Comparison of pre-reducing agents for antimony determination by hydride generation atomic fluorescence spectrometry

Analytica Chimica Acta 511 (2004) 295–302

Comparison of pre-reducing agents for antimony determination by hydride generation atomic fluorescence spectrometry Ricard Miravet, José Ferm´ın López-Sánchez∗ , Roser Rubio Departament de Qu´ımica Anal´ıtica, Universitat de Barcelona, Avda. Diagonal 647, E-08028 Barcelona, Spain Received 18 November 2003; received in revised form 16 January 2004; accepted 4 February 2004

Abstract A systematic study of antimony reduction prior to its determination by hydride generation atomic fluorescence spectrometry (HG-AFS) was carried out. The efficiency of l-cysteine, potassium iodide and potassium iodide/ascorbic acid was studied for this purpose. The hydride generation step was optimised in the presence of those pre-reductors. From the results, l-cysteine was found to be the most suitable pre-reducing agent. Methodology was validated, obtaining detection limits lower than 90 ng l−1 and repeatability and reproducibility better than 3% R.S.D. and 5% R.S.D., respectively, in all cases. In order to evaluate the methodology developed and the influence of the matrix, recovery from waters from different sources was tested by HG-AFS and also by inductively coupled plasma mass spectrometry (ICP-MS). Accuracy was assessed by analysing three water reference materials at different antimony concentration levels. The high sensitivity of the developed methodology enables it to be applied for monitoring drinking waters according to the maximum admissible concentration of antimony established by the EU Directives. © 2004 Elsevier B.V. All rights reserved. Keywords: Antimony; Pre-reduction; Hydride generation; Atomic fluorescence spectrometry

1. Introduction Antimony has received relatively little attention since it is a non-essential element for life and also because its content in most matrices is very low [1]. Nevertheless, antimony is a cumulative element with similar chemical and toxicological properties to arsenic [2]. Thus, the US Environmental Protection Agency (EPA) considers antimony and its compounds as priority pollutants, and the European Union has established a maximum admissible concentration of antimony in drinking water of 5 ␮g l−1 [3]. The determination of antimony needs specific and sensitive analytical techniques. These must be simple and rapid. Among them, inductively coupled plasma mass spectrometry (ICP-MS) [4–8] and ICP atomic emission spectrometry (ICP-AES) [9] are techniques that offer good analytical performance in terms of response linearity, detection limits and selectivity. Atomic absorption spectrometry coupled to hydride generation (HG-AAS) has also been widely applied

∗

Corresponding author. Fax: +34-93-4021233. E-mail address: [email protected] (J.F. L´opez-S´anchez).

0003-2670/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2004.02.014

to antimony determination in several types of environmental samples [10–17]. In contrast, the application of electrothermal atomic absorption spectrometry (ETAAS) [18,19] is scarce, since this technique presents major difficulties involving being coupled “on-line” with sample introduction methods like hydride generation, which is extensively used in atomic spectroscopy. In the last few years, atomic fluorescence spectrometry coupled to hydride generation (HG-AFS) has received growing attention. Thus, the determination of Sb by HG-AFS, has been described in detail [18,20–23]. However, the lack of commercially available spectrometers has restricted the application of this analytical technique. Although the combination of hydride generation with AFS offers good detection limits and selectivity, these analytical figures of merit depend dramatically on the hydride generation step, where the difference in sensitivity obtained for the pentavalent and trivalent forms of antimony is well documented [24]. This difference is partly due to the slower reduction of the pentavalent form. In some studies, the different reduction potentials and hydride forming rates of Sb(III) and Sb(V) have been used for antimony speciation analysis [25]. However, the pre-reduction of Sb(V) to

