- Email: [email protected]
Determination of trace impurities in tantalum by inductively coupled plasma mass spectrometry after removal of the matrix by liquid-liquid extraction
ANALYTICA CHIMICA ACTA
ELSEVIER
Analytica
Chimica Acta 329 (1996) 153-159
Determination of trace impurities in tantalum by inductively coupled plasma mass spectrometry after removal of the matrix by liquid-liquid extraction Vijay K. Panday, Johanna Sabine Becker*, Zentralabteilungfir
Hans-Joachim
Dietze
Chemische Analysen, Forschungszentrum Jdich GmbH, D-52425 Jiilich, Germany
Received 3 October
1995; revised 29 February
1996; accepted 3 March 1996
Abstract Trace impurities in tantalum have been isolated by extracting the tantalum matrix from aqueous solutions containing hydrofluoric and hydrochloric acid into methyl isobutyl ketone. A study on the extractability of the fluoride complex by methyl isobutyl ketone and diisopropyl ketone indicated that the former was much more effective. The extractability of tantalum was higher in a HCl medium as compared to HNOs. Tantalum was easily extracted into the organic phase while most impurities usually accompanying tantalum, e.g. V, Nb, Zr, Hf and W, remained almost completely in the aqueous phase. Trace impurities were analyzed in the aqueous phase by inductively coupled plasma mass spectrometry (ICP-MS) after evaporating the excess acids. The detection limits for most impurities in tantalum were in the sub-l&g range. The procedure could be applied to the determination of spallation products in a tantalum target irradiated with 800MeV proton especially in combination with the sensitive ICP-MS.
Keywords; Mass spectrometry; Matrix separation; Tantalum; Trace analysis
1. Introduction Tantalum and its alloys have found wide applications because of their outstanding properties, e.g. high melting point, great chemical resistance, high hardness, electrical conductivity and good mechanical treatability. Moreover, at the ISIS facility of the Rutherford Appleton Laboratory (UK), tantalum was used as the target material of a spallation neutron source. It was non-continuously irradiated between
* Corresponding
author. Fax: +49 2461 612560
0003-2670/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved PII SOOO3-2670(96)00138-9
1988 and 1994 with 800MeV protons up to a charge of 1.75 Ah [ 11. In order to determine the concentrations of radioactive and stable nuclides in the irradiated tantalum target of a spallation neutron source different analytical methods - such as radiochemical methods after separation of matrix and enrichment of spallation products, instrumental gamma-ray spectrometry, liquid scintillation counting and inductively coupled plasma mass spectrometry (ICP-MS) will be applied. The concentrations of spallation nuclides (estimated by theoretical calculations) varied between 1 rig/g and 50 ug/g. In order to verify the theoretical model for the calculation of
154
VK. Panday et al./Analytica
spallation yields by irradiation of a tantalum target with high energy protons experimental measurements of concentrations of radioactive and stable nuclides formed in this process are planned. Furthermore, many impurities present in original tantalum can, however, give rise to radioactive spallation products when tantalum is irradiated by 800MeV protons. A need thus arose to develop a method for the determination of trace impurities in a tantalum target which had been irradiated with 800MeV protons for 500 days. In order to reduce the high activity of irradiated tantalum, it was, therefore, necessary to separate the matrix tantalum and isolate the trace impurities. In this paper an analytical method for the separation of the tantalum matrix and the determination of trace impurities by ICP-MS will be described. The fluoride complexes of tantalum are known to be extractable into polar organic solvents from aqueous solutions containing hydrofluoric and mineral acids [2]. Preliminary investigations indicated that a large number of impurities present along with tantalum were not appreciably extracted and remained in the aqueous phase making it possible to concentrate and determine them quantitatively using ICP-MS, which often has very low detection limits for aqueous solutions. It was also possible to strip the tantalum from the organic solvent by washing with water and reusing the organic solvent. Among the analytical methods, mass spectrometric techniques distinguish themselves as universal, powerful and very sensitive for the characterization of high-purity metals and alloys. They permit the simultaneous determination of all chemical elements with a wide coverage, a high efficiency of evaporation and ionization of solid or dissolved samples, and with a high absolute and relative sensitivity. Trace impurities in high-melting metals such as tantalum, zirconium, hafnium, niobium or molybdenum can be determined directly by solid-state mass spectrometric methods such as spark source mass spectrometry (SSMS) [3], glow discharge mass spectrometry (GDMS) [4] or laser ionization mass spectrometry (LIMS) [5]. The accuracy and precision of solid-state mass spectrometric methods is limited by the lack of suitable reference materials and by inhomogeneities of trace elements in the solid sample. To avoid the inhomogeneity problem metals or alloys can be analyzed sensitively using ICP-MS after dissolution
