Optimization of a flow injection hydride generation atomic absorption spectrometric method for the determination of arsenic, antimony and selenium in iron chloridesulfate based water treatment chemical

Optimization of a flow injection hydride generation atomic absorption spectrometric method for the determination of arsenic, antimony and selenium in iron chloride/sulfate-based water tr

Optimization of a flow injection hydride generation atomic absorption spectrometric method for the determination of arsenic, antimony and selenium in iron chloride/sulfate-based water

treatment chemical Teemu Näykkia,∗, Paavo Perämäkia, Jyrki Kujalab, Anna Mikkonenb

aDepartment of Chemistry, University of Oulu, P.O Box 3000, FIN-90014 Oulu, Finland

bKemira Chemicals Oy, Oulu Research Centre, P.O Box 171, FIN-90101 Oulu, Finland

Received 26 September 2000; received in revised form 3 March 2001; accepted 19 March 2001

Abstract

The flow injection hydride generation technique together with atomic absorption spectrometry was used for the determi-nation of arsenic, antimony and selenium in the iron-based water treatment chemical FeClSO4 Thiourea, l-cysteine and potassium iodide–ascorbic acid were used as masking agents to diminish the interference caused by the very high iron con-centrations in the samples These reagents act also as prereductants for As(V) and Sb(V) Thiourea andl-cysteine did not prevent the signal depression caused by such high iron content, but potassium iodide–ascorbic acid eliminated iron interference well even up to 2500 mg Fe l−1 The limits of detection (LODS) in aqueous solutions containing no iron were 0.037␮g l−1, 0.121 g l−1and 0.131␮g l−1for As, Sb and Se, respectively The linear dynamic range was 0–10␮g l−1for As and 0–30␮g l−1 for Sb and Se The precision relative standard deviation was expressed as 2.6% for As, 4.4% for Sb and 2.9% for Se The precision determinations were done on the FeClSO4matrix at the level 0.5–0.8␮g l−1for the elements to be analyzed The accuracies of the methods were tested by using two standard reference materials (SRM 361, LA Steel and SRM 2074, river sediment) The concentrations obtained for As, Sb and Se were very close to the certified values © 2001 Elsevier Science B.V All rights reserved

Keywords: Atomic absorption spectrometry; Hydride generation; Flow injection; Arsenic; Antimony; Selenium; Iron interference; Water

treatment chemical

1 Introduction

It is commonly known that the transition metals

in-terfere in the determination of hydride-forming

ele-ments when the hydride generation (HG) technique is

used [1,2] Different mechanisms have been suggested

∗Corresponding author Present address: Finnish Environment

Institute (FEI), Research Laboratory, Hakuninmaantie 4-6,

FIN-00430 Helsinki, Finland Fax: +358-9-4030-0890.

E-mail address: teemu.naykki@vyh.fi (T Näykki).

for the interference effects observed [3,4] The pre-dominant mechanism is probably due to the reaction

of the interfering transition metal ions with the NaBH4

reductant, and the precipitate which is formed is able

to capture and catalytically decompose the evolved hydrides [3] For instance, Bax et al [4] and Bye [5] have stated that the precipitates are probably not el-emental metals, but rather metal borides Lugowska and Brindle [6] investigated the redox processes oc-curring in transition metal solutions during reduction

by NaBH4 Also they found, boron(III) as boride-like

0003-2670/01/$ – see front matter © 2001 Elsevier Science B.V All rights reserved.

PII: S 0 0 0 3 - 2 6 7 0 ( 0 1 ) 0 1 0 0 1 - 7

species causing the suppression of the signal in

hy-dride generation [6]

Iron is a common transition metal that is present

at high concentrations in many type of samples

However, only a few studies have been published

where the iron concentrations of the samples are as

high as in this work Narsito et al [7] found that

the interference caused by 200 mg l−1 iron was

re-moved when 1% thiourea was used as a prereductant

Wickström et al [8] studied hydride generation and

the complexation reactions in an alkaline sample

solution The hydrides formed were evolved by

sub-sequent acidification of the sample solution They

found that 0.3 mol l−1 tartrate completely removed

iron interference at the 500 mg Fe l−1level and when

the iron concentration was 5000 mg l−1, the signal

was depressed by only 15% However, dissociation of

the metal complex can occur during the acidification

of the sample Thus, the formation of free metal ions

and precipitation is possible Boampong et al [9]

used 3% l-cystine in 5 mol l−1 hydrochloric acid as

a masking agent to prevent the interferences of very

high concentrations of iron Welz and Sucmanova [10]

