Microwave-assisted total digestion of sulphide ores for multi-element analysis

Analytica Chimica Acta 638 (2009) 101–105

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Microwave-assisted total digestion of sulphide ores for multi-element analysis M. Al-Harahsheh a,∗ , S. Kingman b , C. Somerfield b , F. Ababneh c a

College of Mining and Environmental Engineering, Al-Hussein Bin Talal University, P.O. Box 20, Ma’an 71111, Jordan Department of Chemical and Environmental Engineering, University of Nottingham, University Park, Nottingham, NG7 2RD, UK c Department of Chemistry, Al-Hussein Bin Talal University, P.O. Box 20, Ma’an, Jordan b

a r t i c l e

i n f o

Article history: Received 21 October 2008 Received in revised form 8 February 2009 Accepted 9 February 2009 Available online 25 February 2009 Keywords: Microwave digestion Sulphide ores Total digestion Inductively coupled plasma atomic emission spectroscopy

a b s t r a c t A new two-stage microwave-assisted digestion procedure using concentrated HNO3 , HCl, HF and H3 BO3 has been developed for the chemical analysis of major and trace elements in sulphide ore samples prior to inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis. In the first stage 0.2 g of the certified reference material (CRM) sample was digested with a combination of acids (HNO3 , HCl, and HF) in a closed Teflon vessel and heated in the microwave to 200 ◦ C for 30 min. After cooling, H3 BO3 was added and the vessel was reheated to 170 ◦ C for 15 min. The precision of the method was checked by comparing the results against six certified reference materials. The analytical results obtained were in good agreement with the certified values, in most cases the recoveries were in the range 95–105%. Based on at least 17 replicates of sample preparation and analysis, the precision of the method was found to be ≤5%. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The speed and efficiency of instrumentation for reliable determination of trace metals in geological and environmental samples have improved dramatically over the last two decades. However, sample preparation techniques have not developed at the same rate and such techniques are still the major factor contributing to the uncertainty in the analytical results. Direct sampling of solids (without pre-treatment) such as laser ablation atomic spectroscopy and electrothermal vaporization/atomization are well known [1–3], but the major challenge in using these techniques is obtaining a representative and homogeneous sample of only a few milligrams from the large (bulk) solid sample. Various analytical methods for trace element determination in environmental and geological samples, such as flame atomic absorption spectroscopy (FAAS) [4] and inductively coupled plasma atomic emission and -mass spectrometry (ICP-AES and ICP-MS) [5,6] require digestion/dissolution of solid samples into solution [7]. Therefore, sample preparation still remains an essential part of atomic spectroscopy; the efficiency of the digestion method is of great significance for obtaining precise and reproducible analytical results. The classical methods of solid sample digestion/dissolution are based on fusion (dry ashing) or wet (acid digestion) procedures [8,9]. In the dry ashing method, the refractory solid material is mixed with fusion reagents such as lithium metaborate and the mixture is heated to a high tem-

∗ Corresponding author. Tel.: +962 777 850164. E-mail address: [email protected] (M. Al-Harahsheh). 0003-2670/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2009.02.030

perature (>600 ◦ C). Nitric acid is then added to prepare a solution ready for metal analysis. This solution contains a high amount of dissolved solids, and this increases the blank value, the dry ashing method also suffers from a loss of volatile metals [10]. The wet digestion method involves the use of a heated mixture of mineral acids such as nitric, hydrochloric, sulfuric, perchloric or hydrofluoric acids and oxidizing agents such as hydrogen peroxide. Both the wet digestion and dry ashing procedures are slow, tedious and time consuming. Hence, new methods for sample preparation have been developed [11]. The improvement of microwave ovens has led to their usage in analytical laboratories for sample digestion. The theory of microwave digestion has been reviewed in detail elsewhere and only brief comment is justified here [12]. The first application of microwaves for sample preparation was reported in 1975 [13]. Since that time many microwave-assisted dissolution methods have been developed to include a variety of sample matrices such as soil [6], fish [14], sediments [15] and biological and environmental samples [16,17]. Microwave digestion procedures are classified according to their operational modes; open vessel microwave-assisted digestion which is more prone to sample contamination and susceptible to losses of volatile metals, and closed (pressurized) vessel procedures which are rapid and efficient digestion techniques. On-line microwave-assisted digestion of solid samples and a combination with ultrasonic radiation also are known [18,19]. Complete microwave-assisted dissolution of the refractory sulphide ore samples can be achieved by varying the operational parameters such as type, concentration and volume of acids and oxidizing agents used and microwave oven settings such as the pressure, microwave power, temperature and heating time.

