% NaBH,-0.6 M HCl is recommended as the optimal compromise decision. In comparison with work with aqueous solutions under these experimental conditions the background signal for Bi does not change, but for As, Se, Sb and Sn it is enhanced 1.5-fold and for Hg it is enhanced 2.3-fold. The relative standard deviations of the measurements in methanol and aqueous media do not differ significantly, varying between 1 and 4%. Table 3 presents the achievable best sensitivity for HG-ICP determination of the investigated elements in aqueous solutions using the optimal experimental conditions for aqueous media (0.5% NaBH,-0.3 M HCl). It can be seen that whereas for As and Hg no substantial change in the sensitivity of their determinations was observed, for selenium the sensitivity in methanol solution of its dithiocarbamate complex is approximately five times worse than by HG from aqueous Se(IV) solutions. This lowering in sensitivity is about a factor of nine for Sn and almost two orders of magnitude for Bi. However, the combination of HG with preliminary 30 fold preconcentration of the traces by column solid phase extraction, as described in the recommended procedure, permits the determination of extremely low analyte concentrations with the exception of Bi (Table 3).
L. Vuchkova,
S. Arpadjan
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485
Table 4 Recovery (%) from spiked water samples (n-number of samples) Sample
Recovery (%)
Tap water (n = 4) Mineral water (n = 3) Ground water (n = 3) Waste water (n = 4) Sea water (n = 7)
3.2. Application
AS
Bi
97 + 5 97 + 3 96 + 6 95*4 96&6
95 + 96 + 97 f 95 + 96 +
5 6 5 7 6
of the method
The recommended procedure was applied to a number of water samples: sea water, waste water, tap water, mineral water and ground water. The recovery studies were performed by adding known amounts of the analytes to the samples prior to adjustment of the pH value to 4.5 + 0.5. The results are shown in Table 4. The high recoveries obtained indicate that the foreign ions present in the samples do not interfere to any significant extent with the determination of As, Bi, Sb, Se and Sn after HG in methanol solutions of the analyte dithiocarbamate complexes. The highest concentrations of coexisting ions in the water samples analysed were 0.4 pg I - ’ Cd, 2 pg I- ’ Cr, 100 pg 1-l Cu, 300 pg 1-r Fe, 200 pg 1-r Mn, 1.2 fig 1-l Ni, 5 pg 1-l Pb and 150 fig 1-l Zn. The alkali and alkaline earth elements, aluminium and the anions are not sorbed and hence could not be present in the methanol solution.
Hg
Se
Sb
Sn
99 i: 2 97 f 3 9854 96 i- 5 97 + 5
97 +3 96k 3 9524 93 * 5 92 f 6
95 +4 97 f 3 96 + 5 97 +4 94 * 4
96 k 5 97*4 98 + 2 95&4 93 * 5
The results for water samples with higher analyte content were compared with those of HG in aqueous solutions as shown for two cases in Table 5. The agreement is good and the difference between the standard deviations of both methods is not statistically significant. HG-ICP in aqueous solutions is undoubtedly a faster and simpler technique, but the proposed column solid phase extraction preconcentration in combination with subsequent HG in methanol solutions for simultaneous determination by ICP-AES allows higher sensitivity for As, Hg, Sb, Se and Sn, and permits better possibilities for the preservation and transportation of the water samples.
4. Conclusion
The conversion of the analytes from their dithiocarbamate complexes in methanol to covalent hydrides or cold vapor (Hg) through direct
Table 5 Results (pg I-‘) of the analysis of waste-water and sea-water samples (n = number of parallel determinations) Element
As Bi Hg Se Sb Sn
Waste water (n = 4)
Sea water (n = 5)
Recommended procedure
HG-ICP in aqueous solutions
Recommended procedure
HG-ICP in aqueous solutions
2.8 f 0.3 9.7 k 0.8 10.001 6.4 5 0.5 17.2 + 1.2 46 f. 3
2.8 + 9.3 + <0.02 6.2 + 16.6 f 47.2 +
1.27 + 0.08
1.32 + 0.07 10.35 2.3 + 0.2 <0.6 <0.5 <0.3
0.2 0.7 0.3 0.8 2.4
486
L. Vuchkotia,
S. Arpadjan
reduction in the organic solvent permits the development of an effective solid phase extraction preconcentration procedure prior to HG for simultaneous determination of As, Bi, Sb, Se and Sn.
