Determination of As, Cd, Cu, Hg and Pb in biological samples by modern electrothermal atomic absorption spectrometry

Spectrochimica Acta Part B 65 (2010) 97–112

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Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b

Review

Determination of As, Cd, Cu, Hg and Pb in biological samples by modern electrothermal atomic absorption spectrometry Jordi Sardans a,⁎, Fernando Montes b, Josep Peñuelas a a b

Ecophysiological and Global Change Unit CSIC-CREAF, Edifici C, Universitat Autònoma de Barcelona, Bellaterra 08193, Barcelona, Spain Departamento de Ciencias Analíticas, Facultad de Ciencias, Universidad Nacional de Educación a Distancia (UNED), C/ Senda del Rey 9. 28040 Madrid, Spain

a r t i c l e

i n f o

Article history: Received 21 April 2009 Accepted 23 November 2009 Available online 5 December 2009 Keywords: GFAAS Biomass Environment Trace elements Pollution

a b s t r a c t Pollution from heavy metals has increased in recent decades and has become an important concern for environmental agencies. Arsenic, cadmium, copper, mercury and lead are among the trace elements that have the greatest impact and carry the highest risk to human health. Electrothermal atomic absorption spectrometry (ETAAS) has long been used for trace element analyses and over the past few years, the main constraints of atomic absorption spectrometry (AAS) methods, namely matrix interferences that provoked high background absorption and interferences, have been reduced. The use of new, more efficient modifiers and in situ trapping methods for stabilization and pre-concentration of these analytes, progress in control of atomization temperatures, new designs of atomizers and advances in methods to correct background spectral interferences have permitted an improvement in sensitivity, an increase in detection power, reduction in sample manipulation, and increase in the reproducibility of the results. These advances have enhanced the utility of Electrothermal atomic absorption spectrometry (ETAAS) for trace element determination at μg L−1 levels, especially in difficult matrices, giving rise to greater reproducibility, lower economic cost and ease of sample pre-treatment compared to other methods. Moreover, the recent introduction of high resolution continuum source Electrothermal atomic absorption spectrometry (HR-CS-ETAAS) has facilitated direct solid sampling, reducing background noise and opening the possibility of achieving even more rapid quantitation of some elements. The incorporation of flow injection analysis (FIA) systems for automation of sample pre-treatment, as well as chemical vapor generation renders (ETAAS) into a feasible option for detection of As and Hg in environmental and food control studies wherein large numbers of samples can be rapidly analyzed. A relatively inexpensive approach with low sample consumption provide additional advantages of this technique that reaches figures of merit equivalent to Inductively coupled plasma mass spectrometry (ICP-MS). Herein is presented an overview of recent advances and applications of (ETAAS) for the determination of As, Cd, Cu, Hg and Pb in biological samples drawn from studies over the last decade. © 2009 Elsevier B.V. All rights reserved.

Contents 1. 2.

3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample pre-treatment . . . . . . . . . . . . . . . . . . . . . . . 2.1. Sample cleaning . . . . . . . . . . . . . . . . . . . . . . . 2.2. Matrix pre-treatments . . . . . . . . . . . . . . . . . . . . 2.2.1. Acid digestion . . . . . . . . . . . . . . . . . . . 2.2.2. Slurry . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. Leaching . . . . . . . . . . . . . . . . . . . . . . 2.2.4. Ashing . . . . . . . . . . . . . . . . . . . . . . . 2.3. Solid sampling . . . . . . . . . . . . . . . . . . . . . . . The use of Flow injection analysis (FIA) and chromatography systems. Temperature programs, modifiers and spectrometer improvements . .

⁎ Corresponding author. Tel.: +34 93 581 29 34; fax: +34 93 581 41 51. E-mail address: [email protected] (J. Sardans). 0584-8547/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2009.11.009

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4.1.

Determination of Cd, Cu and Pb. . . . . . . . . . . 4.1.1. Modifier use and chemical vapor generation 4.1.2. Spectrometer improvements . . . . . . . . 4.2. Determination of As . . . . . . . . . . . . . . . . 4.2.1. Modifiers and chemical vapor generation . . 4.2.2. Thermal programs . . . . . . . . . . . . . 4.2.3. Spectrometer improvements . . . . . . . . 4.3. Determination of Hg . . . . . . . . . . . . . . . . 5. Conclusions and future prospects . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction The great impact trace elements have on natural resources and the quality of the environment justifies the increased interest in their monitoring. Human activities release trace elements into the environment in chemical forms that may increase their bioavailability. Powerful analytical methods are indispensable for evaluating the impact of trace elements in studies of food toxicity [1–5], human health risk assessment [6,7] and environmental pollution by monitoring bioaccumulation in mosses [8–10], mushrooms [11], lichens [12,13], vascular plants [14–16] and animals [17,18] and to follow the processes of phyto-remediation of polluted soils [19]. Knowledge of plant bio-accumulators is also useful for evaluating the effects of trace elements on a geographic and time scale, providing information on trace element distributions, establishing their transport directions and localizing the emission sites [20,21]. The ability of plants to uptake trace elements depends on the water mass flow, on the cation exchange capacity of the plant–soil system, and on chelation with macromolecules. Moreover, and mainly in the case of moss and lichens, direct absorption from the atmosphere to aerial tissues is a significant uptake mechanism. Among the trace elements, arsenic, cadmium, copper, mercury and lead are those that generate the greatest concern for the general public and therefore also for environmental agencies in the majority of states [20]. The monitoring and control of these trace elements in the environment and in food sources require processing large numbers of samples to accurately characterize their abundance and to reach reliable conclusions. Generally, in these types of studies, knowledge of the total trace element content is sufficient, without the need for speciation. The techniques most widely used for these analyses include ICP-OES (Inductively coupled plasma optical emission spectrometry), ICP-MS (Inductively coupled plasma mass spectrometry), FAAS (Flame atomic absorption spectrometry) and ETAAS (Electrothermal AAS). ETAAS is extensively employed for the determination of the total trace element concentration in biological samples. However, several constraints have limited its performance because the response is often perturbed by multiple physical or chemical reactions in the atomizer which are influenced by concomitant elements in the sample matrix. These can cause a greater loss of analyte atoms compared to the case for a reference solution. Additionally, the sample matrix may produce volatile or stable molecule forms of the analyte and influences the absorption signal. Nevertheless, differences in atomization efficiencies between the analyte in samples and in calibration standard solutions can be avoided and the analytical accuracy checked by using a reference standard with a similar matrix to that of the samples [1–3,7–13], or by comparing the slopes derived from aqueous standards with those from certified reference standards [1,6]. More recently, several improvements in ETAAS spectrometers have also contributed to partially or totally solving these problems and by facilitating direct analyses of solids thereby enhancing the

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capacity of ETAAS to compete with atomic emission spectrometry in terms of sensitivity and detection limits, while maintaining the current advantages of absorption techniques, such as their low cost. The main constraints suffered by ETAAS techniques over the past few decades for determination of As, Cd, Cu, Hg and Pb in biological samples are examined in this review along with the progressive improvements introduced to solve them. These include use of modifiers, improvements in graphite furnace design to enhance atomization efficiency, pre-concentration of the analyte by in situ trapping, advances in methodology for spectral background correction and the emergence of new optical systems and light sources which culminated in high resolution continuum source ETAAS (HR-CS ETAAS). When used with modifiers, this approach provides high sensitivity, low detection limits, and ease of sample preparation. In addition to these advances, complimentary improvements in sample manipulation are also highlighted, such as interfacing FIA systems to the graphite furnace to automate sample pre-treatment thereby offering an additional improvement that, together with a reasonably low cost and a relatively short time consumption, make ETAAS a very useful tool for quantitation of As, Cd, Cu, Hg and Pb in biological samples. The various possibilities offered by ETAAS for the determination of As, Cd, Cu, Hg and Pb in biological samples are reviewed with a view to providing an understanding of this technique for scientists in all disciplines of the environmental and human health fields who frequently need to undertake their determination. 2. Sample pre-treatment Due to the heterogeneity of matrices encountered with biological samples, a wide set of publications has been examined so as to include all types of biological materials (Table 1). There is no unique pre-treatment prior to sample introduction into the graphite furnace, and its execution depends on the capabilities of the laboratory and the study objectives. Because the matrix may hinder complete atomization of the analyte and thus interfere with the accuracy of the determination, several methodologies are practiced to separate the analyte from the matrix. However, recent advances in instrumentation now permit direct solid sampling approaches that reduce the necessity for sample preparation. In addition, other aspects have to be taken into account, such as cleaning the sample, if necessary [20]. 2.1. Sample cleaning The necessity (or not) of cleaning the surface of the samples depends on the aims of the study. For example, environmental studies focused on bioaccumulation in tissues require elimination of any solid particles adsorbed on the sample surface [15,20] because the real impact of trace elements on trophic chains and the health of an organism depends mainly on their concentration within the organism.

