Optimisation of volatile elements determination by flow injection hydride generation-inductively coupled plasma spectrometry in electrolytic manganese

Journal of Alloys and Compounds 427 (2007) 73–77

Optimisation of volatile elements determination by flow injection hydride generation-inductively coupled plasma spectrometry in electrolytic manganese R. Antol´ın b , G. Borge b , T. Posada b , N. Etxebarria a , J.C. Raposo a,∗ a

Kimika Analitikoaren Saila, Euskal Herriko Unibertsitatea, 644 PK, E-48080 Bilbao, Spain b Bostlan S.A., Pol´ıgono Industrial Trobika, PK E-48100 Mungia, Spain

Received 9 January 2006; received in revised form 27 February 2006; accepted 2 March 2006 Available online 2 May 2006

Abstract The simultaneous determination of As, Hg, Sb, Se and Sn by flow injection hydride generation-inductively coupled plasma optical emission spectrometry (FI-HG-ICP-OES) can be performed in electrolytic manganese after this work. Volatile species were generated mixing acidified aqueous samples and sodium tetrahydroborate solution in a continuous flow system, and the chemical and instrumental conditions were optimised experimentally. In case of the instrumental variables, the optimisation was achieved following the experimental design approach, while the chemical variables were adjusted following the variation of the signals of the analytes. As a result, the simultaneous determination of all the analytes was achieved at the following conditions: power supplied (P = 1550 W), sample and argon flow rates (F = 4.75 ml min−1 , G = 0.6 ml min−1 ), NaBH4 (≤1.0%) and HCl (=0.6 mol dm−3 ). The spectral interferences between the volatile elements and matrix effects were evaluated. Finally, different detection limits (w/w) were obtained for As (0.0006%), Hg (0.0008%), Sb (0.0008%), Se (0.0005%) and Sn (0.001%) with repeatability R.S.D. < 2.0% for all elements concentrations tried. © 2006 Elsevier B.V. All rights reserved. Keywords: Volatile elements; Electrolytic manganese; Hydride generation; Inductively coupled plasma; Variables optimisation; Aluminium alloying

1. Introduction Electrolytic manganese is used as an alloying agent in which Mn is a minor component in aluminium and steel alloys. In fact, manganese additions in aluminium foundries are required for food and drinking packings, domestic tools, decoration or covering. Manganese (purity near to 99.0%) used for aluminium alloys is produced by an electrolysis procedure wherein different impurities can be associated to the electrolytic process; for instance, the content of Se is not negligible [1]. Manganese added to aluminium alloys can be recovered in the secondary aluminium industry, but metallurgical losses become important anthropogenic inputs of manganese to the environment. Within the operating aluminium furnaces (700–800 ◦ C), volatile elements are supposed to evaporate out of the molten aluminium as metal vapour. As a consequence, the chemical specification

∗

Corresponding author. Tel.: +34 946015543; fax: +34 944648500. E-mail address: [email protected] (J.C. Raposo).

0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.03.001

of the raw materials and products can be a matter of deep revision in order to fulfil all the legal regulations on health, safety and environment [2,3]. Therefore, total contents of several elements have to be monitored in electrolytic manganese in order to control and identify environmental fates and exposures from the volatile elements. Analytical chemistry is consolidating an important position within the framework of the industrial community, i.e. foundries. In that sense, potential risks can be estimated and corrected solely by means of highly efficient analytical methods. Hydride generation (HG) techniques have been successfully used in trace-element determination because they offer several advantages [4,5]: (i) enhancement of transport efficiency (approaching 100% for many elements), (ii) separation of the analyte from the matrix and (iii) possibility of pre-concentration. In addition, gaseous sample introduction promotes more efficient plasma atomisation, excitation and ionisation of the analyte [6]. In a previous work [7], hydride generation (HG) together with inductively coupled plasma optical emission spectrometry (ICP-OES) was concluded as a highly efficient method for the

