Analytica Chimica Acta 547 (2005)

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1 Analytica Chimica Acta 547 (2005) Determination of arsenic(iii) and total inorganic arsenic in water samples using an on-line sequential insertion system and hydride generation atomic absorption spectrometry Aristidis N. Anthemidis, George A. Zachariadis, John A. Stratis Laboratory of Analytical Chemistry, Department of Chemistry, Aristotle University, Thessaloniki 54124, Greece Received 21 March 2005; received in revised form 16 May 2005; accepted 17 May 2005 Available online 24 June 2005 Abstract A simple and robust on-line sequential insertion system coupled with hydride generation atomic absorption spectrometry (HG-AAS) was developed, for selective As(III) and total inorganic arsenic determination without pre-reduction step. The proposed manifold, which is employing an integrated reaction chamber/gas liquid separator (RC-GLS), is characterized by the ability of the successful managing of variable sample volumes (up to 25 ml), in order to achieve high sensitivity. Arsine is able to be selectively generated either from inorganic As(III) or from total arsenic, using different concentrations of HCl and NaBH 4 solutions. For 8 ml sample volume consumption, the sampling frequency is 40 h 1. The detection limit is c L = 0.1 and 0.06 gl 1 for As(III) and total arsenic, respectively. The precision (relative standard deviation) at 2.0 gl 1 (n = 10) level is s r = 2.9 and 3.1% for As(III) and total arsenic, respectively. The performance of the proposed method was evaluated by analyzing the certified reference material NIST CRM 1643d and spiked water samples with various concentration ratios of As(III) to As(V). The method was applied for arsenic speciation in natural waters samples Elsevier B.V. All rights reserved. Keywords: Arsenic speciation; Hydride generation atomic absorption spectrometry; Sequential; Gas liquid separator 1. Introduction The occurrence of arsenic in aquatic environment arises mainly from biogeochemical cycles and anthropogenic processes. The concentration ratio of As(III) to As(V) can be used as a chemical indicator of the red-ox status of underground-water systems [1]. Inorganic compounds of arsenic are far more toxic than the organic ones, and As(III) and As(V) are the most important species released in the environment from mineral deterioration. Because of the different toxicity of the inorganic arsenic species (arsenite > arsenate), their determination in the environment has generated considerable interest. Undoubtedly, hydride generation atomic absorption spectrometry (HG-AAS) is currently the most popular technique for routine determination of trace amounts of arsenic, sele- Corresponding author. Fax: address: anthemid@chem.auth.gr (A.N. Anthemidis). nium, bismuth and other elements, which generate volatile hydrides [2,3]. In this technique, the signal response obtained from As(V) is about 40% lower than that obtained from As(III), thus a pre-reduction of As(V) to As(III) before the formation of the arsine, is recommended [2,4]. Usually, two procedures are demanded for the discrimination of the As(III) content from the total arsenic one; a selective procedure for As(III) and another one, including a pre-reduction step for total arsenic. However, complete volatilization of arsenate in the absence of pre-reducing step could be achieved by using higher HCl and NaBH 4 concentrations [5 8]. It is well known that manual (batch) methods, may lead to operator errors particularly when a considerable number of reagents and repetitive steps is involved and a large number of samples have to be analyzed. Continuous flow (CF) [9,10], flow injection (FI) [8,11], and recently sequential injection or insertion (SI) [12 16] systems were reported in order to decrease or eliminate the above drawbacks, by automated handling of reagents, which also improves the pre /$ see front matter 2005 Elsevier B.V. All rights reserved. doi: /j.aca

