Arsenic in marine tissues — The challenging problems to electrothermal and hydride generation atomic absorption spectrometry

Spectrochimica Acta Part B 62 (2007) 258 – 268 www.elsevier.com/locate/sab

Arsenic in marine tissues — The challenging problems to electrothermal and hydride generation atomic absorption spectrometry☆ Irina B. Karadjova a , Panayot K. Petrov a , Ivan Serafimovski b , Trajče Stafilov c , Dimiter L. Tsalev a,⁎ b

a Faculty of Chemistry, University of Sofia, 1 James Bourchier Blvd., Sofia 1164, Bulgaria Food Institute, Faculty of Veterinary Medicine, Sts. Cyril and Methodius University, P.O. Box 95, MK-1000, Skopje, Macedonia c Institute of Chemistry, Faculty of Science, Sts. Cyril and Methodius University, P.O. Box 162, MK-1000, Skopje, Macedonia

Received 5 September 2006; accepted 11 October 2006 Available online 21 November 2006

Abstract Analytical problems in determination of arsenic in marine tissues are addressed. Procedures for the determination of total As in solubilized or extracted tissues with tetramethylammonium hydroxide and methanol have been elaborated. Several typical lyophilized tissues were used: NIST SRM 1566a ‘Oyster Tissue’, BCR-60 CRM ‘Trace Elements in an Aquatic Plant (Lagarosiphon major)’, BCR-627 ‘Forms of As in Tuna Fish Tissue’, IAEA140/TM ‘Sea Plant Homogenate’, NRCC DOLT-1 ‘Dogfish Liver’ and two representatives of the Black Sea biota, Mediterranean mussel (Mytilus galloprovincialis) and Brown algae (Cystoseira barbata). Tissues (nominal 0.3 g) were extracted in tetramethylammonium hydroxide (TMAH) 1 ml of 25% m/v TMAH and 2 ml of water) or 5 ml of aqueous 80% v/v methanol (MeOH) in closed vessels in a microwave oven at 50 °C for 30 min. Arsenic in solubilized or extracted tissues was determined by electrothermal atomic absorption spectrometry (ETAAS) after appropriate dilution (nominally to 25 ml, with further dilution as required) under optimal instrumental parameters (pyrolysis temperature 900 °C and atomization temperature 2100 °C) with 1.5 μg Pd as modifier on Zr–Ir treated platform. Platforms have been pre-treated with 2.7 μmol of zirconium and then with 0.10 μmol of iridium which served as a permanent chemical modifier in direct ETAAS measurements and as an efficient hydride sequestration medium in flow injection hydride generation (FI-HG)–ETAAS. TMAH and methanol extract 96–108% and 51–100% of As from CRMs. Various calibration approaches have been considered and critically evaluated. The effect of species-dependent slope of calibration graph or standard additions plot for total As determination in a sample comprising of several individual As species with different ETAAS behavior has been considered as a kind of ‘intrinsic element speciation interference’ that cannot be completely overcome by standard additions technique. Calibration by means of CRMs has given only semi-quantitative results. The limits of detection (3σ) were in the range 0.5–1.2 mg kg− 1 As dry weight (wt.) for direct ETAAS analysis of extracts in both TMAH and MeOH. Within-run precision (RSD%) was 5–15% and 7–20% for TMAH and MeOH extracts at As levels 4–50 mg kg− 1 dry wt., respectively. The hydride active fraction of As species in extracts, i.e. the sum of toxicologically-relevant arsenic species (inorganic As(III), inorganic As (V), monomethylarsonate (MMA) and dimethylarsinate (DMA)) was determined by FI-HG–ETAAS in diluted tissue extracts. Arsine, monomethylarsine and dimethylarsine were generated from diluted TMAH and MeOH extracts in the presence of 0.06–0.09 mol l− 1 hydrochloric acid and 0.075 mol l− 1 L-cysteine. Collection, pyrolysis and atomization temperatures were 450, 500, 2100 and 2150 °C, respectively. The LODs for the determination of hydride forming fraction (arsenite + arsenate + MMA + DMA) in TMAH and MeOH extracts were in the range 0.003– 0.02 mg kg− 1 As dry wt. Within-run precision (RSD%) was 3–12% and 3–7% for TMAH and methanol extracts at As levels 0.15–2.4 mg kg− 1 dry wt., respectively. Results for the hydride forming fraction of As in TMAH and MeOH extract as % from the certified value for total As (for CRMs) or vs. the total As in TMAH extract (for real marine samples) are generally in agreement. © 2006 Elsevier B.V. All rights reserved. Keywords: Arsenic determination; Marine tissues certified reference material; Electrothermal atomic absorption spectrometry; Flow injection hydride generation; Insitu trapping

☆ This paper was presented at the VII European Furnace Symposium on Atomic Absorption Spectrometry, Electrothermal Vaporization and Atomization (VII EFS) and XII Solid Sampling Colloquium with Atomic Spectrometry (XII SSC), held in St. Petersburg, Russia, July 2–7, 2006 and is published in the special issue of Spectrochimica Acta Part B, dedicated to that conference. ⁎ Corresponding author. Tel.: +359 2 8161318; fax: +359 2 9625438. E-mail address: [email protected] (D.L. Tsalev).

0584-8547/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2006.10.008

I.B. Karadjova et al. / Spectrochimica Acta Part B 62 (2007) 258–268

1. Introduction Arsenic has become one of the most popular toxic trace elements in recent decades. A Google™ search has offered ca. 31,000,000 records in b1 s (!) [1], while a ScienceDirect® has retrieved 7384 and 7391 articles on ‘Arsenic’ query — an article per day [2]! Most of relevant information has been collected in the last 2–3 decades, owing to the widespread availability and progress in atomic spectrometry and mass spectrometry and their successful hyphenation with chromatographic separations for arsenic quantification and speciation analysis — see recent reviews [3–10] and monographs [11–13]. Arsenic is ubiquitous in nature and humans are subject to numerous exposure sources: environmental, dietary, occupational, accidental etc. There is a great concern about exposure of very large human populations (many millions) to elevated As doses, mainly from natural sources such as contaminated drinking water [14,15] and marine-derived food products [8,16,17]. Seafood could be a major source of total arsenic exposure for man, since it contains mg kg− 1 As levels that could spread up to tens and even hundreds of mg kg− 1 [8,17]. Toxicity of different As species in marine samples is highly dependent on their oxidation states and chemical forms. Therefore sensitive, selective, reliable and inexpensive analytical procedures for laboratory practice are required. The determination of total As and its chemical species or fractions with toxicological, environmental and nutritional relevance is a challenging analytical task, even for modern atomic absorption spectrometry (AAS). Most of the current approaches to As speciation analysis rely on complete (or partial) extraction of As species from solid tissue, with or without previous defatting and clean up of crude extracts, followed by high performance liquid chromatographic (HPLC) separation and element-selective detection. Widely used extractants are water, methanol (MeOH)–water and MeOH–chloroform. HPLC separations with ICPMS detection are mostly used (∼ 40% of application papers [3] with this powerful coupling), followed by hydride generation (HG)atomic fluorescence spectrometry (HGAFS) [18–21], while HGAAS detection is gradually declining because of poorer sensitivity (ca. 10-fold) (reviews [3,5,6,9]). It would appear that in the future AAS will occupy its more optimal application niche in the field of total As quantification [22], nonchromatographic speciation [23–28] and reliable comparative measurements. Modern techniques of AAS: HGAAS, electrothermal (ET)-AAS and HG–ETAAS (reviews [9,10]) are sensitive down to 0.01–0.1 μg l− 1 As concentrations, yet relatively simple, widespread, cost-effective, robust and reliable [10], especially in their HG–ETAAS or flow injection (FI)-HG–ETAAS modes [23,26,28,29]. For total As determination, solid tissues are either brought in solution by wet or dry ashing, which may not completely decompose some stable methylated organoarsenicals [22], although some recent advances in sample pretreatment allow analyses of incompletely digested, leached, extracted, solubilized, slurried and solid samples. Promising application fields for ETAAS should be analyses of incompletely (wet) digested [30,31] and solubilized or