proposed the use of l-cysteine instead of l-cystine

as a masking agent Earlier Chen et al [11] had

ob-served thatl-cysteine removed iron interference up to

1000 mg l−1 even though the solution became turbid

due to the formation ofl-cystine when the iron(III)

concentration was greater than 200 mg l−1

Gener-allyl-cysteine has been proved to be very useful for

preventing iron interferences, but nearly all

previ-ous publications have focused on iron concentrations

below 1000 mg l−1 Finally, it is worth noting that

iron(III) has also been used for minimizing

interfer-ence effects of the other transition metals in the HG

technique The high positive potential of the reduction

of iron(III) to iron(II) suggests a preferential

reduc-tion of this species More seriously interfering metal

ions, such nickel(II) and copper(II), will be reduced

to the metals and precipitated only after all the iron

has been reduced [12,13]

The extent of the transition metal interference

de-pends very much on the HG system used Severe

in-terference was observed when we earlier used the

batch type hydride generation system described by

Siemer and Hagemann [14] The flow injection (FI)

technique is less prone to transition metal

interfer-ence [10,15] There are at least two main reasons for

this observation When using the FI system instead of the batch system, the concentration of the reductant is usually lower and formation of the interfering precip-itates, e.g borides, is decreased Another reason can

be called kinetic discrimination The reduction of the hydride-forming elements is fast and the reaction is completed before the reduction of the transition metal ion to the interfering species Also the separation of the hydrides from the sample matrix is very fast in a gas–liquid separator

In this study, the parameters for the determina-tion of As, Sb and Se from the PIX-110TMchemical (FeClSO4) using flow-injection hydride generation (FI-HG) were optimized The PIX-110TMwater coag-ulant is a FeClSO4solution manufactured by Kemira Chemicals Oy, containing approximately 12.0 wt.% (180 g l−1) of Fe3 +, 22.3% of SO

4 − and 7.5% of

Cl− Since As, Sb and Se are toxic, the European

Committee for Standardization (CEN) has laid down the maximum limits for these elements and therefore

it is essential to monitor their low amounts in water treatment chemicals [16]

2 Experimental

2.1 Instrumentation

A Perkin-Elmer model 5100 Zeeman atomic ab-sorption spectrometer equipped with a Perkin-Elmer model FIAS-400 flow injection system and an AS90 autosampler, controlled by Perkin-Elmer AA WinLab version 2.61 software, was used for the measurements Silicone pump tubes (NaBH4: 1.14 mm i.d.; carrier HCl: 1.52 mm i.d.; sample loading and waste: 3.17 mm i.d.) were used throughout this study, and all other tubing was 1 mm i.d PTFE An electrically heated quartz tube was used as an atomizer Electrodeless dis-charge lamps, operated from an external power supply (Perkin-Elmer EDL system 2) were used for the mea-surements The instrumental parameters are shown in Tables 1 and 2; they were mainly chosen according to the manufacturer’s recommendations

2.2 Reagents and standard solutions

All reagents were of analytical-reagent grade un-less otherwise stated Ultrapure water (18 M cm−1),

Table 1

AA spectrometer settings used for the hydride

generation-AAS-measurements

Sb: 217.6 Se: 196.0 Lamp current (mA) As and Sb: 400

Se: 280 Measured signal Integrated absorbance

Sb and Se: 18

Quartz cell temperature ( ◦C) 900

Quartz cell length/i.d (mm/mm) 180/7.5

Reaction coil length (mm) 300

Background correction No

prepared with a USF Elga Maxima purification

sys-tem, was used throughout All glassware and plastic

containers were soaked in (1+1) nitric acid overnight

and rinsed five times with de-ionized water and five

times with ultrapure water prior to use

The working standard solutions were prepared

daily by diluting the 1000 mg l−1 standard stock

so-lution of As(V) (Merck) and the 4000 mg l−1Sb(III)

solution (Merck) To prepare the selenium standard

stock solution (1000 mg l−1) appropriate amounts

of selenium salts NaHSeO3 (Merck, for Se(IV)) or

Na2SeO4·10H2O (Merck, for Se(VI)) were dissolved

in ultrapure water A few drops of concentrated

hy-drochloric acid were added before dilution

The iron stock solution was prepared from

PIX-110TM, because it was observed to contain less

arsenic than the commercial iron(III) chloride

stan-dard The 20,000 mg l−1stock solution of Fe(III) was

prepared by diluting 16,5289 g (1000 g ≈ 0.667 ml)

of PIX-110TMsolution (purity not certified) to 100 ml

with 4.7 mol l−1HCl.