J. Sardans et al. / Spectrochimica Acta Part B 65 (2010) 97–112

However, if atmospheric emission is of interest, such particles are of prime interest and no cleaning is undertaken. 2.2. Matrix pre-treatments Since the introduction of ETAAS, detection limits have improved significantly, and several technical advances have eliminated many problems linked to interferences from matrix components. Some of these improvements consist of pre-treatments that partially or completely eliminate the matrix prior to the introduction of the sample into the atomizer and rely on the optimization of the atomization process and minimization of spectral interferences. 2.2.1. Acid digestion Matrix destruction based on oxidation with concentrated acids is the most widely used approach for biological samples because of its efficiency [1–4,6–17,20,22–76] (Table 1) in liberating the elements of interest from the molecular structures. Analyses of the resultant digest by ETAAS have several advantages: ease of manipulating the analyte concentrations to be in the optimum range of detection, availability of appropriate calibration standards and the use of less sophisticated instrumentation. However, if inadequate equipment is used to conduct the acid digestion, unacceptable losses of analytes such as As and Hg may arise by volatilization. The widespread use of microwave assisted acid digestion with elevated pressure and temperature control has alleviated these problems [2,4,7–10,12– 14,17,20,27–29,32,35–38,41,43,45–48,50–57,59–62,64–66,69,71– 73,76] and enabled the use of large sample masses (1–2 g) without the danger of vessel rupture and with a minimal chance for contamination. Microwave assisted processes are particularly important when the determination of As and Hg is targeted as complete mineralization of the sample can be achieved at low temperature preventing losses of these elements. Nitric acid is the most widely used [4,8–10,12,15–17,20,24–42,66] and the addition of H2O2 increases its oxidization power [2,7,13,22,43–64]. Mixtures of nitric acid with other acids have also been favored [13,14,22,67–75]. Because biological tissues are generally poor in silicates (particularly if surfaces have been cleaned of soil particles, if necessary), use of HF is not necessary, preventing the health hazard that its use represents. Use of HCl is not generally recommended because interferences can arise from chlorides during the determination of Cd and Pb by ETAAS [28,29,35,67]. Moreover, attention must be paid to sources of contamination arising from use of concentrated acids and from containers used for the digestion process. Additionally, attention must be paid to the microwave assisted temperature and pressure program in order to achieve complete mineralization of all organoforms of the analytes as, for example, organoarsenicals can then be lost during the pyrolysis stage of the thermal program of atomizer [76]. When ultra pure acids and use of appropriate microwave digestion apparatus utilizing optimized programs, digestion using HNO3 alone or with other acids provides the most comprehensive treatment compared to slurry sampling or simple leaching [10,31,53,54] if the aim is to determine the total analyte concentration. 2.2.2. Slurry A slurry is a suspension of insoluble particles, usually in acid solution and/or in other media such as Triton X-100 that permits the homogenization of the sample and the liberation, to some extent, of the trace elements to the solution [6,19,21,28,70,77–98]. Taking into consideration parameters such as safety, the environment, economy, time and low risk of contamination to permit low detection limits, slurry sampling techniques appear advantageous over classical microwave assisted acid digestion. To obtain good precision and recoveries with slurry techniques it is necessary to optimize the influence of particle size, slurry concentration (relation between sample weight and slurry volume) and homogeneity [20,28,35,67,77–

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87]. Use of low concentrations of HNO3 and H2O2 or other reagents that enhance the decomposition of carbonaceous residues inside the atomizer which can reduce detection power [93]. Under optimum conditions, results are frequently comparable to those obtained with acid digestion methods and total analyte recovery is achieved [28,29,70,77–82,84,87–97]. Despite optimization, incomplete liberation of the analyte from the sample matrix is not always achieved, leading to unsatisfactory results [35,85,86]. 2.2.3. Leaching When the aim is to determine that part of the trace element fraction that is bioavailable or mobile by water solubilization, other techniques such as liquid–solid leaching are used [28,70,82,99–102]. Ultrasonic extraction followed by sonolysis in an acidic medium can be a useful method to prevent losses of As, Cd, Cu, Hg and Pb [28,70,99,100] with good recovery of these elements for extraction from animal tissues [28,102]. For determination of Hg, due to its high volatility and low concentration in environmental samples, some sophisticated extraction-pre-concentration methods coupled to ETAAS using Pd as a permanent modifier have been developed [103]. In general, when the objective is to determine the total concentration of analyte, acid digestion and slurry sampling are the most appropriate techniques, whereas when determination of different chemical species of the analyte is of interest, a selective extraction method such as liquid–liquid extraction [103] or a chromatographic technique [102], may be more advisable. 2.2.4. Ashing Whereas sample calcination at temperatures above 400 °C has been used by some authors [10,21,54,104–106] (Table 1) several studies have shown that it may not be the most appropriate method since significant losses of As, Hg, Cd and Pb can occur [105]. In general, ashing methods may present poorer accuracy and provide lower analyte recovery when compared with acid digestion methods [54,104]. 2.3. Solid sampling Direct introduction of solid samples into the graphite furnace (SS ETAAS) provides an alternative methodology [107–110], although it is less frequently used. The method was not fully accepted until recently, owing to technical improvements in spectrophotometer and software capabilities of modern instrumentation. The main arguments against it have been (i) the difficulty of handling and introducing small sample masses; (ii) the high imprecision of the results due to the heterogeneity of some natural samples; (iii) the difficulty in calibration in that similar solid standards with similar matrix composition and structure are required (iv) the limiting linear working range of AAS, and (v) the difficulty in diluting solid samples. In spite of these constraints, direct solid sampling is a feasible alternative when the determination of the total content of analyte is required, since it needs almost no sample preparation. In this way, time and cost are reduced, and higher sensitivity is achieved since no dilution is employed. Another advantage of solid sample introduction is the reduction in the risk of sample contamination due to the absence of both preparation and use of chemical reagents. With classical line source (LS) ETAAS, one of the arguments in favor of preparing slurries or acid digests is the possibility of altering the analyte concentration so that it falls within the linear range of the calibration function. This is true if the recovery is 100%; if not, the slurry or acid digests are subject to the same limitations as SS ETAAS due to the natural heterogeneity of the small subsamples used, which is not always improved by reducing particle size [111]. However, there are other systems that may be more appropriate than dilution, such as the use of a gas flow during atomization [112,113], the use of alternate, less sensitive analytical lines [113,114] or the use of three-

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Table 1 Different analytes, sample matrix types, sample treatments, modifiers and ETAAS spectrometer and background correction used in the determination by ETAAS of biological materials. Analytes

Sample matrix

Sample pre-treatment

Modifiers

Background correction

Spectrometer characteristics and use

Ref.

Cu,Pb,Ni Cd

Vegetable oils Mammal tissues

Acid digestion HNO3 + H2SO4 Acid digestion HNO3 + H2O2

(NH4)2HPO4

Smith–Hieftje Deuterium lamp

[1] [2]

Cd,Cu,Pb Ba,Cd Cr,Cu Fe Ni,Pb,Zn Cd, Pb

Fish tissues Plant tissues

Acid digestion HNO3 + HClO4 Acid digestion HNO3

(NH4)2HPO4 + Mg (NO3)2

Zeeman Zeeman

Rice

Slurry and acid digestion HNO3

No used

Zeeman

Co, Cu, Mn, Ni Cd,Cr,Cu,Pb,Se

Baby food Bovine milk

Slurry and acid digestion HNO3 + H2O2 Acid digestion HNO3 + H2O2

Atomization at 1900 °C. Light line source Atomization at 1200 °C. Light line source. LHA Line source Line source Atomization at 2050 °C Cd, 2100 °C Cu, 1800 °C Pb Line source. Atomization at 1500 °C Cd, 1700 °C P Line source. Atomization at 2000 °C. LHA. Line source. Atomization at 1500 °C Cd, 2300 °C Cu, 1800 °C Pb

Cd Mo,Pb As,Cd Co,Cr,Pb

Lichens and Mosses Mosses

Acid digestion HNO3 Acid digestion HNO3

Cd, Pb

Mushrooms

Acid digestion HNO3 + H2O2

As Co,Cr,Cu Fe, Ni, Pb Cd,Pb Cd, Pb Cd Cd, Cr, Ni, Pb, V Cd, Pb Cd Cu,Fe,Mn,Pb

Mosses

Acid digestion HNO3

Lichens and mosses Lichens Plant tissues Plant tissues Animal and plant tissues Fish tissues

Acid digestion HNO3 Acid digestion HNO3 + H2O2 Acid digestion HNO3 + HCl Acid digestion HNO3 Acid digestion HNO3 Dry ashing, acid digestion HNO3

As

Fish and algae tissue

Leaching and CV in FIA

Mg (NO3)2 + NH4H2PO4 (Cd) PcCl2 + acid ascorbic(Cu, Pb)

Acid digestion HNO3

Al,Cd Cu Ni,Pb,Zn Cd,Pb Cd,Cu,Pb Cu Ni,Mn Pb Cd,Cu,Fe Mn Pb, Zn Hg Cu,Fe Se,Zn Cd,Co,Cu,Ni,Pb

Fish tissues Animal and plant tissues Animal and plant tissues Algae Plant tissues Lichens

Acid Acid Acid Acid Acid Acid

Animal and plant tissues Animal tissues Animal tissues

Acid digestion HNO3 CV of line Acid digestion HNO3 Acid digestion HNO3

Pd + Zr permanent in situ trapping Pd

As

Animal and plant tissues

Ir permanent in situ trapping

Bi,Cd,Pb As Cd,Cr Ni,Pb Pb Cu, Ni

Urine Plant tissues Animal tissues Plant tissues Vegetable oils

Acid digestion HNO3 Slurry. CV in FIA system Acid digestion HNO3 FIA preconcent. Acid digestion HNO FIA CV3 Acid digestion HNO3 Acid digestion HNO3 Acid digestion HNO3

Cd,Co,Cr Pb

Human blood

centrifugation + Acid digestion HNO3

digestion digestion digestion digestion digestion digestion

HNO3 HNO3 HNO3 HNO3 and Slurry HNO3 HNO3

Line source. Atomization at 1650 °C Cd, 1800 Pb. LHA Line source. LHA.