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determination of selenium in electrolytic manganese. In fact, that method is currently used for the routine analysis of raw manganese in Bostlan S.A. [8]. The analytical technique (FIHG-ICP-OES) enables the separation of selenium hydride from the major components of the sample, but a wide study for the identification of the different factors or variables influencing the hydride generation procedure was performed in order to establish a correct analytical method for selenium determination. The multi-element detection capability of ICP-OES makes it an ideal detector to be implemented as a simple and sensitive analytical procedure for volatile element determination. Consequently, one of the main objectives of this study was to assess the utility of the FI-HG-ICP-OES technique for the determination of volatile elements as routine analysis in industrial laboratories. Therefore, the optimal set of operational conditions that allow, together with Se, the simultaneous determination of As, Hg, Sb and Sn in electrolytic manganese was defined. The methodology followed was based on experimental design [9]. As it is already known, experimental design is the most powerful way to make efficient experiments as we get the information we need with minimum efforts. In this sense, a full factorial design was used to screen the effect of each variable while a composite design was used to build the response surface. Chemical (NaBH4 and HCl concentrations) and instrumental variables (radio frequency power, sample flow rate and argon flow rate) were evaluated. 2. Experimental 2.1. Reagents and solutions Calibration solutions of every trace element considered were prepared using ICP standard solutions (1000 mg l−1 ) from Merck (Darmstadt, Germany). All solutions were prepared using analytical grade HCl from Merck and de-ionised water. HNO3 from Merck was also used. NaBH4 (1%, w/v) and NaOH (0.1 mol dm−3 ) from Merck were used as reagents in the hydride generation.

2.2. Instrumentation An inductively coupled plasma optical emission spectrometer IRIS Advantage (Thermo Corporation) with a CID detector was used. The instrument consists on a multi-channel peristaltic pump and gas–liquid separator assembled to the spectrometer (T-PHD). The presence of spectral interference was considered negligible when the contribution of the interfering signal at the selected wavelength was less than 10% [10,11]. Detailed operating conditions for plasma excitation, wavelengths and hydride generation are listed in Table 1.

2.3. Analytical procedure Electrolytic manganese (Xiangxi Autonomous Prefecture, China) was collected in flakes and crushed into a laboratory mill (Retsch S100) and sieved (500 m). An acid digestion of the manganese powder obtained (2 g) was immediately performed using concentrated HCl without heating [12]. The final solution was filtered through Whatman filters (0.45 ␮m) and rinsed to 500 ml with MilliQ water. The acidified sample (in HCl 0.6 mol dm−3 ) and the reductant solutions were mixed at a Y-piece and introduced into the hydride generator using a peristaltic pump. The volatile compounds were separated from the solution in a gas–liquid separator, and a continuous stream of Ar (0.6 ml min−1 ) swept the hydrides directly into the torch. The reading time was 30 s and no background correction was used. The analytical signals obtained were the average of three replicate measurements.

Table 1 Optimum operating conditions for FI-HG-ICP-OES experiments Supplied power (P) Plasma Ar flow rate Auxiliary Ar flow rate Carrier Ar flow rate (G) Pressure on nebuliser As wavelength Hg wavelength Sb wavelength Sn wavelength Integration time Background correction Sample flow rate (F) NaBH4 flow rate (F) NaBH4 concentration Samples acidity

1550 W 2.0 l min−1 0.5 l min−1 0.6 ml min−1 28 psi 193.760 nm 184.850 nm 206.850 nm 189.980 nm 60 s Yes (IRIS/AP) 180 rpm or 4.75 ml min−1 180 rpm or 4.75 ml min−1 1.0% in NaOH, 0.1 mol dm−3 HCl: 0.6 mol dm−3