Chimica Acta 329 (1996) 153-159
and optimal dilution [6,7]. In general, in trace element analysis molecular and cluster ions (e.g. oxide ions, hydride ions, hydroxide ions and argon molecular ions), multiply-charged atomic ions, formed by inductively coupled plasma ionization, can disturb the determination of analyte ions. The trace element analysis of tantalum by ICP-MS is easy because tantalum has two stable isotopes (181Ta and ‘*‘Ta), with an isotopic abundance of 99.988% for 181Ta. Therefore, scarcely any interferences occur (except e.g. 181TaO+ and 19’Au+) resulting in poorer limits of detection (LOD). The isolation of trace impurities from the metallic tantalum matrix through a simple separation step can considerably improve the LOD by suppressing matrix effects (by separation of high salt concentration in solution) and make it possible to quantify many impurities at the sub-ug/g range [8-lo]. Stummeyer and Wiinsch [II] describe the determination of trace elements in high-purity tantalum by inductively coupled plasma atomic emission spectrometry (ICP-AES) and ICP-MS after trace-matrix separation with cation exchange. The isolation of the impurities from the matrix would be particularly necessary in the case of irradiated tantalum. An attempt has, therefore, been made in the present investigation to separate the trace impurities from a tantalum sample through selective extraction of the matrix. Preliminary work indicated that the extraction of tantalum was most effective when methyl isobutyl ketone (MIBK) was used as the extractant. The present report describes the results obtained on the quantification of residual impurities in the aqueous phase after separation of the matrix. The data obtained on the analysis of some tantalum samples using this approach is also reported and compared with that available from other measurements.
2. Experimental 2.1. Instrumentation A Sciex Elan 5000 inductively coupled plasma mass spectrometer (ICP-MS; Perkin-Elmer, ijberlingen, Germany) with a quadrupole mass analyzer was employed in the present work. The applied ICP-MS operating parameters are summarized in Table 1. No
VK. Panday et al./Analytica Chimica Acta 329 (1996) 153-159 Table 1 Operating parameters used for studying tions of various elements by ICP-MS
the elemental
concentra-
ICP operating conditions R.F. power Coolant gas flow rate Auxiliary gas flow rate Nebulizer gas flow rate Nebulizer
1060 W 13.8 l/min 0.78 l/min 0.68 Vmin cross flow nebulizer
ICP-MS inteface Sampling cone Skimmer cone Pressure (quadrupole ICP on
nickel with a 1.Omm orifice nickel with a 0.75 mm orifice analyzer) 7.3 x 10-s Pa
Scanning (peak hopping) Mass resolution (m/Am) Mass range of scan No. of replicats Points per peak
300 6-240 u 10 3
modifications were made to the instrumentation, but the instrument was adjusted to give optimal signals for certain elements by controlling the aerosol flow rate as well as the forward power as given in Table 1. The gas flows were controlled via a built-in mass flow controller. The measured peaks for each element were almost always the most abundant masses for each element except in the case of Hf where Hf-178 was usually employed. An AS-90 autosampler was employed along with a peristaltic pump for sample introduction. A solution of 5% (v/v) HNOs solution was used for washing the sample path before sample and standard introduction into the plasma. The standard deviation of measurements was based on 10 measurements for each sample.
155
Organic solvents used were of analytical grade (E. Merck) and were equilibrated with 6M nitric acid and with deionized water before use. All glassware and PTFE apparatus employed during the work was cleaned in a cleaning apparatus (Ktimer, Rosenheim, Germany) with nitric acid vapour and then rinsed with ultrapure water. It was dried and stored in a dust-free apparatus. Most open operations were performed under a laminar flow hood. 2.3. Dissolution The tantalum samples were washed with acetone and left in 1 M HNOs overnight. They were then stirred, heated to boiling for 5 min, rinsed with deionized water and finally dried. 0.5 g of the sample was accurately weighed and transferred into a PTFE beaker. 3 ml of HF was added. The solution was slowly heated on a hot plate and the sample was dissolved slowly by the addition of a few drops of nitric acid. lOm1 of concentrated HCl (30%) was added and the contents made up to 25 ml with deionized water. 2.4. Recovery
of trace impurities
The recovery of various impurities in the aqueous phase after extraction of tantalum into the organic phase was investigated for various acid compositions by spiking the respective aqueous acidic solutions with known concentrations of impurity elements and tantalum. The tantalum was extracted and the impurities in the aqueous phase quantified using quantitative measurement of the respective element by ICP-MS. The recoveries were calculated as the fraction recovered of the spike added before equilibration in the solutions concerned.
