Highly accurate radiochemical neutron activation ... - Springer Link

Microchim Acta (2010) 168:37–44 DOI 10.1007/s00604-009-0252-1

ORIGINAL PAPER

Highly accurate radiochemical neutron activation analysis of arsenic in biological materials involving selective isolation of arsenic by hybrid and conventional ion exchange Ewelina Chajduk & Rajmund S. Dybczyński

Received: 5 August 2009 / Accepted: 31 October 2009 / Published online: 27 November 2009 # Springer-Verlag 2009

Abstract A highly accurate (definitive) radiochemical neutron activation analysis (RNAA) method was developed for the determination of traces of arsenic (As) in biological materials. It consists of the following steps: (a) irradiation in the nuclear reactor; (b) microwave-assisted sample digestion; (c) quantitative and selective radiochemical separation of arsenates on hydrated ferric oxide nanoparticles dispersed in a macroporous cation exchanger, preceded by a conventional strongly acidic cation exchanger column, and (d) gamma-ray spectrometric measurement of 76As. The suitability and accuracy of the method was demonstrated by analysing several certified reference materials. The detection limit is 8 ng g−1. The standard uncertainty in the determination of As in oriental tobacco leaves is around 3.4%. This, together with its compliance with several other formal requirements, makes the method comparable to primary methods based on isotope dilution mass spectrometry. Keywords Arsenic . Radiochemical neutron activation analysis . Highly accurate method . Hybrid ion exchanger

Introduction The poisonous properties of arsenic have been known for centuries. Obviously, its toxicity strongly depends on the chemical forms in which it is present; inorganic compounds Electronic supplementary material The online version of this article (doi:10.1007/s00604-009-0252-1) contains supplementary material, which is available to authorized users. E. Chajduk (*) : R. S. Dybczyński Institute of Nuclear Chemistry and Technology, Dorodna 16, PL 03-195 Warsaw, Poland e-mail: [email protected]

are more toxic (dangerous) than some organic species, and the trivalent arsenicals have more potent toxic properties than the pentavalent ones [1, 2]. Humans may be exposed to arsenic in food, water and air. Exposure may also occur through skin contact with soil or water that contains arsenic. An exposure to inorganic arsenic can cause various noxious effects, such as irritation of the stomach and intestines, decreased production of red and white blood cells, skin changes and lung irritation. It is suggested that the uptake of significant amounts of inorganic arsenic can intensify the chances of cancer development, especially the chances of developing skin cancer, lung cancer, liver cancer and lymphatic cancer. A very high exposure to inorganic arsenic can cause infertility and miscarriage in women, and it can cause skin disturbances, declined resistance to infections, heart disruptions and brain damage in both men and women. Finally, inorganic arsenic can damage DNA. A lethal dose of arsenic oxide is generally regarded as being 100 mg. Organic arsenic can cause neither cancer nor DNA damage, but exposure to high doses may cause certain effects to human health, such as nerve injury and stomach aches [1, 2]. Various arsenic species are usually determined by hyphenated techniques where HPLC is coupled to a suitable detection system such as ICP-MS or HG-AAS [3, 4]. However, accurate knowledge of the total As content is important in order to judge if all arsenic species have been determined and whether no gross errors occurred during speciation analysis. Thus, except for the identification of appropriate arsenic compounds, accurate determination of the total concentration is necessary. In the last few years different analytical techniques have been improved, nevertheless accurate determination of arsenic traces is still an important problem in analytical chemistry. The main reason is the low and very low concentration of arsenic in typical samples. Due to its volatility, sample