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Table 1 Instrumental operating condition for PerkinElmer Optima® 3300 DV (ICP-AES). Parameter

Instrument operating conditions

RF power Plasma gas flow Auxiliary gas flow Nebulizer gas flow Sample flow rate Wash time Read delay Replicate readings

1300 W 15 L min−1 0.5 L min−1 0.8 L min−1 1 mL min−1 40 s 110 s 3

However, the main aim of this work was to develop a robust, rapid and easy to apply digestion procedure for sulphide ores using closed-vessel microwave digestion system. Fig. 1. The temperature and pressure profile inside the digestion vessel under microwave heating. () Temperature (◦ C) and () pressure (bar).

2. Experimental 2.1. Apparatus and instruments A CEM® Microwave accelerated reaction system, model MARS X® was used for total microwave-assisted digestion of the samples. The MARS® system delivers three levels of microwave power; (300, 600 and 1200 W) at a frequency of 2450 MHz. Each power level can be varied from 1% to 100% of its value. XP-1500® vessels made of a translucent and chemical resistant material (Teflon PFA® ), which are temperature resistant up to 240 ◦ C, were used as the digestion container. The microwave cavity was capable of holding up to 12× 80 mL of these digestion vessels on a turntable at the same time, 11 of the 12 vessels incorporated a pressure release valve and were capable of bearing pressures up to about 55 bars. The 12th vessel – the so-called control vessel – was used to capture and monitor temperature and pressure profiles in that vessel during heating. Profiles were downloaded to an external computer from the MARS X® system in real time. Elemental analysis was performed using ICP-AES (PerkinElmer Optima® 3300 DV). The ICP-AES was equipped with autosampler (AS-90 Plus® ) and controlled with Perkin Elmer Winlab software. The ICP operational conditions are given in Table 1. 2.2. Certified reference materials and sulphide ore sample Various certified reference materials (CRMs) were employed in this study in order to validate the developed microwave digestion protocol for sulphide ores and concentrates. CRMs include: copper concentrate CCU-1c (obtained from Canadian Centre for Mineral

and Energy Technology, CANMET), reference zinc–tin–copper–lead ore–mp-1a (CANMET), copper concentrate BGS100 (obtained from The British Geological Survey, BGS), reference zinc concentrate CZN-1 (CANMET), antimony ore CD-1 (CANMET) and reference lead concentrate CPB-1 (CANMET). A copper sulphide ore (provided by a mineral dealer) sample was also considered in this work to develop the method. The advantage of using this ore sample for method development is that it contained range of minerals including chalcopyrite, pyrite, pyrrhotite, sphalerite, quartz, calcite and minor content of other minerals. A representative sample was ground to a particle size of 100% passing 75 ␮m and manually homogenized for prolonged time. 2.3. Chemical reagents and standards All ICP-AES standards were prepared from ICP single element standard solutions (Aldrich–Sigma and Merck) after appropriate dilution with 10% HNO3 . For calibration, two sets of multielement standards containing all the analytes of interest at five different levels of concentration were prepared and measured at the suitable wavelength. The emission lines used are shown in Table 2. All the acids used were analytical grades with the following concentrations: nitric acid, HNO3 —69.5% (AnalaR® ), hydrochloric acid, HCl—33% (AnalarR® ), hydrofluoric acid, HF—40% (AristaR® ), boric acid, H3 BO3 —99.99% (AristaR® ). All solutions were prepared using Milli-Q ultrapure water (18.2 M cm−1 ). All the plastic and

