Chromium and its speciation in water samples by

Talanta 132 (2015) 814–828

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Talanta journal homepage: www.elsevier.com/locate/talanta

Review

Chromium and its speciation in water samples by HPLC/ICP-MS – technique establishing metrological traceability: A review since 2000 Barbara Markiewicz, Izabela Komorowicz, Adam Sajnóg, Magdalena Belter, Danuta Barałkiewicz n Department of Trace Element Analysis by Spectroscopy Method, Faculty of Chemistry, Adam Mickiewicz University in Poznań, 89b Umultowska Street, 61-614 Poznań, Poland

art ic l e i nf o

a b s t r a c t

Article history: Received 23 July 2014 Received in revised form 25 September 2014 Accepted 2 October 2014 Available online 14 October 2014

Chromium holds a special position among living organisms because depending on its species it can be either essential or toxic. Cr(VI) even at very low concentrations is harmful and carcinogenic, while Cr(III) is a necessary microelement for cellular metabolism. Therefore, a simple analysis of Cr concentration in collected samples will not be able to distinguish these differences effectively: for a proper chemical analysis we need to perform a reliable detection and quantification of Cr species. Separation and detection of chromium can be accomplished with high performance liquid chromatography hyphenated to inductively coupled plasma mass spectrometry (HPLC/ICP-MS) in a one-step. Our review assembles articles published since 2000 regarding chromium speciation in water samples with the use of HPLC/ICPMS. It addresses the following issues: chromium chemistry, the possibilities of dealing with interferences, metrological aspects, analytical performance and speciated isotope dilution mass spectrometry (SIDMS) which is a definitive measurement method. The authors would like to advocate this hyphenated advanced technique as well as the metrological approach in speciation analysis of chromium. & 2014 Elsevier B.V. All rights reserved.

Keywords: Speciation analysis Chromium speciation HPLC/ICP-MS Water samples Review

Contents 1.

2.

3.

4. 5.

n

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 815 1.1. Cr(III) vs. Cr(VI): the good and the evil twin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 815 1.2. Determination of chromium species. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 815 1.3. Legal norms concerning the concentration of chromium in drinking water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 815 Environmental occurrence of chromium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 816 2.1. Chromium chemistry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 816 2.1.1. pH value. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 816 2.1.2. Oxidation/reduction reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 816 2.1.3. Precipitation/dissolution reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 816 2.1.4. Adsorption/desorption reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 817 2.2. Chromium in water systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 817 Analytical problems in quantification by ICP-MS and their manners of reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 818 3.1. Interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 818 3.1.1. Spectral interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 818 3.1.2. Non-spectral interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 819 3.2. Reaction/collision cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 819 3.3. Other approaches to dealing with interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 819 Speciation analysis of chromium by HPLC/ICP-MS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 823 Metrological approach to measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 823

Corresponding author. Tel.: þ 48 61 829 15 73; fax: þ 48 61 829 15 55. E-mail address: [email protected] (D. Barałkiewicz).

http://dx.doi.org/10.1016/j.talanta.2014.10.002 0039-9140/& 2014 Elsevier B.V. All rights reserved.

B. Markiewicz et al. / Talanta 132 (2015) 814–828

815

5.1.

Method validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824 5.1.1. Linearity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824 5.1.2. LOD and LOQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824 5.1.3. Precision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824 5.1.4. Trueness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824 5.2. Traceability of measurement result . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824 5.3. Estimation of uncertainty budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 825 6. Speciated isotope dilution ICP-MS as a definitive method for determining chromium species. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 825 7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 826 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 827 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 827

1. Introduction

among which are an increased incidence of cancer and dermal disease [5].

1.1. Cr(III) vs. Cr(VI): the good and the evil twin 1.2. Determination of chromium species Element toxicity, bioavailability and transport properties depend on the specific form in which element is present in the environment rather than its total amount. Along with an improvement of analytical methods, speciation analysis has attracted more attention [1]. A lot of research focus on chromium, as it is widely used in industrial processes, including: metallurgy (production of stainless steel, ferroand nonferrous alloys), refractory industry (high temperature-resistant materials used as linings for furnaces) and chemical manufacturing (textile dyes, pigments, electroplating, leather tanning and wood preservatives). Large quantities of chromium compounds are released into the environment which has an adverse effect on the ecosystem [2–5]. Additionally, dissolved species of chromium in relatively high concentrations can occur naturally [6,7]. Chromium is an extraordinary example of an element which different species show opposite types of behavior. This phenomenon occurs in the case of some other metals but not to such an extent. The two most widespread oxidation states of chromium: Cr(III) and Cr(VI) differ with respect to their mobility and bioavailability. In general, Cr(III) compounds are relatively immobile and poorly soluble compared to highly mobile, soluble and, consequently, more bioavailable Cr(VI) compounds [8–10]. These differences extend to the chemical and biochemical reactivity of both species. Cr(III) in trace amounts is an essential nutrient for the human body: it is involved in carbohydrate, lipid and protein metabolism [11–13]. Nevertheless, it appears that the free ion of Cr(III) has no biological activity unless bound to a suitable organic ligand. Its properties are not unequivocally established. Additionally, it has also been reported that Cr(III) picolinate, which may be used as a micronutrient and dietary supplement, can cause harmful effects on human cells, although in very rare cases [5]. By contrast, Cr(VI) is explicitly toxic because of its high oxidation potential and ability to penetrate biological membranes. The ability of Cr(VI) (or actually the CrO4 2 ion) to diffuse freely through cell membranes is possible due to its structural similarity to anions such as SO4 2 or PO4 3 , which are transported by suitable anion exchange channels. The reduced form of chromium is practically unable to enter the cell by the same ion channels, thus Cr(VI) compounds are recognized as 1000 times more cytotoxic. However, intracellular Cr(III), which is the terminal product of the Cr (VI)-reduction, can also exhibit toxic effects e.g., by suppressing the activity of some metallo-enzymes or reaction with macromolecules (including DNA) [14]. Since 1990, the International Agency on Research Cancer (IARC) has classified Cr(VI) compounds to Group 1, known as human carcinogens. The US Environmental Protection Agency (USEPA) has also considered this chromium species as a mutagenic and carcinogenic agent. Epidemiological studies have shown that exposure to Cr(VI) can induce a variety of clinical problems

Due to significant differences in the biochemical properties of both Cr species there is an obvious need for their accurate quantification. In consequence, speciation analysis takes an important role in an environmental monitoring. Numerous papers concerning analytical methods for chromium speciation in water samples have been published during recent years. Among these approaches, HPLC/ICP-MS has proved to be a powerful analytical tool. The high diversity of chromatographic separation mechanisms determines various applications of the HPLC technique. The ICP-MS is a preferred detector in elemental speciation analysis owing to its unique analytical capabilities. Currently, as one of the most robust commercially available instruments, ICP-MS presents many advantages that include elemental specificity, multi-isotope detection and extremely low detection limits [15,16]. Furthermore, it offers high sensitivity of measurements along with a wide linear dynamic range. In addition, ICP-MS can be easily coupled online with HPLC [17]. 1.3. Legal norms concerning the concentration of chromium in drinking water The World Health Organization (WHO) has recommended that the concentration of Cr in drinking water should be 0.05 mg L 1 or less [18]; a similar value has also been published by the Council of the European Union [19]. However, according to the drinking water standards laid down by the Environmental Protection Agency (EPA), in the United States the permissible level of chromium is 0.10 mg L 1 [20]. However, the recommended values of Cr concentration in drinking water do not take into consideration particular species of this element. They provide just a temporary solution implemented because no routine or reliable method exists for the determination of low concentrations of chromium in water. The number of articles concerning elemental speciation has increased appreciably over the last two decades, particularly since 1995 [21,22] and many different analytical strategies have been developed to deal with quantification of Cr forms existing in the nature. The papers (also overview articles) have covered various topics such as environmental occurrence [2,23,24], risk assessment, health effects [5,25–29] and analytical approaches (including offline and online methods) [3,21,30–41]. In a recently published comprehensive overview the application of the isotope dilution technique, which provides the highest quality of metrological traceability in chromium speciation analysis in different matrices, was widely described [41]. In this review we present a new perspective for the determination of chromium chemical forms in water. We focus on the problems connected with obtaining reliable and competent results,

816


which are basic requirements for any projected legislation concerning chromium species determination in water. Unlike other reviews, besides presenting several issues related to chromium occurrence in water environments, determination of chromium species by the HPLC/ICP-MS method and accompanying problems, this work raises an important issue regarding the quality assurance of obtained analytical results: method validation, the traceability and uncertainty of the measured results.