Acknowledgement The authors thank the Bulgarian Sciences, Project X-519.
Foundation
of
References [I] J. Aznarez, F. Palacios, M.S. Ortega and J.C. Analyst, 109 (1984) 123. [2] J. Aznarez, F. Palacios, J.C. Vidal and J. Galvan, lyst, 109 (1984) 713.
Vidal, Ana-
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43 (1996)
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[3] J. Aznarez, J.M. Rabadan. A. Ferrer and P. Cipres, Talanta, 33 (1985) 458. [4] J. Aznarez, J.C. Vidal and J.M. Gascon, At. Spectrosc., 7 (1986) 59. [5] J. Aznarez, J.C. Vidal and R. Carnicer, J. Anal. At. Spectrom., 2 (1987) 55. [6] J.R. Castillo, J.M. Mir, J. Val, M.P. Colon and C. Martinez, Analyst, 110 (1985) 1219. [7] M. Chikuna and N. Aoki. J. Anal. At. Spectrom., 8 (1993) 415. [8] S. Zhang, H. Han and G. Ni, Anal. Chim. Acta, 221 (1989) 85. [9] M.R. Rezende, R.C. Campos and A.J. Curtius, J. Anal. At. Spectrom., 8 (1993) 247. [lo] B. Huang, X. Zeng, Z. Zhang and J. Liu, Spectrochim. Acta, Part B, 43 (1988) 381, [I I] A.G. Menendez, J.E.S. Uria and A. Sanz-Medel, J. Anal. At. Spectrom., 4 (1989) 581. [12] A. Alexandrova and S. Arpadjan, Analyst, 118 (1993) 1309. [13] L. Vuchkova and S. Arpadjan, Analyst, (1996) submitted.
Talanta
43 (1996)
479-486
Behaviour of the dithiocarbamate complexes of arsenic, antimony, bismuth, mercury, lead, tin and selenium in methanc with a hydride generator L. Vuchkova, Faculty Received
qf Chemisrry,
Uniuersiry
Z June 1995; revised
S. Arpadjan” oj Sofia.
30 August
I126 Sqfia,
1995; accepted
Bulgaria 9 October
1995
Abstract
A study was carried out with a continuous hydride generator coupled to an atomic emissionspectrometerwith inductively-coupledplasmato determinewhether hydridesof As, Bi. Pb, Sb, Sn and Se and mercury vapor could be generatedin methanol solutionsof their dithiocarbamatecomplexes.It was found that (with the exception of Pb) hydride generationwith sufficient efficiency for simultaneousmulti-elementdetermination is achievedusing 0.25’%1 NaBH4-0.6 mol 1~’ HCl asreaction medium.The detection limit was found to be 0.2 ng ml - ’ for As, 30 ng ml-- ’ for Bi, 0.03 ng ml ’ for Se, Sb and Sn. Kevwords: Inductively-coupled plasma atomic emission _ _ _ mate complexes in methanol; Solid phase extraction
spectrometry;
1. Introduction The generation of volatile covalent hydrides and mercury vapor has become a widely-used technique in atomic spectrometry for analysis of As, Sb, Se, Sn, Ge, Te, Bi, Hg and Pb in various matrices. For the determination of extremely low element concentrations and for the elimination of matrix interferences a preliminary preconcentration is necessary. Liquid-liquid extraction preconcentration methods have been described in combination with subsequent hydride generation * Corresponding 0039-9140/96/Sl5.00 SSDI
author. ‘0 1996 Elsevier
0039-9140(95)01780-1
Science
B.V.