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its trivalent state before hydride generation is mandatory when the total content has to be determined. Potassium iodide [26,27], alone or mixed with ascorbic acid [28], is the reagent most frequently used for this purpose. Nevertheless, a disadvantage of KI is the extremely high acid concentration required if the pre-reduction is to be completed rapidly. In addition, blank values can be increased at high reagent concentrations and they affect detection limits. Thus, Ulrich [9] used KI (1.2%, m/v) in a strong hydrochloric acid medium (5 mol l−1 ) for the pre-reducing process. Moreda-Piñeiro et al. [29] pre-reduced seawater samples by using 0.4 mol l−1 KI but also a high HCl concentration (6.5 mol l−1 ). De Gregori et al. [18] improved the procedure for Sb(V) pre-reduction in a soil sample solution by decreasing the HCl concentration to 1.5 mol l−1 . However, the reduction of Sb(V) to Sb(III) in the presence of 1.5% KI was not complete at room temperature. So, heating the solution to 90 ◦ C for 40 min in a water bath was necessary to perform the reduction in a short time. The instability of KI solutions has been also reported, so they must be freshly prepared before each run [30]. l-Cysteine has been proposed for pre-reducing Sb(V) by several authors [20,21,30,31]. l-Cysteine is particularly attractive for this purpose owing to its low toxicity and the low acid concentrations required in the hydride generation. According to Brindle and Le [32], a possible reaction between the thiol group of l-cysteine and the tetrahydroborate could lead to the formation of a complex between both species. This intermediate could alter the chemical pathway for the hydrid-generating reaction and could be related to the enhancement of signals observed for some hydride-forming elements compared with hydrochloric acid alone. It also can decrease interferences from transition metal ions [30–33]. Here we present a systematic study of antimony determination by HG-AFS, specially focused on the pre-reduction process. Since this step is essential for antimony determination by HG-AFS, the pre-reducing agents most frequently employed in previous studies were rigorously studied and compared. The experimental parameters that influence the pre-reduction and the hydride generation step were carefully optimised. The influence of time on the pre-reducing step was also examined. Quality parameters were assessed. Accuracy was established by analysing three water certified reference materials. The suitability of the proposed procedure for antimony determination was tested by analysing several water samples by HG-AFS and also by HG-ICP-MS.

2. Experimental 2.1. Apparatus The coupled systems HG-AFS and HG-ICP/MS were used in the measurements. In the HG-AFS measurements,

a continuous flow system was used for stibine (SbH3 ) generation. Hydride generation was performed with a Hydride Generator module from PS Analytical (Kent, UK), model 10.004. Detection was carried out in a PS Analytical Excalibur atomic florescence spectrometer equipped with a diffusion flame, a Sb boosted hollow cathode lamp (Super Lamp, Photron, Teknokroma), and a Perma pure drying membrane (Perma Pure Products, Farmingdale, NJ) for drying the generated hydride. A type A gas–liquid separator was also used in conjunction with the PSA Excalibur detector. Data acquisition of the signal from the spectrometer was performed with a microcomputer by using software (Avalon 2.0) from PS Analytical. A Perkin-Elmer ELAN 6000 inductively coupled plasma mass spectrometer was also used for total antimony determination. Hydride generation was carried out with a hydride generation system (Perkin-Elmer FIAS 400). A “ChemifoldTM ” gas–liquid separator was used together with a PTFE membrane, which prevent droplets being transported into the transfer line. Data acquisition of the FIA peaks was carried out with a microcomputer by using software (ELAN 2.3.1) from Perkin-Elmer. A Perkin-Elmer Elmer Optima 3200 RL inductively coupled plasma atomic emission spectrometer was used for standardisation of stock standard solutions of antimony. 2.2. Standard solutions and reagents All chemicals and reagents used were of analytical-reagent grade or higher purity and de-ionized water obtained from a Milli-Q system (Usf Purelab Plus, Ransbach Baumbach, Germany, 18.2 M
cm−1 ) was used throughout. Two 1000 mg l−1 stock standard solutions of Sb(III) and Sb(V) were prepared by dissolving appropriate amounts of potassium antimonyl tartrate (Fluka, Neu-Ulm, Switzerland) and potassium hexahydroxyantimonate (Riedel de-Haën, Seelze, Germany) in Milli-Q water and diluting to 100 ml. All stock standard solutions were stored in polyethylene bottles in a refrigerator at 4 ◦ C. Working solutions were prepared daily by diluting the stock standard solutions. These solutions were standardised using a standard reference material (NIST 3102a, antimony standard solution) by ICP-AES measuring at three resonance wavelengths of antimony (206.8, 217.6, and 231.2 nm). Sodium tetrahydroborate solutions were prepared daily from NaBH4 97% Purum. (Fluka) in NaOH·H2 O Suprapur (Merck, Darmstadt, Germany) aqueous solution. Different concentrations of sodium tetrahydroborate and/or NaOH were used in some experiments. Solutions of HCl were prepared from HCl fuming Pro-analysis 37% (Merck). Diluted solutions of different concentrations were obtained from this acid and assayed in the hydride generation step. Sb(V) was reduced to Sb(III) using KI (99.995% Merck, Suprapur) alone and mixed with ascorbic acid (99.7% Merck, Pro-analysis) or an l-cysteine solution (99.5% Fluka). All these solutions were prepared fresh daily.