2. Environmental occurrence of chromium 2.1. Chromium chemistry Cr can exist in several oxidation states from 0 to VI. The two forms of chromium commonly found in the environment are Cr (III) and Cr(VI). The intermediate states: Cr(II), Cr(IV), Cr(V) are unstable products in oxidation and reduction reactions of trivalent and hexavalent chromium, respectively [2,8,11]. The prevalent forms of chromium might undergo a series of transformations, changing from one into another under the influence of various physicochemical processes. In many cases, disruption of the chemical balance between particular species strongly depends on pH value. Additionally, redox reaction, precipitation/dissolution and adsorption/desorption can cause Cr conversions. Some of these aspects are discussed below [9,10,12,24]. 2.1.1. pH value In aqueous solution (assuming the absence of other complexing agents besides H2 O or OH ), Cr(III) exists as a hexa-aqua complex CrðH2 OÞ33 þ (abbreviated as Cr3 þ ) which has hydrolysis products as demonstrated by equations (Eqs. 1–3). CrðH2 OÞ33 þ þ H2 O2CrðOHÞðH2 OÞ25 þ þ H3 O þ

ð1Þ

CrðOHÞðH2 OÞ25 þ þ H2 O 2 CrðOHÞ2 ðH2 OÞ34 þ þ H3 O þ

ð2Þ

CrðOHÞ2 ðH2 OÞ4þ þ H2 O2CrðOHÞ03 þH3 O þ

ð3Þ

The deprotonated forms of hexaaquachromium(3þ) can be simply abbreviated to CrðOHÞ2 þ , CrðOHÞ2 þ and CrðOHÞ3 0 . The presence of these hydroxo species in the environment is closely related to pH. In more acidic solutions (pHE4), Cr3 þ is the predominant species of Cr(III). Under slightly acidic conditions its hydrolysis generates CrðOHÞ2 þ and CrðOHÞ2 þ . Within a pH range from neutral up to alkaline Cr(III) tends to precipitate as a virtually insoluble CrðOHÞ3 ðsÞ , which is transformed into soluble CrðOHÞ4 in more basic solutions (pH412), because of its amphoteric properties. As a consequence, in a pH range typical for natural waters (approximately 4–9), CrðOHÞ2 þ and CrðOHÞ3 0 are the prevalent aqueous forms of Cr(III). In the case of Cr(VI), pH-value and its total concentration determine the relative fractions of these species occurring in a solution. H2 CrO4 belongs to the strong acids and is only observed below pH 1. As a result of its dissociation the following products are formed: HCrO4 at a pH from about 1–6.5 and at a Cr(VI) concentration below 0.01 mol L 1, whereas Cr2 O7 2 is prevalent in more concentrated solutions (c40.01 mol L 1) and CrO4 2 at pH-value higher than 6.5 over the entire concentration range. All of the above-mentioned Cr(VI) oxyanions are present in a pH range of natural waters [2,8,9]. 2.1.2. Oxidation/reduction reactions Redox potential (Eh) and pH play a crucial role in the interconversion of both stable states of chromium. To understand the behavior of Cr(III) and Cr(VI), please refer to the Eh–pH diagram for chromium species in aquatic environments presented in Fig. 1.

Under acidic conditions, Cr(VI) exhibits a high positive Eh value, which means it is a strong oxidizing agent and is easily reduced by electron donors such as organic matter or appropriate inorganic compounds widespread in water systems. At neutral pH, the Cr(III)–Cr(VI) proportion is related to the oxygenation of solution. By contrast, in more alkaline media, the oxidizing character of hexavalent chromium is much less effective and thus it has a tendency to remain in this state rather than be reduced [2,9]. The concentration of reducers and oxidation mediators also regulates the redox behavior of chromium. Even though the Eh value of the Cr(VI)/Cr(III) couple is high there are several oxidants in the natural aquatic environment that can oxidize Cr(III) into Cr(VI) but only a few of them are found at sufficient levels. Dissolved oxygen does not affect the oxidation of Cr(III) if additional compounds are absent. Only in the presence of manganese oxides Cr(III) can be readily converted to Cr(VI). Although hexavalent Cr reduction is less privileged thermodynamically, this process can be carried out because the concentrations of reductants are high enough. Consequently, the major factors in the water body taking part in the transformation of Cr(VI) into Cr(III) are: Fe2 þ , S2 , dissolved organic matter (e.g., humic acid), as well as the activity of microorganisms [7,10,13,42,43]. Sunlight also plays an essential role in the regulation of chromium chemistry in surface water. It has been proven that photochemical generation of Cr(III) and Cr(VI) is possible. The sunlight contributes to the reduction of iron which affects the formation of hydrogen peroxide and both of these reducing agents favor the reduction of Cr(VI). Additionally, sunlight influences the oxidation of manganese (which determines Cr(III) formation), breaks organic bindings of chromium and thereby releases its inorganic species [7]. However, the problem of Cr(III) oxidation to more toxic Cr(VI) may be intensified owing to the anthropogenic contamination of water and an application of ozone in water treatment systems.

2.1.3. Precipitation/dissolution reactions Precipitation and dissolution are other important aspects that influence the distribution of Cr species in the environment. They are ruled by the solubility of the chromium compounds, which varies between individual Cr species [7]. Under conditions prevalent in surface waters, Cr(III) solubility is limited by the formation of insoluble chromium hydroxides, including mixed ones (e.g., ðCr; FeÞðOHÞ3 ). The amount of soluble Cr(III) is extensively regulated by the precipitation of these hydroxides, especially mixed iron–chromium that have even lower solubility than CrðOHÞ3 [24,42]. However, trivalent chromium exhibits a tendency to form various complexes with naturally

Fig. 1. Trivalent and hexavalent species of chromium in water environments as a function of Eh vs. pH; assuming the presence of H2 O, OH only and the concentration of total Cr¼ 10 6 mol L 1. The dotted lines demonstrate precipitated Cr(III) as CrðOHÞ3 ðsÞ , and the gray zone illustrates the typical range for pH in natural waters [2,9,10].


occurring macromolecular substances such as fulvic or humic acids. This complexation reduces the efficiency of Cr(III) precipitation and affects the mobility of organic ligands. If humic compounds are bound to sediment or soil, we observe the removal of Cr(III) from aquatic environments due to formation of its stationary complexes. A large number of Cr(VI) compounds (chromates and dichromates) are water-soluble at wide range of pH. However, they can also exist as solid minerals containing different divalent cations 2þ (e.g., Ba2 þ , Sr2 þ , Pb , Ca2 þ ). The solubilities of CrO4 2 , Cr2 O7 2 and above mentioned cations are governed by pH, and it has been shown that the dichromates are, in principle, highly soluble as opposed to the chromates [7,8,44]. Therefore, the mobility of hexavalent anionic forms is relatively greater than trivalent chromium, which causes the first ones to be more available for living organisms in soil-water systems [9,42,45]. 2.1.4. Adsorption/desorption reactions The concentration of chromium in natural aquatic systems is also controlled by adsorption and desorption reactions. Sorption causes the migration of Cr from the aqueous phase to the surface of suspended and settled sediments. The equilibrium of this process depends on complex environmental conditions such as pH, surface area or density of active sites [43]. Hexavalent chromium can be adsorbed on particle surfaces (which have protruding –OH groups), including iron and aluminum oxides as well as kaolinite. Iron oxides are the dominant adsorbents in acidic to neutral oxidized soils and ground water. The sorption of dissolved Cr(VI) is higher at lower pH values. However, Cr(VI) anions are generally weakly adsorbed on minerals due to their being repelled by the negative charge of soil particles [13]. On the other hand, Cr(III) species are attracted to partially negatively charged surfaces such as silicates, and organicallybound Cr(III) can also adsorb on or desorb from the organic substances [2,7,8,45]. 2.2. Chromium in water systems Chromium and its compounds present in water come mostly from anthropogenic sources but natural Cr release cannot be neglected. In combination with other elements, Cr naturally occurs