NH4H2PO4 + Mg (NO3)2 Rh + W coated

Zeeman. Zeeman.

Line source Line source Line source. Atomization at 1900 °C Cd, 2100 °C Cu, 2000 °C Pb Line source. THA.

[18]

Line source.

[20]

Line Line Line Line Line

source. source. source .LHA. source. source. Atomization at 2200 °C.

[21] [22] [23] [24] [25]

Line source. Line source. Line source. THA. Line source. Atomization at 1900 °C. Line source. Atomization at 2000 °C. LHA. Light line source. Atomization at 1700 °C Cd, 2100 °C Cu, 1900 °C Pb Line source. Atomization at 1800 °C. Line source. Atomization at 2500 °C. LHA. Light line source. Atomization at 1800 °C Cd, 2300 °C Cu, 2200 °C Pb Line source. Atomization at 2500 °C.

[26] [27] [28] [29] [30] [31]

Line source. Line source. Line source. Line source. Line source. THA. Line source.

Atomization at 2000 °C Cu.

[36] [37] [38] [39] [40]

Atomization at 1400 °C Cd,

[41]

Atomization at 2100 °C

Deuterium lamp. Zeeman. Zeeman.

Zeeman. Zeeman. Deuterium lamp.

Pd (Cd, Pb) Deuterium lamp. Deuterium lamp.

Pd + Zr permanent in situ trapping Mg (NO3)2+ Pd

Deuterium lamp. Zeeman. Deuterium. Zeeman. Zeeman.

NH4H2PO4, Mg (NO3)2 Pd (NO3)2

[11]

Line source. Line source. LHA.

Zeeman.

Mg (NO3)2 + Pd, Ni ∙ (NO3)2 ∙ 6H2O + Pd Cu (NO3)2 ∙ H2O + Pd

[10]

Deuterium lamp Deuterium lamp.

NH4H2PO4 + Mg (NO3)2 Zr + Ir coated in situ trapping + Pd mobile

[6] [7]

Zeeman.

Atomization at 1700 °C. THA. Atomization at 2200 °C. THA.

[12] [13] [14] [15] [16] [17]

[32] [33] [34] [35]

J. Sardans et al. / Spectrochimica Acta Part B 65 (2010) 97–112

NH4H2PO4 + Mg (NO3)2+ Pd

[5]

[8] [9]

Line source

Mg (NO3)2 + NH4H2PO4 (Cd) Mg (NO3)2 + Pd (As), Malonic Acid (Pb) NH4H2PO4 (Pb), Mg (NO3)2+ Pd (Cd)

Al,Ba,Cd,Cr,Cu Ni, Lichens, mosses and Pb plant tissues Pb Mushrooms Cd Mammal tissues Cu Fe Mn Mammal tissues Hg Urine As Animal tissues

Dry ash and slurry Acid digestion HNO3 + H2O2 Acid digestion HNO3 Acid digestion HNO3 Acid digestion HNO3

Deuterium lamp Zeeman

[3] [4]

Table 1 (continued) Analytes

Sample matrix

Sample pre-treatment

Modifiers

Background correction

in different combinations. Cd,Pd Cd Cd, Cu, Mo, V

Mammal tissues Animal tissues Plant tissues

Acid digestion HNO3 Acid digestion HNO3 + H2O2 FIA CV Acid digestion HNO3 + H2O2

As Cd

Animal tissues Animal tissues

Acid digestion HNO3 + H2O2 Acid digestion HNO3 + H2O2

Cd,Cu,Pb As As,Cd,Cr Cu, Ni, Pb Cd,Cr,Pb

Plant tissues Animal tissues Algae Wines

Acid Acid Acid Acid

digestion digestion digestion digestion

HNO3 + H2O2 HNO3 + H2O2 Slurry, extraction (ultrasound) HNO3 + H2O2 HNO3 + H2O2

Human and animal tissues Mosses Human and animal tissues

Cd, Pb

Animal and plant tissues

As Cd,Cr,Fe Pb

W permanent + NH4H2PO4, Pd (NO3)2

Deuterium lamp.

Zeeman. Deuterium lamp.

Mg (NO3)2+ Pd

Zeeman.

Pd (NO3)2

Zeeman.

Mg (NO3)2+ Pd Mg (NO3)2+ Pd (Cd) NH4H2PO4 (Pb) Mg (NO3)2+ Pd, NH4H2PO4

Zeeman. Deuterium lamp. Deuterium lamp.

Ir, Pd, Rh permanent

Deuterium lamp. Zeeman. Deuterium lamp.

Acid digestion HNO3 + H2O2

Mg (NO3)2 + NH4H2PO4

Zeeman.

Animal and plant tissues Plant tissues

Acid digestion HNO3 + H2O2 Acid digestion HNO3 + H2O2

Zeeman. Deuterium lamp.

As, Cd,Cr,Cu,Pb

Honey

Acid digestion HNO3 + H2O2

Pd Mg (NO3)2+ Pd (Cd) NH4H2PO4 (Pb) Mg (NO3)2 + Pd

As, Se Cd As Cd,Co,Cr,Cu,Mn, Ni,Pb Hg

Animal tissues Plant tissues Animal tissues Plant tissues Human hair

Acid digestion Acid digestion Acid digestion Acid digestion Dry ashing Acid digestion

Cd, Pb

Plant tissues

Acid digestion HNO3 dry ashing, slurry

Pb As

Urine Mosses

As,Cd,Cr,Mn, Ni,Pb As

Animal tissues Animal and plant tissues

Cd As

Animal and plant tissues algae

Cd, Pb As, Cd, Pb As Cu Cu, Zn Cd

Plant tissues Plant tissues Animal tissues Plant tissues Milk Fish tissues

Cd,Cu Mn,Pb

Animal and plant tissues

PdCl2 Acid digestion HNO3 + HClO4 Acid digestion HNO3 + H2O2 Mg (NO3)2+ Pd HNO3 + H2O2 + HF Acid digestion HNO3 + H2O2 Slurry, ultrasound assisted leaching Mg (NO3)2, Pd(NO3)2 microwave assisted acid leaching (NH4)2HPO4 Ir permanent Acid digestion HNO3, HNO3 + NaF, HNO3 + NaF, FIA CV in situ trapping Mg (NO3)2 + (NH4)2HPO4 Acid digestion HNO3 + HCl + HF Acid digestion HNO3 + H2O2 + HF Mg (NO3)2+ Pd and with HNO3 + HClO4 Zeeman. Acid digestion HNO3 + HCl Acid digestion HNO3 + HClO4 Deuterium lamp. Acid digestion HNO3 + H2O2 in FIA W permanent + Pd + Ir + Rh Ultrasound slurry and dry ashed Zeeman. Ultrasound slurry (NH4)2HPO4 Ultrasound slurry W + Rh permanent, Mg (NO3)2 + Pd Slurry –

HNO3 + H2O2 FIA CV HNO3 + H2O2 HNO3 + H2O2 HNO3 + H2O2 HNO3

Zeeman.

Rh coated in situ trapping Mg (NO3)2 (NH4)2HPO4 + Pd (Pb) Pd (NO3)2 (Cd) Pd, Rh, Ir, Rh + Pd AgMnO4, KMnO4. HNO3 + HClO4

Deuterium lamp. Deuterium lamp. Zeeman. NH4H2PO4 Zeeman. Zeeman. Zeeman.

Zeeman. Zeeman. Line source. Line source. LHA. Deuterium lamp. Line source. LHA. Deuterium lamp. Zeeman. Zeeman. ,

1800 °C Pb THA Line source Line source. Atomization at 1800 °C. Line source. Atomization at 1900 °C Cd, 2300 °C Cu Line source Line source. Atomization at 1200 °C. THA. Line source. Line source. Atomization at 2100 °C. THA. Line source. Line source. Atomization at 1700 °C Cd, 2200 °C Pb Line source. Line source. Atomization at 2200 °C. Line source. Atomization at 1650 °C Cd, 1800 °C Pb. Line source. Atomization at 1650 °C Cd, 1800 °C Pb. LHA. Line source. Atomization at 1960 °C. THA. Line source. Line source. Atomization at 1500 °C Cd, 1700 °C Pb. Line source. Atomization at 1400 °C Cd, 1600 °C Pb THA. Line source. THA Line source. Atomization at 1650 °C Cd, 1800 °C Pb.LHA. Line source. Simultaneous multielement determination with atomization during 6 s at 2300 °C. THA. Line source. Atomization at 2200 °C. Line source. Atomization at 2300 °C.LHA. Line source. Atomization at 1750 °C Cd, 2400 °C Cu, 1850 °C Pb. Light line source. Atomization at 2000 °C Hg. Furnace with a platform of Boron Nitride. Line source. Atomization at 1600 °C Cd, 1800 °C Pb. Line source. Atomization at 2000 °C. Line source. Atomization at 2200 °C. LHA.