2.2. Chemicals 2.5. Separation Ultrapure mineral acids (E. Merck, Darmstadt, Germany) and reagents were used to dissolve the sample of tantalum. Spex spectra grade solutions or those available from the National Institute of Standards and Technology (NIST) were employed for the preparation of calibration standards and for spikes. Deionized water was obtained from a Millipore Mini-Q Plus water purification system.
of trace impurities from tantalum
An aliquot of the dissolved sample of tantalum (5 ml) was transferred to a polypropylene separatory funnel. 5 ml of the organic extractant (MIBK) was added and the contents stirred vigorously for three minutes. The aqueous phase was transferred to another separatory funnel, treated with another 5 ml of the organic extractant and the process was
VK. Panday et al./Analytica Chimica Acta 329 (1996) 153-159
156
add I
transfer an aliquot into ; repeated
add
1
aqueous phase
I
a separation funnel
collect
:
&-J-F
add I
lii-~l_~~~l Fig.
1. Schematic
diagram
of a procedure
for separating
tantalum
from impurities.
VK. Panday et al./Analytica
repeated. The aqueous phase was collected in a PTFE beaker and Rh was added as an internal standard element for the ICP-MS measurement. The contents were evaporated slowly to near dryness and diluted to the desired volume (usually 10 ml). The procedure is shown schematically in Fig. 1. The organic extracts were combined and washed continually with lOm1 aliquots of water to strip all the tantalum. 2.6. Data acquisition for analysis The data were acquired in the normal resolution mode for all measurements reported here. The initial measurements were made in the TotalQuant mode using a mixture of 36 analyte elements as the external calibrating solution for preliminary surveys. Final data were acquired in the quantitative mode (external calibration) by constructing calibration curves in the corresponding acidic solution. A corresponding blank was employed in all cases. The built-in software was used to calculate the results from each measurement.
3. Results and discussion 3.1. EfJiciency of tantalum removal The efficiency of tantalum extraction was examined in the HNOs medium as well as in the HCl medium at various acid molarities. Since the extractability of tantalum using diisopropyl ketone has been examined previously [2], this solvent was used in the present work. Table 2 summarizes the fraction of tantalum remaining in the aqueous phase after each extraction with diisopropyl ketone using
Table 2 Efficiency of tantalum extraction using diisoprophyl ketone from fluoride solutions (Ta=20 ug/ml; volume=10 ml; HF=0.5 M)
Molarity 1M 2M 4M 6M
of acid
equal volumes of of HNO, and HCl
equal volumes of the organic and aqueous phase. Furthermore, a TotalQuant study on the recovery of other elements also showed appreciable losses for the elements, e.g. Cr, Mn, Co, Ni, Cu, Zn, Ag, Cd, Pb, Bi, Sr, Te in addition to As, Ga, In and Tl, in the two acid media employed here. It was, therefore, decided to examine the extraction efficiency of MIBK, which has also been reported to extract tantalum from fluoride solutions in HNOs [12]. Table 3 gives the data on the extracting efficiency for tantalum by MIBK in HNOs and HCl media. It can be concluded from the data that using acid molarities above 4M HCl, two extractions would bring down the level of tantalum to less than 0.01% of that present in the initial solution. The amount of tantalum remaining in the aqueous phase was in practice somewhat higher. 3.2. Recoveries
of trace impurities
The recovery of various elements was examined quantitatively in HCl and HNOs solutions using MIBK as the extracting agent. The elements spiked to the aqueous acid solutions containing HCl and HNOs included Ag, Al, B, Ba, Bi, Ca, Cd, Ce, Co, Cr, Cu, Dy, Er, Eu, Fe, Ga, Gd, Hf, Ho, In, K, La, Li, Mg, Mn, MO, Na, Nb, Nd, Ni, Pb, Pr, Sm, Sr, Tb, Th, Ti, Tl, Tm, V, U, W, Yb, Zn and Zr. The measurement of the residual concentration of each element in the aqueous phase using both TotalQuant as well as quantitative measurements showed that most elements were quantitatively recovered after extraction with each of the organic solvents. Significant losses were observed only for the elements Cd, Fe, Ga, Th, Tl and Zn, depending upon the mineral acid used. In
Table 3 Efficiency of tantalum extraction using equal volume of MIBK from fluoride solutions of HNOs and HCl (Ta =20 ug/ml; volume 10 ml; HF =0.5 M) % of tantalum remaining
% of tantalum remaining
in aqueous solution
HNOs
HCl
40.012.2 30.5f1.9 24.0f1.8 17.0f1.7
12. 0f1.4 8.0f0.9 3.5f0.8 2.8f0.4
157
Chimica Acta 329 (1996) 153-159
in aqueous solution
Molarity of acid
HNOs
HCl
1M 2M 4M 5M 6M
30.5f2.2 18.0f2.0 13.8f1.9 11.8f1.8 11.011.6
2.25f0.34 1.15f0.28 1,19&0.24 0.99&O. 18 0.85f0.14
158
VK. Panday et al./Analytica
Chimica Acta 329 (1996) 153-159
Fe
Molarity of Acid
1
2
3
4
5
1
2
3
4
5
2
3
4
5
Zn
p_L;
1
2
3
4
5
Cd
_il---_:
01 1
2
3
4
1
5
Ba
Nb 150
150 1
I ‘““E
5:: 1
I 2
I
I
1
4
Ga
2
1
Fig. 2. Recoveries (W HN&
3
3
100
0
50 0 1
2
4
5
I
4
5
I
obtained for different elements in the aqueous phase at different acid molarities
; 0 HW.