817

in the environment mainly as an ore mineral – chromite ðFeCr2 O4 Þ. The primary natural sources of this element, contributing to an increase of its concentration in aquatic systems, are: weathering of rocks, wet precipitation and dry fallout from the atmosphere (e.g., volcanic dust, forest wildfires), as well as run-off from terrestrial areas. As mentioned earlier, Cr contamination of water and soil is caused by intensive industrial activities to a much greater extent than natural release. However, elevated levels of naturally occurring Cr(VI) in ground and surface waters (exceeding the value of total Cr for drinking water recommended by the WHO) have been observed. This is a consequence of a release of Cr(III) to solution from rocks (e.g., ultramafic) and then its oxidation by commonly found MnO2 [6]. The introduction of sewage, which is rich in chromium, into waters (mostly surface waters) has become a serious problem and may be a result of the discharge of industrial waste (e.g., from metallurgical and electroplating plants, tanneries or from dyeing and other chemical factories) as well as sanitary landfill leaching [45,46]. The industrial use of Cr is associated with chromite mining. In 2011 its total world mine output was approximately 24 000 103 metric tons. Large reserves of this ore exist in Kazakhstan, South Africa, India and the United States [10]. The majority of chrome ore production (90%) is consumed by metallurgical industries, while chemical and refractory industries equally account for 5%. Chemical processes in which chromium is used influence the quantity and species of Cr later released in sewage outflows. The above mentioned activities employ an oxidized form of Cr [2,7,12,24]. Typically, lakes are characterized by high levels of biological activity and high ratios of sediment-to-water surface area that control transport of metals. Significant amounts of organic matter, which are responsible for establishing reductive and complexing conditions, favor reduction of Cr(VI). Subsequently, the fate of chromium depends on the various processes discussed above. Chromium enters the ocean in two ways: by rivers or atmospheric fallout. In the ocean waters an equilibrium exists between dissolved and precipitated Cr. The most prevalent form of chromium in seawater is found in chromates (besides estuaries), as a result of the oxidizing character of these waters and the low concentration of suspended solids. When examining the distribution of chromium species in ground water, we should take into account their solubility and ability to be

Fig. 2. The chromium cycle in the water-sediment system [2,9,23].

818


sorbed by soil or aquifer materials. These factors are governed by the chemistry of this type of water and the properties of soil or aquifer material. In general, Cr(III) is immobile in most ground waters due to the formation of poorly soluble compounds (in neutral to alkaline pH values). Hexavalent chromium, in turn, tends to be the most mobile form of Cr in shallow ground water. Chromium species behave similarly in surface waters [7]. The transformations and movement of chromium species in the environment are illustrated in Fig. 2. Natural concentrations of total chromium in surface waters (except regions with substantial chromium deposits) are very low, normally between 1 and 10 mg L 1, and in lake water do not exceed 5 μg L 1 [47]. Total Cr concentration in surface waters in Europe ranges from o 0.01 μg L 1 to 43.3 μg L 1, with a median value of 0.38 μg L 1 [13]. Chromium concentration in sea water is one order of magnitude lower than in rivers and freshwater lakes, most of the samples contain 0.1 μg L 1–0.5 μg L 1 of Cr. In ground water the content of chromium is comparable to the typical amount of this element found in surface waters [45,48]. Total Cr level in drinking waters is predominantly less than 2 μg L 1, however, a higher concentration of chromium (120 μg L 1) has also been noted [18]. Concentrations of specific species of Cr in various water samples, which have been analyzed using HPLC/ICPMS, are summarized in Table 1.

3. Analytical problems in quantification by ICP-MS and their manners of reduction 3.1. Interferences Elemental speciation, as previously discussed, involves the coupling of two analytical tools: a highly selective separation technique with a sensitive detection. Among numerous methods described in the literature concerning the quantification of chromium species, most of the recent ones are based on the ICP-MS as a detector. The ICP-MS instrument provides many advantages, however, it also presents some pronounced difficulties such as

spectral and non-spectral interferences, in particular quadruple ICP-MS (qICP-MS).

3.1.1. Spectral interferences Spectral interferences occur mainly due to polyatomic and isobaric ions, which have the same mass-to-charge ratios (m/z þ ) as the analyte of interest. For the determination of chromium by ICP-MS, four Cr isotopes with natural abundances between 2.4% and 83.8% are available. Nevertheless, all of these isotopes are affected by numerous polyatomic species, which are formed by interaction between constituents of gaseous plasma (Ar), reagents and sample matrix (O, H and less often C, N, Cl, S, P) [49]. The þ polyatomic interferences on 52 Cr þ , including 40 Ar12 C and 35 16 1 þ Cl O H are the most frequent ones for ICP-MS detection of Cr. Despite the fact that determination of chromium has some difficulties, 52Cr is preferred by researchers because it is the most abundant isotope (83.8%) [12,28,32,50,51]. Additionally, isobaric interferences are caused by isotopes of other elements that overlap ions at the masses of interest, but doubly charged ions derive only from elements with a low second ionization energy, therefore their formation depends on the ionization source [49]. Spectral interferences associated with the measurement of chromium ions using ICP-MS are listed in Table 2. A variety of approaches can be applied to effectively overcome interferences caused by the limited resolving power of qICP-MS. In order to eliminate isobaric and doubly charged ions, the usage of alternative isotopes of the analyte is recommended. Moreover, mathematical correction (also as an option in the software of contemporary ICP-MS) can be used for isobaric interferences, while the optimization of operational conditions of the instrument is suitable for doubly charged forms [49,52]. In turn, the application of multipole collision (hexapole or octapole) or reaction cells (in upgraded qICP-MS), mathematical correction, cool plasma conditions, high resolution ICP mass spectrometer or optimization of the chromatographic separation reduces the level of polyatomic interferences to a great extent [17,37,44,53–55].

Table 1 The concentration of chromium in varied water samples. Naturally occurring concentration [mg L 1]

Sample

52

52

o 1.60 7 0.06 0.116 70.007 0.08 0.2727 0.019 0.205 7 0.038 1.08–1.27 or oLOD 0.17 o 1.60 7 0.08 No data 0.066 7 0.006 o 1.60 7 0.02 0.080 7 0.024 o LOQ 0.14–0.56

o 1.20 7 0.08 0.050 7 0.012 o LOD 0.083 7 0.010 o LOD 0.96–1.70 0.87 o 1.20 7 0.08 0.73–1.17 or o LOD o LOD 2.30 7 0.03 0.1357 0.009 0.2977 0.024–1.032 7 0.082 3.7 or o LOD

[44] [3] [71] [3] [74] [70] [77] [44] [50] [3] [44] [69] [72] [71]

Cr(III)

Natural water

Surface water Lake water Pond water SLEW-3 (estuarine water) River water Ground water Porewater Tap water

Municipal water

References

Cr(VI)

Beverage

Bottled water

0.12–0.31 0.005–0.05 0.036 7 0.006–0.060 70.005

o LOD 0.055–0.24 0.054 7 0.003–0.409 7 0.002

[71] [67] [68]

Industrial water

River water/stream (discharge of urban and industrial effluents)

No data 1.65 7 0.09 22.2 7 1.3 26.2 7 1.6 0.82 7 0.10–5.10 7 0.60

o 5.9a o LOD 31.7 7 1.8 48.07 2.3 21.0 7 1.5–2337 16a

[75] [4] [53] [32] [54]

Waste waters Brines from industrial process a

Monitored isotope –

53

Cr.