Table 2 Limits of detection and wavelengths used for each element. Element

Wavelength (nm)

Machine detection limit (mg kg−1 )

Method detection limit (mg kg−1 )

Al As Ca Cd Cr Co Cu Fe Mg Mn Mo Ni Pb S Sb Zn Bi Mo

394 189 317.9 214.4 267.7 228 327.4 259 279 257.6 202 231.6 220.3 180.7 217 334 (206) 223.061 202.031

0.035 0.048 0.021 0.001 0.001 0.002 0.009 0.003 0.027 0.001 0.003 0.004 0.008 0.103 0.012 0.039 (0.0.036) 0.013 0.003

17.5 24.2 10.33 0.37 0.23 0.98 4.56 1.50 13.36 0.24 1.62 1.90 3.76 51.26 5.83 19.62 (18.22) 6.75 1.62

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Fig. 2. Summarized procedure for the total element digestion.

pressure achieved inside the vessels was about 27 bars. The temperature and pressure profiles during heating and cooling are shown in Fig. 1. After cooling to room temperature the contents of digestion vessels were quantitatively transferred to 100-mL polypropylene volumetric flasks and diluted to the mark with 10% (v/v) HNO3 .

glassware were cleaned by soaking in dilute HNO3 and then rinsed with ultrapure water prior to use.

2.4. Microwave digestion procedure Sample aliquots of about 200 mg were weighed to the nearest ±0.1 mg, transferred to digestion vessels and various combinations of acid mixtures were added. Vessels were heated in the microwave cavity choosing a power setting dependant on the number of samples (300 W for 1 or 2 samples, 600 W for 3 or 4 samples or 1200 W for more than 4 samples) according to the following programmed steps: ramp from room temperature to 200 ◦ C in 15 min; holding at 200 ◦ C for 30 min, then cool to room temperature. The maximum

3. Results and discussion 3.1. Optimization of acid volumes and combinations during microwave digestion In order to achieve complete dissolution of the ore samples prior to ICP-analysis, various acid combinations were tested. Firstly, 20 mL of concentrated HNO3 was added to 200 mg of the ore sample

Table 3 Metal concentration in CZN-1, CD-1, and CPB-1 certified reference materials. CZN-1 (N = 20) Min.

Max.

CD-1 (N = 23) Avg.

CV

Ref. value

% Recov.

Min.

4.40 4.92 3.48 2.29 2.91 1.18 1.38 3.74 2.32 2.18 2.32 4.12

0.132 0.026 0.179 0.132 0.144 10.93 7.45 30.20 44.74 0.193 0.219 0.025

97.8 100.9 92.1 96.6 97.9 98.2 98.9 96.7 98.6 93.0 94.9 119.2

– 0.62 – – – 2.61 – 2.93 – – – 3.25

Mass percentage (%) Al As Ca Cd Cu Fe Pb S Zn Mg Mn Sb

0.121 0.024 0.154 0.123 0.135 10.28 7.18 27.93 42.37 0.173 0.201 0.011

0.140 0.029 0.173 0.132 0.149 11.13 7.61 29.34 45.42 0.186 0.216 0.043

0.130 0.026 0.165 0.128 0.141 10.73 7.37 29.20 45.13 0.180 0.208 0.030

Max.

CPB-1 (N = 19) Avg.

CV

Ref. value

% Recov.

– 3.64 – – – 1.57 – 3.02 – – – 2.73

– 0.66 – – – 2.80 – 3.10 – – – 3.57

– 100.0 – – – 95.9 – 99.0 – – – 95.9

Mass percentage (%) – 7.31 – – – 2.99 – 3.48 – – – 3.86

– 0.66 – – – 2.69 – 3.06 – – – 3.43

Min.