Table 2

FIAS-400 flow injection program used for the hydride

generation-AAS-measurements

Flow injection program

Step Time (s) Speed of pump

1 (rpm)

Speed of pump

2 (rpm)

Sodium tetrahydroborate solution (0.1–0.3% (w/v)) was prepared daily by dissolving the appropriate amount of NaBH4powder (Fluka) in ultrapure water The solution was stabilized with NaOH (Merck) Hydrochloric acid (0.03–4.7 mol l−1, Merck) was

used in samples and as a carrier solution Different concentrations of HCl were used in the optimization

of the operating conditions

The stock solution of potassium iodide was pre-pared by dissolving 5 g of potassium iodide (Merck) in

100 ml of ultrapure water The ascorbic acid stock so-lution was prepared by dissolving 5 g ofl(+)-ascorbic acid (Merck) in 100 ml of ultrapure water

Thiourea stock solution (2.0 mol l−1) was prepared

by dissolving 38 g of thiourea, (J.T Baker, purity >

99%) in 250 ml of 1.5–4.7 mol l−1HCl.

l-Cysteine stock solution was prepared by dis-solving 5 g of l-cysteine (Merck, for biochemistry, purity > 99%) in 100 ml of 0.03–0.09 mol l−1 HCl.

Different concentrations of l-cysteine and/or HCl were used to optimize the operating conditions The standard reference materials (SRMs) 361 Low-alloy Steel and 2704 Buffalo River Sediment were obtained from the National Institute of Stan-dards and Technology (NIST, Gaithersburg, MD) The samples were prepared by dissolving 1 g of the SRMS in 30 ml of aqua regia (HCl:HNO3, 3:1 (v/v))

2.3 Procedure

The FI system used consisted of two peristaltic pumps: one was used for pumping the carrier and re-ductant solutions (HCl and NaBH4, respectively) and the other for pumping the sample solution (Fig 1) The carrier flow rate was 9 ml−1min and the

reduc-tant flow rate was 5 ml−1min The 500␮l sample loop was used in all measurements

The optimal instrumental parameters for As, Sb and

Se are given in Tables 3 and 4 Potassium iodide (0.5% (w/v)) and ascorbic acid (1.0% (w/v)) were added as prereductants for arsenic and antimony prior to anal-ysis The prereduction time used was 15 min Sele-nium(VI) was reduced to selenium(IV) by heating the sample in a beaker with HCl (4.7 mol l−1) at 90◦C for

20 min

The standard addition method was used when PIX-110TMwas analyzed Dilutions of 1–225 ml (As)

or 1–72 ml (Sb and Se) were made before analysis

Fig 1 Flow schematic of FIAS-400.

Iron was added to calibration standards to match

the iron content in the certified reference materials

when the accuracy of the method was tested A more

detailed description of the method optimization and

validation is given in the next section

Table 3

Optimal experimental conditions for the measurement of As and

Sba

values

HCl (carrier and samples) (mol l −1) 4.7 1.5

Carrier gas (Ar) flow rate (ml min −1) ∼65 ∼100

a 0.5% (w/v) KI, 1.0% (w/v) ascorbic acid, 0.3% (w/v) NaBH 4

in 0.05% (w/v) NaOH.

Table 4

Optimal experimental condition for the measurement of Se

values HCl (carrier and samples) (mol l −1) 4.7

Carrier gas (Ar) velocity (ml min −1) ∼65

Prereduction temperature ( ◦C) 90

Prereduction time (min) 20

(w/v) NaOH

3 Results and discussion

3.1 Selection of the prereduction agent 3.1.1 Arsenic and antimony

At the beginning of the study, the instrumental pa-rameters were optimized using only aqueous As stan-dards The same parameters were used for Sb because

of the similar chemical nature of As and Sb Opti-mum levels for carrier gas flow, reductant concentra-tion, carrier-HCl concentration and the concentrations

of the prereductants were investigated using two-level factorial designs Measurements were performed with-out an interfering iron matrix The results were pro-cessed with Modde for Windows version 3.0 software (Umetri AB) [17] Later, iron was added to the stan-dard solutions and the optimization was repeated using the parameters obtained from the previous stage The optimal concentrations of the various prereductants were studied using iron contents up to 5000 mg l−1.