819

Table 2 Spectral interferences, including the most common (in bold) associated with chromium detection by ICP-MS [65,97–99]. Isotope

Interferences Isobaric

50

50

52

Cr (4.3%)a Cr (83.8%)a 53 Cr (9.5%)a

– –

54

Cr (2.4%)a a

54

V þ ,50Ti þ

Polyatomic S O þ , 36Ar14N þ , 35Cl15N þ , 36S14N þ , 32S18O þ , 33S17O þ Ar12C þ , 35Cl16O1H þ , 36Ar16O þ , 37Cl15N þ , 34S18O þ , 36S16O þ , 38Ar14N þ , 36Ar15N1H þ , 35Cl17O þ 37 16 þ 38 Cl O , Ar15N þ , 38Ar14N1H þ , 36Ar17O þ , 36Ar16O1H þ , 35Cl17O1H þ , 35Cl18O þ , 36S17O þ , 40Ar13C þ 34 16

40

Fe þ

37

Cl16O1H þ ,

Ar14N þ ,

38

Ar15N1H þ ,

Ar18O þ ,

36

Ar16O þ ,

38

Ar17O1H þ ,

36

Cl17O þ ,

37

F216O þ

19

Abundance of naturally occurring chromium isotope recommended by IUPAC.

3.1.2. Non-spectral interferences Non-spectral interferences called matrix-effects are induced by the sample matrix or mobile phases when organic compounds as well as inorganic salts are contained. They contribute to clogging the nebulizer, the sampling and skimmer ICP-MS cones, they can also accumulate on the torch and the ion lens, causing distortions by increasing or decreasing signal intensities. Incorrect measurements stemming from non-spectral interferences are usually overcome with the use of appropriate calibration procedures e.g., standard addition method or internal standardization [49,52]. 3.2. Reaction/collision cells Among currently available approaches to cope with the problem of spectral interferences, dynamic reaction cells (DRC) as well as multipole collision cells (MCC) are one of the most effective. The DRC is a rf/DC quadrupole placed inside the enclosed reaction chamber which is positioned between the single lens ion optics and the mass filter. The DRC is pressurized with a reactive gas in order to carry out ion-molecule reactions intended to suppress plasma-based polyatomic interferences [56]. The bandpass of the DRC is selected in a manner so as to control the generation of new interfering species within the reaction cell. Removal or reduction of polyatomic interferences has been achieved by different processes including neutralization or exchange reaction between interference ions and gaseous molecules pressurizing the cell. Products of these reactions are swept from the chamber with a continuous fresh reaction gas stream [57–61]. As a consequence of these interactions, the obtained chemical forms are characterized by another m/z þ than the analyzed ion or they are converted into neutral species which cannot be recognized by the detector. Preferred reaction gases as well as the mixtures (e.g., with argon) used in the DRC are: ammonia, methane, hydrogen or oxygen [62,63]. The selection of gas depends on its ability to undergo a gas phase chemical reaction with the polyatomic species and to remove them. An exemplary interaction between NH3 , as an extensively employed reaction gas, and argon plasma ion was demonstrated as follows [56]: Ar þ þ NH3 -NH3 þ þAr ðneutralizationÞ According to scheme presented above, the major interference þ 40 Ar12 C , which occurs at the same m/z þ as 52 Cr þ , can also be transformed into neutral species by ammonia [58]: 40

40

Ar12 C

þ

þ NH3 -NH3 þ þ 40 Ar þ 12 C

Carbon-based interferences react much faster with NH3 than chromium, and thus the interfering ions will be effectively reduced. As can be seen, the ionization energy of Cr (6.8 eV) is lower than NH3 (10.2 eV), therefore, the electron transfer from ammonia to chromium ion is endothermic and the reaction is not observed to proceed [64]. MCC similar to the DRC employ rf to focus the ion beam instead of separating ions according to their m/z þ like a typical mass analyzer. However, in the case of collision technology, non-reactive gases (e.g., He, H2 , Xe) are introduced into the cell and they can attenuate or even eliminate polyatomic interferences by an ion–

molecule collision mechanism. In order to separate the collision product from the analyte, kinetic energy discrimination (KED) is used. Particular ions will be distinguished, while the analyte ions will have higher energy than the interfering ions [62,65,66]. In the reviewed articles the application of DRC/MCC technology was one of the most frequently adopted approaches used to overcome spectral interferences. Sakai et al. used H2 as a collision þ cell gas for attenuating 40 Ar12 C and 35Cl16O1H þ , primary basedþ matrix interferences at m/z 52 [67]. Chen et al. reported that an octopole reaction system can be used for determination of 52 Cr þ in the presence of chloride, using either an He or H2 mode [53]. However, the authors also recommended H2 cell gas to efficiently remove polyatomic spectral interferences. By contrast, Wang et al. suppressed polyatomic interferences from chlorine and carbon by application of a mixture of 7.28% H2/He, while McSheehy et al. utilized a mixture of 8% H2/He as the collision cell gas for þ preventing 40 Ar12 C [28,68]. An alternative approach to the above mentioned system was collision/reaction interface (CRI), where a collision/reaction gas had been introduced through the sampler or skimmer cones [69]. The use of H2 CRI gas at 70 mL min 1 appreciably eliminated the problem of chlorine-based and carbon-based interferences and so enabled a reliable determination of 52 Cr þ . Bednar et al. employed methane as a reaction gas in þ DRC for attenuating 40 Ar12 C interferences at m/z þ 52 and 53 [44]. Tsoi and Leung applied DRC technology to overcome polyatomic ions in multi-elemental speciation analysis of seven forms of arsenic, chromium, and selenium altogether [70]. They tested the efficiency of the interference reduction by optimizing the following DRC conditions: gas flow rate and rejection parameter q (Rpq) using various reaction gases including NH3 , CH4 and O2 . The experiments revealed that the preferable gas for determination of all seven analytes of interest is CH4 . Its capability of dealing with the problem of chloride interferences, under the optimal conditions of the reaction cell, was demonstrated by the analysis of 250 mg L 1 NaCl solution. However, Neubauer et al. suppressed interferences encountered during trace element speciation of þ arsenic and chromium, using O2 , which reduces the 40 Ar12 C 52 75 16 þ þ ions on Cr and forms a new species As O free from interfering ions [71]. Other authors selected NH3 as the DRC gas to remove a great part of the chlorine-based and carbon-based polyatomic interferences, predominantly occurring in chromium determination. [50,72,73]. They studied influential operating parameters for the DRC system (gas flow rate and Rpq) to achieve the optimal signal to noise (S/N) ratio. In particular Chang et al. and Wolf et al. effectively reduced the interfering background peaks at m/z þ 52 as well as 53 (under the optimized DRC conditions) by three or even four orders of magnitude [3,12]. 3.3. Other approaches to dealing with interferences An alternative approach to resolve the problem of spectral interferences is the use of mathematical correction [49,52,55]. As previously mentioned (Section 3.1.1), corrections may be applied þ for evaluating the formation of 40 Ar12 C and 35Cl16O1H þ , being the

820


Table 3 Analytical and metrological aspects concerning chromium species determination in water matrices by HPL/ICP-MS based on the literature review since 2000. No. Sample

1. Species 2. Matrix

1

1. Cr(III), Cr(VI) 2. Brines from industrial process

Sample storage and Chromatographic separation preservation conditions 1. Column 1. Type of 2. Elution type container 3. Mobile phase 2. Temperature (1C) 4. Complexing agent 3. Acidification

1. No data 2. No data 3. No data

1. Anion exchange: ANX3202 CETAC Tech. (2 mm 200 mm), guard disc 2. Isocratic 3. 60 mM HNO3 4. No data

HPLC operating conditions

Validation characteristicsa

1. Mobile phase flow rate (mL min 1) 2. Column temperature (1C) 3. Injection volume (mL) 4. Retention time (min) i. Cr(III) ii. Cr(VI) iii. Total analysis time

1. i. ii. 2.