259

extracted aquatic tissues by means of TMAH [28,32,33] (review [34]), formic acid [33], enzymatic hydrolysis [35], sonicated slurries [36] and (crude) methanolic extracts [28,37]. Determination of the hydride forming fraction (‘first-order speciation’ of hydride reactive As [23] or ‘hydride active’ [3]; ‘toxicologicallyrelevant As’ [28]) in acid leached [23], extracted/solubilized [28] or (partitioned) slurries [26] by HGAAS and HG–ETAAS [23,26,28] also appears an interesting approach. Procedures involving hydride generation are by themselves prone to under- or overestimation of the ‘toxicologically-relevant As’ due to possible contribution from some As species which are unknown, rarely addressed or unavailable as calibrants in these studies, such as trimethylarsine oxide (yielding (CH3)3As with b.p. 70 °C [13]) and arsenosugars (forming volatile unidentified products under certain experimental conditions) [38]. The main objectives of this study have been formulated on the basis of our previous experience with TMAH solubilization of hair and nail [39] and recent promising results for As in fish and oyster certified reference materials (CRM) [28] after mild microwave extraction by means of ETAAS and HG–ETAAS for the total and hydride active As fraction in TMAH and MeOH extracts, respectively [28]. In an attempt to extend the scope of previous procedure (denoted in text as Procedure B) to other marine tissues, including representatives of aquatic flora, other CRMs and other calibrants such as arsenobetaine (AsBet) and arsenosugars (AsSug) [40] have been involved. To the authors' best knowledge, calibration issues and behavior of these species in the graphite atomizer and problems with applying CRMs as calibrants have not attracted proper attention until now. 2. Experimental 2.1. Instrumentation ETAAS measurements were carried out with a Perkin-Elmer Model AAnalyst 600 atomic absorption spectrometer equipped with a Model 4100 ZL transverse-heated graphite atomizer (THGA®), longitudinal Zeeman effect background corrector and an AS-800 autosampler [41–43]. ‘End-capped’ THGA® graphite tubes with integrated platforms (Part no. B300-0653) were either used as purchased or otherwise pre-treated with Zr and Ir (referred as Zr–Ir treated platform). The experimental procedure for treatment of integrated platforms and deposition of Ir (8 μg) on zirconium (250 μg) as permanent chemical modifier has been described elsewhere [44,45]. Instrumental parameters for ETAAS and FI-HG–ETAAS measurements are presented in Tables 1–3. Sample aliquots of 20 μl and 5 μl modifier injections (300 mg l- 1 Pd) were performed successively. Washing solution for autosampler capillary between samples and modifier injections was 0.2% v/v HNO3. Precautions to minimize tube memory effects and crosscontamination have been taken: in all cases modifiers were applied from the lowest to higher concentration levels and individual graphite tubes were used for each modifier. Some preliminary and comparative ETAAS measurements with modifiers other than permanent Zr–Ir or Zr–Ir with Pd addition

260

I.B. Karadjova et al. / Spectrochimica Acta Part B 62 (2007) 258–268

were performed using the temperature Program B with settings given in parentheses in Table 2. FI-HG–ETAAS measurements were performed by PerkinElmer FIMS® 100 Mercury Analysis System with an AS 93 Plus Autosampler [41–43]. THGA® graphite tubes with Zr–Ir treated platforms were used. An electrodeless discharge lamp for As (EDL System II, Perkin-Elmer) was used as radiation source. All measurements were in integrated absorbance mode (peak area, Aint). The standard gas–liquid separator (GLS) made of polymethylpenten with an internal volume of 2.15 cm3 and PTFE membrane was replaced in this work with a larger custom-made GLS (20 cm3) produced from borosilicate glass with tubular shape (height 155 mm, i.d. 16 mm, o.d. 19 mm), which shows better tolerance towards aerosol formation, flooding and foaming, as observed elsewhere for urine samples [46]. Optimized instrumental parameters and temperature programs for FI-HG–ETAAS are given in Tables 1 and 3. Some continuous flow (CF) HGAAS measurements were carried out with a Varian AA 240 atomic absorption spectrometer equipped with a continuous flow VGA-77 Vapor Generation Accessory and externally heated quartz tube atomizer at 900 °C, controlled with an ETC-60 Electrothermal Temperature Controller [47,48]. No background correction was required in this mode of operation. Sample solutions containing HCl (0.05 mol l− 1)-L-cysteine (0.07 mol l− 1) were introduced via both sample channel (6.2–7 ml min− 1) and acid channel (0.9–1 ml min− 1), while reductant solution (0.24 mol l− 1 NaBH4 in 0.02 mol l− 1 NaOH) was introduced at a flow rate of 0.9–1 ml min− 1. A CEM Model MARSx closed vessel microwave solvent extraction system with a HP500 rotor (CEM Corporation, Matthews, NC, USA), equipped with a pressure and temperature control option was used for microwave assisted extraction of arsenic species from marine tissues as well as for some wet digestions with HNO3 and H2O2.

Table 2 The optimized temperature program for ETAAS determination of As in MeOH and TMAH extracts by Procedure A Step

Temp. (°C)

Ramp time (s)

Hold time (s)

Internal Ar flow (ml min− 1)

Read

1 2 3

110 150 900 (1400) a 2100 (2000) a 2150 (2300) a

10 (30) a 10 (15) a 15 (25) a

20 10 (20) a 20 (30) a

250 250 250

– – –

0

5

1

3 (2) a

4 5 a

0

Yes –

250

Values for Procedure B in parentheses.