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2.3. Samples and standard reference materials

Table 2 Instrumental conditions of the HG-ICP-MS system

Waters from several sources (tap, mineral, well, river freshwater) from the Barcelona area (Spain) were spiked with different concentrations of Sb(V) standard solution. The river freshwater was the sample with the most complex matrix since the river crosses a highly industrial area and had the following composition: CO3 2− , 404; SO4 2− , 194; Cl− , 509; Ca, 112; Mg, 30; Na, 314 (all in mg l−1 ). All the samples were stored in polypropylene bottles at 4 ◦ C until analysed. The standard reference material SRM 1640 from the National Institute of Standards and Technology (Gaithersburg, MD) as well as TMDA-54.3 and TM-28.2 from the National Water Research Institute (Burlington, Ontario, Canada) were analysed for method validation. The certified antimony values in the standard reference materials are 13.79 ± 0.42 ␮g Sb kg−1 and 25.1 ± 3.38 and 3.1 ± 0.74 ␮g Sb l−1 , respectively.

Hydride generation Sample flow rate (ml min−1 ) HCl flow rate (ml min−1 ) NaBH4 flow rate (ml min−1 ) HCl concentration, % (v/v) NaBH4 concentration, % (m/v) Ar flow rate gas–liquid separator (ml min−1 ) Loop (␮l)

2.4. Analytical procedures 2.4.1. HG-AFS determination Aliquots of antimony(V) standard solution were placed in 50 ml volumetric flasks to give final antimony concentration of 2 and 15 ␮g l−1 . The solutions were diluted to 1 ml with water, and finally diluted to the mark with the pre-reducing solution prepared by dissolving the appropriate pre-reducing agent in hydrochloric acid. The pre-reducing solution must be in the same acid medium as that used in the hydride generation step. Finally, the solutions were immediately analysed by HG-AFS. The measuring conditions of the HG-AFS system are reported in Table 1, in which HG conditions (reagent concentration and flows), as well as detection conditions, are reported.

Table 1 Instrumental conditions of the HG-AFS system

ICP-MS Forward rf power (W) Auxiliary Ar flow (l min−1 ) Nebulizer Ar flow (ml min−1 ) Integration time (s) Isotope monitored Measuring time (s) Signal processing

7.6 6 ml min−1 3.6 ml min−1 10 0.2 in NaOH 0.05 % (m/v) 600 200 1000 1.2 600 0.4 121 Sb 12 Peak area