Max.

Avg.

CV

Ref. value

% Recov.

3.632 5.511 2.544 2.377 3.188 2.280 1.917 2.217 1.840 2.585 2.042 2.94

0.148 0.056 0.629 0.014 0.254 8.430 64.74 17.80 4.420 0.09 0.039 0.360

100.4 104.5 99.4 102.2 101.1 98.4 96.1 98.8 95.8 96.8 96.3 94.7

Mass percentage (%) 0.138 0.053 0.586 0.014 0.241 7.705 60.12 17.01 4.034 0.082 0.036 0.319

0.158 0.065 0.647 0.015 0.270 8.513 64.05 18.32 4.359 0.091 0.039 0.357

0.149 0.059 0.626 0.015 0.257 8.295 62.21 17.59 4.234 0.088 0.038 0.341

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Table 4 Metal concentration in BGS100, CCU-1c, and MP-1a certified reference materials. BGS100 (N = 17) Min.

Max.

CCU-1c (N = 19) Avg.

CV

Ref. value

Elements below are measured in mg kg−1 As 61 103 77 15.3 94 Cd 4.8 7.5 5.8 10.9 5.2 Co 83 91 86 2.76 99 Cr 8.6 9.6 9.2 2.57 8.1 Mn 207 238 218 3.50 214 Ni 27 33 28 5.10 28 Zn 581 661 624 2.99 665 Bi – – – – – Mo – – – – – Elements below are measured in mass% As – – – – – Al 0.94 1.01 0.96 2.02 0.97 Ca 0.27 0.28 0.27 2.30 0.27 Cu 34.1 37.58 35.9 2.22 35.9 Fe 18.5 20.42 19.6 1.84 20.0 Mg 0.48 0.523 0.49 1.99 0.51 S 26.0 27.8 26.9 1.81 28.1 Zn Pb – – – – –

% Recov. 81.6 111.2 87.3 113.4 101.9 103.1 93.9 – –

– 99.1 99.6 99.8 97.9 97.0 95.7 –

MP-1a (N = 19)

Min.

Max.

Avg.

CV

Ref. value

% Recov.

133 16 28 120 10

143 19 31 121 13

138 18 29 120 11

2.11 3.8 2.5 1.8 7.2

136 18 30 120 11

101 97.1 98.0 97.9 105.5

– –

– –

– –

– –

– –

– 0.13 0.10 25.01 27.67 0.59 30.9 3.79 0.34

– 0.19 0.112 26.89 29.41 0.62 32.59 4.07 0.38

– 0.15 0.104 25.87 28.42 0.60 31.7 3.91 0.35

– 8.9 3.87 2.67 2.31 1.68 1.67 1.86 2.75

– 0.18 0.107 25.62 29.34 0.62 33.3 3.99 0.34

in the control vessel. The vessel was then heated in the microwave cavity according to the program described in Fig. 1. Concentrated nitric acid is a powerful oxidizing agent that forms soluble metal nitrates. However, in this work, nitric acid alone did not dissolve all minerals associated with the ore sample and a white residue remained. This result was in good agreement with previous studies [20,21] which show that HNO3 alone could not completely extract metals from peat or plant matrices, or samples with siliceous fractions. Acid mixtures composed of HNO3 in combination with another reagent such as HCl, H2 SO4 , H2 O2 or HClO4 have been investigated and found to be very effective in microwave digestion of biological and environmental samples [22–24]. However, for complete digestion of samples with alumina-silicate matrixes such as rocks, ores and other geological samples the use of hydrofluoric acid (HF) is a must for total digestion [25,26]. Therefore, in a second experiment, 5 mL of hydrofluoric acid (HF) was used in addition to 20 mL nitric acid (under the same heating and cooling conditions), and it was found that some residual remained which was noted to have a blue to black colour. In a third experiment the volume of HF was increased to 10 mL but again complete dissolution was not achieved. Concentrated HCl forms soluble chloride-complexes with most metals. Thus, in the fourth trial, 5 mL of concentrated HCl was added to a mixture of 10 mL HNO3 and 10 mL HF which then used as the digestion media. This last acid combination resulted in total dissolution with no visible residue. Due to the toxicity of HF, and the low recovery of rare earth metals extracted from plants due to excess fluoride [26] the quantity of HF was subsequently reduced to 5 mL. After digestion, boric acid (H3 BO3 ) was then added to the