2.2. Standards and reagents Analytical grade reagents were used. Stock standard solutions for As were: 1000 μg ml− 1 As(III) (AAS standard solution, No. 11082. Fluka); 1000 μg ml− 1 As(V), As standard solution traceable to SRM from NIST, H3AsO4 in 0.5 mol l− 1 HNO3 (CertiPUR®, Merck, Darmstadt, Germany); 1000 μg ml− 1 monomethylarsonate (MMA) prepared by dissolving of sodium methylarsonate (MMA), CH3AsO(ONa)2·6H2O (Carlo Erba, Milan, Italy); 1000 μg ml− 1 dimethylarsinate (DMA) prepared by dissolving sodium cacodylate, C2H6O2AsNa3·H2O (Carlo Erba) in doubly distilled water; and BCR (Community Bureau of Reference, Geel, Belgium) CRM 626 BCR Arsenobetaine Calibrated Solution 1031 μg ml− 1 as AsBet. Arsenosugar 1, 3-[5′-deoxy-5′-(dimethylarsinoyl)-β-ribofuranosyloxyl]-2-hydroxypropylene glycol, was used as a calibrant, Table 3 The optimized program for determination of hydride forming As in MeOH and TMAH extracts by FI-HG–ETAAS (a) FIAS

Table 1 Instrumental parameters for ETAAS and FI-HG–ETAAS measurements Parameter

Setting

Wavelength Bandpass EDL System 2 power Signal measurement Smoothing Baseline offset correction (BOC) time Read delay Sample coil volume Sample load conduit

193.7 nm 0.7 nm (low) 360 mA AA–BG (peak area) 5 points 2s

Carrier conduit Reductant conduit Gas–liquid separator (GLS) Waste from GLS Reaction coil Argon flow rate

0s 500 μL, PTFE tubing, 1 mm i.d., 64 cm Tygon® tubing, “yellow/blue”, 1.52 mm i.d., 7.0 ml min− 1 at 80 rev min− 1 Tygon tubing, “yellow/blue”, 1.52 mm i.d., 4.0 ml min− 1 at 50 rev min− 1 Tygon tubing, “red/red”, 1.14 mm i.d., 2.0 ml min− 1 at 50 rev min− 1 Custom-made GLS, 16 mm. i.d., length 155 mm [46] Tygon tubing, “white/black”, 3.18 mm i.d., 10.5 ml min− 1 at 80 rev min− 1 PTFE tubing, 1.3 mm i.d., 100 cm 125 ml min− 1

Step

Time (s)

Pump speed (rev min− 1)

Valve position

Prefill 1 2 3

10 10 5 40

80 80 80 50

Fill Fill Fill Inject

(b) Furnace Program FI-HG–ETAAS Step

Temp. (°C)

Ramp time (s)

Hold time (s)

Internal Ar flow (ml min− 1)

Read

1 2 3 4

450 500 2100 2150

1 1 0 1

50 20 4 2

0 250 0 250

– – Yes –

(c) Autosampler and Furnace Sequence for FI-HG–ETAAS Step

Actions and parameters

A B C D E F

Run FIAS steps 1 to 1 Run furnace step 1 with FIAS step 2 Stop FIAS pumps Move autosampler arm into furnace for FIAS steps 3 to 3 Move autosampler arm out of furnace Run furnace steps 2 to 4

I.B. Karadjova et al. / Spectrochimica Acta Part B 62 (2007) 258–268

while other arsenosugars 2–4 were employed in optimization studies [40] (kindly provided by Prof. K. A. Francesconi). The working standard solutions were prepared weekly and kept refrigerated at 4 °C. Stock standard solution of Pd 10,000 μg ml− 1 in 5% HCl, stock standard solution of Mg 10,000 μg ml− 1 in HNO3 and stock standard solution of Ir 1000 μg ml− 1 in 20% HCl were used for modifier solutions preparation. Tetramethylammonium hydroxide (25%, p.a., Merck) was used for alkaline solubilization/extraction of tissues. Solution of sodium tetrahydroborate, NaBH4 (Fluka) (0.5% m/v) in NaOH (0.1% m/v) was prepared daily. Silicon antifoaming agent (Merck) (1 ml l− 1) was added to sodium tetrahydroborate solution on the basis of previous experience [28,46]. Aqueous solutions of L-cysteine, 0.7 or 0.9 mol l− 1, were prepared fresh daily from solid reagent (N99.5%, Fluka) and diluted as required. Doubly distilled water was used in all operations. The following SRM® and CRM were used during development and validation of experimental procedures: NIST SRM® 1566a ‘Oyster Tissue’ from the National Institute of Standards and Technology (NIST, Gaithersburgh, MD, USA); BCR-60 CRM ‘Trace Elements in an Aquatic Plant (freshwater angiosperm Lagarosiphon major)’ and BCR-627 ‘Forms of As in Tuna Fish Tissue’ from the Institute for Reference Materials and Measurements (IRMM, Geel, Belgium), IAEA-140/TM ‘Sea Plant Homogenate’ from the International Atomic Energy Agency (IAEA, Vienna, Austria), DOLT-1 ‘Dogfish Liver’ from the National Research Council of Canada (NRCC, Ottawa). Two typical representatives of the Black Sea biota were used as well: Mediterranean mussel (Mytilus galloprovincialis) and Brown algae (Cystoseira barbata). 2.3. Sample preparation 2.3.1. Procedure A (this work) A nominal 0.3 g of lyophilized marine tissue or CRM was directly weighed in a plastic CEM extraction vessel. Two extraction media were used: either 1 ml of aqueous 25% m/v solution of TMAH and 2 ml of water or 5 ml of 80% v/v methanol. Samples were left for 30 min, capped and placed in the rotor of MW extraction module. Extraction program consisted of two steps: 10 min ramp to 50 °C and 20 min hold time at 50 °C. After cooling, extracts were transferred to 25 ml flasks and diluted with doubly distilled water, resulting in 1.2% m/v of tissue in diluted TMAH (1% m/v) or aq. 16% (v/v) MeOH, respectively. This dilution is referred in text as ‘nominal dilution’. Extracts were further diluted with water as required, typically 4–20 fold for ETAAS and 0–5 fold for FI-HG–ETAAS. 2.3.2. Procedure B (detailed in Ref. [28]) The difference from Procedure A is that the amount of aq. TMAH is smaller (0.3 ml), microwave extraction hold time is shorter (10 min), pyrocoated integrated platform of THGA® is not treated with permanent modifier and different settings in temperature program are used as given in parentheses in Table 2. Some preliminary experiments (Section 3.1) and results in Fig. 1 and Table 4 were performed following Procedure B,

261

which finally resulted in adoption and recommendation of Procedure A. 3. Results and discussion 3.1. Preliminary back-up studies An attempt has been made to adapt Procedure B [28] to various matrices of marine tissues, including aquatic plants. Procedure B has been found to yield accurate results for oyster and tuna fish CRMs solubilized in TMAH, provided that calibration is performed by means of standard additions with AsBet or otherwise after bomb decomposition with HNO3– H2O2. vs. acid-matched calibration with i-As(V) (Table 4). Admittedly, the latter pressurized wet decomposition mode does not completely digest AsBet and methylated arsenicals [20,29] but rather transforms them to species that are thermally stable and could be quantified in ETAAS in the presence of Pd or Zr– Ir modifiers. Significant underestimation (− 28% and − 34%, respectively) could be involved with other calibration modes, as demonstrated by tabulated values in Table 4. Endogenous arsenic in diluted, non-spiked extracts from CRM BCR-627 has been thermally stabilized on pyrocoated, untreated platform of the THGA to vaporization temperatures (Tvap) of 1400 °C with 10 μg Pd and 10 μg Pd + 6 μg Mg modifiers (Fig. 1) and so did other typical As species (i-As, MMA, DMA and AsBet). The other two examined modifiers, 10 μg Pd + 10 μg citric acid and 2 μg Ir were excluded from further experiments because of worse sensitivities. The known As species in this CRM are AsBet (∼ 81% as As) and DMA (∼ 3% as As) from the certified value for the total As, 4.8 ± 0.3 mg kg− 1, therefore the prevailing substance AsBet would appear to be a suitable calibrant. Several important observations were made in preliminary examinations which justified further studies and ultimately lead to the development of Procedure A: (i) Arsenosugars are poorly stabilized, therefore Tvap has to be confined to 900 °C, while atomization temperature (Tat) should be increased to the optimal 2200 °C for AsSug;

Fig. 1. Pyrolysis-atomization curves for As in CRM BCR-627 (0.3% m/v) in 0.225% m/v TMAH extracts using different modifiers: 10 μg Pd ( ), 10 μg Pd + 6 μg Mg (▴), 10 μg Pd + 10 μg citric acid (•) and 2 μg Ir (★).