2 238 A.N. Anthemidis et al. / Analytica Chimica Acta 547 (2005) cision, diminishes drastically the reagents consumption and increases the sampling frequency [17]. Unfortunately improvements in the sensitivity of the automated HG-AAS methods are limited due to small sample volumes which are usually used. In contrast, batch methods employing high sample volumes (up to 50 ml) can achieve high sensitivity. However, batch hydride generation technique suffers from inselectivity due to the interference mainly from transition metals and the slow reaction kinetics [18]. To the best of our knowledge only one FI method for inorganic arsenic speciation in water samples without prereduction step has been reported, which makes use of a complicated merging zone manifold with limited sensitivity [8]. In the present work a simple and robust on-line sequential insertion system coupled with hydride generation atomic absorption spectrometry was developed, for arsenic speciation without pre-reduction step. For this purpose, an integrated reaction chamber/gas liquid separator was utilized, which has been manufactured in our laboratory and its performance has been studied for mercury determination elsewhere [16]. The proposed RC-GLS facilitates the use of a wide range of sample volume (5 25 ml) and thus high sensitivity is achieved. The experimental conditions for the arsine generation from As(III) and As(V) solutions were investigated separately. The accuracy of the developed method was demonstrated by analyzing a standard reference material and natural waters spiked with mixtures of As(III) and As(V) in various concentration ratios. 2. Experimental 2.1. Instrumentation A Perkin-Elmer model 5100 PC atomic absorption spectrometer equipped with a deuterium arc background corrector was used as detector. Arsenic electrodeless discharge lamp (EDL) was used as light source operated at 8 ma. The wavelength was set at nm resonance line and the monochromator spectral bandpass at 0.7 nm. A time-constant of 0.2 s was used for peak height evaluation. A Perkin-Elmer Hydride System MHS-1 with an electrothermal quartz flow throw cell atomizer was used for the hydride atomization at 950 C. The atomic absorption flow throw cell (AAC) was sealed with removable quartz windows at either ends. Two nipples were at the extreme ends to permit exit of the gas flow. The conduit between the outlet of the reaction chamber/gas liquid separator (RC-GLS) and the inlet of the AAC was as sort as possible (15 cm length, 0.5 mm i.d.) in order to keep the dead volume at low levels for achieving small values of the hydride vapour dispersion. The sequential insertion manifold is shown schematically in Fig. 1. It was consisted of two peristaltic pumps (Watson Marlow model 205U/BA, and Gilson Minipuls-3) a multiposition selection valve (Valco, C25Z) and two six-port twoposition injection valves (Labpro, Reodyne, USA). The two positions A and B of the injection valves (IV 1,IV 2 ), are presented schematically in Fig. 1a and b, respectively. The whole system was controlled by a personal computer and the LabVIEW software (National Instrument). The central port of the selection valve (SV) was connected with the inlet of the reaction chamber/gas liquid separator (RC-GLS) via a PTFE tubing (10 cm length, 0.8 mm i.d.), while the ports 1 and 2 were connected with the sample + HCl and NaBH 4 streams, respectively. Port 4 was connected with a short conduit (15 cm length, 1 mm i.d.) for the discarding of the waste. Nitrogen gas (N 2 ) was used as purge gas for the release and transportation of the generated arsine. Two displacement bottles DB1, DB2 (Tecator, Hoganas, Sweden) were serially combined and used to propel the strong HCl solution (9 mol l 1 ), due to the incompatibility of Tygon peristaltic tubes with the high acid concentration [19]. Fig. 1. (a) Optimized manifold for total arsenic and (b) the manifold for As(III) determination; A1, 1.5 mol l 1 HCl; A2, 9.0 mol l 1 HCl; R1, 0.5% m/v NaBH 4 ; R2, 3.0% m/v NaBH 4 ; purge gas, N l min 1 ; W, waste; P1, P2, peristaltic pumps; IV 1,IV 2, injection valves; SV, selection valve; RC-GLS, integrated reaction chamber/gas liquid separator; AAC, heated atomic absorption flow through cell.