3
after extraction
with diisopropyl
ketone
VK. Panday et al./Analytica
the case of HCl, the losses were serious only for Fe, Ga and Tl, which were significantly extracted into the organic phase at all acid concentrations. Fig. 2 summarizes the recovery of these elements at each acid concentration and indicates that it is possible to apply the procedure proposed here for the determination of all the elements listed above except Fe, Ga, and Tl. The determination of these elements, if required, could be carried out in the HNOs medium. 3.3. Determination
of trace impurities
in tantalum
The above separation procedure was applied to two different types of tantalum samples and the impurity content of these materials determined using the quantitative external calibration approach. The same procedure was applied for the determination of trace impurities in the blank. The results for the two samples are shown in Table 4. Analysis results of
Chimica Acta 329 (1996) 153-159
ICP-MS after matrix separation are comparable to measurements by SSMS. An agreement of measured concentrations was found for V, Co, Cu, Zn, Ag, MO, W and Th in both mass spectrometric methods. The purity of the tantalum samples I and II was determined to be 99.96% and 99.9992%, respectively. Limits of the detection (LOD) in ICP-MS for pure original tantalum metal sample after matrix separation by extraction are in the 20-lOOng/g range. An improvement of LOD - this is necessary for the determination of spallation products in irradiated tantalum - can be achieved up to the low rig/g and sub-rig/g range, using the double-focusing ICP-MS “Element” (Finnigan MAT) in the low resolution mode. Investigations of trace and ultratrace analysis of tantalum by ICP-MS “Element” are in progress.
References 111 H. Ullmaier and F. Carsughi,
Table 4 Concentrations of impurity elements in two samples of tantalum obtained after removal of matrix by liquid-liquid extraction and ICP-MS (concentration pg/g)
Element Li Al Ti V Cr Mn co Ni cu Zn Sr Zr Nb MO Ag Cd Ba La Ce Hf W Pb Bi Th U
Sample I ICP-MS
Sample II ICP-MS
0.26f0.08 1.37f0.47 1.41 f0.39 0.28f0.11 0.55*0.03 0.2110.11 0.17*0.02 0.66f0.09 0.72f0.18 0.92f0.31 0.11*0.02 38.7f5.5 120.7flO.l 16.3f1.06 0.38+0.11 0.12f0.02 <0.02
0.10f0.02 2.15f0.18 0.62f0.16 0.22f0.08 3.07f0.17 0.12&0.03 0.72f0.08 8.3510.28 0.35f0.05 0.32+0.05 0.07f0.02 29.3&l .O 18.3f1.8 4.2f0.8 0.52f0.05 0.10f0.02 0.11 f0.02 <0.02
159
t21 [31
141 [51 L61 171
PI [91 [lOI L111 Cl21
Nucl. Instrum. Meth. Phys. Res., B 101 (1995) 406. P.C. Stevenson and H.G. Hicks, Anal. Chem., 25 (1953) 1517-1519. H.-J. Dietze, Massenspektrometrische Spurenanalyse (1975). Akademische Verlagsgesellschaft Geest and Portig K.-G., Leipzig. X. Feng and G. Horlick, J. Anal. At. Spectrom., 9 (1994) 823. H.-J. Dietze and J.S. Becker, Fresenius’ 2. Anal. Chem., 302 (1985) 490. SK. Luo and EC. Chang, Spectrochim. Acta, 45B (1990) 527. J.S. Becker, V.K. Panday and H-J Dietze, Colloquium Spectroscopicum Intemationale, June 29-July 4 (1993) York, UK. I. Tsuyoshi, E Noriko and K. Masaaki, Analyst, I15 (1990) 1185. V.K. Panday, J.S. Becker and H.-J. Dietze, Fresenius’ J. Anal. Chem., 352 (1995) 3027. V.K. Panday, J.S. Becker and H.-J. Dietze, Atomic Spectroscopy, May/June (1995) 97. J. Stummeyer and G. Wilnsch, Fresenius’ J. Anal. Chem., 342 (1992) 203. S. Kallman, in Lolthoff and Elving (Eds.), Treatise on Analytical Chemistry, NY, 1964, Part II, Vol. 6, Ch. 3.