3. Results and discussion The efficiency of hydride generation is strongly dependent on the oxidation state of the species, the experimental conditions and the specific instrumental assembly in use [13,14]. The design and the optimisation of the operating conditions determine the performance of the FI-HG-ICP-OES coupling. In case of the oxidation state, the chemical speciation of the analytes in the acidified samples was thermodynamically studied taking into account all the aqueous species described in the literature [15,16]. Although it is difficult to foresee the redox potential of the acid solutions, a slightly oxidant media can be estimated if there are tiny concentrations of Mn(III, IV) in solution. As a consequence, As(V), Hg(II), Sb(IV) and Sn(IV) species are highly compatible with acid Mn(II) solutions. On the one hand, the oxidation states of mercury and tin are considered in the literature as the most efficient forms to the hydride generation reactions [4,5]. On the other hand, arsenic and antimony were not at the most efficient state, but both can be readily reduced to render the corresponding hydrides [17]. 3.1. Chemical variables The efficiency of hydride generation depends also on the NaBH4 concentration. However, NaBH4 concentrations higher than 1.1% destabilised the plasma discharge due to excessive hydrogen evolution. Thus, NaBH4 concentration of 1.0% was considered the highest operating value in all the analysis performed and it was kept constant. Hydrochloric acid has been found to be the most satisfactory medium to generate hydrides. In general, the efficiency of hydride generation increased slightly with the increasing HCl concentration or reached a plateau above 0.6 mol dm−3 . As an example Fig. 1a shows the influence of the HCl concentration on the direct response of As. However, the influence of HCl concentration is completely different for Sn(IV). The optimal value could be easily fixed at HCl 0.6 mol dm−3 (Fig. 1b) as the maximum response obtained varying with the HCl concentration. This optimal value of HCl concentrations was exactly the same

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Fig. 2. Influence of Se (0.1%) on the (×) As, () Hg, () Sb and (䊉) Sn responses.

Fig. 1. Influence of HCl concentration on As (a) and Sn (b) response at 200 ␮g l−1 .

as the one optimised for the Se determination in a previous work [7]. Spectral interference between the hydride forming elements like As(V), Hg(II), Sb(III) or Sn(IV) on the hydride generation efficiency was also studied. It has been shown in the literature [18] that hydride generation interference does not depend on the analyte-to-interferent ratio but on the interfering concentration in the solution for measurement. In the samples where these analyses are required, the selenium contents (0.03–0.16%) are commonly higher than the rest of analytes, due to additions for improving the current efficiency of the electro-winning of electrolytic manganese [19]. Fig. 2 shows the hydride generation efficiency (%R) for this real situation defined as: %R =

Sx,i × 100 Sx,0

(1)

where Sx,i is the signal of each volatile element with interferences and Sx,0 without interferences. As it can be seen, influence of other forming volatile element (selenium at higher concentrations) on each hydride response was negligible in this case. 3.2. Instrumental variables: experimental designs The instrumental variables of the FI-HG-ICP-OES system were studied using aqueous standard solutions at 80 ␮g l−1 of each element.

Several variables, such as power supplied (P), sample flow rates (F) and auxiliary Ar flow rate (G) could affect the hydride detection. A two-level full factorial design (23 + 4 replicates of central point) was carried out using The Unscrambler Program ˚ Norway, v.7.5) [20]. Table 2 lists the experimental (Camo, As, design matrix defined by The Unscrambler Program. The analysis of the effects was carried out with The Unscrambler Program, and it was concluded that all the variables showed a significant effect on the response (p level < 0.05). The previous design was extended to a central composite design to build the response surface and to obtain the instrumental conditions that define the maximum response. The CCD designs were carried out using The Unscrambler Program. Table 3 shows the central composite design defined by The Unscrambler Program. The response surface was calculated with The Unscrambler for all the analytes, and the significant effect of G and P variables was reassured (p < 0.05). The sign of their main effects for each of the analytes was positive for F and P and negative for G. These facts mean as when F and P are higher more effective in the reduction and atomisation steps was obtained. However, this efficiency was higher when G was lower because the atomisation time in the torch was higher. If the F variable is fixed at the maximum level of the CCD (180 rpm or 4.75 ml min−1 ), it is possible to plot the response surface as a function of the most significant variables, as can be seen in Fig. 3. From those plots, we could deduce the optimum Table 2 Experimentation proposed by the full factorial design Experiment

F (rpm)

G (ml min−1 )

P (W)

(−,−,−) (+,−,−) (−,+,−) (+,+,−) (−,−,+) (+,−,+) (−,+,+) (+,+,+) (0,0,0) (0,0,0) (0,0,0) (0,0,0)