1. No data 2. No data 3. 20 4. i. 1.30 ii. 3.30 iii. 5.00

1.

References

LOD (mg L 1) Cr(III)b Cr(VI) Precision (%) i. Cr(III) ii. Cr(VI)

3. Recovery (%) i. Cr(III) ii. Cr(VI)

[54] i.

ii.

52

Cr, Rc ¼ 300: 0.080 (HP)c, 0.040 (CP)c, 52Cr, R¼ 3000: 0.050 (HP), 53Cr, R ¼300: 0.060 (HP), 0.140 (CP), 53Cr, R¼ 3000: 0.170 (HP), 52 Cr, R ¼300: 0.190 (HP), 0.070 (CP), 52Cr, R¼ 3000: 0.120 (HP), 53Cr, R¼ 300: 0.150 (HP), 0.280 (CP), 53Cr, R¼ 3000: 0.360 (HP)

2. No data 3. No data 2

3

1. Cr(III), Cr(VI) 2. Reference Material (synthetic and estuarine water)

1. No data 2. No data 3. HNO3

1. Cr(VI), As(III), As (V), MMA, DMA, Se(IV), Se(VI) 2. Surface water (located in an industrial area)

1. Polyethylene bottles 2. Cooling (4) 3. Without

1. Anion exchange: IonPac AS7 Dionex (4 mm 250 mm, 10 μm), guard column: IonPac AG7 Dionex (4 mm 50 mm, 10μm) Isocratic 2. 50 mM NH4NO3, pH¼ 8.0 3. EDTA

1. 1.5 2. No data 3. 1000 i. 4.30 ii. 10.80 iii. 11.50

1. Anion exchange: PRP-X100 Hamilton (4.1 mm 250 mm, 10 μm) 2. Gradient 3. A: 20 mM NH4NO3 B: 60 mM NH4NO3, pH¼8.7 for A and B 4. No data

1. 1.0 2. No data 3. 100 i. 9.40 ii. 14.00

1.

[74] i. ii.

52

Cr: 0.005 53Cr: 0.007 Cr: 0.012 53Cr: 0.016

52

2. 4.8 for 0.05 mg L 1, for Cr species i. 94–97 ii. 98–101

[75]

1. ii.

0.2

ii.

2.6 (ST)c for 0.45 mg L 1 1.6 (ST) for 10 mg L 1 0.6 (ST) for 100 mg L 1 3.6 (LT)c for 10 mg L 1

ii.

80–100

i. ii.

52

2.

3.

4

5

1. Cr(III), Cr(VI) 2. Surface water (located in an industrial area)

1. Cr(III), r(VI) 2. Ground water

1. No data 2. Freezing ( 20) 3. No data


1. Ion exchange: IonPac CS5A Dionex (4 mm 250 mm, 9 μm), guard column: IonPac CG5A Dionex (4 mm 50 mm, 9 μm) 2. Gradient 3. A: 0.35 M HNO3 B: 1 M HNO3 4. No data

1. 1.0 2. No data 3. 100 i. 7.15 ii. 2.60 iii. 9.00

1. Anion exchange: ANX4605 CETAC Tech. 2. Isocratic 3. 0.06 M HNO3 4. No data

1. 1.0 2. No data i. 200 ii. 0.90 iii. 2.00 iv. 2.90

1.

[4] Cr: 0.32; 53Cr: 0.38 Cr: 0.19; 53Cr: 0.20

52

2. for 10 mg L 1 52 i. Cr: 3.8 (ST) 53Cr: 4.0 (ST) 52 ii. Cr: 1.8 (ST) 53Cr: 2.5 (ST) iii. 0–56 iv. 101–102


[77]


821

Table 3 (continued ) No. Sample

6

7

8

1. Cr(III), Cr(VI) 2. Mineral water

1. Cr(III), Cr(VI) 2. Waste water, lake water and seawater

1. Cr(III), Cr(VI) 2. Waste water

Sample storage and preservation conditions 1. No data 2. No data 3. No data



Chromatographic separation


1. Anion exchange: G3268A Agilent (4.6 mm 30 mm) 2. Isocratic 3. 5 mM EDTA, pH¼ 7.0 4. EDTA

1. 1.2 2. Ambient i. 500 ii. 1.00 iii. 2.40 iv. 3.00

1. Anion exchange: G3154A/101 Agilent, guard column: G3154A/102 Agilent 2. Isocratic 3. 20 mM NH4NO3, pH¼ 5.9 4. EDTA

1. 1.0 2. 30 3. 50

1. Anion exchange: G3154A/101 Agilent, guard column: G3154A/102 Agilent 2. Isocratic 3. 20 mM NH4NO3, 10 mM NH4H2PO4, pH¼ 7.0 4. EDTA

1. 1.0 2. 30

i. ii. iii.

i. ii. iii. iv.


References

1.

[67] i. ii.

0.0132 0.0158

2. for 0.5 mg L 1 i. 0.80 (RT)c 1.87 (PA)c 2.28 (PH)c ii. 0.57 (RT); 1.79 (PA); 3.01 (PH) iii. 100–105 iv. 95–102

1.

[53] i. ii.

4.50 8.59 10.00

0.3 0.4

2. for 20 mg L 1 i. 1.9 ii. 2.8 iii. 92.3–98.5 iv. 91.1–94.5

1. 50 4.21 5.77 7.00

[32] i. ii.

0.2 0.1

2. for 10 mg L 1 i. 2.8 ii. 1.9 3. 91.5–104.3 for Cr species

9

10

11

1. Cr(III), Cr(VI), Se (IV), Se(VI) 2. Surface, ground and tap water, synthetic laboratory sample (high salted)

1. No data 2. Cooling (4), 10 (AT)c 3. No data

1. Cr(III), Cr(VI) 2. Tap water

1. Polyethylene bottles 2. No data 3. Without

1. Cr(III), Cr(VI), As (III), As(VI), MMA, Se(IV), Se (VI) 2. River water

1. Polypropylene bottles 2. Cooling (4) 3. Without

1.

1. Anion exchange: IonPac AS11 Dionex (4.1 mm 250 mm), guard column: IonPac AG-11 Dionex (4.1 mm 50 mm) 2. Isocratic 3. 20 mM NaOH 4. EDTA

1. 1.25 2. 25 3. 35 i. ii. iii.

1. Ion exchange: IonPac AG7 Dionex (4 mm 50 mm, 10 μm) 2. Gradient 3. A: 0.1 M NH4NO3 (pH ¼ 4.0) B: 0.8 M HNO3 4. No data

1. 1.5 2. Ambient 3. 50 i. 3.40 ii. 1.30 iii. 7.50

1. Anion exchange: PRP-X100 Hamilton (4.6 mm 150 mm, 5 μm), guard column: PRPX100 Hamilton 2. Gradient 3. A: 0.5 mM NH4H2PO4, 10 mM NH4NO3 B: 30 mM NH4NO3, pH¼ 6.0 for A and B 4. EDTA

1. 2.0 2. No data 3. 200 i. 6.01 ii. 8.35 iii. 11.00

[44] i. ii.

0.40 2.80 4.00

52

53

52

53

Cr: 0.8; Cr: 0.6;

Cr: 1.5 Cr: 0.7

2. No data 52 Cr: 78–114; i. 52 ii. Cr: 85–114;

53

Cr: 80–111 Cr: 85–113

53

[69]

1. i. ii.