▪

262

I.B. Karadjova et al. / Spectrochimica Acta Part B 62 (2007) 258–268

Table 4 Results for arsenic extracted with TMAH in marine CRM or SRM® with different calibration approaches (mg kg− 1) a CRM or SRM®

Certified value

Found vs. aq. standards as As (III) (R%)

Found vs. standards as As (III) in 0.15% m/v TMAH (R%)

NIST SRM 1566a 14.0 ± 1.2 10.2 ± 0.6 (72 ± 4) 13.1 ± 0.4 (92 ± 3) ‘Oyster Tissue’ BCR-627 ‘Forms of As in 4.8 ± 0.3 3.2 ± 0.2 (66 ± 5) 3.8 ± 0.2 (79 ± 4) Tuna Fish Tissue’ a

Found by standard addition calibration with As(III) in 0.15% m/v TMAH medium (R%)

Found by standard addition calibration with AsBet in 0.15% m/v TMAH medium (R%)

Found, after MW wet digestion by HNO3 and H2O2 vs. acid-matched standards as As(V) (R%)

13.6 ± 0.4 (96 ± 3)

13.8 ± 0.8 (99 ± 4)

14.2 ± 0.8 (101 ± 6)

4.4 ± 0.1 (92 ± 2)

4.9 ± 0.3 (102 ± 6)

4.7 ± 0.5 (98 ± 11)

Certified value ± 95% confidence range or uncertainty; experimental data ± 2 SD (n = 3).

(ii) Both TMAH and methanolic extracts need further dilution (N4-fold) in direct ETAAS (in addition to nominal 0.3 g → 25 ml) in order to better tolerate solvent/diluent effect; (iii) ‘Hot injection’ mode brings improvement in precision in CH3OH medium in direct ETAAS; (iv) Zr–Ir treated platforms performed better for methanolic extracts as well as for AsBet and AsSug species; further improvement is obtained by applying Pd additions (1.5 μg) with each sample; (v) Compromise Tat and clean temperatures (Tcl) are adopted in direct ETAAS with permanently-treated platforms (2100 and 2150 °C, respectively) in order to protect permanent modifier from re-distribution and gradual loss of Ir; (vi) There are no problems with (Zeeman) background correction in analyses of diluted extracts; (vii) Amount of TMAH should be increased for aquatic flora matrices (from 0.3 ml [28] to 1 ml 25% m/v (CH3)4NOH, as adopted for all matrices in further work); (viii) The hold time for microwave assisted extraction was increased from 10 to 20 min; (ix) AsBet and AsSugar 4 are not transformed to hydride active species during both extraction procedures; HG conditions used in this procedure are relatively mild (pH ∼2 and NaBH4 concentration 0.5% m/v), hence no contribution from volatile derivatives from arsenosugars [38] could be detected; (x) Sample extracts at nominal dilution are stable in TMAH and MeOH at least on the second day after preparation at room temperature; (xi) There is no major difference between results for shaken and not shaken slurries (except for mussel M. galloprovincialis) for both extractants; hence all further work was performed with centrifuged slurries, thus providing better precision and lower matrix load to the graphite atomizer and the HG system.

manently pre-treated with Zr–Ir was employed as atomizer, with and without 1.5 μg Pd addition to each sample aliquot (wet addition, modifier on top of sample). Sensitivity is higher with Pd modifier for both media. The thermal behavior and signals of As species in both extraction media are different, AsBet and AsSug being the most vulnerable As species. In methanol– water mixture, palladium modifier (1.5 μg) stabilizes i-As and DMA up to 1200 °C, but pyrolysis temperature (Tpyr) is confined to 900 °C for AsBet and AsSug. Therefore, Tpyr and Tat were set at compromise values of 900 and 2100 °C, respectively. In TMAH medium, the behavior of AsSug and AsBet differs strongly as compared to DMA and i-As (ca. 2-fold depression vs. ≤ 14% in MeOH medium). The thermal behavior of the most critical species, AsBet and AsSug, was also studied with spiked extracts of CRMs. It was found that 900 and 2200 °C are optimal Tpyr and Tat, respectively, for marine tissue extracts. Therefore in the presence of various As species with different behavior, a crucial consideration for calibration and quantification of As in sample extracts becomes the chemical form of As compounds used for spiking, standard additions, calibration and quantification. In direct ETAAS measurements, superposition of several effects could be expected to take place: (i) species-dependent sensitivity; (ii) some mismatch of optimal conditions for each specie hence resorting to compromise thermal conditions; (iii)

3.2. ETAAS determination of total As The normalized integrated absorbance signals (Aint) for four representative arsenic species, i-As(III), DMA, AsBet and AsSug (i-As(III) = 100%) are summarized in Fig. 2 for two extractants with final concentrations 1% m/v TMAH and 8% v/ v MeOH. Such extracts would result from Procedure A after a nominal dilution (0.3 g → 25 ml) for TMAH or further 2-fold dilution for CH3OH. THGA® with integrated platform, per-

Fig. 2. Normalized integrated absorbance (vs. arsenite) for four representative As species in two solvents (1% m/v TMAH or 8% v/v MeOH) on integrated platform permanently pre-treated with Ir–Zr, with and without 1.5 μg Pd addition.

I.B. Karadjova et al. / Spectrochimica Acta Part B 62 (2007) 258–268

263

3.3. Extraction with TMAH and MeOH ETAAS results for As in TMAH and MeOH extracts and the hydride forming fraction in these extracts by FI-HG–ETAAS are shown in Fig. 3. All these assays are based on standard additions calibration with AsBet or AsSug for aquatic fauna and flora, respectively, and with DMA in FI-HG–ETAAS. There is a general agreement between the certified and experimental values for the total As in TMAH extracts. Under conditions of Procedure A, TMAH serves as an efficient tissue solubilizer, leaving a small residue with insignificant As content. Smaller amounts of TMAH (Procedure B) are less efficient, especially for plant materials. Aqueous 80% v/v MeOH did not extract quantitatively As from ‘Oyster Tissue’ SRM (51% from the certified value) and mussel (M. galloprovincialis) (48% from the TMAH result by standard addition calibration). On the other hand, methanol (80% v/v) yielded quantitative extraction of As from BCR-60 (100% from the indicative value of 8 mg kg− 1), DOLT-1 (95% from certified value) and brown algae C. barbata (108% from the result by TMAH extraction by standard addition calibration). 3.4. Calibration for total As determination by direct ETAAS