3 A.N. Anthemidis et al. / Analytica Chimica Acta 547 (2005) Table 1 Operating sequences of the proposed method for total arsenic determination (IV1 and IV2 are in A position, as it is shown in Fig. 1a) Step SV P1 P2 Delivered medium Flow rate (ml min 1 ) Time (s) Operation 1 1 ON OFF Sample, HCl 16.0, Sample and HCl insertion 2 2 OFF ON NaBH Reductant insertion 3 3 OFF OFF N AsH 3 vapor separation, transportation and measurement 4 4 OFF OFF Waste RC-GLS evacuation The proposed integrated reaction chamber/gas liquid separator (RC-GLS), is characterized by simplicity and small dead volume, operating both as reaction chamber and gas liquid separator with minimum contribution in vapour dispersion. The RC-GLS was constructed in our laboratory of a cylindrical glass tube (100 mm length; 26 mm i.d.), while the push-fit connections at the two ends were made of polytetrafluoroethylene (PTFE). The above connections have internal conical cavities as they are illustrated in Fig. 1. This construction facilitates the effective separation and transportation of the released arsine. The upper connection is a Tee type confluence connector with a 0.5 mm i.d. horizontal channel and a 1.0 mm i.d. vertical one. The lower connection has a 0.8 mm i.d. channel. A significant advantage, which arises from this configuration, is that, during step 2 the purge gas flows out from the RC-GLS (by-pass), and thus the generated arsine vapor is trapped for few seconds into the RC-GLS, eliminating the dispersion and increasing the peak height of the measurement. Using the above GLS a wide range of sample volumes from 5 to 25 ml can be successfully managed, in order to increase method sensitivity Reagents and samples All chemicals were of analytical reagent grade and were provided by Merck (Darmstadt, Germany). Ultra-pure quality water was used throughout which was produced by a Milli-Q system (Millipore, Bedford, USA). Working standard solutions of As(III) were prepared by appropriate stepwise dilution of a 1000 mg l 1 As(III) to the required gl 1 levels just before use. The solution of 1000 mg l 1 As(III) was made dissolving g of As 2 O 3 in 25 ml of 20% m/v potassium hydroxide solution, followed by neutralization with 20% m/v sulphuric acid and diluting to 1000 ml with 1% m/v sulphuric acid. Standard solutions of As(V) were prepared by appropriate stepwise dilution of a 1000 mg l 1 As(V) stock standard solution (Titrisol, Merck). Sodium tetrahydroborate solutions in 0.2% m/v NaOH were freshly prepared from NaBH 4 (Fluka, As < % m/m). Solutions of HCl were prepared by adequate dilution of concentrated HCl (Fluka, As < % m/m, 1.16 g ml 1, 32 % m/m). Water samples were collected from Axios River, Prespa Lake and Thessaloniki tap water, Northern Greece during October All samples were filtered through 0.45 m membrane filters, acidified to ca. ph 2.5 with dilute HCl and stored at 4 C in polyethylene bottles. Certified reference material NIST CRM 1643d (National Institute of Standard and Technology) containing trace elements in water and total arsenic at a certified concentration ± 0.73 gl 1, was also analyzed Procedure The proposed method is operated in two modes for total arsenic and As(III) determination, respectively. The operation sequences for total arsenic determination are presented in Table 1. For As(III) determination the injection valves IV 1 and IV 2 are actuated to B position (Fig. 1b) following the same operation sequences. The use of a time-based injection mode enables the metering of the sample volume as a function of time and is a very advantageous way for sample introduction. During step 1, sample and HCl solution were loaded into the RC-GLS through port 1 of the SV by actuating pump P1 for appropriate loading time. During step 2, NaBH 4 solution was inserted into RC-GLS for 10 s and the produced nascent hydrogen generates the arsine vapor. In step 3, SV is actuated to 3rd position thus N 2 flows through the RC-GLS resulting to a flash release and transportation of AsH 3 towards the AAC. During all other steps, N 2 flows out (by-pass) from the RC- GLS, purging the AAC. In step 4, the resulted solution into the RC-GLS is evacuated to waste throw port 4, aided by the purge gas. The recorded peak height of the absorbance signals was proportional to arsenic concentration in the sample, and it is used throughout. Five replicates were made in all measurements. 3. Results and discussion 3.1. Influence of atomization temperature and purge gas N 2 flowrate In hydride generation systems the temperature of the heated atomic absorption flow through cell (AAC) and the purge gas flow rate affects the height and the width of the transient absorption signal. Using the manifold presented in Fig. 1b for As(III) determination, the atomization temperature for the AsH 3 was investigated in the range C, while the purge gas flow rate in the range l min 1.By increasing the temperature the sensitivity increases, while by increasing the N 2 flow rate, the recorded signal was becoming higher and narrower. However, an occurred deterioration of signal reproducibility was observed for flow rate higher than 0.5 l min 1 probably due to the droplets transportation in the AAC. Thus, for higher sensitivity and reproducibility 950 C and 0.4 l min 1, was adopted for further experiments.