0.04 0.02

2. No data 3. 101.5 for Cr species

[70]

1. i. ii.

0.24 0.29

2. No data 3. 92.7–108.2 for Cr species

822



12

1. Cr(III), Cr(VI) 2. River sediment pore water

Sample storage and preservation conditions 1. No data 2. No data 3. No data


1. Anion exchange: IonPac AS7 Dionex (4 mm 250 mm, 10μm) 2. Isocratic 3. 60 mM NH4NO3, pH¼ 8.0 4. EDTA (in the 2nd option)c


1. 2. 3. 4.

1.0 No data 100 No data


References

[78]

1. i. ii.

0.23, 0.10 (with ETDA) 0.14, 0.11 (with ETDA)

i. ii.

12, 9.2 (with ETDA) 11, 5.7 (with ETDA) for 12– 13 QC calibration check samples

2.

3. i. ii.

13

14

1. Cr(III), Cr(VI) 2. Lake, pond and tap water



1. No data, glass HPLC vials 2. No data 3. No data

1. Reverse phase: CR C8 PerkinElmer (3 mm 30 mm, 3 mm) 2. Isocratic 3. 0.6 mM EDTA, 2 mM TBAP, 2% (v/v) MeOH, pH¼ 6.9 4. EDTA

1. 1.0 2. Ambient 3. 100 4. i. 1.90 ii. 3.10 iii. 4.00

1. Reverse phase: 3CR C8 PerkinElmer (4.6 mm 33 mm, 3 μm) 2. Isocratic 3. 1 mM TBAH, 0.6 mM EDTA, 2% MeOH, pH¼ 6.9 4. EDTA

1. 1.5 2. Ambient 3. 50 i. 1.15 ii. 1.80 iii. 3.00

76–140, 1.3–180 (with ETDA) 62–90 but in majority of samples Cr(VI) was not recovered, all spiked samples had zero (or virtually zero) percent recovery of Cr(VI) (with ETDA)

1.

[3] i. ii.

2.

52

Cr: 0.063; 52 Cr: 0.061;

53

Cr: 0.117 53 Cr: 0.115

52 Cr: o2 (ST) for 1 mg L 1, for Cr species i. 90.9–95.3 ii. 89.6–109.8

1.

[73] i. ii.

0.050 mg L 1 0.050 mg L 1

2. No data 3. No data

15

1. Cr(III), Cr(VI), As (III), As(V) 2. Lake, river, municipal and bottled water

1. Polyethylene/ polypropylene bottles 2. No data 3. Without

1. Reverse phase: Pecosphere C8 PerkinElmer (30 mm, 3 μm) 2. Isocratic 3. 1.0 mM TBAH, 0.5 mM EDTA, 5% MeOH, pH¼ 7.2 4. EDTA

1. 1.5 2. No data 3. 50 i. 1.10 ii. 1.80 iii. 2.50


[71]

16

1. Cr(III), Cr(VI) 2. Mineral and spring water


1. Reverse phase: Hypersil GOLD™ Thermo (4.6 mm 150 mm, 5 μm) 2. Isocratic 3. 0.25 mM TBAP, mM EDTA, pH¼ 6.9 4. EDTA

1. 0.8 2. No data 3. 50 and 200 i. 3.58 ii. 4.33 iii. No data

1.

[68]

1. Reverse phase: 3CR C8 PerkinElmer (4.6 mm 33 mm, 3 μm) 2. Isocratic 3. 2.0 mM TBAH, 0.5 mM EDTA, 5% (v/v) MeOH, pH¼ 7.6 4. EDTA

1. 1.5 2. 35

17

1. Cr(III), Cr(VI) 2. Natural water

1. No data, glass HPLC vials 2. Cooling (10), 10 (AT) 3. Without

i. ii.

0.039 (Ic – 50 mL) 0.017 (I – 200 mL) 0.017 (I – 50 mL) 0.009 (I – 200 mL)

2. No data i. 90–110 ii. 98–106

i. ii. iii. iv.

1. 50 1.10 1.70 2.00

[12] i. ii.

0.09 0.06

2. 3 for 2 mg L 1, for Cr species i. 107.0–138.8 ii. 55.2–108.5


823


Sample storage and preservation conditions




References

18

1. Cr(III), Cr(VI) 2. Sediment and porewater

1. Polyethylene bottles 2. Cooling (4) 3. Without

1. Reverse phase: 3CR C8 PerkinElmer (4.6 mm 33 mm, 3 μm) 2. Isocratic 3. 2.0 mM TBAH, 0.6 mM EDTA, pH¼ 6.9-7.0 4. EDTA

1. 1.5 2. No data 3. 50 i. 0.90 ii. 1.25 iii. No data

1. LOQc: 0.30–0.50 for Cr species 2. No data 3. No data

[50]

19

1. Cr(III), Cr(VI) 2. Human urine

1. Polyethylene bottles 2. Freezing (-80) 3. Without

1. Reverse phase: PRP-1 Hamilton, (4.6 mm 150 mm, 5μm) 2. Isocratic 3. 2.0 mM TEAH, pH ¼ 3.5 4. EDTA

1. 1.5 2. Ambient 3. 100 i. 1.09 ii. 2.02 iii. 4.00

1. 0.03 for Cr species 2. No data ii. 94.8–105

[28]

20


1. Polyethylene bottles, glass HPLC vials 2. Freezing, Peltiercooled autosampler tray 3. No data

1. Reverse phase: 3CR C8 PerkinElmer (4.6 mm 33 mm, 3 μm) 2. Isocratic 3. 0.8 mM TBAH, 0.6 mM EDTA, pH¼ 6.9 4. EDTA

1. 1.2 2. 25 3. 50

1.

[72]

i. ii. iii.

i. ii. 1.42 1.92 3.00

0.094 0.10

2. for 2 mg L 1 i. 1.5 (ST) 3.4 (LT) ii. 1.6 (ST) 3.5 (LT) iii. 100–115 iv. 93–106

a

The parameters typically found in the reviewed articles. Determination of 52Cr, unless otherwise stated. R – mass resolution; HP – hot plasma conditions; CP – cool plasma conditions; ST – short-term precision; LT – long-term precision; RT – retention time; PA – peak area; PH – peak height; AT – autosampler temperature; LOQ – limit of quantification; I – injection volume; in the 2nd option – two ways of chromium species determination were performed without and with EDTA, in the first and the second analysis, respectively. b c

most common interferences in total chromium determination at m/z þ 52 by ICP-MS. Even though it is cost-effective, taking into consideration all the interfering ions is not an easy task, especially in complicated matrices [69]. This is probably the reason why mathematical equations were less popular as solutions in the reviewed papers. Another way to avoid polyatomic interferences is the optimization of chromatographic separations, especially in the case of ion exchange chromatography because it enables interfering ions to be isolated from chromium species. Chlorides were not applied as components of mobile phases in order to reduce chlorine-based spectral interferences. Different solutions with high ionic strength such as NH4 NO3 , NH4 H2 PO4 and EDTA were often used for the Cr(III) and Cr(VI) elution [53,74,75]. However, the presence of the above-mentioned reagents in mobile phases causes an instrumental instability of ICP-MS which can be decreased by using diluted HNO3 [4]. Additionally, Gürleyük and Wallschläger as well as Séby et al. reported that a separation between the chromium species and carbon- or sulfur-based polyatomic interferences is achievable þ [4,74]. Effective elimination of 40 Ar12 C , which co-elutes with Cr (VI), was also performed under cool plasma conditions. In contrast, the 35Cl16O1H þ , which interferes with the determination of Cr(III), was not reduced [54]. However, those spectral interferences generated by carbonate and chloride ions were separated with a high resolution ICP-MS instrument; unfortunately, its main disadvantage is a significant loss in sensitivity [54,52,76,77].