When optimizing the prereduction conditions, the sample matrix contained 1500 mg l−1 of iron The

optimal concentration of the ascorbic acid was in-vestigated first and then kept at a constant level (0.8% (w/v)) when optimizing the KI concentra-tion (Fig 2A) When the sample matrix contained

5000 mg l−1 iron 1.0% (w/v) ascorbic acid gave the

Fig 2 The effect of KI and ascorbic acid concentrations in the determination of As, when the iron concentration of the samples was: (A) 1500 mg l −1 and (B) 5000 mg l−1 Arsenic concentration in the samples was 1␮g l −1 HCl concentration of samples and carrier was

4.7 mol l −1.

maximum sensitivity The concentration of ascorbic

acid was kept at a constant level and the optimal KI

concentration was found to be 0.8% (w/v) (Fig 2B)

The sensitivity obtained with thiourea andl-cysteine

was worse than that with KI Using an iron

concen-tration, <200 mg l−1 and l-cysteine as the masking

agent, the recovery of the signals of arsenic and

anti-mony were excellent, as, for example, Welz [10] and

Chen [11] have reported The use of concentrated iron

solutions caused a severe signal depression despite

the presence ofl-cysteine; using a 1500 mg l−1 iron

Fig 3 Variation of peak area for 1 ␮g l −1As as a function of prereduction time.

matrix, the depression of the As signal was ony 50%

of the signal measured without iron

The prereduction times needed with the different prereductants were investigated by measuring the re-sponse of 1␮g As(V) l−1solutions as a function of the

prereduction time (Fig 3) For each measurement the amount of As in the matrix/prereduction agents was subtracted from the results When thiourea was used, the prereduction of As(V) occurred immediately With l-cysteine and KI–ascorbic acid the maximum sensi-tivity was achieved within 15 min Fig 3 shows also

the influence of the prereductants on the sensitivity.

The highest sensitivity was obtained with KI–ascorbic

acid When KI–ascorbic acid was used a higher

sen-sitivity (7%) was obtained in the presence of an iron

matrix The reasons for this effect were not studied

further

Memory effects were investigated by performing

about 10 successive atomizations from the samples

containing 1␮g As l−1 and 5000 mg Fe l−1

Immedi-ately after, 10 successive atomizations were performed

without the iron matrix For each measurement, the

amount of As in the matrix/prereduction agents was

subtracted from the results Memory effects were not

observed in the presence of any prereduction agent

Once again a dramatic decrease in the sensitivity

was observed when using prereductants other than

KI–ascorbic acid When KI–ascorbic acid was used

in the presence of 5000 mg Fe l−1, the sensitivity was

only 14% lower than that for the aqueous As standard

Using thiourea or l-cysteine to prevent 5000 mg l−1

iron interference, depression of As-signals were over

75% compared to the signal measured without iron

The existence of the matrix effect was studied by

observing the slopes of the calibration graphs obtained

with and without the iron matrix The matrix effect

was observed when using thiourea and l-cysteine in

the presence of very high iron concentrations The

de-crease in the calibration slope caused by the iron

ma-trix was from 0.32 to 0.17 when thiourea was used

as prereductant A similar effect was observed

us-ing l-cysteine, but with KI–ascorbic acid the

mea-surements were practically free of the matrix effect

(change of slope 0.31–0.30)

3.1.2 Selenium

At first the optimal HCl concentration,

prereduc-tion time and prereducprereduc-tion temperature without an

interfering iron matrix were studied using two-level

factorial designs Selenium(VI) was used in these

experiments Prereductants, e.g thiourea, can reduce

Se(VI) to the elemental state and the formation of

hy-dride becomes impossible [18] Therefore, different

prereduction agents for selenium were not examined

The heating of the sample solution together with

hydrochloric acid has been observed to be the most

practical way to reduce Se(VI) to Se(IV) Using the

optimal parameters obtained from the previous stage,

the optimization was repeated in the presence of iron

Table 5 The factors and their levels used for the optimization of parameters for As and Sb by CCI design a

Factor Low level ( −) High level ( +)

Argon (ml min −1) 50 (75) 100 (125)

KI (% (w/v)) 0.1 (0.5) 1.0 (1.5) Ascorbic acid (% (w/v)) 0.1 (0.5) 1.0 (1.5)

a The star distance used was 0.35; values in brackets are for Sb The axial points of the design can be calculated using the equation: (mean level) ± [(high level) − (mean level)] × star distance.