All rights
reserved
Continuous
hydride
generator;
Dithiocarba-
(HG) directly in non-aqueous media without any mineralisation or re-extracting procedures [ 1- 111. However, the solvent extraction method offers limited enrichment factors and is unsuitable for automation of the analysis and for preservation and transportation of pretreated samples. This is the reason for the recent enhanced interest in preconcentration methods using column solid phase extraction. An effective preconcentration technique allowing higher concentration factors than with liquid extraction was developed using a water-insoluble ligand, such as ammonium hexahydroazepine- 1-dithiocarboxylate (ammonium hexamethylenedithiocarbamate) (HMDC), physically immobilized on polyurethane foam, and dis-
480
L. Vuchkoua,
S. Arpadjan
posable syringes as microcolumns [12]. In a preliminary study optimum conditions were established for quantitative solid phase extraction of As, Bi, Hg, Sb, Se, Sn and Pb. A complete elution was achieved by total dissolution of the sorbed analyte dithiocarbamate complexes in organic solvents [12,13]. The application of a water-miscible solvent such as methanol simplifies the analytical procedure due to the possibility of using the conventional hydride generator for aqueous solutions without any changes. However, no data are available on whether elemental hydrides and mercury vapor could be generated from methanol solutions of chelate complexes of the analytes and then followed by inductively-coupled plasma atomic emission spectrometry (ICP-AES) measurements. The aims of the present work are: (i) to investigate the hydride generation of As, Sb, Sn, Se, Bi and Pb when present as dithiocarbamate complexes in methanol solutions; (ii) to investigate the behaviour of Hg under the same conditions; and (iii) to optimize the working conditions for combination of the HG in methanol solutions of the analyte dithiocarbamate complexes with an inductively-coupled plasma optimized for simultaneous determination of the above elements by atomic emission spectrometry. 2. Experimental 2.1. Reagents and apparatus
All reagents used were of analytical-reagent grade. Redistilled water was used throughout. Stock solutions of 0.25% and 0.5% NaBH, (Merck) in 0.4% NaOH were prepared daily and stored in a polyethylene bottle before use. The multielement standard solutions for all the studied elements were prepared from Titrisol (Merck, Germany). HMDC (Merck) was used as received. The solvent methanol (Merck) was additionally purified by distillation. The ICP-AES measurements were performed with a simultaneous ICP spectrometer Spectroflame combined with a continuous hydride generator 341-ARL. The ICP and HG operating conditions are given in Table 1.
/ Talanta
43 (1996)
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The HMDC-immobilized polyurethane foam and the sorbent columns for solid phase extraction were prepared as described earlier [12]. 2.2. Preparation
of test solutions
To 20.00 ml of a standard solution containing 50 ng ml ~ ’ As(III), Sb(III), Bi(III), Se(IV); 60 ng ml ~ ’ Sn( IV); 20 ng ml - ’ Hg(II) and 100 ng ml - ’ Pb 5 ml acetic buffer at pH 4.66 was added. This solution was pumped through a sorbent column filled with HMDC-immobilized polyurethane foam using the conditions reported previously [12]. Then the organic chelating ligand, HMDC, together with the analyte-dithiocarbamate complexes formed were completely dissolved in 9.0 ml methanol or in 9.0 ml aqueous methanol solution with 66% methanol (6.0 ml CH,OH + 3.0 ml H,O). 2.3. Recommended procedure
In 150 ml of a water sample 3 g KI is dissolved and heated gently at 60°C for 1 h to ensure that the analytes are present in their lower oxidation states. The pH is adjusted to 4.5 + 0.5 with acTable ICP
1
operating parametersand
Incident power (kW) Argon flow rates (I min-‘): coolant carrier sheath Observation height (mm) Pre-integration time (s) Integration time (s) Wavelengths (nm): As 1 Bi I Hg 1 Pb II Sb 1 Se 1 Sn II Reductant Reductant flow rate (ml min-‘) Acid flow rate (ml min-‘) Sample flow rate (ml min ‘)
HG
conditions 1.25 17 0.9 0.85 15 30 5 189.04 230.0 184.95 220.35 206.8 196.09 189.98 NaBH, solution 0.4% NaOH 2.4 1.2 6.0
in
L. Vuchkooa,
S. Arpadjan
1 Talanta
43 (1996)
481
479-486
0.25% NaBH4 - 0.6M HCI
As
Ha :
Fig. 1. Analyte line intensity generation: + L in presence
sb
Pb
tza 66(HcH3OH-L %%% CEJOH-L
LF2*tHcL
relative to aqueous reference solutions using 0.25% of HMDC; - L in absence of HMDC. 0.25%
sn
SO
NaBH4
NaBH,-0.6
M HCI as reaction
medium
for hydride
- l.ZM HCI
R * 1
1 n t 0 n I I t Y
m Fig. 2. As Fig.