ELSEVIER
Analytica
Chimica Acta 329 (1996) 153-159
Determination of trace impurities in tantalum by inductively coupled plasma mass spectrometry after removal of the matrix by liquid-liquid extraction Vijay K. Panday, Johanna Sabine Becker*, Zentralabteilungfir
Hans-Joachim
Dietze
Chemische Analysen, Forschungszentrum Jdich GmbH, D-52425 Jiilich, Germany
Received 3 October
1995; revised 29 February
1996; accepted 3 March 1996
Abstract Trace impurities in tantalum have been isolated by extracting the tantalum matrix from aqueous solutions containing hydrofluoric and hydrochloric acid into methyl isobutyl ketone. A study on the extractability of the fluoride complex by methyl isobutyl ketone and diisopropyl ketone indicated that the former was much more effective. The extractability of tantalum was higher in a HCl medium as compared to HNOs. Tantalum was easily extracted into the organic phase while most impurities usually accompanying tantalum, e.g. V, Nb, Zr, Hf and W, remained almost completely in the aqueous phase. Trace impurities were analyzed in the aqueous phase by inductively coupled plasma mass spectrometry (ICP-MS) after evaporating the excess acids. The detection limits for most impurities in tantalum were in the sub-l&g range. The procedure could be applied to the determination of spallation products in a tantalum target irradiated with 800MeV proton especially in combination with the sensitive ICP-MS.
Keywords; Mass spectrometry; Matrix separation; Tantalum; Trace analysis
1. Introduction Tantalum and its alloys have found wide applications because of their outstanding properties, e.g. high melting point, great chemical resistance, high hardness, electrical conductivity and good mechanical treatability. Moreover, at the ISIS facility of the Rutherford Appleton Laboratory (UK), tantalum was used as the target material of a spallation neutron source. It was non-continuously irradiated between
* Corresponding
author. Fax: +49 2461 612560
0003-2670/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved PII SOOO3-2670(96)00138-9
1988 and 1994 with 800MeV protons up to a charge of 1.75 Ah [ 11. In order to determine the concentrations of radioactive and stable nuclides in the irradiated tantalum target of a spallation neutron source different analytical methods - such as radiochemical methods after separation of matrix and enrichment of spallation products, instrumental gamma-ray spectrometry, liquid scintillation counting and inductively coupled plasma mass spectrometry (ICP-MS) will be applied. The concentrations of spallation nuclides (estimated by theoretical calculations) varied between 1 rig/g and 50 ug/g. In order to verify the theoretical model for the calculation of
154
VK. Panday et al./Analytica
spallation yields by irradiation of a tantalum target with high energy protons experimental measurements of concentrations of radioactive and stable nuclides formed in this process are planned. Furthermore, many impurities present in original tantalum can, however, give rise to radioactive spallation products when tantalum is irradiated by 800MeV protons. A need thus arose to develop a method for the determination of trace impurities in a tantalum target which had been irradiated with 800MeV protons for 500 days. In order to reduce the high activity of irradiated tantalum, it was, therefore, necessary to separate the matrix tantalum and isolate the trace impurities. In this paper an analytical method for the separation of the tantalum matrix and the determination of trace impurities by ICP-MS will be described. The fluoride complexes of tantalum are known to be extractable into polar organic solvents from aqueous solutions containing hydrofluoric and mineral acids [2]. Preliminary investigations indicated that a large number of impurities present along with tantalum were not appreciably extracted and remained in the aqueous phase making it possible to concentrate and determine them quantitatively using ICP-MS, which often has very low detection limits for aqueous solutions. It was also possible to strip the tantalum from the organic solvent by washing with water and reusing the organic solvent. Among the analytical methods, mass spectrometric techniques distinguish themselves as universal, powerful and very sensitive for the characterization of high-purity metals and alloys. They permit the simultaneous determination of all chemical elements with a wide coverage, a high efficiency of evaporation and ionization of solid or dissolved samples, and with a high absolute and relative sensitivity. Trace impurities in high-melting metals such as tantalum, zirconium, hafnium, niobium or molybdenum can be determined directly by solid-state mass spectrometric methods such as spark source mass spectrometry (SSMS) [3], glow discharge mass spectrometry (GDMS) [4] or laser ionization mass spectrometry (LIMS) [5]. The accuracy and precision of solid-state mass spectrometric methods is limited by the lack of suitable reference materials and by inhomogeneities of trace elements in the solid sample. To avoid the inhomogeneity problem metals or alloys can be analyzed sensitively using ICP-MS after dissolution