Fig. 3. Results for different As fractions vs. certified values for total As contents in CRMs and typical Black Sea marine tissues.

some effect of diluent (alkaline TMAH or organic solvent CH3OH) on processes in the graphite atomizer and on the efficiency of modifier etc. Examined As species tend to give more uniform responses in dil. CH3OH rather than in alkaline TMAH (Fig. 2). As a result, the combined matrix effect for each examined media is speciesdependent, hence the choice of appropriate spike compound becomes essential for obtaining accurate results. For example, BCR-60 ‘Aquatic Plant’ CRM extracts (with nominal dilution) were analyzed by standard addition calibration with known amounts of AsSug (16 μg l− 1 As) or AsBet (17.2 μg l− 1 As) employed as standard addition calibrants. Lower results by 50 and 54% were obtained in MeOH and TMAH extracts with AsBet standard addition, respectively, while results for As in both extracts were quantitative with AsSug standard addition calibrant, 100.5 and 99.7%, respectively. Sample dilution (at least 4-fold) is required for reliable ETAAS measurements, together with standard addition method for calibration. Matrix load to the graphite atomizer from aquatic tissues matrix is higher after TMAH treatments. Generally, AsSug performed as better standard addition for aquatic plants, while AsBet calibrant was used for marine fauna matrices (fish, fish liver, mussel and oyster).

The species-dependent slope of calibration curve or standard addition curve for total As determination in a sample comprising of several individual As species with different ETAAS behavior could be considered as a cause of specific kind of ‘intrinsic element speciation interference’ or ‘intrinsic speciation interference’. Some possibilities for calibration are depicted in Fig. 4. Integrated absorbance signals for direct ETAAS measurements in diluted TMAH extracts are plotted vs. mass of As (in pg) in 20 μl injections. Individual calibration curves for each specie are given as lines with different slopes declining in the order: i-As (III) N DMA N AsBet N AsSug, accompanied by their corresponding characteristic masses for integrated absorbance measurements (mo): 29, 35, 57 and 65 pg, respectively. Aint signals

Fig. 4. Integrated absorbance for direct ETAAS measurements in TMAH extracts vs. mass of As in 20 μl injections of four representative calibrants, indicating declining slope of calibration graph in the order: i-As(III) N DMA N AsBet N AsSugar. Corresponding characteristic masses mo and signals from 4 marine CRMs (solid dots) are shown.

264

I.B. Karadjova et al. / Spectrochimica Acta Part B 62 (2007) 258–268

obtained from four representative CRMs are shown as solid dots as well: 40, 57, 61 and 68 pg for NIST SRM 1566a ‘Oyster Tissue’, IAEA-140/TM ‘Sea Plant Homogenate’, NRCC DOLT-1 ‘Dogfish Liver’ and BCR-60 CRM ‘Trace Elements in an Aquatic Plant (L. major)’. Calibration with i-As(III/V) is obviously ruled out, while the rest of three relevant calibrants (DMA, AsBet and AsSug) confines a sector within which should be very reasonable to fall Aint signals from solubilized (or completely extracted) aquatic tissue extracts. The exact slope (respective mo) for each individual sample (or otherwise calibrant, CRM, laboratory reference material, quality control material and so on) will depend on the mass fraction of each constituent species, their individual slopes (mo) and the imposed (residual) matrix effect of another origin. From the Beer law one could expect that the composite signal for a given sample (As) will depend on the contribution from all present As species. As ¼ as cs ¼

m X

ai g i c s ;

i

where: As as cs m ai gi

The absorbance signal for a given sample. The slope for a given sample. The total As concentration in a given sample. The number of individual As species in a given sample. The slope for individual specie i. The mass fraction (as As) for individual specie i.

The successful application of the standard addition technique in cases of species-dependent sensitivity will depend on the exact matching of the ratio of species in standard addition blend and in real sample. For example, if a sample containing 80% As as AsBet and 20% As as DMA is considered, the bias resulting from standard addition calibration could be estimated as +12, 0 and − 30% for calibration performed with 100% AsBet, 80% AsBet-20% DMA blend and 100% DMA addition, respectively. In real cases (and in most CRMs with certified total As content!) these actual ratios are unknown; moreover unidentified species with unpredictable behavior could be present; therefore such a multivariate calibration task becomes impossible. In practice, results with small bias could be eventually obtained just because of mutual compensation of numerous small errors with different signs. An attempt to imagine that one or a few more CRMs are employed as calibrants for analytical samples or for another CRM, considered as ‘sample’ is presented in Table 5. Obviously, analytical estimations obtained by this approach are only semi-quantitative. It could be seen that the recovery of CRM certified value ranges from 45 to 172%, thus involving intolerable bias, e.g. from − 55% for BCR-60 as ‘sample’ vs. the other three CRMs as calibrants to +72% for SRM® 1566a as ‘sample’ vs. BCR-60 as calibrant. Supposedly, better results could be obtained when only similar matrices are compared but improvement gained is not satisfactory — cf. − 17% and +20% in cross estimations between for aquatic flora CRMs representatives, yet − 35% and +54% for aquatic fauna CRMs (Table 5).

Table 5 Semi-quantitative analysis by employing CRM for calibration CRM as ‘sample’, % found from certified value CRM as ‘calibrant’ BCR-60 Aquatic Plant DOLT-1 Dogfish Liver NIST 1566a Oyster Tissue IAEA-140/TM Sea Plant Homogenate All CRMs except for the 4th one considered as ‘Sample’

BCR-60 Aquatic Plant – 89 (− 11) 58 (− 42) 83 (− 17) 45 (–55)

DOLT-1 Dogfish Liver 112 (+12) –

NIST 1566a Oyster Tissue 172 (+72)

IAEA-140/TM Sea Plant Homogenate 120 (+20)

154 (+54)

107 (+7)

65 (− 35) –

70 (–30)

93 (− 7)

–

144 (+44)

72 (–28) 148 (+48)

56 (–44)

Bias in parentheses.

3.5. ETAAS determination of hydride forming fraction in extracts Optimum concentrations of HCl (0.06–0.09 mol l− 1) and Lcysteine (0.075 mol l− 1), leveling off the sensitivities of As(III), As(V), MMA and DMA, found in recent optimization studies [28,46] were adopted in current work. The criterium for optimization was the rate of coincidence (RC), i.e. the signal matching factor for these four species of As, calculated in the following steps: (i) calculating the average value of Aint for each of As species at current chemical condition, i.e. the L-cysteine and HCl concentrations; (ii) calculating the sum of squares of deviation of the forms from mean values at the same chemical conditions (DEVSQij), where i and j are indexing the L-cysteine and HCl concentrations, respectively; (iii) finding the highest deviation (DEVSQmax); (iv) dividing the DEVSQij values by DEVSQmax for normalization; and (v) expressing the RCi,j value in % for the current chemical medium (i.e. the i, j subscripts):  RCij ¼

DEVSQij 1− DEVSQmax

  100

The worst sensitivity leveling between the most and least sensitive species, i-As(III) and DMA, respectively, was between 6 and 12%, as demonstrated by overlayed peaks for four As species in TMAH and MeOH media (Fig. 5). After neutralization of TMAH extracts, the concentration of HCl was adjusted within the robust range of 0.06–0.09 mol l− 1, with a target value of 0.075 mol l− 1. TMAH extracts were diluted at least 2-fold before hydride generation because of slight depressive matrix effect as seen from compiled data for characteristic masses: mo 64–96 pg in TMAH extracts from marine flora samples vs. 43–46 pg in marine fauna extracts and 42–44 pg in DMA standards. Methanolic extracts could be analyzed without dilution — cf.: mo 46–76 pg in MeOH extracts from marine flora samples vs. 42– 45 pg in marine fauna extracts and 41–42 pg in DMA standards. Calibration was performed by means of standard additions with DMA, which is anticipated to be the main hydride active species

I.B. Karadjova et al. / Spectrochimica Acta Part B 62 (2007) 258–268

265

Fig. 6. Comparison of hydride forming As fraction (arsenite + arsenate + MMA + DMA) extracted in 80% v/v MeOH vs. 25% m/v TMAH for 5 CRMs and two other marine tissue samples (% from total arsenic). The dotted line indicates a slope value = 1.