a m
66HCH30H+L C!H3OH+L
I but with
0.25%
etate buffer or if necessary with some drops of dilute ammonia or HCl. The sample is pumped through the sorbent column at a flow rate of 2 ml min’. Then 5.0 ml CH,OH is placed in a dry quartz beaker and passed six times through the sorbent with the aid of the syringe plunger. This methanol solution of the analyte-dithiocarbamate complexes is connected to the sample channel of the hydride generator. The calibration solutions were prepared from aqueous multielement standard solutions by appropriate dilution with a methanol solution of HMDC (10 g l- I).
NaBH,-1.2
66%CH3OH-L CHjOH-L
M HCI
as reaction
medium.
3. Results and discussion
The influence of methanol and of the chelateforming ligand HMDC on the yield of hydride was investigated for the following reaction systems: A, 0.25% NaBH,-0.6 M HCl; B, 0.25% NaBH,- 1.2 M HCl; C, 0.25% NaBH,-0.6 M HNO,; D, 0.25% NaBH,- 1.2 MHNO,; E, 0.25% NaBH,-2 M HNO,; F, 0.5% NaBH,-2 M HNO,. The intensity of the emission signal of the analytes when present as dithiocarbamate complexes
L. Vuchkooa,
482
S. Arpadjan
: Talantu
43 (1996)
479-486
0.25%NaBH4-0.6MHN03 2.50 R e I I n t e n s 1 t
2.00
1.50
1.00
050
Y 0.W
As
Fig. 3. As Fig.
Bi
Hg
m
66HCHJOH+L
m
CH30H+L
1 but with
0.25%
-Pb
sb m m
NaBH,-0.6
Se
Sn
66%CE3OH-L CHJOH-L
M HNO,
as reaction
medium.
0.25?hNaBH4-1.2MHN03 350, R * I
I s.oof
J Pb
m
Fig. 4. As Fig.
66% CH3OH CHjOHtL
I but with
+ L
0.25’1/0 NaBH,-1.2
in methanol (Z,) was compared with that obtained for aqueous solutions (I,) at the same experimental conditions. The results obtained for the relative intensity, Z, ( = IJZ,), are shown in Figs. l-6. The chelate-forming ligand HMDC affects the process of hydride generation. The HG reaction was realised with higher efficiency in methanol solutions without ligand than in those with ligand. The influence of HMDC depends on the stability of the dithiocarbamate complexes of the analytes in methanol and on the rate of hydride formation or reduction to the elemental state (Hg). In the
Sb m m
Se
Sn
66YCH3OH-L CIDOH-L
M HNO,
as reaction
medium
presence of methanol and HMDC Pb cannot form a volatile covalent hydride at all. The ligand HMDC strongly suppresses the formation of bismuthine in all the investigated reaction systems (Figs. l-6) due to the high stability of the Bidithiocarbamate complex and the relatively slow rate of bismuthine formation. The determination of Bi in the presence of HMDC is possible only in 0.25% NaBH,-0.6 M HCl with twice as poor sensitivity (Z, = 0.5). Arsenic forms a volatile hydride independent of the presence of ligand and in the applied reaction
L. Vuchkma,
S. Arpadjan 8.25%
16.00,R e 1 1 n t 6 n
14.00
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483
NaBH4 - 2IU HNO3
.. I
lZ.al 10.00 8.00~ 6.00.
sb
Pb m 0
Fig. 5. As Fig.