Ref. [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61]

[62] [63] [64] [65] [66] [67] [67] [69]

Line source. Atomization at 1600 °C Cd, 1600–1900 °C Pb. LHA. Line source. THA.

[70]

Line source. Atomization at 1900 °C. Line source. Atomization at 2300 °C. THA.

[72] [73]

Line source. THA. Line source. Atomization at 2400 °C. LHA. Line source. Atomization at 1250–1400 °C. THA. Line source. Atomization at 1100 °C Cd, 1800 °C Cu, 1500 °C Pb. THA.

J. Sardans et al. / Spectrochimica Acta Part B 65 (2010) 97–112

As Cd Cd, Pb

Acid digestion HNO3 + H2O2 Acid digestion HNO3 + H2O2 Acid digestion HNO3 + H2O2 HNO3 + H2O2 + H2SO4 and dry ashing Acid digestion HNO3 +H2O2 and dry ashing Acid digestion HNO3 + H2O2 Acid digestion HNO3 + H2O2 Acid digestion HNO3 + H2O2

Cd, Se Mushrooms As Mosses Cd,Co,Cr Mn Ni, Mushrooma and plant Pb tissues Cd, Pb Mushrooms

W permanent in situ trapping Mg (NO3)2 + Pd

Spectrometer characteristics and use

[71]

[74] [75] [76] [77] [78] [79] [80] 101

(continued on next page)

102

Table 1 (continued) Sample matrix

Sample pre-treatment

Modifiers

Background correction

Spectrometer characteristics and use

Ref.

Animal and plant tissues

Ultrasound slurry

Mg + Pd

Atomization at 1700 °C. Line source. THA.

[81]

Hg Cd, Pb

Human hair Blood and urine

Slurry and leaching Slurry

Pb + Ascorbic acid (NH4)2HPO4

Deuterium lamp and Zeeman Deuterium lamp. Zeeman.

at 1400 °C. at 1300 °C Cd,

[82] [83]

Cu Co,Cu,Ni Cd Al, Cu,Li,Mn Pb Cd, Pb,Se

Animal and plant tissues Plant tissues Animal tissues Plant tissues Fish tissues Animal and Plant tissues

Slurry Slurry Slurry Slurry Ultrasound slurry Slurry

Mg (NO3)2

at 2500 °C. at 2300 °C. LHA. at 1200 °C. at 2300 °C. LHA. at 2100 °C. LHA. at 1300 °C Cd,

[84] [85] [86] [87] [88] [89]

Cd, Pb Cd, Pb Cd, Pb

Human blood Plant tissues Animal and plant tissues

Mg (NO3)2, (NH4)2HPO4 Mg (NO3)2 + (NH4)2HPO4 W + Rh coated + (NH4)2HPO4

Zeeman.

at 1300 °C Cd,

[90] [91] [92]

Al, Cd, Cu

Plant tissues

Slurry Slurry with FIA Acid digestion HNO3 + H2O2 Ultrasound slurry Slurry

(NH4)2HPO4

Zeeman.

at 1500 °C Cd,

[93]

Cd, Pb Pb Cd, Pb

Slurry Slurry Slurry

Mg (NO3)2 + (NH4)2HPO4 Mg (NO3)2+ Pd

Zeeman. Zeeman. Deuterium lamp.

at 1500 °C. THA. at 1800 °C. at 1200 °C Cd,

[94] [95] [96]

As,Cd,Pb,Se Pb Cd,Cu,Mn,Pb

Human blood Animal tissues Platt decoctions and beverages Animal tissues Wine Animals and plant tissues

Slurry Slurry Leaching with ultrasounds

Ir + Rh permanent, Mg (NO3)2+ Pd Zeeman. Mg (NO3)2+ Pd Zeeman. Zeeman.

at 1700 °C. at 2000 °C. THA. at 2100 °C Cd,

[97] [98] [99]

As

Animal tissues

Hg

Human urine

Leaching with ultrasounds or ozonolysis, FIA CV Leaching and preconcntration

Line source. Atomization Line source. Atomization 1600 °C Pb. Line source. Atomization Line source. Atomization Line source. Atomization Line source. Atomization Line source. Atomization Line source. Atomization 1700 °C Pb LHA. Line source.THA. Line source. Line source. Atomization 1600 °C Pb THA. Line source. Atomization 2000 °C Cu Line source. Atomization Line source. Atomization Line source. Atomization 1800 °C Pb. Line source. Atomization Line source. Atomization Line source. Atomization 2500 °C Cu, 2500 °C Pb Line source.

As

Animal tissues

Hg As,

Human hair Plant tissues

Cd, Pb

Plant tissues

Pb Pb Fe, Mn, Pb

Aninal and plant tissues Animal tissues Cereals

Cu, Zn Cu, Fe, Zn Co, Cu, Mn Hg

Leaching, Chromatographic separation of different As species Microleaching Acid digestion HClO4, slurry and dry ashing Dry ashing

Dry ashing Solid sampling Solid sampling and acid digestion with HNO3 Bovine liver Solid sampling Blood serum Solid sampling Green coffee Solid sampling Diverse standard solutions Separation and pre-concentration with chelating resin in FIA system

(NH4)2HPO4 Triton X-100 EDTA, Pd, NH4NO3, NH4NO3 + Pd (NH4)2HPO4

Pd (NO3)2, KMnO4 + HCl, Pd (NO3)2, + KMnO4 + HCl

Pd Ir + Mg

Deuterium lamp. Zeeman. Deuterium lamp. Zeeman. Zeeman. Deuterium lamp.

Zeeman.

Zeeman.

Line source. Atomization at 2000 °C.

[101]

Zeeman.

Line source. THA.

[102]

Zeeman.

Line source. Line source. Atomization at 2500 °C.

[103] [104]

Line source. Atomization at 1700 °C Cd, 2200 °C Pb Line source. Atomization at 1600 °C. LHA. HR-CS ETAAS. Line source. Atomization at 1800 °C.

[105] [106] [107] [111]

Line source. Atomization at 2300 °C. THA. Line source. Atomization at 2100 °C. THA. Atomization at 2100 °C Line source.

[114] [115] [116] [119]

Deuterium lamp. Mg (NO3)2+ NH4H2PO4 Ru permanent

Deuterium lamp. Deuterium lamp. Zeeman. Zeeman. Deuterium lamp.

Pd permanent

[100]

J. Sardans et al. / Spectrochimica Acta Part B 65 (2010) 97–112

Analytes Pb

Table 1 (continued) Sample matrix

Sample pre-treatment

Cu,Ni,Mn,Pb,V Cu

Xylem sap Bovine serum

Cu

Milk

Cd Cu Mn,Pb

Honey

Chromatographic separation Liquid–solid extraction and size-exclusion chromatography Fractionation and chromatography by SEC-HPLC Acid digestion HNO3 + HClO4

Pb

Plant tissues

Pb

Human hair

Cd As

Urine Animal and plant tissues

Cd Cd, Pb

Human urine and plant tissues Animal and plant tissues

Acid digestion HNO3 + HCl CPE in FIA system Acid digestion HNO3 + H2O2 CPE in FIA system Acid digestion HNO3 +HCl LLE in FIA system Slurry, FIA CV and chromatographic separation of As species Acid digestion HNO3 + H2O2 preconcen. in Lab-on-valve in FIA system Coprecipitation

Pb Pb Cd Pb Pb Pb

Animal tissues Rice and human blood Animal and plant tissues Animal and plant tissues Animal and plant tissues. Human urine

Acid digestion Acid digestion Acid digestion Acid digestion Slurry Dilution

Pb Pb As, Cd, Cu, Pb

Sugar Blood Wines

Dissolution in 0.2 HNO3 v/v Slurry Direct sampling

Cd, Pb Cd, Pb Cd Cd Cd, Pb As, Cd, Pb

Animal tissues Human urine Blood Orujo spirit Animal tissues Sugar-cane spirits.

Extraction Direct sampling Slurry Slurry Acid digestion HNO3 + HClO4 Direct sampling

As, Se Cd, Fe, Ni Cd, Fe As, Co, Mn, Ni

Urine Diverse solutions Cereals Urine

Direct sampling Direct sampling Solid sampling Direct sampling

W + Ir permanent Pd

As Hg Hg

Human urine Human blood serum Human and animal tissues

Direct sampling Direct sampling Solid sampling

(NH4)3 Rh + Citric acid Au coated KMnnO4

HNO3 + FIA coprecipitation HNO3 + FIA coprecipitation HNO3Ni + HCl HNO3

Modifiers

Background correction

Spectrometer characteristics and use

Ref.