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digestion and analytical treatment may result in arsenic losses. Detection of low concentrations of As using various instrumental techniques is not devoid of interferences. Atomic absorption spectrometry with hydride generation (HG-AAS) is a well-known technique for the determination of trace amounts of arsenic, especially in small routine laboratories. In this technique, the possibility of inaccurate determination may occur as a result of interference from other elements forming hydrides. Also, a large concentration of metals could decrease the analytical signal from arsenic down to 65% of the true value [5]. Similarly, the volatile NOx gives the same effect, i.e. a lowered As signal. Arsenic has only one stable isotope, and hence arsenic cannot be determined by the isotope dilution mass spectrometry technique. The inductively coupled plasma mass spectrometry (ICP-MS) is increasingly applied to the determination of arsenic owing to the high sensitivity and high sample throughput. It is, however, extremely difficult to accurately determine arsenic in biological samples by ICP-MS due to isobaric overlap between the 75As and the 40 Ar35Cl molecular ion [6]. In various analytical procedures, to assure good sensitivity and to minimize the influence of interferences, the separation of As was carried out by anion exchange chromatography [6], for instance. In the case of determination of As by neutron activation analysis, one has to maintain the delicate balance between the desire to achieve the lowest possible detection limit and a safe working environment involving radioactive material. The Institute of Nuclear Chemistry and Technology (INCT) has already been organizing worldwide interlaboratory comparisons for more than 20 years. Data collected in 2003–2004 show that in the case of materials with low or very low concentrations of trace elements, assigning a certified (or even information) value for arsenic on the basis of results obtained from the participating laboratories was impossible [7]. In the case of Corn Flour (INCT-CF-3), 19 laboratories (out of a total of 92) sent results for As which varied between 0.367 and 220 ng g−1 [8]. Similarly for Soya Bean Flour, the arsenic results delivered were between 0.215 and 140 ng g−1 [9]. The highly accurate methods developed by the Institute of Nuclear Chemistry and Technology (INCT) are based on a combination of neutron activation with selective and quantitative separation of the determined element by column chromatography followed by γ-ray spectrometric measurement according to several rules described previously in detail [10, 11]. Also, each result must simultaneously fulfil several criteria before it is approved as obtained by this method [10, 11]. Usually, these methods are not intended for routine determinations, but rather for checking the accuracy of other analytical methods and certifying reference materials or proficiency testing samples. The high-accuracy radiochemical neutron activation

E. Chajduk, R.S. Dybczyński

analysis (RNAA) methods for the determination of cadmium [12], cobalt [13], copper [14] and selenium [15] in biological materials have been elaborated. The methods have detection limits of the order of ng g−1 and provide results traceable to SI units with very low values of uncertainty (the expanded uncertainty being less than 5%). Such values are characteristic of the methods of trace analysis of the highest metrological quality. The highly accurate RNAA procedures are also fully described mathematically, thus they have a potential to be termed Primary Methods of Measurements (PMM) [10, 11, 16]. In the past these methods found practical application in the process of certification of Polish certified reference materials produced by INCT [7, 17]. The intent of the present work is to provide a new, highly accurate method for As determination in biological samples at trace level. This method could serve as a primary method of measurement, which would be of significance considering that in the case of As, ID-MS cannot be used.

Experimental Reagents, ion-exchange resins, standards, radioactive tracers Lewatit SP 112 [H+] and Lewatit TP 208 [Na+], 20–50 mesh (Bayer-http://www.lewatit.com/ion/en/products/) were ground and sieved to get a fraction of the particle size of 0.10 mm≤ϕ≤0.12 mm. Dowex 50 WX4 [H+], 100– 200 mesh (Serva- www.serva.de) and Chelex 100 [Na+], 200–400 mesh (Bio-Rad Laboratories-www.bio-rad.com) were used as received. The following salts were selected to prepare appropriate column fillings: FeCl3 ×6 H2O, LaCl3 × n H2O (n ≈ 6), ZrCl4 and Ce(SO4)2. A standard of arsenic was obtained from 99.99%±0.01% As2O3, (Sigma) by weighing an appropriate amount of oxide, dissolving in concentrated ammonia, and weighing the solution obtained. As standards for irradiation were prepared by weighing aliquots of standard solutions in a PE capsule and evaporating to dryness before encapsulation. The following radioactive tracers were used in the elaboration and optimization of the separation scheme: 75Se (T1/2 =120 d), 59 Fe (T1/2 =44.5 d), 51Cr (T1/2 =27.7 d), 181Hf (T1/2 =42.4 d), 46 Sc (T1/2 =83.8 d), 86Rb (T1/2 =18.7 d), 65Zn (T1/2 =244 d), 133 Ba (T1/2 =10.5 y), 60Co (T1/2 =5.27 y), 134Cs (T1/2 = 2.06 y), 73As (T1/2 =80 d), 122Sb (T1/2 =65 h), 186Re(T1/2 = 3.8 d), where T1/2 is the half-life time of radionuclide. All tracers were prepared by neutron irradiation of spectrally pure oxides or salts (mostly nitrates) in a Polish nuclear reactor MARIA (neutron flux ~1014 n cm−2 s−1), except for 73 As which was produced in a Cracow cyclotron by nat Ge(p,xn)73–75As reaction. All reagents were of the highest grade commercially available. 18 MΩ cm grade water from a