66%CH3OH+L C!ZUOH+L
1 but with
0.25%
m m
NaBH,-2
se
66% CH3OH CHJOH-L
M HNO,
Sn
-L
as reaction
medium.
0.5% NaJSH4 - ZM HNO3 2.00, R e 1 I n t * n I i t
1.50
1.00
0.50
Y 0.00
As
B1
m
Fig. 6. As Fig.
Se
sll 66HCH?JoH+L CHjOHtL
1 but with
0.5% NaBH,-2
system in pure CH,OH as well as in 66% CH,OH (Fig. 1). Selenium forms a volatile hydride in methanol solutions of HMDC with all the studied reduction systems. The efficiency of H,Se formation is higher in HCl medium (I, z 1.9-2.4, Figs. 1 and 2) than in HNO, medium (Z, z 0.85-1.15, Figs. 3, 4 and 6) except for the system 0.25% NaBH,-2 M HNO, where 1, = 4.3 (Fig. 5). In the case of antimony the efficiency of stibine formation is higher (I, z 2) in hydrochloric acidcontaining reaction systems (Figs. 1, 2) than in nitric acid medium (Z, < 1, Figs. 3, 4 and 6).
El tiii%
Sn
66WCH3OH-L CIDOH-L
M HNO,
as reaction
medium.
A strong depression of SnH, formation was observed in the presence of HMDC in the methanol solution for all reaction systems with HNO,. Mercury forms stable dithiocarbamate complexes. Nevertheless, the efficiency of mercury vapor formation is high in the reaction systems containing HCl (I, z 4.5) and HNO, (I, z 1.13). This is probably due to the high reduction rate of mercury to the elemental state. The higher intensity signals for As, Hg, Sb, Se and Sn in methanol in the absence of the chelate-
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Table 2 Detection limits (3 0) for HG with ICP-AES for methanol solutions of dithiocarbamate Reaction medium
0.25% NaBH,-0.6 M HCl 0.25% NaBH,-1.2 M HCI 0.25% NaBH,-0.6 M HNO, 0.25% NaBH,- 1.2 M HNO, 0.25% NaBH,-2 M HNO, 0.5% NaBH,-2 M HNO,
complexes of the analytes
Detection limit (ng ml-‘) As
Bi
Hg
Sb
Se
Sn
0.2 0.5 0.8 0.7 0.4
30 -
0.03 0.1 0.2 0.1 0.2 0.2
3
3.3
1.8 3
9 7 9 8.5
2.1 4 -
-
-
forming ligand HMDC (Figs. 1 and 2) than for their aqueous reference solutions could be explained by a lowering of the surface tension, promoting a higher rate of hydride or cold vapor liberation from the liquid phase. This is presumably the explanation for the higher efficiency in pure methanol than in 66% methanol. An exception is observed for bismuth and lead. The generation of bismuthine in HCl medium does not depend (I, z 1) on the replacement of water by CH,OH. 3.1. Choice of working conditions
A higher sensitivity and a more stable plasma were obtained for methanol solutions of HMDC in comparison to aqueous methanol solutions (66% CH,OH). Table 3 Comparison of the detection limits (3 c) of HG-ICP in aqueous solutions and in methanol solutions of the analyte dithiocarbamate complexes after preconcentration by column solid phase extraction (recommended procedure) Element
HG-ICP in aqueous solutions (ng ml-‘)
Recommended procedure (ng ml-‘)
As Bi Hg Sb Se Sn
0.2 0.35 0.02 0.5 0.6 0.3
0.008 1 0.001 0.1 0.1 0.1
The optimum conditions for single element determination can be seen from the calculated limits of detection presented in Table 2. For simultaneous multi-element analysis, elution by total dissolution of the analyte-dithiocarbamate complexes in pure methanol and HG using the reaction system 0.25% NaBH,-0.6 M HCl is recommended as the optimal compromise decision. In comparison with work with aqueous solutions under these experimental conditions the background signal for Bi does not change, but for As, Se, Sb and Sn it is enhanced 1.5-fold and for Hg it is enhanced 2.3-fold. The relative standard deviations of the measurements in methanol and aqueous media do not differ significantly, varying between 1 and 4%. Table 3 presents the achievable best sensitivity for HG-ICP determination of the investigated elements in aqueous solutions using the optimal experimental conditions for aqueous media (0.5% NaBH,-0.3 M HCl). It can be seen that whereas for As and Hg no substantial change in the sensitivity of their determinations was observed, for selenium the sensitivity in methanol solution of its dithiocarbamate complex is approximately five times worse than by HG from aqueous Se(IV) solutions. This lowering in sensitivity is about a factor of nine for Sn and almost two orders of magnitude for Bi. However, the combination of HG with preliminary 30 fold preconcentration of the traces by column solid phase extraction, as described in the recommended procedure, permits the determination of extremely low analyte concentrations with the exception of Bi (Table 3).
