1、Optimisation of flow-injection-hydride generation inductively coupled plasma spectrometric determination of selenium in electrolytic manganese Abstract Flow-injection-hydride generation procedure for Se in electrolytic manganese was optimized by means of the experimental design ap- proach. Instrumen
2、tal variables like power supplied (P), sample (F) and argon (G) flow rates together with chemical variables like NaBH4 and HCl concentrations were studied. In case of the chemical variables, it was concluded that sodium tetrahydridoborate concentrations higher than 1.0% extinguished the plasma while
3、 HCl concentration should always be higher than 0.6 mol dm3 . The analysis of effects suggested that all the instrumental variables are significant factors, and the optimum conditions were P = 1550 W, F = 4.75 mL min1 and G = 0.6 mL min1 . The influence of Mn was specially studied and it was conclud
4、ed that the interferences were negligible if Mn is be- low 2.0 g L1 . In the same sense, the interferences of antimony(III), arsenic(V) and mercury(II) were also considered negligible. In the same sense, the interferences of antimony(III), arsenic(V) and mercury(II) were also considered negligible.
5、Finally, a detection limit of 0.0005% (w/w) was obtained (a repeatability R.S.D. 2.0% for all Se concentrations tried). Some manganese samples were also spiked with different concentrations of Se(IV) and the results demonstrated to be in good statistical agreement with expected values.1. 1. Introduc
6、tion Manganese is essential to metallurgical industries (alu- minium and steel foundries) with important applications. Manganese additions in aluminium are required for food and drinking packings, domestic tools, decoration or covering. Manganese used for aluminium alloys is produced by elec- trolys
7、is wherein selenium additions are purposefully added to improve the electrical current efficient 1. This leads to the contamination of manganese as a result of co-deposition of selenium at the cathode. It is believed that over 90% of the selenium used for improving the current efficiency of the elec
8、tro-winning of electrolytic manganese enters the cathode in the elemental form and, thus, some of the electrolytic grade of manganese currently used can contain variable quantities of selenium (0.030.16%) Within the operating aluminium furnaces (700800 C), selenium is supposed to evaporate out of th
9、e molten alu- minium as metal vapour. Therefore, as a consequence of toxicity and environmental hazard in high concentrations of selenium and its compounds 3,4, furnace stack emissions and occupational airborne exposures must be monitored. In that sense, total selenium concentration in aluminium al-
10、 loying process, raw and waste materials must be also con- trolled to identify the Se environmental fate and exposures in aluminium processing. Hagelstein 2 studied the environmental management of selenium in aluminium processing and indicated that the main environmental issues were se- lenium produ
11、ced in operating aluminium furnaces. The aluminium and steel in- dustries may limit their future environmental liabilities due to selenium accumulation in processing facilities and waste streams by avoiding or controlling raw material inputs con- taining selenium. Thus, different confident selenium
12、analyses should be developed for implementation in cast-houses and metallurgical industries 5,6. Great selectivity and sensibility are obtained in aqueous samples by using different atomic techniques as inductively coupled plasma optical emission spectrometry (ICPOES), ETAAS or AFS. Besides, the imp
13、lementation of cold vapour or hydride generation previous to electrothermal atomic ab- sorption detection increases this selectivity minimising spec- tral interference caused by other matrix component 7. In the last decade, on-line flow-injection (FI) separation techniques have become increasingly p
14、opular for the de- termination of trace elements in different kind of sample Hydride generation (HG) together with inductively cou- pled plasma optical emission spectrometry (ICPOES) is a widely used method for the determination of selenium 910. However, there is lack of information in the literatur
15、e on this procedure for Se determination in electrolytic manganese. This analytical technique (FIHGICPOES) enables sepa- ration of selenium from the major components of the sample but, in fact, is prone to interference from several transition metal ions and other volatile elements. The transition me
16、tal interference occurs in the liquid phase and leads to a signifi- cant inhibition of selenium signal. Specifically, manganese ions present in the sample solution in hydride generation conditions yield manganese borides, which absorb hydrides and cause their decomposition 11. Manganese species may
17、catalyse decay of reducing agent as well. This chemical inter- ference is well documented in a review article by Nakahara and Kikui 12. However, the treatment of mutual interference by hydride-forming elements is relatively scarce and only few works discuss the possible interference mechanism 13,14.
18、 Therefore, the study and the identification of the dif- ferent factors or variables influencing the hydride generation procedure is clearly required in order to establish a correct analytical method for selenium determination. As a consequence, the main purpose of the present study is to find the o
19、ptimal set of operational conditions that allow a simple and sensitive method for the determination of Se in electrolytic manganese by FIHGICPOES. The experi- mental approach to accomplish this aim was the experimental design 15. As it is already known, experimental design is the most powerful way t
20、o make efficient experiments as we get the information we need with the minimum effort. 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. 2.2. Instrumentation An inductively coupled plasma optical em
21、ission spectrometer IRIS advantage (Thermo Corporation) with a CID detector was used. The instrument consists on a multi-channel peristaltic pump and gasliquid separator assembled to the spectrometer (T-PHD). A 196.090 nm line for Se was used and no background correction system was used. The presenc
22、e of spectral interference was considered negligible when the contribution of the interfering signal at the selenium wave- length was less than 10% 16,17. Detailed operating conditions for plasma excitation and hydride generation are listed in Table 1. 2.3. Digestion of electrolytic manganese Electr
23、olytic 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 per- formed using concentrated HCl without heating 18. This procedure was perf
24、ormed both with a water cooling (refriger- ant) and without it. Similar efficiency (signals) was obtained for all samples in both procedures (Fig. 1). Thus, analyti- cal determination of Se(IV) was independent on the use of a cooling column in the digestion step. 2.4. Thermodynamic selenium speciati
25、on The two common inorganic oxidation states of selenium are IV and VI, but only the lower state (IV) is successfully reduced to hydrogen selenide by NaBH4 19. Thus, it is important to verify that the main species of Se after HCl digestion is IV state. This fact has been both thermodynamically and e
26、xperi- mentally confirmed (Fig. 2a and b). Fig. 2a shows the dis- tribution diagram of Se against pH while Fig. 2b shows the predominance diagram of Se against pH and redox potential,using the medusa program. 2.5. Hydride generation procedure For selenium hydride generation, the sample (in HCl 0.6 m
27、ol dm3 ) and sodium tetrahydridoborate (1.0%) solu- tions are introduced into the FI system using a peristaltic pump. The solutions are then pumped through a gasliquid separator in a continuous stream. The released hydrides are supported by the carrier gas flow (Ar flow rate of 0.6 mL min1 ) to the
28、plasma. The reading time is 30 s and no background correction is used. 3. Results and discussion The efficiency of selenide generation depends directly on the NaBH4 concentration. However, NaBH4 concentrations higher than 1.1% destabilised the plasma discharge due to excessive hydrogen evolution and
29、 concentrations higher than 2.0% extinguished the plasma. 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 satis- factory medium to generate hydrides. The efficiency of selenide generation was constant at HCl concentrations above 1 mol dm3 (Fig. 3).