The most suitable prereduction agent, KI–ascorbic acid, was used in the final optimization of the oper-ating conditions for As and Sb by central composite inscribed (CCI) design In this design, each factor had five-levels The low and high levels of the design are given in Table 5 The factor levels used for the opti-mization of Se parameters by CCI design are given in Table 6

3.2 Optimal parameters for As, Sb and Se

The final optimal parameters for As, Sb and Se are shown in Tables 3 and 4 The memory effects and the matrix effects for selenium were investigated us-ing the optimal parameters as previously described for

As and Sb As in the determinations of As and Sb,

a memory effect was not observed in the presence of

5000 mg Fe l−1 However, the matrix effect was

ob-served and, according to the results, the iron matrix increased the sensitivity of the selenium determina-tion by about 45% when the iron concentradetermina-tion of the samples was 2500 mg l−1 Using higher iron

concen-trations, the signals of the selenium determinations

Table 6 The factors and their levels used for the optimization of parameters for Se by CCI-design a

Factor Low level ( −) High level ( +)

a The star distance used was 0.50; the axial points of the design can be calculated using the equation: (mean level) ± [(high level)

− (mean level)] × star distance.

were depressed and the sensitivity was lower The

in-creased sensitivity was probably due to an oxidizing

and complexing effect of Fe(III) It is known that

el-emental chlorine can sometimes be produced in the

sample solutions According to the investigations

car-ried out by Krivan et al Se(IV) was slowly converted

to non-reactive Se(VI) due to back-oxidation by

ele-mental chlorine [19,20] If some iron is present in the

sample solution, it will form complexes with chloride,

thus preventing the formation of elemental chlorine

This may partly explain the better sensitivity obtained

for selenium in the samples-containing iron

The results also showed that when a lower

prereduc-tion temperature (50◦C) was used, the selenium signal

from the aqueous standard solution was greater to that

from the solution-containing iron When the

tempera-ture was raised to 70–90◦C, the selenium signal from

the iron solution was larger Thus, there is some

evi-dence that selenium may be lost more easily from an

aqueous solution (as a volatile chloride or hydride, for

example), than from the solution having an oxidizing

iron(III) Anyway, prereduction of selenium with HCl

is a common practice in HG [18]

To minimize the matrix effects, the determination of

selenium in PIX-110TMwas carried out using the

stan-dard addition method by adding 0.5–2.0␮g l−1Se(VI)

prior to prereduction

3.3 Validation of the methods

Detection limits (blank+3σ) and quantitation limits

(blank+ 10σ ) for As, Sb and Se were determined

in aqueous solutions that contained no iron matrix

The detection and quantitation limits are presented in

Table 7 The linear dynamic ranges for As, Sb and

Se (Table 8) were monitored in aqueous solutions

Precision and reproducibility was determined using

the standard addition method

Table 7

Detection limits (blank+3σ) and quantitation limits (blank+10σ )

in aqueous solutions for As, Sb and Se

Element Limit of detection

( ␮g l −1) Limit of quantitation(␮g l −1)

Table 8 Linear dynamic ranges for As, Sb and Se in aqueous solutionsa Element Linear range

( ␮g l −1) Calibration equation R

2 (n= 6) Arsenic 0–10 y = 0.3516x + 0.0557 0.9981

Antimony 0–30 y = 0.1173x − 0.0008 0.9997

Selenium 0–30 y = 0.1055x − 0.0307 0.9964

ay: integrated absorbance, x: concentration in␮g l −1.

3.4 Precision

The precision of the methods was tested using one-way variance analysis The determinations were performed according to the arrangement in Fig 4 The concentrations of As, Sb and Se in the samples were between 0.5 and 0.8␮g l−1 In the determination

of As the samples prepared contained 800 mg Fe l−1.

In the determination of Sb and Se the iron content of the samples was 2500 mg l−1.

The total variance of the results was composed of the residual variance and the influence of the day on which the measurement was made (s2

tot= s2 res+ s2 day)

A one-way F-test was used to test if the two variances

differed significantly from each other [21] According

to the F-tests, the variance arising from the

measure-ment day (s2

day) appeared to be significantly greater than the variance of the residual error for arsenic and selenium (Table 9) Considering the low concentra-tions of the element determined it was concluded that the precisions obtained (2–4% relative standard devi-ation, R.S.D.) were excellent

3.5 Reproducibility

The reproducibilities of the methods when car-ried out by more than one analyst were evaluated

Fig 4 Experimental arrangement used in the determination method precision.