2.4.2. HG-ICP-MS determination An appropriate aliquot of the antimony(V) standard solution was diluted to 1 ml with water. The solution was finally diluted to 10 ml with the pre-reducing solution prepared following the same procedure as for HG-AFS determination. All the measuring solutions were prepared in 10% (v/v) hydrochloric acid, the same acid concentration as used in the hydride generation step. The solution was immediately analysed by HG-ICP-MS. The operating conditions are given in Table 2. 2.4.3. Sample procedure Mineral and tap water were not subjected to any pre-treatment. Well water and river freshwater were filtered through a 0.45 ␮m nylon filter (Osmonics, USA) and then through a C18 cartridge (Lida, USA) pre-conditioned with 5 ml of methanol and then with 10 ml of water. 1 ml of sample was spiked with between 2 and 5 ␮g l−1 antimony(V) standard solution and then diluted to 1.5 ml with water in the HG-ICP-MS determination. The solution was finally diluted to 10 ml with the pre-reducing solution. In the HG-AFS determination, 4 ml of sample was spiked with 0.1 ml of antimony(V) standard solution to give final concentration of 5 and 10 ␮g l−1 antimony. The solution was finally diluted to 20 ml with the pre-reducing solution.

Hydride generation Sample flow rate (ml min−1 ) HCl flow rate (ml min−1 ) NaBH4 flow rate (ml min−1 ) HCl concentration rangea (mol l−1 ) NaBH4 concentration, % (m/v) Ar flow rate (ml min−1 ) Air flow rate (l min−1 )

7.6 7.6 3.2 0.5–3.5 1–1.5 300 2.5

Injection program Delay time (blank) (s) Rise time (sample) (s) Measure time (sample) (s) Memory time (blank) (s)

10 20 20 30

3.1. Hydride generation in the presence of pre-reducing agents for antimony determination by HG-AFS

Atomic fluorescence detector Lamp primary energy (mA) Lamp boost energy (mA) Signal processing

17.5 15 Peak height

Conditions were optimised for the HG-AFS determination in order to obtain the maximum yield of Sb hydride, as well as to generate the hydrogen that maintains the flame in the AFS detector. Flow rates of 7.6 ml min−1 for the acidic solution channel, and 3.2 ml min−1 for the NaBH4 solu-

a

According to the pre-reducing agent studied.

3. Results and discussion

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tion channel, were used. Sb(V) standard solutions at two concentrations were used to optimise the recovery in antimony hydride generation. Standard solutions of Sb(V) were pre-reduced separately with l-cysteine, potassium iodide and potassium iodide with ascorbic acid just before the measurement to assess the influence of the pre-reducing agent on the hydride generation step. All the pre-reducing agents were studied at two concentrations. Three independent replicates of solutions containing 2 and 15 ␮g l−1 of Sb(V) were analysed under every condition tested. The net fluorescence signal was compared with that obtained for Sb(III) standard solution at the same concentration which was considered as a reference since this species generates stibine quantitatively. Solutions of NaBH4 ranging from 1 to 1.5% (m/v) in NaOH 0.1 mol l−1 were studied in order to establish the maximum fluorescence signal. A lower NaBH4 concentration extinguished the flame while a higher concentration made it unstable. The effect of HCl concentration was also studied. In the range 2.5–3.5 mol l−1 when using KI and KI + ascorbic acid and in the range 0.5–2 mol l−1 when using l-cysteine. As an example, the effect of NaBH4 and HCl concentration on the relative net fluorescence signal intensity of 15 ␮g l−1 Sb(V) solution for each pre-reducing agent tested is shown in Fig. 1. The relative net intensity was calculated as the ratio between the signal of an Sb(V) and a Sb(III) standard solution at the same concentration. The Sb(III) standard solution for measurement was also treated with pre-reducing agent in order to establish a possible influence of the pre-reducing agent on its determination, but no effect was observed. The relative net intensity of Sb(V) increased with increasing acid concentration in all cases (Fig. 1a–c); 3.5 mol l−1 HCl gave the highest results when using KI and KI + ascorbic acid and 2 mol l−1 HCl was the best suitable acid concentration in the presence of l-cysteine. The highest relative net fluorescence signal intensity and the best overall performance were obtained for the highest concentration of sodium tetrahydroborate when using l-cysteine (Fig. 1c). In contrast, NaBH4 at 1.3 and 1.5% (m/v) gave similar results in the presence of the optimum concentration of HCl (3.5 mol l−1 ) when using KI and KI + ascorbic acid (Fig. 1a and b). Thus, 1.3% NaBH4 was found to be optimum for those pre-reducing agents as a slight fluorescence signal instability was observed at the highest sodium tetrahydroborate concentration tested. The influence of the pre-reducing agent concentration on the hydride generation optimised conditions is also shown in Fig. 1. In all cases, similar results were obtained for both pre-reducing agent concentrations tested. So, 0.5% (m/v) l-cysteine, 1% (m/v) KI, and 1% (m/v) KI 1% + 0.2% (m/v) ascorbic acid were used for further investigations in order to avoid high blank signals and to decrease the overall reagent consumption. According to these results, Sb(V) pre-reduction is quantitative when using any of the pre-reducing agents tested under the optimised conditions. However, by using l-cysteine the hydrochloric acid consumption is substantially reduced.