Highly accurate radiochemical neutron activation analysis of arsenic

Milli-ORG Millipore Co. purification system was used throughout. Apparatus To prepare samples, CRMs and standards for irradiation, calibrated analytical and micro-analytical balances Sartorius MC5 and Sartorius BP221S were used. To digest samples, a microwave high-pressure system (Plazmatronika, Poland) was used. To perform gamma-ray spectroscopic measurements, the following detectors were used: -180 cm3 HPGe well-type (Canberra-www.canberra. com) with associated electronics (resolution 2.09 keV for 1,332 keV 60Co line, efficiency ca. 30%) coupled to the multichannel analyzer TUKAN (The Andrzej Soltan Institute for Nuclear Studies, Świerk, Poland), -255 cm 3 HPGe well type detector (Canberra) with associated electronics (resolution 2.15 keV for 1,332 keV 60 Co line, efficiency ca. 50%), coupled to the multichannel analyzer and spectroscopy software Genie-2000 (Canberra). Procedures Column filling preparation Column filling preparation consisted of the following steps: a) loading of Men+ onto the ion exchanger by passing the appropriate salt solution through the resin bed, b) desorption of Men+ with simultaneous precipitation of Men+ hydroxides within the gel and/or pore phase of the exchanger while passing through the resin bed of a 1– 2 mol L−1 NaOH solution. In addition, in the case of Lewatit SP 112 loaded with Fe (III) ions, desorption of Fe3+ with simultaneous precipitation of iron hydroxide was carried out with 5% NaOH and NaCl. An additional operation (rinsing and washing with a 50/50 ethanol-water solution followed by a mild thermal treatment (50–60 °C)), similar to that used earlier by DeMarco et al. [18] was also carried out. The hybrid sorbent thus obtained will be referred to as HFEIX sorbent henceforth. Column studies Column experiments were performed using glass columns of 6 mm I.D. containing the resin bed supported on a quartz wool plug or fritted glass disc. Arsenic tracer plus carrier was introduced at the top of the column in a small volume of initial solution (depending on experiment), next the remaining volume of the mobile phase was added. Most of the column investigations were carried out at room

39

temperature. In the case of checking the influence of temperature on the elution efficiency, special columns with water jackets were used. An initial solution containing arsenic radiotracer was loaded onto the column at room temperature. The column temperature was raised to 70 °C, and eluent solutions of appropriate composition and of 70 °C were passed through the column. The effluent was collected in fractions of several drops by means of a fraction collector and measured using the HPGe detector. The count rate measurements of the respective fractions were done using counting times in a live-time mode varying between 1 min and 15 min. Separation scheme and RNAA procedure Biological samples (150–200 mg), arsenic standards (10 μg) and a blank in PE vials were irradiated at a thermal neutron flux of 9.7×1013 n cm−2 s−1 for 1 h. After approximately 20–40 h of cooling, the samples were quantitatively transferred into special teflon vessels. Then 50 μg of As carrier (in arsenate form), 3 mL of concentrated HNO3 and 1 mL of concentrated HF were added. Digestion of the sample was carried out in a microwave high-pressure system under controlled conditions. The resulting solution was transferred into a teflon evaporating dish and evaporated to wet salts. The residue was dissolved in concentrated HCl and evaporated near to dryness; dissolution in concentrated HCl and evaporation was repeated twice. Finally, the sample was dissolved in 5 mL of 0.1 mol L−1 HCl and introduced onto the column with Dowex 50 WX4 [H+] (f=6 mm, h=6 cm), 100–200 mesh, equilibrated previously with 0.1 mol L−1 HCl. The column was washed with 12 mL 0.1 mol L−1 HCl; elements like sodium, iron, cobalt, scandium and hafnium stayed on the ion-exchanger. Next, the eluate was diluted to pH ≈ 1, and the solution obtained was quantitatively transferred onto the column (f=6 mm, h=5 cm) with HFEIX sorbent. The retained arsenate (V) was quantitatively eluted with 15 mL of 6 mol L−1 HCl. The arsenic content was determined by γ-ray specrometry via 559 keV line. Blank (empty PE container) and one or two standards were processed identically to the samples. One standard was measured directly (after bringing to the same geometrical conditions). Depending on the analyte content, the measurement time varied between 1,500 and 20,000 s.