Zeeman.

Line source. Atomization at 1800 °C. Line source. Atomization at 2100 °C. LHA.

[120] [121]

–

Deuterium lamp.

Line source. Atomization at 2100 °C.

[122]

Mg (NO3)2+ Pd (Cd) (NH4)2HPO4 (Pb)

Deuterium lamp.

[123]

Deuterium lamp.

Line source. Atomization at 1650 °C Cd, 2300 °C Cu, 1800 °C Pb.LHA. Line source.

Deuterium lamp.

Line source. Atomization at 2100 °C. THA.

[125]

Zeeman.

Line source. Line source.

[128] [129]

Zeeman.

Line source. THA.

[131]

Zeeman.

Line source. Atomization at 1500 °C Cd, 1600 °C Pb Line source. Atomization at 2000 °C. LHA. Line source. Line source. Atomization at 1400 °C. Line source. Atomization at 1700 °C. Line source. Atomization at 1700 °C. THA. W filament used as atomizer. Line source. THA. Line source. Line source. Atomization at 1400 °C. THA. Line source. Atomization during 6 s at 2200 °C allowing sequential multielement determination. THA. Line source. THA. Line source. THA. Line source. Line source. Atomization at 1200 °C. Line source. Line source. Sequential multielement determination with atomization at 2100 °C. THA. HR-CS ETAAS. THA. HR-CS ETAAS. THA. HR-CS ETAAS. Atomization at 1700 °C. Line source. Sequential multielement determination. THA. Line source. Atomization at 2600 °C. THA. Line source. HR-CS ETAAS.

[135]

Ir permanent in situ trapping

Mg (NO3)2+ NH4H2PO4

Deuterium lamp.

W + Rh permanent W + Rh permanent Rh

Smith–Hieftje. Zeeman. Zeeman. Zeeman

W + Rh permanent W + Rh coated Pb + Mg (NO3)2

Zeeman. Zeeman. Zeeman.

W coated (NH4)2HPO4 Ir coated + Pb + Mg (NO3)2

Zeeman Zeeman. Zeeman. Zeeman. Zeeman. Zeeman.

Zeeman. Zeeman. Deuterium lamp.

[124]

[137] [138] [145] [146] [148] [149] [151] [152] [154]

[155] [156] [158] [160] [162] [163]

J. Sardans et al. / Spectrochimica Acta Part B 65 (2010) 97–112

Analytes

[164] [169] [175] [176] [178] [179] [180]

103

104

J. Sardans et al. / Spectrochimica Acta Part B 65 (2010) 97–112

field Zeeman effect [114,115]. Another advantage of SS ETAAS is the possibility of introducing samples into the atomizer in almost two orders of magnitude greater mass than with other sampling techniques, hence improving the sensitivity and detection limits [108]. Moreover, some techniques reduce spectral interferences, such as use of graphite platforms to introduce solid samples into the furnace [116]. Another concern of the SS ETAAS approach is calibration. It has been noted that while accurate analytical results can generally only be achieved using solid standards, and a certified solid standard similar to the sample is not always available, calibration with aqueous standards has been successfully obtained for biological samples [117,118]. Another argument that has been brought up in favor of slurry and acid digest sampling is that chemical modifiers have more effective contact with analyte than with solid samples. However, the fact that analytes can be stabilized by use of a permanent modifier when introduced as a solid sample has been demonstrated in several studies, including the determination of Hg in biological samples [117]. The recent introduction of high resolution continuum source ETAAS (HR-CS ETAAS) (see below) has opened a number of possibilities for the development of previously unavailable direct solid sampling methods. This technique permits simpler optimization of furnace programs and the availability of the spectral environment makes it easy to avoid spectral interference, improving the detection limits and opening up the possibility for simultaneous determination of several elements [107–109]. For instance, Borges et al. [107] have applied this technique to determination of Pb in biological samples [107]. Their use of Ru as a permanent modifier allowed a pyrolysis temperature of 900 °C to completely eliminate the sample matrix and hence the continuous background that preceded the analyte signal. Interference-free determinations could be achieved by setting integration limits adjusted to integrate only the atomic signal and calibration against an aqueous standard was successful. In most studies that introduce solid samples directly into the graphite furnace, the samples have been ground to yield particles having diameters in the range of 0.3–50 µm [107–109], which results in marked improvements in sample homogeneity [110]. 3. The use of Flow injection analysis (FIA) and chromatography systems Flow injection analysis (FIA) systems are frequently used for sample pre-treatment, especially for processing large numbers of samples (Table 1). A variety of approaches has been proposed to alleviate interferences from matrix components, ranging from instrument modification to the use of chemical modifiers (see below). However, the most effective approach is usually an appropriate sample pretreatment prior to analysis to achieve matrix separation or its destruction along with analyte pre-concentration [36,37,91,119] (Table 1). More recently, it has become apparent that the hazardous effects of trace elements should not be evaluated solely on the basis of their total concentration but rather by accounting for their chemical form as individual species may exhibit widely different mobility and toxicity. In this sense, chromatography is a powerful tool for speciation [119–121] and can be applied off line [122] or incorporated on line in automated FIA systems and, with an autosampler, be directly injected into the atomizer. The processes most commonly implemented on line with FIA systems are pre-concentration by liquid–solid extraction (LSE) [36,47,91,119], liquid–liquid extraction (LLE) [124– 128], or coprecipitation [108,109], separation of different analytes [47] and chemical vapor generation [35,37,43,62,71,100,129]. To pre-concentrate metal species and remove interfering matrix species using LSE, the digested sample is passed through a column that is frequently packed with an ion exchange resin as sorbent and subsequently eluted directly into the graphite furnace in a more concentrated solution than the original sample [36,47,91,119], thus

improving the limits of detection. However, on line LSE performed with permanent packed-columns may suffer from sufficient reliability as a consequence of the progressive contamination of the solid surfaces of the beads and the potential stripping of surface functional moieties. Recently, so-called bead-injection (BI) analysis has overcome such shortcomings as renewal of the sorbent material occurs in each analytical cycle by flow programming [130,131]. Lab-on-valve (LOV) is a novel and powerful method in flow analysis which has introduced new advances as all operations can be achieved in a miniaturized, automated fashion under strictly controlled conditions [132]. The microconduit unit is a single monolithic structure mounted atop a multi-position valve of a sequential injection assembly. This unit incorporates all necessary laboratory facilities for a variety of analytical applications, including in-valve manipulation of sorbent materials. Microchannels serve as the bead-retention cell along with mixing points for chemical derivation of the analyte and a multipurpose flow-through cell for real-time control of analytecontaining beads [133]. Thus, packed-column reactors are generated by aspirating beads from a peripheral section of the valve. The use of reverse phase sorbents with good bead size homogeneity such as poly (styrene-divinylbenzene) have allowed high analytical reproducibility in the use of LOV-ETAAS systems for the determination of Cd from acid digests of sea lettuce [131], permitting large enrichment factors (7.4–17.2) and low detection limits (5–135 ng L− 1) for a sample loading volume of 1.25 mL [87]. In this way, detection limits of 0.001 mg L− 1 and 0.07 mg L− 1 for Cd(II) and Pb(II), respectively, have been obtained by one-line sequential injection LOV-ETAAS analysis of acid digests from marine plants by injecting 1.8 mL of sample solution into the LOV FIA system [134]. The possibility of coupling on line extraction or chemical speciation [135] to a LOV-ETAAS has been achieved using a sophisticated FIA design to determine Cr(VI) in soil extracts. It has proved to be a reliable process that can also be applied to the pre-concentration and speciation analyses of other trace elements, including As, Cd, Cu, Hg or Pb in leachates or acid digests from biological samples. Coprecipitation is a useful method for the concentration of metal ions and their separation from matrices, and has been combined with ETAAS to determine Cd and Pb [136] and, more recently, Pb [137,138] in digested biological and environmental samples. A chelating reagent (2mercaptobenzothiazole, dithiocarbamate or diethydithiophosphate) complexes with the analyte, which is subsequently immobilized onto an absorbing agent (silica gel sorbent or polytetrafluoroethylene) with a co-precipitated carrier. After this, quantitative separation of the analyte and its transfer to the ETAAS occurs with the main requirement being that the sorbent be separated from the matrix solution (by filtering or centrifuging, followed by washing the precipitate). With SLE, apart from the use of ionic exchange resins, application of the most specific and effective sorbents for pre-concentration of trace elements from solutions remains to be investigated to improve figures of merit by increasing enrichment factors (EF). Several new sorbents employed to pre-concentrate trace elements from water or urine could also be applied to As, Cd, Cu, Hg and Pb in acid digests or slurries of biological samples, such as thioureasulfonamide resin yielding an EF of 90 for collection of Cd and Pb from water [139]; , the determination of Co in human urine [140] and V in waters [141] using multiwalled carbon nanotubes which yield an EF of 20; the determination of Cu in waters using microcolumns containing 8-hidroxyquinoline azo-inmobilized on controlled pore glass [142], and the determination of Cd and Pb [143] and of As [144] in waters using activated carbon which yields EFs between 22 and 50. Another possibility for using FIA systems for pre-concentration or separation purposes is the application of modern LLE methods. Cadmium, Cu and Pb have been determined by FI-LLE-ETAAS in various acidic digests of biological samples or water standards using novel techniques such as cloud point extraction (CPE) [125,143], supported liquid membrane extraction (SLME) [126], single drop microextraction (SDME) [127]