Chimica Acta 329 (1996) 153-159
and optimal dilution [6,7]. In general, in trace element analysis molecular and cluster ions (e.g. oxide ions, hydride ions, hydroxide ions and argon molecular ions), multiply-charged atomic ions, formed by inductively coupled plasma ionization, can disturb the determination of analyte ions. The trace element analysis of tantalum by ICP-MS is easy because tantalum has two stable isotopes (181Ta and ‘*‘Ta), with an isotopic abundance of 99.988% for 181Ta. Therefore, scarcely any interferences occur (except e.g. 181TaO+ and 19’Au+) resulting in poorer limits of detection (LOD). The isolation of trace impurities from the metallic tantalum matrix through a simple separation step can considerably improve the LOD by suppressing matrix effects (by separation of high salt concentration in solution) and make it possible to quantify many impurities at the sub-ug/g range [8-lo]. Stummeyer and Wiinsch [II] describe the determination of trace elements in high-purity tantalum by inductively coupled plasma atomic emission spectrometry (ICP-AES) and ICP-MS after trace-matrix separation with cation exchange. The isolation of the impurities from the matrix would be particularly necessary in the case of irradiated tantalum. An attempt has, therefore, been made in the present investigation to separate the trace impurities from a tantalum sample through selective extraction of the matrix. Preliminary work indicated that the extraction of tantalum was most effective when methyl isobutyl ketone (MIBK) was used as the extractant. The present report describes the results obtained on the quantification of residual impurities in the aqueous phase after separation of the matrix. The data obtained on the analysis of some tantalum samples using this approach is also reported and compared with that available from other measurements.
2. Experimental 2.1. Instrumentation A Sciex Elan 5000 inductively coupled plasma mass spectrometer (ICP-MS; Perkin-Elmer, ijberlingen, Germany) with a quadrupole mass analyzer was employed in the present work. The applied ICP-MS operating parameters are summarized in Table 1. No
VK. Panday et al./Analytica Chimica Acta 329 (1996) 153-159 Table 1 Operating parameters used for studying tions of various elements by ICP-MS
the elemental
concentra-
ICP operating conditions R.F. power Coolant gas flow rate Auxiliary gas flow rate Nebulizer gas flow rate Nebulizer
1060 W 13.8 l/min 0.78 l/min 0.68 Vmin cross flow nebulizer
ICP-MS inteface Sampling cone Skimmer cone Pressure (quadrupole ICP on
nickel with a 1.Omm orifice nickel with a 0.75 mm orifice analyzer) 7.3 x 10-s Pa
Scanning (peak hopping) Mass resolution (m/Am) Mass range of scan No. of replicats Points per peak
300 6-240 u 10 3
modifications were made to the instrumentation, but the instrument was adjusted to give optimal signals for certain elements by controlling the aerosol flow rate as well as the forward power as given in Table 1. The gas flows were controlled via a built-in mass flow controller. The measured peaks for each element were almost always the most abundant masses for each element except in the case of Hf where Hf-178 was usually employed. An AS-90 autosampler was employed along with a peristaltic pump for sample introduction. A solution of 5% (v/v) HNOs solution was used for washing the sample path before sample and standard introduction into the plasma. The standard deviation of measurements was based on 10 measurements for each sample.
155
Organic solvents used were of analytical grade (E. Merck) and were equilibrated with 6M nitric acid and with deionized water before use. All glassware and PTFE apparatus employed during the work was cleaned in a cleaning apparatus (Ktimer, Rosenheim, Germany) with nitric acid vapour and then rinsed with ultrapure water. It was dried and stored in a dust-free apparatus. Most open operations were performed under a laminar flow hood. 2.3. Dissolution The tantalum samples were washed with acetone and left in 1 M HNOs overnight. They were then stirred, heated to boiling for 5 min, rinsed with deionized water and finally dried. 0.5 g of the sample was accurately weighed and transferred into a PTFE beaker. 3 ml of HF was added. The solution was slowly heated on a hot plate and the sample was dissolved slowly by the addition of a few drops of nitric acid. lOm1 of concentrated HCl (30%) was added and the contents made up to 25 ml with deionized water. 2.4. Recovery
of trace impurities
The recovery of various impurities in the aqueous phase after extraction of tantalum into the organic phase was investigated for various acid compositions by spiking the respective aqueous acidic solutions with known concentrations of impurity elements and tantalum. The tantalum was extracted and the impurities in the aqueous phase quantified using quantitative measurement of the respective element by ICP-MS. The recoveries were calculated as the fraction recovered of the spike added before equilibration in the solutions concerned.