Results and discussion Separation scheme One of the assumptions for highly accurate RNAA procedures is the quantitative and selective isolation of the

40

E. Chajduk, R.S. Dybczyński 100 As(V) retention [%]

determined elements from the irradiated sample by column chromatography. Thus, it is necessary to find an appropriate column filling and to elaborate the separation scheme with a guaranteed 100% recovery and very high radiochemical purity of the analyte fraction. It is well recognized that arsenic in the arsenate form is strongly sorbed by iron oxides and oxide hydroxides such as goethite and ferrihydrite or sorbents containing iron compounds [18–20]. Also, materials like zirconium oxide, zirconium-coated materials [21], lanthanide compounds [22] etc. are proposed for arsenic removal from aqueous solutions. The idea of arsenic separation on the sorbents chemically modified as described in the literature seemed the most promising for use during the elaboration of highly accurate RNAA methods. Because the hydrated (Men+) oxides or hydroxides are not suitable for column usage due to an excessive pressure drop and column blockage, preparation of a special sorbent, where suitable oxides or hydroxides are precipitated within the pore and/or gel phase of the ion exchanger, was tested. Three different resins were used as a base for the sorbents prepared: a strongly acidic, macroporous cation exchanger Lewatit SP 112, as well as chelating resins with iminodiacetic groups: Chelex 100 gel type and macroporous Lewatit TP 208. The resins were loaded with Fe (III), La(III), Zr(IV) and Ce(IV) ions respectively. Figure 1 presents the sorption of AsO43- on the prepared sorbents at pH=7. As can be seen, only iron and zirconium loaded resins assure quantitative arsenate sorption. Further column experiments have shown, that zirconium-loaded resins are suitable only for neutral solutions. In the case of ironloaded resins, independently of the ion-exchanger used and the way of preparation, AsO43− sorption was quantitative in the case of neutral and weakly acidic solutions. Several researchers emphasized the role of the Donnan Membrane Effect on the effectiveness of sorption of various ions on nanosized hydrated ferric oxide (HFO) particles dispersed within the resin phase and pores of conventional ion exchangers [23–25]. The Donnan Membrane Effect should

80 60 40 20 0 H2O

0.05 0.1 0.5 0.01 -1 HCl concentration [mol L ]

1.0

Fig. 2 Influence of HCl concentration on retention of As(V) on HFEIX sorbent. Column: 0.126 cm2 ×5 cm, eluate volume—20 mL

in general act towards the exclusion of anions from the cation exchanger phase and exclusion of anions from the cation exchanger phase [26]. Cumbal and SenGupta [23] observed an overwhelmingly better sorption of arsenates on HFO dispersed within the strongly basic Purolite A-400 resin than when HFO was dispersed within the resin phase of the strongly acidic Purolite C-100 cation exchanger (both exchangers being gel type). In our study we used macroporous sulfonic resin Lewatit SP 112 and did not observe any problems due to the Donnan Membrane Effect. It seems that in the case of macroporous resin with an average pore diameter of the order of 18.5 nm [27], the anions can diffuse freely into the pores of the macroporous strongly acidic cation exchanger, and in this case the Donnan Membrane Effect is not an obstacle. Perhaps more surprising is the fact that arsenates were also effectively sorbed on HFO dispersed within Chelex 100, which is a gel-type resin. Apparently, Fe(III) is strongly bound by the functional grouping of Chelex 100, so that the resin phase practically contains no ions and the Donnan Membrane Effect is then not operative. Sorption of arsenates probably occurs by ligand exchange with hydroxyl groups, which originally compensate the +1 charge of 1:1 Fe(III)-Chelex complexes.

100

Table 1 Recovery of arsenic retained on the column with Lewatit SP112-Fe. Column: 0.28 cm2 ×5 cm, eluate volume: 15 mL Lewatit TP 208-Zr

Lewatit TP 208-La

Chelex 100-La

Chelex 100-Zr

HFEIX sorbent

Lewatit TP 208-Ce

0

Eluent

Chelex 100-Ce

20

Lewatit SP 112-Fe

40

Lewatit TP 208-Fe

60

Chelex 100-Fe

arsenate(V) sorption [%]

80

Fig. 1 Sorption of arsenate (V) on prepared sorbents at pH=7. Column: 0.126 cm2 ×5 cm, eluate volume—20 mL

10% NaOH 20% NaOH 1 mol L−1 2 mol L−1 4 mol L−1 6 mol L−1

% of eluted arsenic

K2HPO4 HCl HCl HCl

Temp. 25°C

Temp. 70°C

86 90 40 55 96 100

80 – – 62 99 100


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Table 2 Confirmation of 100% efficiency of arsenic isolation Standard: radiotracer 73As+10 μg carrier in 10 mL 0.1 mol L−1 HCl