4. Speciation analysis of chromium by HPLC/ICP-MS HPLC/ICP-MS technique enables speciation analysis of chromium at very low concentration levels. It may be achieved by involving the

DRC mode, which significantly decreases the background signal. The quality of separation is provided by selection of the mobile and stationary phase. Using HPLC/ICP-MS online system chromium speciation analysis takes only several minutes. Additionally, the measurement consumes very little sample volume and usually requires minimal sample pretreatment. More detailed information about this online method applied for chromium speciation is given in Table 3, which contains data from papers concerning Cr speciation in water samples published since 2000. The data include: (i) type of sample, its storage and preservation conditions; (ii) parameters of the separation process; (iii) HPLC operation conditions and (iv) figures of merit.

5. Metrological approach to measurement The authors would like to emphasize the importance of chemical metrology in the analytical process. The metrological approach has been introduced by international organizations such as the ISO or BIPM through numerous documents, standards and guidelines describing the analytical processes in detail and providing an opportunity to compare measurement results worldwide. Three main concepts need to be taken into consideration while discussing the metrological approach, namely: (i) analytical validation; (ii) metrological traceability and (iii) uncertainty of the measurement result. Unfortunately, the above mentioned aspects are not always presented in all of the papers. The last column in Table 3 contains the validation parameters from articles regarding Cr speciation, namely the limit of detection (LOD), precision and recovery. The validity of methods and procedures used for Cr speciation in water samples, industrial processes brines, waste waters and human urine has been tested by various authors.

824


A description and commentary of method validation (linearity, LOD, limit of quantification (LOQ), precision and trueness), traceability and uncertainty of measurement result is presented below. 5.1. Method validation 5.1.1. Linearity Linear range is a range of concentrations in which the output signal from the detector is linearly correlated with the concentration of the analyte in standard solutions. The HPLC/ICP-MS technique allows a wide range of linearity to be achieved. The presented papers show the linear calibration range up to 4 or 5 orders of magnitude: 1 mg L 1–1000 mg L 1 and 0.1 mg L 1–1000 mg L 1, respectively [32,78]. The lowest concentrations were chosen by Sakai et al., (0.05 mg L 1–1.0 mg L 1), Chang and Jiang as well as Neubauer et al. (0.1 mg L 1–10 mg L 1) and Tsoi and Leung (0.1 mg L 1– 100 mg L 1), in contrast, the highest were chosen by Gürleyük and Wallschläger (50 mg L 1–5000 mg L 1) and Chen et al., (1 mg L 1–1000 mg L 1) [3,32,67,70,71,74]. For the majority of calibration graphs the correlation coefficients were very good, usually up to 0.999 and higher. 5.1.2. LOD and LOQ One of the most important parameters in the speciation analysis is LOD. It is defined as the concentration or amount of analyte which can be reliably determined with a given level of confidence. In general, the most popular method of LOD estimation is 3 times standard deviation (SD) of a certain analytical response, specifically: (i) 3 times SD of blank [68,78], (ii) 3 times S/N ratio [32,53,67,71,75], 3 times SD of the background signal [3,54,74] and 3 times SD of a low level analyte sample [70]. Barałkiewicz et al. presented three approaches to LOD estimation which are: the modified blank determination method, the graphical method and linear regression method [72]. The above methods; 3 times SD, 3 times S/N and 3 times of the background signal; do not take into consideration the influence of the parameters of the calibration curve, namely the slope, interception and their errors. Also, usually little is known about the number of replicates of samples or blank used for estimating LOD. A knowledge of these factors is important for the selection of an appropriate LOD estimation approach. Methods relying on 3 times SD of blank or S/N determine the ability of the apparatus or detector to recognize the analyte on the lowest possible level. Measurements are usually carried out using blank samples or calibration standards without matrix. The LOD estimated by such methods is called the instrumental detection limit. Another type of LOD is the method detection limit where the lowest analyte level is determined in real samples and includes the possible effects caused by matrix and interferences, besides instrumental noise. It is crucial to remember to always present the LOD value with a description of method and numbers of replicates used for its estimation in order to compare it with different research [79]. The LOD values presented in the reviewed papers vary to a great extent in the range 0.005 mg L 1–1.5 mg L 1 with the most common values being around 0.04 mg L 1–0.4 mg L 1. Vanhaecke et al. calculated LOD for different detector conditions [54]. The application of cool plasma conditions or increasing resolution to R¼3000 cause the LOD at m/z þ 52 to decrease slightly and at m/z þ 53 cause a slight increase. The authors of the papers presented in Table 3 obtained higher LOD values at m/z þ 53 than at m/z þ 52 due to much lower 53Cr isotope abundance [3,4,44,54,74]. A smaller volume of injected sample generates higher LOD, a similar effect is observed in precision values [68]. Application of DRC/MCC technology enables the background count rate to decrease significantly, which translates to the lower LOD values [12,70]. The LOQ is usually omitted in papers, however, several authors have presented it as either an LOQ or a practical quantification

limit (PQL). McSheehy et al. and Burbridge et al. calculated LOQ as 10 times the SD of the blank while Barałkiewicz et al. used 3 times the LOD [68,72,78]. To calculate PQL, Wolf et al. measured several samples with low concentrations until the result was within the 720% of the true concentration value [12]. 5.1.3. Precision Precision is a parameter describing the spread of values obtained by performing repeated analysis of a sample. Depending on the time period that separates subsequent analysis, there is repeatability, an analysis performed during one day and intermediate precision, one over the course of several days or longer. Precision is usually presented as SD or relative standard deviation (RSD). The values of RSD for both Cr species ranged from 0.6% to 12% with the most frequent values lying between 1.5% and 3.6% in the papers presented in Table 3. Precision should be stated with the number of replicates, which was not always the case in the reviewed articles. Several authors have evaluated the intermediate precision by analyzing samples over three different days which resulted in a more than two-fold higher long-time precision value compared to short-time precision [72,75]. Chang and Jiang have shown that there is no significant difference in the precision between the standard and DRC mode [3]. It is also noticeable that higher concentrations of analyte contribute to better precision values [75]. 5.1.4. Trueness To confirm if the obtained result is in accordance with the true value the trueness is determined. In the reviewed papers this was achieved by calculating recoveries of real samples spiked with a known amount of analyte. The lowest and highest recoveries for all Cr species are 0% and 180%. Such extremely low and high values are considered unacceptable and are probably due to interconversions of Cr species in the presence of Mn, Fe, sulfides or organic carbon such as humic substances, plant exudates as well as heat treatment of the samples [12,78]. Most of the recoveries fall within the range of 90%– 110% which is generally considered as very good, as presented in Table 3. The values that are usually acceptable are 75%–125% [72]. By assessing the recoveries one can determine whether the sample matrix, container material or eluent could cause conversions of Cr species in the sample. Seby et al. has noticed that low recoveries of Cr(III) can be explained by its precipitation as CrðOHÞ3 in an alkaline medium [4]. 5.2. Traceability of measurement result Traceability is a property of a measurement result whereby the result of the measurement can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty [80]. One of the best ways to establish traceability is to apply CRM. Unfortunately, there are no appropriate CRMs that possess the specified species concentration that would match the matrix of the samples. Usually CRMs are certified for total chromium concentration without distinguishing for Cr(III) and Cr(VI). BCR CRM 544 was one of the few CRMs with certified Cr(III) and Cr(VI) content available in the past and applied for Cr speciation in surface waters [4,68,81]. Currently no CRM with certified concentration values of Cr(III) and Cr(VI) forms is available due to difficulties with maintaining the stability of Cr species, which is the most important property of CRMs. One way to overcome the lack of CRMs is to spike the real samples with a known amount of Cr species. This method is widely applied in speciation analysis [53,82–84]. The result is then compared to the unspiked sample value and presented as the recovery. One must keep in mind that standard spiking is not a fully metrologically accepted method of establishing traceability in measurements. SIDMS has been


825

Table 4 SIDMS for determination of Cr(III) and Cr(VI) in water. No. Sample

1. Species/ isotope ratio 2. Matrix

Optimization of ICP- Concentration Preparation of isotopic spike Technique solutions separation, detection and MS for ID interference elimination measurements 1. Cr(III) 2. Cr(VI)

1

1.