J. Sardans et al. / Spectrochimica Acta Part B 65 (2010) 97–112

and single step-LLE [128]. New approaches using new and more efficient sorbents together with LOV FIA systems open new scenarios for advancement in sensitivity and limits of detection by coupling these pre-concentration systems to ETAAS. This is a powerful line of research to improve analytical capacity for the determination of As, Cd, Cu, Hg and Pb in solutions of acid digests or leachates of biological samples. 4. Temperature programs, modifiers and spectrometer improvements Although most studies report determination of Cd, Cu and Pb alone or together (Table 1) and As or Hg is determined separately due to their higher volatilities, some studies have reported determination of Cd, Pb and As [9,70,97], or As, Cd, Pb and Cu [49]. For this reason, the use of the ETA to determine Cd, Cu and Pb and then its use for determination of As and for Hg separately is described. 4.1. Determination of Cd, Cu and Pb Although Cd, Cu and Pb are frequently determined by ETAAS, two issues required solutions to aid their determination (i) concentration levels below detection limits of ETAAS, and (ii) interferences from matrix components that perturb detection limits. Use of improved atomizer designs and background correction systems has alleviated most of these constraints and some recent spectrometer improvements suggest the possibility of simultaneous determination of more than one element and of the enhanced suitability of ETAAS for solid sample analysis through use of new light sources and detectors. In recent years, several improvements have progressively appeared which have enhanced the use of ETAAS for the determination of Cd, Cu and Pb in biological samples, the most significant being use of appropriate modifiers, advances in atomizer designs, background correction systems that ensure the separation of analyte and nonspecific response under exactly repeatable conditions, the development of in situ trapping methods and improvements in the light source and detector which, together with the appearance of advanced software, have produced a HR-CS spectrometer that reaches low detection limits, improves the use of AAS for application to direct analysis of solids and opens the possibility for the determination of several elements at once. 4.1.1. Modifier use and chemical vapor generation One of the first approaches to improve the determination of Cd, Cu and Pb in biological samples arises from an investigation into the use of chemical modifiers. Modifiers are introduced into the furnace with the sample to enable more efficient thermal separation of the analyte and concomitants during the pyrolysis stage. Generally, the analyte is stabilized and the matrix is removed by volatilization. Such procedures have been particularly relevant to the determination of Cd and Pb for which six different modifiers used alone or mixed are popular: Pd(NO3)2/Pd [31,33,41,50], PdCl2/Pd [68], PdCl2/Pd +ascorbic acid [7], Pd(NO3)2/Pd + Mg(NO3)2 [10,13,38,44,53,54,60,81,96– 98,120,123], (NH4)3PO4 [1], NH4H2PO4 [10,13,53,54,60,67,72,78,93,123], Mg(NO3)2 + (NH4)2HPO4 [7,17,27,41,58,63,90,91,94,106,135], Mg (NO3)2 +(NH4)3PO4 [9,41], Triton X-100+NH4H2PO4. [43,46,83,86,89]. Correct use of these modifiers has reduced interferences and improved the atomization process. In the case of Cu, determination is frequently accomplished without use of modifiers due to its sufficient stability during atomization. However, some studies have used Pd+Mg(NO3)2 [44] and Mg (NO3)2 [84] as modifiers; other have highlighted some important aspects of the use of conventional modifiers, such as prevention of interferences by chloride by use of Ni+ HCl modifiers [145] or NH4F [146]. Phosphate modifiers were shown to be inadequate for Cd because they cause a decrease in sensitivity [46] and produce PO derivatives during atomization which increase background absorption.

105

Although use of modifiers has been widespread, two drawbacks arise: (i) contamination degrades detection limits and (ii) atomizer lifetime may be reduced. These shortcomings have been solved by the use of permanent chemical modifiers [147], which are generated by introduction of several aliquots of the chemical modifier solution onto the atomizer platform, followed by stepwise drying and pyrolysis to obtain a coating of the chemical modifier thermally deposited onto the surface [148]. During thermal treatment, volatile impurities in these reagent solutions are eliminated such that high purity permanent chemical modifiers are not required [148]. Better detection limits can be reached since blank values are usually lower than 0.01 As [147,148]. Moreover, permanent modifiers prolong atomizer lifetime, providing enhanced thermal stability and repeatability [28,43,46,79,97,107,147–150]. Currently, no single chemical compound can be identified as having a broad application for the determination of other elements. However, some trends can be noted, such as the use of Pd as a traditional chemical modifier and the use of W, Ir and Rh as permanent chemical modifiers. Moreover, the use of permanent chemical modifiers seems to hold the most promise for the near future. The use of W+ Rh [28,43,79,151,152], W [15,46,97], Ir + Rh [97] or Ru [107] coatings on an integrated transversely heated GF atomizer (THGA) platform has been successful for the determination of Cd and Pb in slurries and acid digests of biological samples. For tungsten coil atomizers (TCA), the use of Ir as a permanent modifier yields the best detection limits for Cd [149] whereas Rh permanent modifiers generate the best analytical results for Pb [148]. Another line of research is to find optimum thermal programs for each type of sample. For Cd, Cu and Pb, most studies use a temperature program typically consisting of 4 steps; one or two drying steps, one pyrolysis, one atomization and a final cleaning step. Purge gas flow is stopped during atomization to increase residence time of atomic vapor. Atomization temperatures of 1800–2600 °C have been used for Cu [4,6,7,17,23,31,33,34,44,77,80,85,87,93,99,120,121], slightly higher than the atomization temperatures used for the determination of Cd and Pb. When conventional spectrometers were not able to detect more than one element at a time, most studies reported unique temperature programs for each element. As Cd is more volatile than Cu and Pb, its atomization is normally lower than for the latter two elements. Temperatures most widely employed for atomization of Cd are 1100– 2050 °C, i.e., 1100 [80], 1200 °C [46,86,96], 1300 °C [83,89,92], 1400 °C [41], 1500 °C [7,93,94], 1600 °C [27,67,70], 1650 °C [10,53,54], 1700 °C [31,50,81,97], 1800 °C [34,36,43], 1900 °C [1,17,44,72] and 2100 °C [99]. For Pb, they are 1500–2200 °C, i.e., 1500 °C [80,94], 1600 °C [27,70,83,92], 1650 °C [106] 1700 °C [81,89,97,107], 1800 °C [4,7,10,36,41,53,54,67,95, 96,120], 1850 °C [68], 1900 °C [1], 2100 °C [88] and 2200 °C [34,50,98]. Temperature programs used for samples directly introduced as solid powders are similar to those used for solution samples [107]. In spite of the differences between the optimum atomization temperatures of the various elements, use of the same temperature program for Cd and Pb yields good analytical results [1,27,36,70,94,97]. An alternative approach for analysis is use of chemical vapor generation (CVG) with in situ collection in the atomizer. In situ trapping of the generated vapors enables analyte pre-concentration in the atomizer in a reproducible manner with the pre-concentration factor being selected by the operator based on the volume of solution taken for CVG, improving detection limits [43]. Lead exhibits serious problems with CVG and currently Cu is practically not generated. In situ trapping using on line CVG from acid digested solutions has been used for the determination of Cd in animal tissues [43] with the volatile analyte (possibly the hydride or the atomic cold vapor) being transferred to a pyrolytic graphite coated graphite furnace in a 200 mL min− 1 flow of Ar and sequestered at 200 °C. 4.1.2. Spectrometer improvements Progressive changes in the design of electrothermal atomizers have improved the sensitivity and detection limits offered by ETAAS. During the 1990s, use of platforms inserted into the graphite furnace