These optimised hydride generation and pre-reducing conditions were adopted for further investigations in Sb determination by HG-AFS. 3.2. Contribution of pre-reducing agents to the background signals In the HG-AFS measurements it is difficult to evaluate the contribution of the pre-reducing agents to the background signal, which is of paramount importance when determination at sub microgram per litre levels is mandatory. This difficulty lies in the characteristics of the continuous fluorescence signal and the almost imperceptible increase of the signal observed when the pre-reductor is introduced as blank. The high sensitivity and selectivity of HG-ICP-MS and its transient signal enable the contribution of the pre-reducing agent to the background signal to be assessed. In this study, all the pre-reducing agents studied in the Sb determination by HG-AFS were tested by HG-ICP-MS at low nanogram per litre level. The blank values for the pre-reducing agents tested and their efficiency at ultratrace levels was assessed by establishing the signal-to-noise ratio obtained in the measurement. Hydride generation conditions were adopted in the measurements according to the conditions suggested by the HG-ICP-MS instrumentation manufacturer. Then, 10% (v/v) HCl and 0.2% (m/v) NaBH4 in 0.05% (m/v) NaOH were used in all the conditions tested. The signal of antimony was monitored at mass 121 (57.25% isotopic abundance) without any isobaric or polyatomic interference. Peak area was used for quantification. Three independent replicates of solutions containing 50, 100, and 200 ng l−1 Sb as Sb(V) were pre-reduced separately with l-cysteine, KI, and KI + ascorbic acid just before the analysis. All the pre-reducing agents were tested at two concentrations. The peak areas measured for Sb(V) standard solutions were compared with those obtained for Sb(III) standard solutions at the same concentration. No difference was observed, so quantitative pre-reduction was obtained when using any of the pre-reducing agents studied as was also observed by HG-AFS measurements. Fig. 2 shows the results obtained for all the conditions tested. The best signal-to-noise ratio was obtained by using l-cysteine at any antimony concentration tested. Similar results were obtained by using 0.5 and 1% l-cysteine. Therefore, the lower concentration was chosen. These results were in agreement with those obtained in Sb determination by HG-AFS. l-Cysteine was the pre-reducing agent which gave the best signal-to-noise ratio, working even at the ng l−1 level. This behaviour agrees with that reported by several authors mentioned previously [20,21,30,31], although such behaviour is in contrast with the information reported by Welz and Sucmanová [30] about the suitability of l-cysteine as a pre-reducing agent.

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Fig. 1. Sb determination by HG-AFS. Optimisation of the HG and the pre-reducing steps in the presence of (a) KI; (b) KI + ascorbic acid; and (c) l-cysteine.