4 240 A.N. Anthemidis et al. / Analytica Chimica Acta 547 (2005) Effect of total liquid volume The total volume of the liquid mixture in the reaction chamber, affects significantly the sensitivity of the vapor generation systems, as it has been reported in the literature [16,20,21]. On the other hand, the successful manipulation of various sample volumes in a large range is beneficial for a hydride generation method in the frame of variable sensitivity. The effect of total volume was studied in the range 5 25 ml, firstly using a sample solution with a fixed concentration of 2.5 gl 1 As(III), and secondly in another study preserving constant at 25 ng the amount of As(III) into the liquid mixture. In any case the HCl concentration in the loaded sample solution was fixed at 1.5 mol l 1. The 1.5% m/v NaBH 4 solution was loaded for 10 s at 6 ml min 1 flow rate. As it is shown in Fig. 2, larger sample volumes offer increased overall sensitivity, however, by increasing the total liquid volume, the absorbance of the fixed amount of arsenic was decreased, showing that large volumes hamper the arsine release. Therefore, the proposed RC-GLS can be used for total volume ranged in from 5 to 25 ml with proportional sensitivity. For all subsequent experiments, 8 ml sample volume and 4 ml HCl solution volume (12 ml total loading volume) was adopted, as a compromise between sensitivity and consumption of HCl and sample Effect of HCl and NaBH 4 concentration on hydride generation The majority of the proposed hydride generation methods for arsenic determination make use of arsine, which is generated from As(III), after a pre-reduction step of arsenate to arsenite. In batch methods arsine can be also directly generated from arsenate, with slower reaction rate, than that from arsenite [2,10]. The effect of the arsenic oxidation state on the measured signal can be decreased by using higher NaBH 4 and HCl concentrations, longer reaction times and integrated measurements [10,22 25]. The concentration of HCl and NaBH 4 solutions was studied for direct arsine gen- Fig. 3. Effect of HCl concentration on the absorbance (mean ± S.D.); ( ) 4.0 gl 1 As(III); ( ) 4.0 gl 1 As(V); [NaBH 4 ] = 1.5% m/v. eration either from As(III) or from As(V), employing the manifold presented in Fig. 1a, using separately 4.0 gl 1 As(III) and 4.0 gl 1 As(V) solutions. The NaBH 4 flow rate was 6.0 ml min 1 and was loaded for 10 s. The effect of HCl concentration on the absorbance from As(III) and As(V) was studied in the range mol l 1 HCl, using a medium concentration 1.5% m/v of NaBH 4 solution. As it is shown in Fig. 3, the signal related to As(III) increases rapidly up to 1.5 mol l 1 and for higher HCl concentrations it remains constant. On the other side the absorbance from As(V) increases with slower rates up to 9.0 mol l 1 and in any case is lower than that obtained from As(III). The effect of NaBH 4 concentration on the absorbance related with both oxidation states, was studied in the range % m/v NaBH 4, using HCl solution at two concentration levels: 1.5 and 9.0 mol l 1 HCl. The results are presented in Figs. 4 and 5, respectively. As it is shown in Fig. 4, the absorbance of As(III) increases with increase of NaBH 4 concentration up to 1.5% and levels off for higher NaBH 4 concentrations, while the absorbance of As(V) increases up to 9.0%, and always is lower than that from As(III). In addition the absorbance obtained from As(V) is not remarkable (<5% of the signal obtained from As(III)), when 0.5% m/v NaBH 4 Fig. 2. Effect of total liquid volume on the absorbance (mean ± S.D.); ( ) fixed concentration of 2.5 gl 1 As(III) in the studied volume; ( ) absolute amount 25 ng As(III) into the studied volume. Fig. 4. Effect of NaBH 4 concentration on the absorbance (mean ± S.D.); ( ) 4.0 gl 1 As(III); ( ) 4.0 gl 1 As(V); [HCl] = 1.5 mol l 1.