Min.

Max.

Avg.

CV

Ref. value

% Recov.

– –

310 290

370 340

340 310

4.89 4.14

320 290

105.5 107.4

– 80.9 97.4 100.9 96.8 97.7 95.3 98.1 101.6

0.88

0.98

0.94

3.84

0.84

111.8

1.33

1.5

1.4

4.8

1.44

97.4

17.7 4.17

18.9 4.48

18.3 4.31

3.1 2.43

19.02 4.33

96.1 99.5

mixture to prevent the precipitation of fluorides [27]. The solution was then heated in the microwave system to 170 ◦ C within 15 min and kept for a further 15 min at this temperature. The procedure developed and used to digest samples is summarized in Fig. 2. 3.2. Method validation using CRMs The optimum digestion method described in the previous section was applied for five CRMs obtained from CANMET and one CRM obtained from BGS. In attempt to avoid errors related to matrix inconsistencies, reagent blank and calibration standards were prepared matrix matched to the sample solution. At least 17 separate samples from each CRM were analyzed. The results of elemental analysis for major elements Al, As, Ca, Cd, Cu, Fe, Pb, S, Zn, Mg, Mn, and Sb in the reference materials CZN-1, CD-1 and CPB-1 are given in Table 3. The results were validated in terms of their accuracy and precision. Accuracy was determined by comparing the measured concentrations with the certified values and was expressed as a percentage recovery (% recov.). The recoveries of all elements of interest were in the range from 95% to 105% except for Mg (93.1%), Ca (92.1%) and Sb (119.2%) in the CZN-1 reference material. It has been reported that elemental recovery of Mg in CRM of urban particulate matter after microwave digestion with HNO3 /HF/H2 O (20:1:20) was 92% [28]. A poor recovery of 45% also was reported for Mg in CRM of marine sediment using microwave-assisted digestion by the acid mixture (HNO3 :HF:HCl, 3:2:1) [21]. For calcium, a microwave digestion of marine sediment using only HNO3 was resulted in very low recovery of 38% [29]. Recoveries of 79% and 86% for calcium in

Table 5 Analysis of natural pure minerals as compared with CRM. Material Chalcopyrite crystal

Cu (%)

Fe (%)

S (%)