3.3. Influence of pre-reducing time in the measurement In all the conditions tested, the standard Sb(V) solutions were pre-reduced just before the analysis. The assessment of the influence of the time lapsed between the addition of the pre-reducing agent and the measurement could be essential, especially at high antimony concentrations. For this purpose, two Sb(V) standard solutions, 50 and 100 ␮g l−1 , were pre-reduced with l-cysteine, KI

and KI + ascorbic acid separately and analysed in triplicate by HG-AFS at different times (up to 30 min) after preparation. All measurements were performed under the optimised hydride generation conditions for all the pre-reducing agents tested. From the results, no influence of the pre-reducing time was observed on the fluorescence signal intensity for all the pre-reducing agents studied. No difference was obtained at the two concentration levels tested. Most probably, Sb(V) was instantly

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Fig. 2. Signal-to-noise ratio in Sb determination by HG-ICP-MS in the presence of pre-reducing agents.

reduced to Sb(III) under the pre-reducing conditions tested. 3.4. Method validation The method was validated by establishing the quality parameters for all the pre-reducing agents studied at their optimum concentration. The measurements were done under hydride generation optimised conditions. Linear range, verified using peak height from the fluorescence signal was assessed. Linearity was proved in a range wider than 500 ␮g l−1 for all the pre-reducing agents studied. Detection and quantification limits were calculated by analysing, in triplicate, 10 blank solutions containing both hydrochloric medium and the pre-reducing agent. The regression line was calculated from the mean values of the peak heights. The concentrations at the detection limit were calculated from 3σ of the background signal and then referred to those regression lines (Table 3). Detection limits at the low nanogram per litre level were obtained for all the pre-reducing agents studied. These values can be considered satisfactory and show that the established conditions

permit Sb determination at very low concentration levels by HG-AFS. Precision in terms of repeatability and reproducibility was also established. Repeatability was calculated as the %R.S.D. from five measurements of two independent standard solutions containing Sb at concentrations of about 5 and 10 times those of the detection limits, respectively. Repeatability was better than 3% R.S.D. in all cases. Reproducibility on three non-consecutive days was also assessed. Two independent standard solutions containing Sb at a concentration about 5 and 10 times those of the detection limit were measured five times each day. Reproducibility was calculated as the %R.S.D. from all the measurements made over the 3 days. Reproducibility was better than 5% R.S.D. in all cases. 3.5. Recovery studies in water samples Several water samples were analysed to evaluate the methodology developed in this study and to assess the influence of the matrix. Waters from several sources (mineral, tap, well (I and II) and river) were spiked at two different antimony concentrations and then analysed by HG-AFS

Table 3 Quality parameters for antimony determination by HG-AFS with a pre-reducing step Quality parameter

(␮g l−1 )a

l-Cysteine 0.5%

KI 1%

KI 1% + ascorbic acid 0.2%

0.5 1 0.5 1

37 107 2.0 0.8 3.2 1.4

13 91 2.0 0.4 4.1 1.6

87 160 0.8 1.0 2.7 1.7

(ng l−1 )

Detection limit Quantification limit (ng l−1 ) Repeatabilityb (%R.S.D.) Reproducibilityc (%R.S.D.) a b c

Concentration of the Sb(V) standard solution measured. Calculated as the %R.S.D. from 5 measurements. Calculated as the %R.S.D. from 15 measurements.

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Table 4 Recovery studies in spiked water samples Sample

HG-AFS

HG-ICP-MS

(␮g l−1 )a

l-cysteine recovery (%)

KI recovery (%)

KI + ascorbic acid recovery (%)

(␮g l−1 )a

l-Cysteine recovery (%)