Table 9

F-test values, the variance components and the precisions for As, Sb and Se using the experimental arrangement in Fig 4

Element Mean (n= 25)

( ␮g l −1) Variance of residualerrors (s2

res )

Variance of days (s2 day )

Precision F-test (MSdays /Ms res )

␮g l −1 R.S.D (%) Fcalc Ftheor(4, 20)a

a Degrees of freedom (numerator, denominator).

Fig 5 Experimental arrangement used in determination of

repro-ducibility.

according to Fig 5 The total variance of the results

is composed of the residual variance, the influence

of the day of measurement and the influence of the

analyst (s2

tot= s2

res+ s2

day+ s2 ana) According to F-tests

(Table 10), the variance arising from the

measure-ment day was significantly greater than the variance

of the residual error only for arsenic The effect of the

analyst was insignificant For antimony and selenium,

the only significant source of deviation was found to

be the residual errors Thus, the total variance of the

determinations equaled that of the residual error The

Table 11

The variance components and the overall reproducibilities for As, Sb and Se using the experimental arrangement presented in Fig 5

Element Mean value(n= 20) Variance of residual errors Variance of days Variance of analysts Reproducibility

(s2 res ) (s2

day ) (s2

ana ) ␮g l −1 R.S.D (%)

a ins.: insignificant.

Table 10

F-test values for As, Sb and Se using the experimental arrangement

in Fig 5

Element F-test (MSdays /MS res ) F-test (MSanalyst /MS days )

Fcalc Ftheor (2, 16)a Fcalc Ftheor (1, 2)a

As 29.48 3.63 5.3 × 10 −4 18.51

a Degrees of freedom (numerator, denominator).

variance components and the reproducibilities for As,

Sb and Se are presented in Table 11 The discrepancies observed in the results were obviously due to the small number of measurement days and analysts (two)

3.6 Accuracy

The accuracies of the methods were tested using the standard reference materials SRM 361 Low-alloy Steel and Buffalo Rivel Sediment SRM 2704 Three replicates were prepared for all determinations and 5–8 successive atomizations were performed from each sample The certified and measured concentrations are shown in Table 12 The results obtained are in good agreement with the certified values

Table 12

Analytical results of SRMs (mg kg −1) Mean± 95% confidence limits

Certified value Found Certified value Found Certified value Found

2704 Sediment 23.4 ± 0.8 22.8 ± 0.7 3.79 ± 0.15 3.77 ± 0.32 1.12 ± 0.05 1.09 ± 0.06

a Value uncertified.

Table 13

Limits for As, Sb and Se laid down by CEN for type 1 (highest

purity) iron chloride sulfate solutions

Element Limits for FeClSO 4 mg kg −1Fe(III)

3.7 Contents of antimony, arsenic and selenium in

PIX-110TM

The methods developed were used to measure the

As, Sb and Se concentrations in the PIX-110TM

The concentration of arsenic in the diluted sample

(1 ml/225 ml) was higher than the limit of

quantita-tion, and the arsenic content of 52␮g l−1was obtained

(0.29 mg kg−1Fe(III); calculated using a PIX-110TM

density of 1.5 g ml−1and Fe(III) content of 12 wt.%).

The measured concentrations for antimony and

sele-nium in diluted samples (1 ml/72 ml) were lower than

the limits of quantitation, so it was concluded that

results for selenium were<0.12 mg kg−1Fe(III) and

for antimony<0.10 mg kg−1Fe(III).

The European Committee for Standardization

(CEN) has laid down certain limits for toxic

sub-stances in different types of iron chloride sulfate

solutions used for the treatment of water intended for

human consumption The measured arsenic, antimony

and selenium concentrations in PIX-110TM were

lower than the limits shown in Table 13 and therefore

it fulfills the requirements for a type 1 (chemical of

highest purity) iron chloride sulfate solution [22]

4 Conclusions

KI–ascorbic acid was found to be the most

ef-fective prereductant/masking agent for As and Sb

interference when the iron content was very high The use of thiourea or l-cysteine did not prevent the signal depression caused by high iron content KI–ascorbic acid eliminated iron interference even

up to 2500 mg Fe l−1 The FI-HG system used was

found to be very sensitive and the samples could be diluted more than with the batch type hydride gener-ation system used earlier The absence of the memory effects also showed the excellent performance of the FI-system

The methods developed for As, Sb and Se are rapid, sensitive and can be used in routine analysis of sam-ples containing high iron concentrations The results obtained for the standard reference materials showed that the methods are accurate when very low analyte levels are measured in high iron matrix concentrations

Acknowledgements

The corresponding author is grateful to Kemira Chemicals Oy for financial support for this work

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