Stages of procedure

15,250±123

After microwave digestion Solution after sample evaporation and dissolution in 0.1 mol L−1 HCl Solution after column with Dowex 50 Wx4 Effluent from column Lewatit SP112-Fe Final elution 73As using 6 mol L−1 HCl

Radiotracer 73 As+10 μg carrier

Radiotracer 73As+10 μg carrier+ 250 mg inactive CRM CTA-VTL-2

– 15,230±123

15,190±123 15,300±124

15,200±123 0 15,270±124

15,240±123 0 15,200±123

Results are presented in the form of n±n1/2 , where n is the number of counts

Although the performance of Lewatit SP 112 with precipitated HFO prepared by simple NaOH treatment and the HFEIX hybrid sorbent prepared in a way similar to that recommended by DeMarco et al. [18] did not differ significantly, HFEIX was used in all further experiments and was incorporated into the RNAA procedure. As can be seen from Fig. 2, the retention of As(V) on HFEIX is quantitative even for acidic solutions up to HCl concentrations of 0.1 mol L−1. Several solutions were tested as possible eluents for 100% removal of the retained arsenate. Theoretically, the best situation would be if AsO43− could be quantitatively eluted, while the properties and structure of the sorbent used is kept intact. Attempts to achieve this goal proved unsuccessful. Elution with NaOH and K2HPO4 solutions do not change the sorbent structure, but the recovery of arsenic varies between 40 and 90%. This is in disagreement with the results obtained by

Fig. 3 RNAA procedure for arsenic determination in biological materials

DeMarco et al. [18], who reported quantitative elution of arsenates with 10% NaOH solution. In the case of strong acid solutions, the elution efficiency increases, but together with the analyte, iron (III) hydroxides are also removed from the cation exchanger. Table 1 shows the recovery efficiency for selected solutions at two temperatures: 25 °C and 70 °C. Only 6 mol L−1 HCl assured the quantitative recovery of arsenic, hence this solution was used in the final analytical procedure. During the elaboration of the separation scheme, the selectivity and quantitative isolation of arsenic was tested with the aid of various radioactive tracers added to several biological materials (inactive) and later also with the irradiated real samples. Tracer experiments using 73As confirmed practically 100% efficiency of isolation (Table 2). The final scheme of the elaborated method is presented in Fig. 3. To assure high purity of the isolated arsenic fraction, the column with the strongly acidic cation exchanger Dowex 50 WX4 [H+] was added to

Preparation of samples, standards, CRMs and blank for the irradiation

Sorption of arsenate (V) on the column with HFEIX sorbent Elution of As 6 mol L-1 HCl

Gamma-ray measurements of 76 As

Irradiation in nuclear reactor at the neutron flux ∼1x1014 n cm-2s-1 for 1 hour

arsenate elution

Separation of As from common cations, column: Dowex 50Wx4 [H+], (100-200 mesh)

Addition of non-active carrier of As, digestion of sample with 3 mL HNO3 +1 mL HF,

Removal of SiO2, evaporation to wet salts conversion of the obtained solution to chlorides

42


Table 3 Results of determination of As in the certified reference materials Certified reference material

Certified value and its confidence limits

Arithmetic mean* and its confidence limits

Apple leaves NIST 1515 Peach leaves NBS 1547 Mixed Polish herbs INCT-MPH-2 Lichen IAEA-336 Virginia tobacco leaves CTA-VTL-2 Oyster tissue NBS 1566a

38±7 ng g−1 60±18 ng g−1 191±23 ng g−1 630±82 ng g−1 969±69 ng g−1 14±1.2 μg g−1

34±7 ng g−1 (n=4) 56±10 ng g−1 (n=4) 180±15 ng g−1 (n=5) 625±30 ng g−1 (n=4) 968±64 ng g−1 (n=3) 15.2±0.6 μg g−1 (n=5)

*

Results are presented in the form of pffiffiffi x t0:05 s= n where x is the arithmetic mean, t0.05 the parameter of t-Student’s distribution for significance level α=0.05, n-1 degrees of freedom, s is the standard deviation, and n the number of individual results

the separation scheme. Arsenic present in dilute solutions of mineral acids in anionic form is not adsorbed by cation exchangers, whereas interfering cations are strongly retained under these conditions. Method validation The results of arsenic determination in biological CRMs obtained by the highly accurate RNAA method are presented in Table 3. The main principle formulated for these methods (which are usually planned as single element methods) is selective and quantitative post-irradiation separation of analytes. Figure 4 shows that the radiochemical purity of the arsenic fraction is very high. Except for radioarsenic, no other γ-emitters can be detected in the spectrum. The presence of β-emitter 32P is visualized by the increasing background in the low energy range of the spectrum (Bremsstrahlung effect). In the case of 76As, when measuring its gamma peak energy of 559 keV, the negative influence of radioactive 32P during measurements of isolated arsenic fractions is negligible.