5 A.N. Anthemidis et al. / Analytica Chimica Acta 547 (2005) (at 95% confidence level) are: slope = (0.981 ± 0.028), intercept = (0.002 ± 0.005), showing that the calibration curves for the two inorganic species determination, did not differ significantly. Consequently, the strong hydride generation conditions (9.0 mol l 1 HCl and 3.0% m/v NaBH 4 ) can be used for total arsenic determination Analytical performance of the proposed method Fig. 5. Effect of NaBH 4 concentration on the absorbance (mean ± S.D.); ( ) 4.0 gl 1 As(III); ( ) 4.0 gl 1 As(V); [HCl] = 9.0 mol l 1. concentration is used. Thus, 1.5 mol l 1 HCl and 0.5% m/v NaBH 4 can be used for selective As(III) determination. As it is presented in Fig. 5, when high HCl concentration (9.0 mol l 1 ) is used, the absorbance obtained from the two inorganic species are equal when 3.0% m/v NaBH 4 solution is used. This observation shows that at the above conditions (HCl: 9.0 mol l 1, NaBH 4 : 3.0% m/v) the arsine generation from As(III) or As(V) is complete, and they can be adopted for total arsenic determination Estimation of hydride generation conditions In order to estimate the possibility of using the above strong hydride generation conditions (9.0 mol l 1 HCl and 3.0% m/v NaBH 4 ) for total arsenic determination, the calibration curve of aqueous As(III) standard solutions was statistically compared with the calibration curve of aqueous As(V) standard solutions. The statistical test, [26] which uses the regression lines, was applied in order to estimate possible systematic differences between the two calibration methods. According to this approach, the responses obtained from a series of As(III) standards are plotted against the responses obtained from an identical series of As(V) standards. If the calculated slope and intercept from the regression line do not differ significantly from the ideal values of 1 and 0, respectively, then there is no evidence for systematic differences between the two calibration methods. The calculated values of slope and intercept with their confidence interval The analytical performance data of the proposed method for selective determination of As(III) and total arsenic using the optimized chemical conditions are presented in Table 2. The accuracy was evaluated, by determining the arsenic concentration of a certified reference material NIST CRM 1643d. The recovery obtained was 96% (53.79 ± 2.43 gl 1, n = 5). The result was in good agreement with the certified value and the calculated recovery was satisfactory. The method was also applied to the analysis of local natural water samples including river, lake and tap water and validated spiking the samples with known amounts of As(III) and As(V). The obtained results are presented in Table 3. The recoveries from spiked solutions were varied in the range % Interference studies An outstanding advantage of on-line vapour generation methods is the improved tolerance to interferences against batch methods [17]. On the other hand, the influence of different NaBH 4 and HCl concentrations on the interference of some typical transition metals during the arsenic determination has been investigated by Welz and Schubert- Jacobs [27]. The authors pointed out that an increase in HCl and a decrease in NaBH 4 concentration improves the range of interference-free determination. The effect of potential interferents encountered in natural waters samples on As(III) determination was examined using 2.0 gl 1 As(III) solution under the optimum conditions ([HCl] = 1.5 mol l 1, [NaBH 4 ] = 0.5% m/v). Differences of the signals higher than 10%, in the presence of the studied ion, were considered as interference. The results showed that Al(III), Cu(II), Cr(III), Fe(II), Fe(III), Mn(II) and Pb(II) can be tolerated up to 10 mg l 1. Co(II), Ni (II), Zn(II) and Sn (II) can be tolerated Table 2 Analytical performance data of the proposed method for As(III) and total arsenic determination Characteristics As(III) a Total arsenic b Regression equation ([As] in gl 1 ) ( ± ) [As] + ( ± ) ( ± ) [As] + ( ± ) Correlation coefficient, r Detection limit, c L (3s) ( gl 1 ) Precision, s r (R.S.D., n = 10; 2.0 gl 1 ) (%) Linear range ( gl 1 ) Sample consumption (ml) 8 8 Sampling frequency (h 1 ) a [HCl] = 1.5 mol l 1, [NaBH 4 ] = 0.5% m/v. b [HCl] = 9.0 mol l 1, [NaBH 4 ] = 3.0% m/v.