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provided a more reproducible and constant atomization process [153]. When the temperature of the atomizer walls increases quickly, sample atomization from the platform is retarded because the sample is primarily radiationally heated, providing a more isothermal environment in which reproducibility is improved. With the classical longitudinally heated atomizer (LHA), the temperature reaches a maximum in the center and a minimum at the cooled extremities. To improve this shortcoming, transversally heated atomizers (THA) were designed which provided a more uniform distribution of temperature over the length of the tube, decreasing the rate of atom loss by condensation, thereby increasing sensitivity and decreasing the detection limits. In spite of this, several studies reporting the determination of Cd, Cu and Pb in biological samples in the last 10 years have been conducted with ETAAS spectrometers equipped with LHAs [2,6,10,13,23, 30,33,54,69,70,75,77,78,85,87–89,106,121,123,137]. The most recent have been based on measurements with improved equipment (Zeeman background correction systems, multielement determination system) conducted with THAs [28,36,38,40,41,47,58,61,79–81,90,92,94,98,114, 115,125,127,128,131,134,147–149,152,154–157]. When the two atomizer configurations have been used to determine the same analytes in the same matrix, the THA has shown better analytical figures of merit [158]. As noted above, spectral interferences from matrix components are frequently the main cause of poor detection limits. To alleviate this problem, several different background correction systems have been used with ETAAS [159]. The continuous source method has been the first used, having some limitations, i.e., if the sample contains another element having an absorption line located too close to the analyte resonance line (within the bandpass) spectral interferences may be undercompensated because only the average non-atomic absorption which occurs over the spectral bandpass is effectively “measured”. Despite these constraints, this methodology has been widely used for the determination of Cd, Cu and Pb in biological samples since the mid-1990s. [2,6,10,13,30,43,54,57,60,65,67,75,78,81,83,84,86,89,93, 105,108,123,125,143,146]. Background correction based on the Smith–Hieftje effect is very useful in the presence of strong molecular spectral interferences and over all wavelengths. This correction system has only infrequently been used for the determination of Cd, Cu and Pb in biological samples by ETAAS [1,145]. Currently, the best results have been obtained using Zeeman effect background correction [83] (until the advent of HR-CS ETAAS) because the background is always measured at the same wavelength as the atomic absorption of the analyte. [5,7,16,21,27,28,33,34,36,40,41,43,50,56,58,61, 68–70,73,74,77,79–81,83,85,87,88,90,92–95,97–99,102,114,115,121,125, 127,128,131,134,135,144,148,149,152,154,155,160–162]. Simultaneous determination of As, Cd, Cu and Pb in wines using electrodeless discharge lamps of As, Cd and Pb operating at 380, 200 and 450 mA, respectively and a hollow-cathode lamp of Cu at 15 mA has been reported [61,154]. Spectral lines of As 193.7 nm, Cd 228.8 nm, Pb 283.3 nm and Cu 324.8 nm were selected with a spectral bandpass of 0.7 nm. Response was evaluated using signal integration times of 5, 3, 3, and 6 s during a 6 s atomization at 2200 °C. Cadmium and Pb were similarly determined in freshwater mussels [38] and, more recently, As, Cu and Pb in sugar-cane spirits [163]. Thus, from 1996 to date, establishment of the most appropriate modifiers, use of the THGA together with the use of more sophisticated spectrophotometers, i.e., Zeeman background correctors, produced an improvement in the analytical figures of merit for the determination of Cd, Cu and Pb in biological samples by decreasing the detection limits and permitting their simultaneous determination (Table 1). In the past decade, several technical advances have permitted commercialization of HR-CS ETAAS. This approach maintains use of conventional atomizer parameters (modifiers, temperature programs, use, gas flow) but involves the following novelties: a continuum radiation source (CS) such as a Xenon short-arc lamp, a modern

transversely heated graphite furnace module, a double èchelle monochromator and high resolution electronic “photoplate” array detectors [164–166]. These advances provide a number of improvements: (i) as only a few pixels of the array detector are necessary to measure atomic absorption, the rest of them are available for other purposes, including correction for all spectral events that occur simultaneously on all pixels, including lamp flicker noise and changes in the transmission of the atomizer that are independent of the wavelength. This also allows observation of the temporal and spectral high resolution characteristics of the analyte signal and the background absorption surrounding the analytical wavelength; (ii) the optimization of the AAS measurements with respect to the furnace temperature programs, the use of optimal modifiers and the control of gas flows result in maximum sensitivity; (iii) data can be manipulated by software to permit subtraction of background (matrix) spectra from that recorded for the sample using least square algorithms; (iv) the much higher radiation intensity of the CS compared to conventional line sources typically improves detection limits fivefold. System optimization (temperature program, modifiers, gas flow, etc.) follows the same process as with conventional line source instruments. This new approach has achieved several tangible improvements, including detection limits enhanced by an order of magnitude to reach figures of merit comparable to those of ICP-MS while requiring considerably less complexity and cost while extending the calibration ranges to levels limited only by memory effects [167–170]. The compact design of the double monochromator system (equipped with a prism, for predispersion of radiation and an echelle grating) requires high precision active wavelength stabilization which is attained using an internal neon lamp. Welz et al. have summarized such features in detail [171]. Since the first combinations of a linear photodiode array detector (PDA) with an échelle spectrometer [170], the characteristics of CSAAS have improved primarily due to advances in the technology of multi-channel detectors [108,169,172–174]. Despite such advances, the state-of-the-art is: (i) that true simultaneous multielement determinations by HR-CS ETAAS with adequate analytical performance will require the use of custom designed detectors which would be prohibitively expensive at this time. The HR-CS ETAAS makes possible a truly simultaneous multielement AAS determination of only certain groups of elements when an appropriate two dimensional CCD array is used, as for example that of Fe [175] and of Fe and Ni [169]. In these cases, nearby analytical lines lying within a narrow spectral bandwidth (0.4 nm) were used. (ii) These novel spectrometers will provide new advantages for the determination of trace elements in complex matrices, such as solid environmental samples, as noted for the determination of Pb in coal [166]. To date, few studies have used HR-CS with ETAAS for the determination of Cd, Cu and Pb in solid biological samples although there are several recent publications on the use of HR-CS ETAAS for Cd and Pb. Use of Ru as a permanent modifier for direct solid sampling of animal tissues to determine Pb permitted a pyrolysis temperature of 900 °C, sufficient to eliminate the continuous background that preceded the analyte signal due to incomplete elimination of the matrix when a lower pyrolysis temperature was used [107]. Cadmium has been determined with a HR-CS ETAAS spectrometer directly from solid ground cereals with a detection limit of 0.6 mg kg− 1 [175]. A new possibility for advancement is, as noted earlier, the use of in situ trapping with conventional line source ETAAS but utilizing a graphite filter atomizer (GFA). The GFA was fabricated from a Perkin Elmer (Bodenseewerk, Perkin Elmer, Uberlingen, Germany) transversely heated graphite furnace by fitting it with a “filter” tube and a collector instead of a platform [169]. The GFA provides for a reduction of interferences without need of chemical modifiers and permits use of large sample volumes. When coupled with LS ETAAS spectrometers, this type of atomizer has already been used for the determination of As, Cd, Cu and Pb, among other trace elements, in organic liquids and can accommodate acidic solutions of digested biological samples [169].

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4.2. Determination of As ETAAS has been widely used for the determination of As in biological tissues [52,64]. However, the relatively low concentrations of As in most such samples and its high volatility together with potential interferences means that its determination is frequently difficult. For example, several phosphorus compounds are reported to produce particularly severe interferences [25]. Nevertheless, several technical advances in ETAAS have recently succeeded in removing various constraints by application of appropriate modifiers, use of hydride generation coupled with in situ trapping techniques, optimization of temperature programs and atomizer design, advances in background correction systems and, above all, the availability of modern HR-CS ETAAS spectrometers. As a consequence, lower detection limits have been successfully achieved. As recent examples, As has been successfully determined directly in acid digests, slurries or liquid–solid leachates [9,11,25,45,48,49,55,59–61,64,70,73,76,97, 102,154,163,176,177] and in biological fluids [45] with or without use of a modifier [75]. For direct liquid sampling, detection limits between 0.24 and 10 µg L− 1 were achieved. The use of modifiers is especially pertinent to prevent volatilization of As during the pyrolysis stage. In most cases, modifiers such as Pd [59,177], Rh [76], (NH4)2HPO4 [70], Ir + Mg [104] and Pd/Mg(NO3)2 [9,25,48,52,61,69,70,73,97,132] have been applied. Pd + MgNO3 is most commonly used and was proven to decrease interferences produced by phosphates [52] as well as allowing better thermal stability of As to yield the best results for digested marine tissues [25]. For the analysis of urine, use of (NH4)3RhCl2 + citric acid as a mixed modifier permitted a pyrolysis temperature of 1600 °C, high enough to drive off the urine matrix without loss of As [178]. The citric acid evolved active gases and carbon during decomposition, facilitating creation of a favorable reduction atmosphere for the formation of a Rh–As alloy or intermetallic compounds which stabilized arsenic at high temperature [55,176]. A furnace coated with W [76] or Ir + Rh [55,97] has also been used as a modifier with good results for As in animal, plant and moss tissues. 4.2.1. Modifiers and chemical vapor generation CVG based on hydride generation is being widely used for the determination of total AsI in biological samples after acid digestion, slurry or liquid–solid leaching. CVG can be conducted on line with a FIA system [18,35,62,71,100,129] and when coupled with in situ trapping, the possibility of pre-concentrating the hydride generated from a large sample volume simultaneously reduces interferences and enhances detection limits. Thus, the GF serves as a single thermal source for both steps, allowing a clean, rapid separation from the matrix without need of a pyrolysis step [35,114]. Metal coil atomizers have also been used for this purpose, along with modifiers, including Ir [35,71,129], Rh [62], Ir + Zr [18] or Pd + Zr [37]. A Rh coil atomizer can be used to reduce the temperatures of the furnace program for collection of As by in situ trapping [62]. Use of permanent modifiers [18] yields detection limits of 0.1–1.5 µg L− 1 in the solution samples introduced to the hydride generation system. 4.2.2. Thermal programs A further means of improving detection limits for As is the use of suitable temperature programs. When determining As in biological digests, slurries and extracts, it is likely that some arsenic will remain in the organic matrix that has not mineralized. ETAAS permits pyrolysis of the sample and thereafter the removal of the remaining organic matrix. The use of STPF (Stabilized Temperature Platform Furnace) and the incorporation of chemical modifiers with the sample have enabled the optimization of furnace temperature programs to more efficiently remove matrix compounds before atomization, thus allowing use of lower temperatures.