Fig. 5. Overlaid peaks of four As species (2 ng of As as arsenite, arsenate, MMA and DMA in 500 μl solution) in FI-HG–ETAAS in the presence of L-cysteine (0.075 mol L− 1) and HCl (0.075 mol L− 1) in 0.5% m/v TMAH or 8% v/v MeOH.

in extracts. Standard addition calibration has been adopted as a conservative yet reliable approach for TMAH extracts because of critical pH adjustment within a relatively narrow HCl concentration range after neutralization of TMAH extracts. From analytical viewpoint, analysis of methanolic extracts is more robust and could be performed even in nominally diluted extracts. Employing a large, custom-made GLS [46] and further dilution of extracts (N 5-fold) allowed smooth operation even without antifoam addition. Skipping antifoam cannot be recommended, however, because of troublesome consequences in case of foam or aerosol droplets reaching the graphite tube or quartz tube atomizer. Results for the hydride forming fraction of As in TMAH and MeOH extract [As(III) + As(V) + MMA + DMA] expressed as % from the certified value for total As (for CRMs) or % from the total As in TMAH extract (by standard addition calibration) are in general agreement with each other (Fig. 6), except for higher concentrations in MeOH for DOLT-1 and C. barbata. If concentrations of hydride active fraction are related to As content in MeOH extracts, then percentage of this fraction in methanolic extract raises to 17.6, 5.2, 9.9, 3.5 and 9.1% for BCR-60, DOLT-1, NIST 1566a, M. galloprovincialis and C. barbata, respectively. Our data for the hydride forming As could be compared with some published data on the sum of these four arsenic species in extracts [24,27,49–54] or acid leached CRMs [23] (Fig. 7). Results are generally somewhat lower than other published values and discrepancies could be explained by employing different extraction procedures. Minor contribution to the hydride active fraction due to eventually generated (CH3)3As from trimethylarsine oxide (if present in tissue extracts) could be supposed but cannot be evaluated in this study because of lack of this organoarsenical in the authors' laboratories.

3.6. Figures of merit The limits of detection (3σ) of Procedure A are in the range (0.5–1.2) mg kg− 1 dry wt. for direct ETAAS analysis of extracts in both TMAH and MeOH depending mainly on dilution factor. Within-run precision (RSD%) is in the range 5–15% and 7–20% for TMAH and MeOH extracts at As levels 4–50 mg kg− 1 dry wt., respectively. The LODs for the determination of hydride forming fraction (arsenite + arsenate + MMA + DMA) in TMAH and MeOH extracts are in the range (0.003–0.02) mg kg− 1 dry wt. Within-run precision (RSD%) is in the range (3–12% and 3–7% for TMAH and MeOH extracts at As levels 0.15–2.4 mg kg− 1 dry wt. in sample, respectively.

Fig. 7. Comparison of our data for the hydride forming As in TMAH and MeOH extracts with some published results on the sum of these four arsenic species in various extracts [24,27,49–54] or acid leached [23] CRMs: JLGA [53], TN [54], PACM [27], SNW [23], AC [49], ZS [50] and YS [52].

266

I.B. Karadjova et al. / Spectrochimica Acta Part B 62 (2007) 258–268

4. Conclusions The outcome of this work has happened to pose more questions than giving answers on the application of analytical techniques (ETAAS, HGAAS) which are considered routine for this analyte for samples with relatively high As contents in the μg g− 1 range. Under relatively mild conditions, TMAH serves rather as a ‘tissue solubilizer’ than as an extractant, being able to recover the total As from all tested matrices for subsequent ETAAS measurements. The widely used methanol–water mixture extracts only part of As from most matrices and therefore its usefulness for total As quantification is questionable. In direct ETAAS measurements, TMAH solutions involve more pronounced differences in species-dependent sensitivities (slopes) in the order i-As(III/V) ≥ DMA N AsBet N AsSug, as reflected in increased characteristic masses from i-As to AsSug. This problem is partly solved by employing permanently modified (Zr–Ir) platforms and Pd additions to each sample aliquot. Depressive matrix effects and species-dependent sensitivity are less pronounced for methanolic extracts which are better analyzed in ‘hot injection’ mode. Standard additions calibration and higher dilution factors are beneficial for both extracts. Standard additions approach is inefficient in cases of species-dependent slope of calibration graph. This effect could be considered an ‘intrinsic element speciation interference’; it could be corrected to some extent by using standard additions based on those As compounds which are expected to be predominating in types of samples to be analyzed: generally, AsBet for marine fauna and AsSug for marine algae. Perfectly and imaginary, calibration should be performed by a blend of dominating species (say, AsBet + DMA) for a given set of similar samples, however the individual profile/pattern of As speciation in each individual sample is unknown, hence such a complicated (expensive and time consuming) calibration strategy cannot be realized in practice. An attempt to calibrate by means of one or more matrix CRMs which are known for being high in metrological hierarchy (or perhaps by means of matrix-matched laboratory reference materials?) has yielded only semi-quantitative results. The hydride active fraction (i.e. the sum of species with higher toxicological relevance, i-As(III) + As(V) + MMA + DMA) can be determined in both TMAH and MeOH extracts in dilute HCl-Lcysteine medium. Results for both extracts are in general agreement with each other and are somewhat lower vs. scarce literature data for these CRMs. This fraction represents higher percentage when related to methanol-extracted As rather than to total As in marine tissue. Extracts in TMAH, and particularly those from aquatic plants, bring higher matrix load to the hydride generation system and are more critically dependent on pH adjustment, foam control and signal depression than methanolic extracts. Additional several-fold dilution and utilization of the dominating As form for calibration (DMA) has proved a good remedy in FI-HG–ETAAS measurements for this fraction. Successful further development in this important analytical field would call for (at least): (i) wider (commercial) availability