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Table 4 Recovery (%) from spiked water samples (n-number of samples) Sample
Recovery (%)
Tap water (n = 4) Mineral water (n = 3) Ground water (n = 3) Waste water (n = 4) Sea water (n = 7)
3.2. Application
AS
Bi
97 + 5 97 + 3 96 + 6 95*4 96&6
95 + 96 + 97 f 95 + 96 +
5 6 5 7 6
of the method
The recommended procedure was applied to a number of water samples: sea water, waste water, tap water, mineral water and ground water. The recovery studies were performed by adding known amounts of the analytes to the samples prior to adjustment of the pH value to 4.5 + 0.5. The results are shown in Table 4. The high recoveries obtained indicate that the foreign ions present in the samples do not interfere to any significant extent with the determination of As, Bi, Sb, Se and Sn after HG in methanol solutions of the analyte dithiocarbamate complexes. The highest concentrations of coexisting ions in the water samples analysed were 0.4 pg I - ’ Cd, 2 pg I- ’ Cr, 100 pg 1-l Cu, 300 pg 1-r Fe, 200 pg 1-r Mn, 1.2 fig 1-l Ni, 5 pg 1-l Pb and 150 fig 1-l Zn. The alkali and alkaline earth elements, aluminium and the anions are not sorbed and hence could not be present in the methanol solution.
Hg
Se
Sb
Sn
99 i: 2 97 f 3 9854 96 i- 5 97 + 5
97 +3 96k 3 9524 93 * 5 92 f 6
95 +4 97 f 3 96 + 5 97 +4 94 * 4
96 k 5 97*4 98 + 2 95&4 93 * 5
The results for water samples with higher analyte content were compared with those of HG in aqueous solutions as shown for two cases in Table 5. The agreement is good and the difference between the standard deviations of both methods is not statistically significant. HG-ICP in aqueous solutions is undoubtedly a faster and simpler technique, but the proposed column solid phase extraction preconcentration in combination with subsequent HG in methanol solutions for simultaneous determination by ICP-AES allows higher sensitivity for As, Hg, Sb, Se and Sn, and permits better possibilities for the preservation and transportation of the water samples.
4. Conclusion
The conversion of the analytes from their dithiocarbamate complexes in methanol to covalent hydrides or cold vapor (Hg) through direct
Table 5 Results (pg I-‘) of the analysis of waste-water and sea-water samples (n = number of parallel determinations) Element
As Bi Hg Se Sb Sn
Waste water (n = 4)
Sea water (n = 5)
Recommended procedure
HG-ICP in aqueous solutions
Recommended procedure
HG-ICP in aqueous solutions
2.8 f 0.3 9.7 k 0.8 10.001 6.4 5 0.5 17.2 + 1.2 46 f. 3
2.8 + 9.3 + <0.02 6.2 + 16.6 f 47.2 +
1.27 + 0.08
1.32 + 0.07 10.35 2.3 + 0.2 <0.6 <0.5 <0.3
0.2 0.7 0.3 0.8 2.4
486
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reduction in the organic solvent permits the development of an effective solid phase extraction preconcentration procedure prior to HG for simultaneous determination of As, Bi, Sb, Se and Sn.
Acknowledgement The authors thank the Bulgarian Sciences, Project X-519.
Foundation
of
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