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4.2.3. Spectrometer improvements Few studies have used continuum background correction systems [35,37,64,76] in recent years, as most are based on Zeeman effect systems [18,25,45,48,52,59,61,69,70,73,79,97,102,104,154,163, 176,178], providing figures of merit similar to those obtained by ICP-MS for the determination of As in food [52]. With Zeeman background correction and electrodeless discharge lamp sources, As, Cd and Pb, were determined simultaneously in honey and wine [61,152]. Similarly, the application of Zeeman background correction and the use of suitable modifiers enabled the simultaneous determination of As, Co, Ni and Mn [176] and of As, Cu and Pb [163]. In each of these studies, THGA atomizers coupled with Zeeman effect background correction allowed detection limits of 0.78–5 µg L− 1 to be achieved, adequate for the determination of As in most biological samples. Application of HR-CS ETAAS removes several constraints on the determination of As in biological samples, offering good sensitivity with short determination time when applied to solid samples. To date, few studies have been undertaken using this technique. For human urine, good results have been attained for As [164], but more studies are necessary to adapt HR-CS ETAAS for the determination of As in biological samples, including the coupling of CVG with in situ collection to enhance figures of merit for the trace elements such as As and Hg. Recently, it has been noted that arsenobetaine exhibits a 30% higher sensitivity than inorganic As, possibly due to its slightly lower enthalpy, which permits better atomization [177]. Moreover, this study also reported that several other organic species yield enhanced sensitivity over inorganic As [177], highlighting the necessity of further studies to improve the atomization process in order to increase efficiency and obtain similar sensitivities for all chemical species of the same element. 4.3. Determination of Hg Until recently, ETAAS was not very suitable for the determination of Hg as it was very difficult to manage in complex matrices since its high volatility impaired application of high pyrolysis temperatures, even if a modifier is used, and losses led to low sensitivity. CVG is thus the method of choice for the determination of Hg. However, biological samples are complex matrices in which Hg is strongly bound by chelation to organic molecules (sulfur linkages) or by the formation of species such as methyl-Hg. For this reason, prior treatment of samples by mineralization is required, or application of a selective extraction method if only the available or more mobile Hg is to be determined. In both cases, errors may arise due to incomplete mineralization or extraction. Moreover, with digestion, losses of Hg by volatilization can be significant. For this reason, and taking into account the possibility of solid sample analysis using the pyrolysis step for destruction of the matrix, several attempts have been conducted recently to overcome these constraints. These problems have been solved by taking advantage of hardware and method improvements. Application of CVG with in situ trapping using various permanent modifiers has achieved good results for acid digested samples and used to concentrate mercury vapor [32,179] or for detection of mercury directly generated from digested or slurried solutions [24,66,82,101,103,119]. Modifiers frequently permits elimination of the pyrolysis stage and use of low atomization temperatures [24,32,82], allowing volatilization of Hg before the corresponding permanent modifier reaches its atomization temperature. When an acid digestion is necessary, it can be conducted at low temperature in closed vessels [32] to prevent losses of Hg. It is also possible to preconcentrate Hg present in solutions of biological matrices by chelation or chromatographic pre-treatment before resorting to in situ trapping [24]. For this purpose, various matrix modifiers (Table 1), mainly from the Pt group metals, have been used for the collection of CV generated Hg onto Pd–Zr [32], or Au [179], or when utilizing solutions from slurries

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or extractions relying on Pd [66,119], Pd–Au [66], Pd + Rh [66] or Pd + Ir [66] as permanent modifiers. When several modifiers (Pd, Ph, Ir, Pd + Rh, KMnO4 and Ag MnO4) were tested for the determination of Hg in human hair using a furnace coated with boron nitride, best recoveries and lowest detection limits were obtained using Pd [66]. The in situ trapping technique together with the optimum use of permanent modifiers, pre-concentration processes and optimum temperature programs provides detection limits of 0.27–600 µg L− 1 [24,32,101,119], i.e., in the range of detection limits achieved with CVAAS and ICP-MS. The possibility of using HR-CS ETAAS for determination of Hg has only recently been contemplated [180]. Hg has been determined directly from solid biomass samples with calibration against aqueous standards. Detection limits of 100 ng g− 1 make this method suitable for routine monitoring purposes due its simplicity. The possibility of coupling CVG with HR-CS ETAAS or conventional LS ETAAS using a GFA suggests an interesting approach which requires further study. In this manner, lower detection limits and shorter determination cycles can be expected because: (i) the vaporization surface is separated from the absorption volume by a porous graphite wall and (ii) the sample is distributed over a larger surface for collection on the vaporization area. 5. Conclusions and future prospects Many studies have used ETAAS for the determination of trace concentrations of As, Cd, Cu, Hg and Pb in biological samples since commercialization of the first spectrometers. Primarily over the last 15–20 years, there has been a continuous improvement in the methodology yielding enhancements in precision, reproducibility and sensitivity, thereby reducing detection limits. These improvements have implied changes at different stages of the analytical process, from sample pre-treatment to advances in the graphite furnace design as well as spectrophotometer optical components and detectors. The most effective advances have been achieved in the field of pre-concentration and sample pre-treatment, in the use of transversally heated atomizers with platforms, in the application of modifiers and in background correction systems which have significantly improved the determination capacity in complex samples, including solids. In addition, progress has also been made via use of new methods to introduce the sample into the furnace, and in the establishment of optimum furnace temperature programs. With regard to sample pre-treatment, acid digestion using HNO3 alone or in combination with other acids is the most useful approach to matrix destruction if the aim is to determine the total amount of analyte. Alternatively, slurry sampling is another approach to be considered. However, if the more mobile or bioavailable analyte fraction is of interest, extraction by liquid–solid leaching is another possibility. ETAAS allows automation of all sample pre-treatment, including CVG, by use of on line FIA systems. An effective line of advances in improvements in detection limits arises from the use of most modern polymers and FIA systems (Lab-on-valve) for preconcentration of the analyte. Moreover, FIA systems are especially relevant for studies of environmental monitoring, food quality control or human health, when a large number of samples have to be analyzed over long periods. Advances in in situ trapping methods coupled with improvements in methods to supply the sample to the furnace and with appropriate thermal programs have allowed a significant reduction in detection limits for As and Hg. Moreover, recent improvements in graphite furnace design, such as the modern THGA, in the optical and detector components of the spectrometer and in the background absorption correction systems, have jointly enabled detection limits to reach near to or below 1 ng g− 1 in the sample and have increased the suitability of ETAAS for direct analyses of solid samples. Moreover, these advances, together with those in data processing, have allowed the

sequential multielement determination of some elements using LS ETAAS. From the present to the future, use of modern HR-CS ETAAS spectrometers has already achieved notable success with solid sampling analyses, but improvements are needed to achieve faster multielement determinations in biological samples because generally, this approach is focused on the determination of large sample sets. Although expected multielement detection with HR-CS ETAAS appears to have materialized, this method has great possibilities for solving background correction problems which remain the main drawback of LS ETAAS, and which is the main reason for the complexity of some current ETAAS instruments. The use of a GFA with HR-CS spectrometers will offer new prospects for overcoming the slowness of element determination, one of the most serious limitations of ETAAS. Moreover, the possibility of coupling HG or CV generation to HR-CS GFAAAS spectrometers to pre-concentrate the analyte prior to atomization is self-evident. This would the broaden capacity to determine volatile elements such as As and Hg and should promise enhanced limits of detection for these elements in biomasses. The use of platforms in transversely heated atomizers, specific chemical modifiers and long temperature programs is typically not compatible with fast sequential multielement determinations. This situation can be alleviated with the search for new atomization systems. One possibility is the use of GFAs based on the introduction of on line sources that may be particularly suitable for high and medium volatility elements such as As and Hg, thereby avoiding use of chemical modifiers while coping with large sample sources. Following this approach, a transversely heated GFA has been used with good results for determination of Cd and Pb in biological samples [156,181], opening a new means for further advances. Additional improvements in the atomizer (particularly those related to GF or GFA composition), in software and in detectors, can prevent vapor condensation on the atomizer surface, improving long-term repeatability of analytical signals, and further reducing background interferences and the time between sequential measurements by more efficient cooling of the atomizer. At present, and in the near future, the possibility of combining improvements in efficient sample pre-concentration with advances in spectrometer design suggests that the ETAAS technique can compete in figures of merit with other atomic spectroscopic techniques. It should be noted that the low cost and analytical strengths, particularly as regards samples with complex matrices such as biological samples, are particularly attractive features.

Acknowledgements This research was supported by the European project NEU NITROEUROPE (GOCE017841), by the Spanish Government projects CGL2006-04025/BOS and Consolider-Ingenio Montes CSD200800040, and by the Catalan Government project SGR 2009-458.

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