2.2. Chemicals 2.5. Separation Ultrapure mineral acids (E. Merck, Darmstadt, Germany) and reagents were used to dissolve the sample of tantalum. Spex spectra grade solutions or those available from the National Institute of Standards and Technology (NIST) were employed for the preparation of calibration standards and for spikes. Deionized water was obtained from a Millipore Mini-Q Plus water purification system.
of trace impurities from tantalum
An aliquot of the dissolved sample of tantalum (5 ml) was transferred to a polypropylene separatory funnel. 5 ml of the organic extractant (MIBK) was added and the contents stirred vigorously for three minutes. The aqueous phase was transferred to another separatory funnel, treated with another 5 ml of the organic extractant and the process was
VK. Panday et al./Analytica Chimica Acta 329 (1996) 153-159
156
add I
transfer an aliquot into ; repeated
add
1
aqueous phase
I
a separation funnel
collect
:
&-J-F
add I
lii-~l_~~~l Fig.
1. Schematic
diagram
of a procedure
for separating
tantalum
from impurities.
VK. Panday et al./Analytica
repeated. The aqueous phase was collected in a PTFE beaker and Rh was added as an internal standard element for the ICP-MS measurement. The contents were evaporated slowly to near dryness and diluted to the desired volume (usually 10 ml). The procedure is shown schematically in Fig. 1. The organic extracts were combined and washed continually with lOm1 aliquots of water to strip all the tantalum. 2.6. Data acquisition for analysis The data were acquired in the normal resolution mode for all measurements reported here. The initial measurements were made in the TotalQuant mode using a mixture of 36 analyte elements as the external calibrating solution for preliminary surveys. Final data were acquired in the quantitative mode (external calibration) by constructing calibration curves in the corresponding acidic solution. A corresponding blank was employed in all cases. The built-in software was used to calculate the results from each measurement.
3. Results and discussion 3.1. EfJiciency of tantalum removal The efficiency of tantalum extraction was examined in the HNOs medium as well as in the HCl medium at various acid molarities. Since the extractability of tantalum using diisopropyl ketone has been examined previously [2], this solvent was used in the present work. Table 2 summarizes the fraction of tantalum remaining in the aqueous phase after each extraction with diisopropyl ketone using
Table 2 Efficiency of tantalum extraction using diisoprophyl ketone from fluoride solutions (Ta=20 ug/ml; volume=10 ml; HF=0.5 M)
Molarity 1M 2M 4M 6M
of acid
equal volumes of of HNO, and HCl
equal volumes of the organic and aqueous phase. Furthermore, a TotalQuant study on the recovery of other elements also showed appreciable losses for the elements, e.g. Cr, Mn, Co, Ni, Cu, Zn, Ag, Cd, Pb, Bi, Sr, Te in addition to As, Ga, In and Tl, in the two acid media employed here. It was, therefore, decided to examine the extraction efficiency of MIBK, which has also been reported to extract tantalum from fluoride solutions in HNOs [12]. Table 3 gives the data on the extracting efficiency for tantalum by MIBK in HNOs and HCl media. It can be concluded from the data that using acid molarities above 4M HCl, two extractions would bring down the level of tantalum to less than 0.01% of that present in the initial solution. The amount of tantalum remaining in the aqueous phase was in practice somewhat higher. 3.2. Recoveries
of trace impurities
The recovery of various elements was examined quantitatively in HCl and HNOs solutions using MIBK as the extracting agent. The elements spiked to the aqueous acid solutions containing HCl and HNOs included Ag, Al, B, Ba, Bi, Ca, Cd, Ce, Co, Cr, Cu, Dy, Er, Eu, Fe, Ga, Gd, Hf, Ho, In, K, La, Li, Mg, Mn, MO, Na, Nb, Nd, Ni, Pb, Pr, Sm, Sr, Tb, Th, Ti, Tl, Tm, V, U, W, Yb, Zn and Zr. The measurement of the residual concentration of each element in the aqueous phase using both TotalQuant as well as quantitative measurements showed that most elements were quantitatively recovered after extraction with each of the organic solvents. Significant losses were observed only for the elements Cd, Fe, Ga, Th, Tl and Zn, depending upon the mineral acid used. In
Table 3 Efficiency of tantalum extraction using equal volume of MIBK from fluoride solutions of HNOs and HCl (Ta =20 ug/ml; volume 10 ml; HF =0.5 M) % of tantalum remaining
% of tantalum remaining
in aqueous solution
HNOs
HCl
40.012.2 30.5f1.9 24.0f1.8 17.0f1.7
12. 0f1.4 8.0f0.9 3.5f0.8 2.8f0.4
157
Chimica Acta 329 (1996) 153-159
in aqueous solution
Molarity of acid
HNOs
HCl
1M 2M 4M 5M 6M
30.5f2.2 18.0f2.0 13.8f1.9 11.8f1.8 11.011.6
2.25f0.34 1.15f0.28 1,19&0.24 0.99&O. 18 0.85f0.14
158
VK. Panday et al./Analytica
Chimica Acta 329 (1996) 153-159
Fe
Molarity of Acid
1
2
3
4
5
1
2
3
4
5
2
3
4
5
Zn
p_L;
1
2
3
4
5
Cd
_il---_:
01 1
2
3
4
1
5
Ba
Nb 150
150 1
I ‘““E
5:: 1
I 2
I
I
1
4
Ga
2
1
Fig. 2. Recoveries (W HN&
3
3
100
0
50 0 1
2
4
5
I
4
5
I
obtained for different elements in the aqueous phase at different acid molarities
; 0 HW.