As mentioned above, not every result produced by the elaborated procedure is automatically qualified as obtained by the highly accurate RNAA method. Each result classified as obtained by this method has to fulfil all previously defined criteria [10, 11, 14] simultaneously, among others the agreement of standards. The results obtained for standards which have passed the whole analytical procedure should agree within predetermined limits with those obtained by direct measurements. Figure 5 presents a comparison of the normalized analytical signal (expressed as net peak area) from two standards, for the last three series of analysis of As in biological samples. As can be seen, the measured values together with their confidence limits overlap in all cases. The detection limit calculated according to the Currie convention for NAA analysis [28] for a sample mass of 150 mg, a neutron flux of 9.7×1013 n cm−2 s−1, an activation time of 1 h, and a measurement time of 3 h, was 8 ng g−1.

55000

standard after analytical procedure standard measured directly

16000

76

net peak area

50000

As

14000

counts

12000 10000

45000 40000 35000

8000

30000

6000 4000 76

2000

25000

As

1

2

3

set of the results

0 25

225

425 625 energy [keV]

825

Fig. 4 Gamma ray spectrum of the separated 76As obtained from 150 mg Oyster Tissue NBS 1566a; measurement time 2,500 s

Fig. 5 Comparison of the normalized analytical signal (net peak area) for two standards, one of which has passed the whole procedure and the other was measured directly (in the same geometrical conditions). Results are presented in the form: n±3n1/2, where n is the number of counts, for the three sets of results


Evaluation of uncertainty budget The uncertainty budget takes into account all possible sources of uncertainty starting from sampling up to measurements. Recognition of the largest contribution defines the most sensitive step in the elaborated procedure, and can initiate appropriate corrections of the procedure. In the case of highly accurate RNAA methods, estimation of expanded uncertainty should prove the significance of these methods from a metrological point of view. For radiochemical neutron activation analysis, the sources of uncertainty (u) are divided into four groups [29]: ▪ preparation of samples, standards and monitors to the irradiation in the reactor u1 ; ▪ irradiation in the neutron flux in the nuclear reactor u2; ▪ radiochemical separations u3; ▪ gamma-ray spectrometric measurements u4. The detailed procedure of the evaluation of uncertainty budget is available as an electronic supplementary file. The combined standard uncertainty calculated according to the uncertainty propagation law amounted to 1.7%. The obtained value for As determination in CTA-OTL-1 is 543 ng g−1, combined standard uncertainty: 9.2 ng g−1, expanded uncertainty for coverage factor k=2 (a level of confidence of approximately 95%): 18.5 ng g−1. The final result with expanded uncertainty is equal to (543±19) ng g−1, where the certified value is (539±59) ng g−1.

Conclusions A new efficient method for the quantitative separation of arsenates from all elements except phosphates, utilizing a combination of the hybrid sorbent and conventional cation exchange resin was devised. This procedure was incorporated into the RNAA analytical scheme. As can be seen, the described method fulfils all the criteria for highly accurate RNAA methods. The presented method for arsenic determination delivers accurate results with very low levels of uncertainty, which is characteristic of the primary methods of measurement. The expanded uncertainty of ca. 3.4% for this and other definitive methods based on RNAA make these methods comparable with Isotope Dilution Mass Spectrometry [30]. In the case of monoisotopic elements, like arsenic, ID-MS cannot be used, and RNAA seems to be the only very accurate method available. It could be emphasized that the position of neutron activation analysis (NAA) among the analytical techniques is still strong despite some inherent weaknesses (the necessity of having access to the nuclear reactor, long analysis time etc.), and NAA is without doubt a valuable method in inorganic trace analysis, especially important in

43

the certification of reference materials and solving problems with quality control/quality assurance (QC/QA). The described high-accuracy RNAA method, is generally not suitable for routine analysis, but it is recommended for checking the accuracy of other techniques of inorganic trace analysis and for certification of new reference materials as well as assigning values in proficiency tests. Acknowledgements The authors would like to thank Dr. Agnieszka Jakowicz (Bayer Chemicals) for the donation of the Lewatit ion exchangers used in this study.

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22.

23.

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25.

26. 27.

28.

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