140 180 140 180 140 180 140 180 160 160 160 160

0.6 0.6 1.0 1.0 0.6 0.6 1.0 1.0 0.8 0.8 0.8 0.8

1150 1150 1150 1150 1550 1550 1550 1550 1550 1350 1350 1350

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Table 3 Experimentation proposed and results obtained by CCD Experiment

Wavelength (nm) and responses (kcnt) (ml min−1 )

#

G

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

0.63 0.97 0.80 0.80 0.80 0.80 0.70 0.90 0.70 0.90 0.70 0.90 0.70 0.90 0.80 0.80 0.80 0.80

F

(ml min−1 )

160 160 140 180 160 160 150 150 170 170 150 150 170 170 160 160 160 160

P (W)

As (193.759)

Sn (189.989)

Sb (206.833)

Hg (184.950)

1350 1350 1350 1350 1014 1686 1150 1150 1150 1150 1550 1550 1550 1550 1350 1350 1350 1350

10.20 4.14 4.38 5.48 2.73 7.71 6.41 2.98 6.22 3.30 8.35 3.51 9.22 4.04 4.96 5.03 5.00 4.99

16.15 2.67 2.82 3.86 1.02 9.02 4.39 0.62 4.50 0.76 11.03 1.88 12.64 2.24 2.96 3.19 3.20 3.28

19.23 8.25 8.74 11.45 5.86 13.94 13.03 6.07 13.22 6.78 15.53 6.58 17.74 7.78 9.88 10.01 10.07 10.17

42.61 12.33 13.02 17.55 5.43 29.23 19.81 5.28 19.47 6.39 32.04 9.81 36.90 11.62 15.15 15.58 15.44 15.45

conditions within the factor space, i.e. without any extrapolation. As a consequence, the determination of As, Hg, Sb and Sn can be carried out with a power of (P) 1550 W and an Ar flow rate of (G) 0.6 ml min−1 . These instrumental parameters coincide with those obtained previously in the determination of Se [7]. 3.3. Analytical performance Using optimised conditions, analytical figures of merit including detection limit and precision of replicate measure-

ments were established. Detection limit was calculated following the IUPAC rules, defined as blank signal + 3S.D., where S.D. is the standard deviation of five measurements of a blank: As (0.0006%), Hg (0.0008%), Sb (0.0008%) and Sn (0.001%). The linear response range was 500 ␮g l−1 for each of the four analytes. Precision of the method (%R.S.D.) within-a-day (n = 6) and among days (n = 6) for five replicate measurements was lower than 2.0% for all concentrations tried in presence of Mn 2 g l−1 .

Fig. 3. Response surfaces (power supplied vs. Ar flow rate) obtained for optimisation of hydrides determination As (a), Hg (b), Sb (c) and Sn (d).

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4. Conclusions This study has demonstrated the possibility of multi-element determination in electrolytic manganese by FI-HG-ICP-OES. The use of experimental designs shows that the three variables studied (power supplied, Ar flow rate and pump flow rate) are significant in this determination. In addition, the optimal variables defined in this study are exactly the same that those defined for Se previously. Therefore, As, Hg, Sb and Sn can be determined together with Se in electrolytic manganese. In terms of selectivity, this study shows the lack of interferences from other analytes. In any case, the main source of interferences is due to the high concentration of Mn, which should be kept lower than 2.0 g l−1 [7]. Under the optimum conditions found (P = 1550 W, F = 180 rpm or 4.75 ml min−1 and G = 0.6 ml min−1 ), the developed method shows adequate analytical performance for the direct determination in electrolytic manganese. The LOD defined in this work is sufficient to guarantee the goodness and quality of use of the electrolytic manganese as a raw material in aluminium foundries for food industrial applications. Acknowledgements Bostlan S.A. is grateful to the Ministry of Science and Technology of the Spanish Government (programs PROFIT and Torres Quevedo) and to the Basque Government (program INTEK) for financial support of different parts of this work. R. Antol´ın is grateful to the Basque Government for the fellowship granted to complete her Ph.D. work. References [1] M.R. Aboutalebi, M. Isac, R.I.L. Guthrie, Light Met. (TMS) (2003) 787–793.

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