Theoretical 38–53 ␮m

34.62 34.84 ± 0.19

30.43 29.98 ± 0.0.04

34.95 35.59 ± 0.10

Zn (%) 0 0.14 ± 0.00

Chalcopyrite concentrate

38–53 ␮m

28.78 ± 0.27

29.84 ± 0.55

34.04 ± 0.42

0.07 ± 0.00

CRM CCU-1c

Analyzed Certificate

25.78 ± 0.20 25.62 ± 0.12

28.73 ± 0.28 29.34 ± 0.68

33.58 ± 0.41 33.30 ± 0.50

4.05 ± 0.06 3.99 ± 0.19

Sphalerite crystal

Theoretical 38–53 ␮m

– 0.03 ± 0.00

– 0.06 ± 0.00

32.90 31.73 ± 0.15

67.09 66.58 ± 0.48

CRM CZN-1

Analyzed Certificate

0.12 ± 0.00 0.144 ± 0.00

11.57 ± 0.07 10.93 ± 0.06

29.35 ± 0.20 30.20 ± 0.20

43.47 ± 0.32 44.74 ± 0.11

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CRMs of sewage sludge and marine sediment, respectively, has been reported [21]. These results show that recoveries for all elements under investigation were satisfactory. It was also shown that in terms of reproducibility, the coefficient of variation (CV) values obtained were lower than 5% except for As (5.5%) in CBP-1 CRM. Another set of CRMs (BGS100, CCU-1c and MP-1a) were analyzed for major and trace elements. The results of the analysis are shown in Table 4. For major elements, the recoveries range was 96–101% and the coefficient of variation values of <5% for all elements were produced except for Al (recovery of 80.9% and CV of 8.9%) in the CCU-1c reference material and As (recovery of 111.8%) in the MP-1a CRM. It has been reported that the percentage recovery of aluminum is sample matrix dependent; microwave-assisted digestion of sewage sludge CRM using the acid combination (HNO3 :HF:HCl, 3:2:1) resulted in percentage recovery of 121%. However, under the same condition the percentage recovery dropped to 93.2% for marine sediment CRM [21]. The analytical results for trace elements (As, Cd, Co, Cr, Mn, Ni, Zn, Bi, and Mo) in the three CRMs show that the percentage recovery ranged from 94% to 105% except for As (81.6%), Cd (111.2%), Cr (113.4%) in the BGS-100 CRM and Mo (107%) in the MP-1a CRM. The coefficient of variation values were also in the range ≤5%, except for As (15.3%) and Cd (10.9%) in the BGS-100 CRM. The volatility of arsenic (As) and its low concentration may be responsible for the relatively high CV value. In the present study, both the accuracy and precision determined for almost all elements analyzed by ICP-AES were acceptable with a recovery of 95–105% and precision of <5%. 3.3. Elemental composition in sulphide ore samples The developed method was used to digest and analyze various samples used by the authors in several hydrometallurgical studies [30–33]. Examples of these samples included high purity natural chalcopyrite crystals, high purity natural sphalerite crystals, and chalcopyrite concentrate prepared from a chalcopyrite ore by multiple stage froth flotation. To ensure the accuracy of the method, 3 replicates from a relevant CRM (CCU-1c for chalcopyrite and CZN-1 for sphalerite samples) were digested simultaneously with samples and the results of analyses were compared with the standard values as shown in Table 5. The analytical values of Cu, Fe, S and Zn for both chalcopyrite and sphalerite crystals were in comparatively good agreement with the theoretical values. More significantly, the percentage recoveries of the selected elements in CRMs (CCU-1c and CZN-1) were ranged from 97% to 105% with excellent reproducibility represented by CV values of ≤1.5%. 4. Conclusions Elemental analysis of sulphide ore samples has been successfully accomplished by a two-stage microwave acid digestion-prior to ICP-AES. A careful choice of suitable digestion procedures for sulphide ores is of great importance to assure that correct results are obtained. The method described, entails the use of acid