6 242 A.N. Anthemidis et al. / Analytica Chimica Acta 547 (2005) Table 3 Analytical results (mean ± S.D., n = 5) of As(III) total arsenic and calculated As(V) determination in natural waters samples and certified reference material Sample As(III):As(V); added ( gl 1 ) As(III) Total arsenic; found ( gl 1 ) As(V) Found ( gl 1 ) R (%) Calculated ( gl 1 ) R (%) River-water 1.20 ± ± ± : ± ± ± Lake-water 0.95 ± ± ± : ± ± ± Tap-water n.d ± ± : ± ± ± CRM 1643d ± 0.73 a 9.86 ± ± ± n.d.: not detected. a Certified value of total arsenic. up to 2 mg l 1, while Se (IV) up to 1 mg l 1. Other common matrix elements such as Ca, Mg, Ba are tolerated at least up to 1000 mg l Conclusions An on-line sequential insertion hydride generation atomic absorption system with a newly designed reaction chamber/gas liquid separator has been evaluated for selective As(III) and total arsenic determination without prereduction step. The proposed RC-GLS operates as reaction vessel and gas liquid separator resulting thus to minimum vapour dispersion, and also facilitating the successive manipulation of a large range of sample volumes with proportional sensitivity. Two different pairs of HCl and NaBH 4 concentrations were proved to be convenient for selective determination of As(III) and total arsenic without pre-reduction step. The simplicity, easy handling, low cost and the good sensitivity of the proposed method, make it attractive for routine determination of the two main species of arsenic in natural waters. The proposed manifold is also beneficial for all the hydride forming elements. References [1] W.M. Mok, V.M. Wai, Anal. Chem. 59 (1987) 233. [2] J. Dedina, D.L. Tsalev, Hydride Generation Atomic Absorption Spectrometry, Wiley, Chichester, [3] Z.L. Fang, G. Tao, S. Xu, X. Liu, J. Wang, Microchem. J. 53 (1996) 42. [4] J. Muller, Fresenius J. Anal. Chem. 363 (1999) 57. [5] P. Bermejo-Barrera, J. Moreda-Pineiro, A. Moreda-Pineiro, A. Bermejo-Barrera, Anal. Chim. Acta 374 (1998) 231. [6] A. Lopez, R. Torralba, M.A. Palacios, C. Camara, Talanta 39 (1992) [7] L. Ebdon, J.R. Wilkinson, Anal. Chim. Acta 136 (1982) 191. [8] N.M.M. Coelho, A. Cosmen da Silva, C. Moraes da Silva, Anal. Chim. Acta 460 (2002) 227. [9] B. Chen, M. Kracher, Z.I. Gonzalez, W. Shotyk, J. Anal. At. Spectrom. 20 (2005) 95. [10] H. Narasaki, M. Ikeda, Anal. Chem. 56 (1984) [11] S. Nielsen, E.H. Hansen, Anal. Chim. Acta 343 (1997) 5. [12] N.V. Semenova, L.O. Leal, R. Forteza, V. Cerda, Anal. Chim. Acta 455 (2002) 277. [13] N.V. Semenova, F.M. Bauza de Mirabo, R. Forteza, V. Cerda, Anal. Chim. Acta 412 (2000) 169. [14] H.B. Ma, S.K. Xu, H.Y. Zhou, S.L. Wang, Z.L. Fang, Spectrosc. Spectrom. Anal. 20 (4) (2000) 529. [15] P. Ek, S.G. Hulden, A. Ivaska, J. Anal. At. Spectrom. 10 (1995) 121. [16] A.N. Anthemidis, G.A. Zachariadis, J.A. Stratis, Talanta 64 (2004) [17] Z. Fang, Flow Injection Atomic Absorption Spectrometry, Wiley, West Sussex, England, [18] M.B. de La Calle-Guntias, R. Torralba, Y. Madrid, M.A. Palacios, M. Bonilla, C. Cámara, Spectrochim. Acta Part B 47 (1992) [19] A.N. Anthemidis, G.A. Zachariadis, J.A. Stratis, Talanta 60 (2003) 929. [20] H-B. Ma, Z-L. Fang, J-F. Wu, S-S. Liu, Talanta 49 (1999) 125. [21] C.P. Hanna, J.F. Tyson, Anal. Chem. 65 (1993) 653. [22] P. Carrero, A. Malave, J.L. Burgera, M. Burgera, C. Rondon, Anal. Chim. Acta 438 (2001) 195. [23] B. Welz, M. Schubert-Jacobs, Atomic Spectrosc. 12 (1991) 91. [24] J. Sanz, F. Gallarta, J.R. Castillo, Anal. Chim. Acta 255 (1991) 113. [25] D.L. Tsalev, P.B. Mandjukov, J.A. Stratis, J. Anal. At. Spectrom. 2 (1987) 135. [26] J.C. Miller, J.N. Miller, Statistics for Analytical Chemistry, Wiley, New York, 1986, p.102. [27] B. Welz, M. Schubert-Jacobs, J. Anal. At. Spectrom. 1 (1986) 23.