of calibrants for greater number of relevant arsenic compounds than those six As species (arsenite, arsenate, MMA, DMA, AsBet, AsChol) that are already well exploited but obviously do not cover the existing variety of over thirty relevant organoarsenicals; good candidates to supplement to this expanding list would be trimethylarsineoxide, tetramethylarsonium [55] and some emerging arsenosugars, thioarsenosugars [56] and arsenolipids [57,58]; (ii) better assortment of CRMs with certified contents for individual species and within broader concentration intervals; (iii) attempts to standardize extraction procedures to a certain rational stage so as to improve reliability and comparability of ample data. Acknowledgments Thanks are due to the Black Sea Ecotoxicity Assessment (BSEA) Joint Research Project IB7320-110933/1 within the framework of the SCOPES Program, Swiss National Science Foundation. Arsenosugars were kindly donated by Prof. K. A. Francesconi (Karl Franzens University, Graz, Austria). The authors are grateful to the Reviewer 2 for comments on possible vapor generation from arsenosugars and trimethylarsineoxide under certain experimental condition. References [1] http://www.google.com, visited 29.08.2006. [2] http://www.sciencedirect.com, visited 29.08.2006 and 04.09.2006. [3] K.A. Francesconi, D. Kuehnelt, Determination of arsenic species: a critical review of methods and applications, 2000–2003, Analyst 29 (2004) 373–395. [4] M. Leermakers, W. Baeyens, M. De Gieter, B. Smedts, C. Meert, H.C. De Bisschop, R. Morabito, Ph. Quevauviller, Toxic arsenic compounds in environmental samples: speciation and validation, Trends Anal. Chem. 25 (2006) 1–10. [5] M. Burguera, J.L. Burguera, Analytical methodology for speciation of arsenic in environmental and biological samples, Talanta 44 (1997) 1581–1604. [6] Z. Gong, X. Lu, M. Ma, C. Watt, X.C. Le, Arsenic speciation analysis, Talanta 58 (2002) 77–96. [7] S. McSheehy, J. Szpunar, R. Morabito, P. Quevauviller, The speciation of arsenic in biological tissues and the certification of reference materials for quality control, Trends Anal. Chem. 22 (2003) 191–209. [8] D.J.H. Phillips, Arsenic in aquatic organisms: a review emphasizing chemical speciation, Aquat. Toxicol. 16 (1990) 151–186. [9] D.L. Tsalev, Hyphenated vapour generation atomic absorption spectrometric techniques, J. Anal. At. Spectrom. 14 (1999) 147–162. [10] D.L. Tsalev, Vapor generation or electrothermal atomic absorption spectrometry? — Both! Spectrochim. Acta Part B 55 (2000) 917–933. [11] B. Welz, M. Sperling, Atomic Absorption Spectrometry, 3rd edn.WileyVCH, Weinheim, 1998. [12] J.A. Caruso, K.L. Sutton, K.L. Aukley (Eds.), Elemental Speciation, New Approaches for Trace Element Analysis, Comprehensive Analytical Chemistry, vol. XXXIII, Elsevier, Amsterdam, 2000. [13] J. Dědina, D.L. Tsalev, Hydride Generation Atomic Absorption Spectrometry, Wiley, Chichester, 1995, pp. 182–245, ch. 8. [14] J. Mattusch, R. Wennrich, Novel analytical methodologies for the determination of arsenic and other metalloid species in solids, liquids and gases, Mikrochim. Acta 151 (2005) 137–139. [15] J.C. Ng, Environmental contamination of arsenic and its toxicological impact on humans, Environ. Chem. 2 (2005) 146–160. [16] W.R. Cullen, K.J. Reimer, Arsenic speciation in the environment, Chem. Rev. 89 (1989) 713–764. [17] J.S. Edmonds, K.A. Francesconi, Arsenic in seafoods: human health aspects and regulations, Mar. Pollut. Bull. 27 (1993) 665–674.

I.B. Karadjova et al. / Spectrochimica Acta Part B 62 (2007) 258–268 [18] X.C. Le, M. Ma, N.A. Wong, Speciation of arsenic compounds using HPLC at elevated temperature and selective hydride generation atomic fluorescence spectrometry, Anal. Chem. 68 (1996) 4501–4506. [19] B. He, G.-b. Jiang, X.-b. Xu, Arsenic speciation based on ion exchange high-performance liquid chromatography hyphenated with hydride generation atomic fluorescence and on-line UV photo oxidation, Fresenius' J. Anal Chem. 368 (2000) 803–808. [20] Z. Šlejkovec, J.T. van Elteren, U.D. Woroniecka, Underestimation of the total arsenic concentration by hydride generation as a consequence of the incomplete mineralization of arsenobetaine in acid digestion procedures, Anal. Chim. Acta 443 (2001) 277–282. [21] S. Simon, H. Tran, F. Pannier, M. Potin-Gautier, Simultaneous determination of twelve inorganic arsenic compounds by liquid chromatography–ultraviolet irradiation–hydride generation atomic fluorescence spectrometry, J. Chromatogr. A 1024 (2004) 105–113. [22] D.L. Tsalev, Atomic Absorption Spectrometry in Occupational and Environmental Health Practice, Vol. III: Progress in Analytical Methodology, CRC Press, Boca Raton, Florida, 1995, pp. 19–31, ch. 3. [23] S.N. Willie, First order speciation of As using flow injection hydride generation atomic absorption spectrometry with in-situ trapping of the arsine in a graphite furnace, Spectrochim. Acta Part B 51 (1996) 1781–1790. [24] O. Muñoz, D. Vélez, R. Montoro, Optimization of the solubilization, extraction and determination of inorganic arsenic [As(III) + As(V)] in seafood products by acid digestion, solvent extraction and hydride generation atomic absorption spectrometry, Analyst 124 (1999) 601–607. [25] O. Muñoz, D. Vélez, M.L. Cervera, R. Montoro, Rapid and quantitative release, separation and determination of inorganic arsenic [As(III) + As(V)] in seafood products by microwave-assisted distillation and hydride generation atomic absorption spectrometry, J. Anal. At. Spectrom. 14 (1999) 1607–1613. [26] H. Matusiewicz, M. Mroczkowska, Hydride generation from slurry samples after ultrasonication and ozonation for the direct determination of trace amounts of As(III) and total inorganic arsenic by their in situ trapping followed by graphite furnace atomic absorption spectrometry, J. Anal. At. Spectrom. 18 (2003) 751–761. [27] P.A. Cava-Montesinos, K. Nilles, M.L. Cervera, M. de la Guardia, Nonchromatographic speciation of toxic arsenic in fish, Talanta 66 (2005) 895–901. [28] I. Serafimovski, I.B. Karadjova, T. Stafilov, D.L. Tsalev, Determination of total arsenic and toxicologically relevant arsenic species in fish by using electrothermal and hydride generation atomic absorption spectrometry, Microchem. J. 83 (2006) 55–60. [29] S. Ringmann, K. Boch, W. Marquardt, M. Schuster, G. Schlemmer, K. Kainrath, Microwave-assisted digestion of organoarsenic compounds for the determination of total arsenic in aqueous, biological, and sediment samples using flow injection hydride generation electrothermal atomic absorption spectrometry, Anal. Chim. Acta 452 (2002) 207–215. [30] M. Deaker, W. Maher, Determination of arsenic in arsenic compounds and marine biological tissues using low volume microwave digestion and electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 14 (1999) 1193–1207. [31] K. Julshamn, A. Maage, E.H. Larsen, Studies of critical factors in the determination of arsenic in Standard Reference Materials of marine origin by ETAAS: NMKL interlaboratory study, Fresenius' J. Anal. Chem. 355 (1996) 304–307. [32] M.B.O. Giacomelli, M.C. Lima, V. Stupp, R.M. de Carvalho Jr., J.B.B. da Silva, P. Bermejo Barrera, Determination of As, Cd, Pb, and Se in DORM-1 dogfish muscle reference material using alkaline solubilization and electrothermal atomic absorption spectrometry with Ir + Rh as permanent modifiers or Pd + Mg in solution, Spectrochim. Acta Part B 57 (2002) 2151–2157. [33] C. Scriver, M. Kan, S. Willie, C. Soo, H. Birnboim, Formic acid solubilization of marine biological tissues for multi-element determination by ETAAS and ICP-AES, Anal. Bioanal. Chem. 381 (2005) 1460–1466. [34] J.A. Nóbrega, M.C. Santos, R.A. de Sousa, S. Cadore, R.M. Barnes, M. Tatro, Sample preparation in alkaline media, Spectrochim. Acta Part B 61 (2006) 465–495.