Well water I

5 10

103 ± 1 94 ± 4

91 ± 3 96 ± 2

94 ± 3 92 ± 1

2 5

102 ± 1 102 ± 4

98 ± 1 99 ± 3

88 ± 6 104 ± 1

Well water II

5 10

101 ± 2 101 ± 5

95 ± 6 95 ± 3

90 ± 1 92 ± 1

2 5

95 ± 4 102 ± 2

108 ± 1 106 ± 4

88 ± 4 106 ± 2

River water

5 10

105 ± 1 97 ± 1

94 ± 1 97 ± 1

92 ± 2 94 ± 1

2 5

99 ± 3 102 ± 2

107 ± 3 102 ± 1

85 ± 6 103 ± 2

Tap water

5 10

97 ± 6 98 ± 1

98 ± 5 98 ± 3

92 ± 1 89 ± 3

2 5

96 ± 2 107 ± 1

105 ± 3 103 ± 1

92 ± 1 106 ± 2

Mineral water

5 10

97 ± 4 98 ± 3

95 ± 2 92 ± 1

88 ± 1 90 ± 1

2 5

96 ± 3 101 ± 1

101 ± 3 105 ± 4

89 ± 1 101 ± 1

a

KI recovery (%)

KI + ascorbic acid recovery (%)

Sb(V) standard solution added.

and also by HG-ICP-MS. The absence of antimony was verified in well water II and also in mineral water when measuring by both analytical techniques. On the contrary, the presence of Sb was found in well water I, tap water and river water. River water showed the highest concentration of Sb (12 ␮g l−1 ). In HG-AFS determinations, the spiked concentrations were 5 and 10 ␮g l−1 Sb as Sb(V). Water samples spiked with 2 and 5 ␮g l−1 Sb(V) were also analysed by HG-ICP-MS. In both cases, the water samples were pre-reduced with each of the pre-reducing agents tested and were analysed in triplicate. The results expressed as the %recovery ± σ are shown in Table 4. Good recovery from all the water samples spiked at both concentration levels was found when using l-cysteine for pre-reducing Sb(V). Even in the most complex matrix (river water), l-cysteine gave recoveries close to 100% measured by both analytical techniques. However, lower recoveries were found when using potassium iodide/ascorbic acid, especially at low concentrations. 3.6. Accuracy This study was carried out by using only 0.5% l-cysteine as pre-reducing agent, since it was considered the most appropriate reagent for this purpose according to all the results obtained in the present work. Three water certified reference materials NIST SRM 1640, NWRI TMDA-54.3, and TM-28.2, were measured by HG-AFS and HG-ICP-MS in order to assess accuracy. The materials were analysed in triplicate, in order to establish the precision of the measurement. Results and standard deviations are presented in Table 5. For the three water certified reference materials measured and both analytical techniques used, the antimony concentrations quantified match the certified values considering the associated uncertainties. A direct comparison between the data reported in

Table 5 Determination of Sb in water certified reference materials Certified value (␮g l−1 ) TMDA-54.3 25.1 ± 3.38 TM-28.2 3.1 ± 0.74 NIST SRM 1640 13.79 ± 0.42a a

HG-AFS

HG-ICP-MS

Quantified Sb (␮g l−1 ) (n = 3)

Quantified Sb (␮g l−1 ) (n = 3)

25.0 ± 0.8 3.0 ± 0.3 14.1 ± 0.4

23.4 ± 0.6 2.8 ± 0.1 13.5 ± 0.6

Expressed as (␮g kg−1 ).

Tables 4 and 5 seems to indicate that HG-AFS gives results slightly higher than those obtained by HG-ICP-MS in the CRMs measurements, whereas the reverse appears to be the case in the recovery study. A two-tailed Student t-test was applied in order to check if there were significant differences between the results obtained by both techniques. Only in the case of the results obtained for the TMDA-54.3 reference material a significant difference was observed. However, when both results were compared with the certified value and its associated uncertainty, the accuracy obtained by both analytical techniques was similar.

Acknowledgements We thank the DGICYT (BQU 2003-02951) for financial help received in support of this study. The authors also thank Dr. Antoni Padró, for his help with the HG-ICP-MS measurements. R. Miravet wishes to thank the Universitat de Barcelona for support through a pre-doctoral grant.

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