3
after extraction
with diisopropyl
ketone
VK. Panday et al./Analytica
the case of HCl, the losses were serious only for Fe, Ga and Tl, which were significantly extracted into the organic phase at all acid concentrations. Fig. 2 summarizes the recovery of these elements at each acid concentration and indicates that it is possible to apply the procedure proposed here for the determination of all the elements listed above except Fe, Ga, and Tl. The determination of these elements, if required, could be carried out in the HNOs medium. 3.3. Determination
of trace impurities
in tantalum
The above separation procedure was applied to two different types of tantalum samples and the impurity content of these materials determined using the quantitative external calibration approach. The same procedure was applied for the determination of trace impurities in the blank. The results for the two samples are shown in Table 4. Analysis results of
Chimica Acta 329 (1996) 153-159
ICP-MS after matrix separation are comparable to measurements by SSMS. An agreement of measured concentrations was found for V, Co, Cu, Zn, Ag, MO, W and Th in both mass spectrometric methods. The purity of the tantalum samples I and II was determined to be 99.96% and 99.9992%, respectively. Limits of the detection (LOD) in ICP-MS for pure original tantalum metal sample after matrix separation by extraction are in the 20-lOOng/g range. An improvement of LOD - this is necessary for the determination of spallation products in irradiated tantalum - can be achieved up to the low rig/g and sub-rig/g range, using the double-focusing ICP-MS “Element” (Finnigan MAT) in the low resolution mode. Investigations of trace and ultratrace analysis of tantalum by ICP-MS “Element” are in progress.
References 111 H. Ullmaier and F. Carsughi,
Table 4 Concentrations of impurity elements in two samples of tantalum obtained after removal of matrix by liquid-liquid extraction and ICP-MS (concentration pg/g)
Element Li Al Ti V Cr Mn co Ni cu Zn Sr Zr Nb MO Ag Cd Ba La Ce Hf W Pb Bi Th U
Sample I ICP-MS
Sample II ICP-MS
0.26f0.08 1.37f0.47 1.41 f0.39 0.28f0.11 0.55*0.03 0.2110.11 0.17*0.02 0.66f0.09 0.72f0.18 0.92f0.31 0.11*0.02 38.7f5.5 120.7flO.l 16.3f1.06 0.38+0.11 0.12f0.02 <0.02
0.10f0.02 2.15f0.18 0.62f0.16 0.22f0.08 3.07f0.17 0.12&0.03 0.72f0.08 8.3510.28 0.35f0.05 0.32+0.05 0.07f0.02 29.3&l .O 18.3f1.8 4.2f0.8 0.52f0.05 0.10f0.02 0.11 f0.02 <0.02
159
t21 [31
141 [51 L61 171
PI [91 [lOI L111 Cl21
Nucl. Instrum. Meth. Phys. Res., B 101 (1995) 406. P.C. Stevenson and H.G. Hicks, Anal. Chem., 25 (1953) 1517-1519. H.-J. Dietze, Massenspektrometrische Spurenanalyse (1975). Akademische Verlagsgesellschaft Geest and Portig K.-G., Leipzig. X. Feng and G. Horlick, J. Anal. At. Spectrom., 9 (1994) 823. H.-J. Dietze and J.S. Becker, Fresenius’ 2. Anal. Chem., 302 (1985) 490. SK. Luo and EC. Chang, Spectrochim. Acta, 45B (1990) 527. J.S. Becker, V.K. Panday and H-J Dietze, Colloquium Spectroscopicum Intemationale, June 29-July 4 (1993) York, UK. I. Tsuyoshi, E Noriko and K. Masaaki, Analyst, I15 (1990) 1185. V.K. Panday, J.S. Becker and H.-J. Dietze, Fresenius’ J. Anal. Chem., 352 (1995) 3027. V.K. Panday, J.S. Becker and H.-J. Dietze, Atomic Spectroscopy, May/June (1995) 97. J. Stummeyer and G. Wilnsch, Fresenius’ J. Anal. Chem., 342 (1992) 203. S. Kallman, in Lolthoff and Elving (Eds.), Treatise on Analytical Chemistry, NY, 1964, Part II, Vol. 6, Ch. 3.