267

[35] P. Bermejo-Barrera, S. Fernández-Nocelo, A. Moreda-Piñeiro, A. BermejoBarrera, Usefulness of enzymatic hydrolysis procedures based on the use of pronase E as sample pre-treatment for multi-element determination in biological materials, J. Anal. At. Spectrom. 14 (1999) 1893–1900. [36] C. Santos, F. Alava-Moreno, I. Lavilla, C. Bendicho, Total As in seafood as determined by transverse heated atomic absorption spectrometry-longitudinal Zeeman background correction: an evaluation of automated ultrasonic slurry sampling, ultrasound-assisted extraction and microwave-assisted digestion methods, J. Anal. At. Spectrom. 15 (2000) 987–994. [37] V.I. Slaveykova, F. Rastegar, M.J.F. Leroy, Behaviour of various As species in ETAAS, J. Anal. At. Spectrom. 11 (1996) 997–1002. [38] E. Schmeisser, W. Goessler, N. Kienzl, K.A. Francesconi, Volatile analytes formed from arsenosugars: determination by HPLC–HG-ICPMS and implications for arsenic speciation analyses, Anal. Chem. 76 (2004) 418–423. [39] D.L. Tsalev, E.I. Tserovski, A.I. Raitcheva, A.I. Barzev, R.G. Georgieva, Z.K. Zaprianov, Analytical scheme for the direct graphite-furnace/flame AAS determination of fifteen trace elements in toenails for biological monitoring, Spectrosc. Lett. 26 (1993) 331–346. [40] A.D. Madsen, W. Goessler, S.N. Pedersen, K.A. Francesconi, Characterization of an algal extract by HPLC-ICP-MS and LC-electrospray MS for use in arsenosugar speciation studies, J. Anal. At. Spectrom. 15 (2000) 657–662. [41] The THGA Graphite Furnace: Techniques and Recommended Conditions, Publication B3210.20, Part No. B050-5538, Release 3.0, Information Publishing Group, Bodenseewerk Perkin-Elmer, Ueberlingen, 1999. [42] The FIAS-Furnace Technique: User's Guide, Technical Documentation, Release 2, Part No. 0993-5250, Bodenseewerk Perkin-Elmer, Ueberlingen, 1999. [43] Flow Injection Mercury/Hydride Analysis: Recommended Analytical Conditions and General Information, Release B, Part No. B050-1820, PerkinElmer, Norwalk, Connecticut, 2002. [44] D.L. Tsalev, A. D'Ulivo, L. Lampugnani, M. Di Marco, R. Zamboni, Thermally stabilized iridium on an integrated, carbide-coated platform as a permanent modifier for hydride-forming elements in electrothermal atomic absorption spectrometry Part 1. Optimization studies, J. Anal. At. Spectrom. 10 (1995) 1003–1009. [45] E. De Giglio, L. Sabbatini, L. Lampugnani, V.I. Slaveykova, D.L. Tsalev, Surface investigation on chemically modified platforms for electrothermal atomic absorption spectrometry, Surf. Interface Anal. 29 (2000) 747–753. [46] P.K. Petrov, I. Serafimovski, T. Stafilov, D.L. Tsalev, Flow injection hydride generation electrothermal atomic absorption spectrometric determination of toxicologically relevant arsenic in urine, Talanta 69 (2006) 1112–1117. [47] Varian AA140/240/280 Series Including Zeeman Operation Manual, Publication No. 8510154700, May 2004, Varian Australia Pty Ltd., Mulgrave, Victoria, 2004. [48] Vapor Generation Accessory VGA-77 Operation Manual, Publication No. 8510104700, May 2004, Varian Australia Pty Ltd, Mulgrave, Victoria, 2004. [49] A. Chatterjee, Determination of total cationic and total anionic arsenic species in oyster tissue using microwave-assisted extraction followed by HPLC-ICP-MS, Talanta 49 (1999) 619–627. [50] Z. Šlejkovec, J.T. van Elteren, A.R. Byrne, Determination of arsenic in reference materials by HPLC-(UV)-HG-AFS, Talanta 49 (1999) 619–627. [51] A. Suñer, V. Devesa, O. Muñoz, D. Vélez, R. Montoro, Application of column switching in HPLC with on-line thermo-oxidation and detection by HGAAS and HGAFS for the analysis of organoarsenical species in seafood samples, J. Anal. At. Spectrom. 16 (2001) 390–397. [52] Y. Shibata, M. Morita, Exchange of comments on identification and quantitation of arsenic species in a dogfish muscle reference material for trace elements, Anal. Chem. 61 (1989) 2116–2118. [53] J.L. Gómez-Ariza, D. Sánches-Rodas, I. Giráldez, E. Morales, Comparison of biota sample pretreatments for arsenic speciation with coupled HPLCHG-ICP-MS, Analyst 125 (2000) 401–407. [54] T. Nakazato, T. Taniguchi, H. Tao, M. Tominaga, A. Miyazaki, Ionexclusion chromatography combined with ICP-MS and hydride generation-ICP-MS for the determination of arsenic species in biological matrices, J. Anal. At. Spectrom. 15 (2000) 1546–1552.

268

I.B. Karadjova et al. / Spectrochimica Acta Part B 62 (2007) 258–268

[55] K. Hanaoka, W. Goessler, H. Ohno, K.J. Irgolic, T. Kaise, Formation of toxic arsenical in roasted muscles of marine animals, Appl. Organomet. Chem. 15 (2001) 61–66. [56] S.D. Conklin, P.A. Creed, J.T. Creed, Detection and quantification of a thio-arsenosugar in marine mollusks by IC-ICP-MS with an emphasis on the interaction of arsenosugars with sulfide as a function of pH, J. Anal. At. Spectrom. 21 (2006) 869–875.

[57] E. Schmeisser, W. Goessler, N. Kienzl, K.A. Francesconi, Direct measurement of lipid-soluble arsenic species in biological samples with HPLC-ICPMS, Analyst 130 (2006) 948–955. [58] E. Schmeisser, W. Goessler, K.A. Francesconi, Human metabolism of arsenolipids present in cod liver, Anal. Bioanal. Chem. 385 (2006) 367–376.