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combinations (HNO3 , HCl and HF) in the first stage and H3 BO3 in the second stage of microwave digestion prior to ICP-AES analysis. The accuracy and precision of the method was confirmed by crosscheck and comparison with six certified reference materials. The recovery (95–105%) and precision (5%) for both major and trace elements demonstrating the applicability of the digestion method combined with ICP-AES for elemental analysis in sulphide ores. The method described has considerable advantages including; it is inexpensive (moderate), it is simple and has a high degree of reproducibility. The required time for total digestion was less than 80 min for a total of 12 samples. References [1] M.D. Axelsson, I. Rodushkin, Journal of Geochemical Exploration 72 (2001) 81. [2] M.A. Belarra, M. Resano, F. Vanhaecke, L. Moens, TrAC Trends in Analytical Chemistry 21 (2002) 828. [3] T. Kántor, Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 1523. [4] C. Micó, L. Recatalá, M. Peris, J. Sánchez, Chemosphere 65 (2006) 863. [5] J. Bozic, D. Maskery, S. Maggs, H. Susil, F.E. Smith, Analyst 114 (1989) 1401. [6] S.R. Dams Melaku, L. Moens, Analytica Chimica Acta 543 (2005) 117. [7] M. Hoenig, A.-M. de Kersabiec, Spectrochimica Acta Part B: Atomic Spectroscopy 51 (1996) 1297. [8] M. Blanusa, D. Breski, Talanta 28 (1981) 681. [9] J.F. Parr, C.J. Lentfer, W.E. Boyd, Journal of Archaeological Science 28 (2001) 875. [10] R. Pöykiö, H. Torvela1, P. Perämäki, T. Kuokkanen, H. Rönkkömäki, Analusis 28 (2000) 850. [11] Y. Sun, P. Chi, M. Shiue, Analytical Science 17 (2001) 1395–1399. [12] J.L. Luque-Garcia, M.D.L. De Castro, TrAC Trends in Analytical Chemistry 22 (2003) 90. [13] A. Abu-Samra, S.J. Morris, S.R. Koirtyohann, Analytical Chemistry 47 (1975) 1475. [14] F.J.S. López, M.D.G. Garcia, N.P.S. Morito, J.L.M. Vidal, Ecotoxicology and Environmental Safety 54 (2003) 223. [15] J.M. Lo, H. Sakamoto, Analytical Science 21 (2005) 1181. [16] J. Sastre, A. Sahuquillo, M. Vidal, G. Rauret, Analytica Chimica Acta 462 (2002) 59. [17] S. Ayrault, P. Bonhomme, F. Carrot, G. Amblard, M.D. Sciarretta, L. Galsomies, Biological Trace Element Research 179 (2001) 177. [18] M. De la Guardia, V. Carbonell, A. Morales-Rubio, A. Salvador, Talanta 40 (1993) 1609. [19] S. Chemat, A. Lagha, H. Ait Amar, F. Chemat, Ultrasonics Sonochemistry 11 (2004) 5. [20] P.A. Tanner, L.S. Leong, S.M. Pan, Marine Pollution Bulletin 40 (2000) 769. [21] V. Sandroni, C.M.M. Smith, Analytica Chimica Acta 468 (2002) 335. [22] C.Y. Zhou, M.K. Wong, L.L. Koh, Y.C. Wee, Analytica Chimica Acta 314 (1995) 121. [23] R.E. Sturgeon, S.N. Willie, B.A. Methven, J.W.H. Lam, H. Matusiewicz, Journal of Analytical Atomic Spectrometry 10 (1995) 981. [24] I. Harrison, D. Littlejohn, G.S. Fell, Journal of Analytical Atomic Spectrometry 10 (1995) 215. [25] C.S.E. Papp, L.B. Fischer, Analyst 112 (1987) 337. [26] M. Krachler, C. Mohl, H. Emons, W. Shotyk, Journal of Analytical Atomic Spectrometry 17 (2002) 844. [27] A.G. Coedo, M.T. Dorado, I. Padilla, F.J. Alguacil, Journal of Analytical Atomic Spectrometry 13 (1998) 1193. [28] A. Robache, F. Mathé, J.-C. Galloo, R. Guillermo, Analyst 125 (2000) 1855. [29] P.A. Tanner, L.S. Leong, Analytica Chimica Acta 342 (1997) 247. [30] M. Al-Harahsheh, S. Kingman, Chemical Engineering and Processing: Process Intensification 46 (2007) 883. [31] M. Al-Harahsheh, S. Kingman, N. Hankins, C. Somerfield, S. Bradshaw, W. Louw, Minerals Engineering 18 (2005) 1259. [32] M. Al-Harahsheh, S. Kingman, S.M. Bradshaw, Hydrometallurgy 84 (2006) 1. [33] M. Al-Harahsheh, S. Kingman, F. Rutten, D. Briggs, International Journal of Mineral Processing 80 (2006) 205.