50 Cr(III), 53Cr (VI); Four isotope ratios: (50Cr/52Cr) for Cr(III) and Cr (VI), (53Cr/52Cr) for Cr(III) and Cr (VI) 2. Drinking, river and deionized water

1.

50 Cr-enriched spike (Isotec Inc. Lot # 2691) for 50 Cr(III); 50Cr metal dissolved in HCl 2. 53Cr-enriched spike (lsotec Inc. Lot # 2692) for 53Cr(VI); 53Cr oxide dissolved in hot HClO4, followed by adjusting pH to 12 with concentrated NH4OH and oxidizing with H2O2

ANX4605Cr anion exchange column (CETAC Tech.) coupled to a VG PlasmaQuad system (Fisons) Interference elimination: 40 Ar12C þ to 52Cr by subtracting background from sample spectrum; no other isobaric interferences were observed

2

1.

50 Cr(III), 53Cr (VI); Two isotope ratios: (50Cr/52Cr) for Cr(III), (53Cr/52Cr) for Cr(VI) 2. Ground and surface water

1. No data 2. No data

No data ANX4605Cr anion exchange column (CETAC Tech.) coupled to a VG PlasmaQuad 3 ICP-MS (ThermoElemental)

3

50 Cr(III), 53Cr (VI); Four isotope ratios: (50Cr/52Cr) for Cr(III) and Cr (VI), (53Cr/52Cr) for Cr(III) and Cr (VI) 2. Local nullah spiked water samples

50 Cr-enriched spike (Lot# I1-8991B) for 50Cr(III); 50 Cr metal dissolved in HCl, the solution was gently heated without boiling 2. 53Cr-enriched spike (Lot# I1-8991A) for 53Cr(VI); 53 Cr oxide dissolved in HClO4, the solution was gently heated without boiling, then it was oxidizing by NH4OH and H2O2; the excess of the last one was removed by boiling for 30 min.

Species separation: iron hydroxide coprecipitation Detection: Elan 6100 DRC-ICP-MS system (PerkinElmer SCIEX) Interference elimination:– DRC technology: Rpq¼ 0.45; Rpa¼ 0.0

1.

a

1.

Dead time – daily optimization Mass discrimination – every 4 h Dead time and mass discrimination factors were optimized with use natural isotopic abundance Cr standard SRM 979

Dead time – 50 ns Mass discrimination – correction factor K: 50 Cr – 1.08; 53Cr – 0.96 Dead time and mass discrimination factors were optimized with use natural isotopic abundance Cr standard SRM 979

Figures of merit

References

Cr(III): oLOD in all matrices Cr(VI): (0.52–51.1) ng g 1 for river water (1.31–46.5) ng g 1 for drinking water ( oLOD–42.5) ng g 1 for deionized water

LOD: 0.21 ng g 1 for Cr(III) 0.37 ng g 1 for Cr(VI)

[91]

Cr(III): ( o 0.01–5.8) mg L 1 for ground water (0.10–0.13) mg L 1 for surface water Cr (VI): ( o 0.01–95.2) mg L 1 for ground water (1.32–1.70) mg L 1 for surface water

LOD: [94] 0.01 mg L 1 for each species Precision: 7 1% (for 50 mg L 1) o 7 10% (for 5 mg L 1)

Cr(III): Sa: (5.2–103.3) mg L 1, Ma: (5.2–97.8) mg L 1 Cr(VI): Sa: (5.4–103.2) mg L 1, Ma: (6.2–106.6) mg L 1

LOD: 0.4 mg L 1 for Cr(III) 0.04 mg L 1 for Cr(VI) Recovery: 90.7–97.9 for Cr(III) 101.4–107.3 for Cr(VI)

[64]

S – spike concentration [mg L 1] and M – measured concentration [mg L 1].

effectively used for Cr speciation. As a definitive measurement method, the SIDMS provides the highest quality of metrological traceability. This issue is described more extensively in Section 6. 5.3. Estimation of uncertainty budget Uncertainty is one of the most important parameters to prove the usefulness and reliability of the analytical method. It is defined by the limits around the obtained analytical result where the true value can be found with a given level of confidence. Depending on the method, uncertainty is calculated using numerous components of the analytical process, parameters connected with preparation of the sample and the measurement itself. There are several approaches to assess uncertainty, two of them are applied by Barałkiewicz et al. [72], namely, the modeling approach and the single-laboratory validation approach. The uncertainties presented in papers from Table 3 are based only on standard deviation of repeated measurements of sample, which is called instrumental uncertainty [3,4,12,32,53,67,74,75,78].

6. Speciated isotope dilution ICP-MS as a definitive method for determining chromium species The combination of two analytical tools: HPLC with ICP-MS in recent years has played an important role in the characterization of element species. These techniques working as an online coupled system enable the isotope dilution technique to be applied. SIDMS allows a highly accurate and precise determination of numerous elemental forms including Cr species in various sample matrices [37,39,41,84,86]. For IDMS analysis, accurate and precise results can be obtained by using the isotope ratio of the isotope-diluted sample for determining the analyte concentration, rather than the absolute intensity measurement [87]. The principle of this method relies on the alteration of natural isotopic abundances of an endogenous form of an analyte by the addition of an accurately known amount of an enriched isotopic spike [76,88–90]. According to data from the literature, the optimum performance of the IDMS analysis is usually achieved by using the isotope ratio of the isotope-diluted sample in a range of 0.1–10 [85,89]. However, it is

826


recommended that the isotope ratio of added enriched spike to the reference isotope (analyte naturally occurring in the sample) should be 1:1 [88,91,92]. Considering that IDMS determinations are based on an isotope ratio measurement, at least two isotopes must remain free from spectral interferences. Therefore, particular care should be taken with regard to the ICP-IDMS analysis because, as mentioned previously, the problem of interferences is present and thus they might disturb the obtained results [85,90]. In order to obtain unbiased chromium isotope ratio measurements in water samples DRC technology was used (Table 4). Additionally, optimization of a detector dead time and mass discrimination factors have to be performed to get accurate and precise results. The application of multiple spiking procedures enables detection of the species transformations that may occur at every step of the analytical process, during sampling, storage, sample preparation and measurement. This means that the original concentrations of different species in a sample can be accurately determined, which cannot be achieved by any other analytical method [64,91]. Species interconversions in environmental matrices commonly found during sample pretreatment are a large problem in trace and ultratrace element speciation. For this reason, the isotope-dilution step should take place before Cr(III)–Cr(VI) transformations, so potential loss of part of an analyte of the isotope-diluted sample will not have any influence on the final result because the isotope ratio of the sample will remain constant [87,89,90]. The prerequisite for accurate results using IDMS is to achieve effective equilibrium between the added enriched isotopic spike and the natural abundant species in the sample [92]. In the case of total element analysis, fulfillment of the foregoing condition is easier as only the equilibration of the spike with all determined species of the analyte is necessary, whereas in element speciation performed by SIDMS it is required that no species interconversion occurs prior to their complete separation [85]. Comparing quantification strategies including the isotope dilution technique, external calibration or standard additions, it can be observed that only the first takes no account of instrumental sensitivity. Consequently, the isotope ratio measurement is largely unaffected by instrumental instabilities such as signal drift or matrix effects and therefore the instabilities will have no impact on the species concentration in a sample [87,88]. The basis of the isotope dilution technique and general equations for determination of the species concentration are detailed in overall reviews as well as in the US EPA method 6800 [85,90,93]. In world literature, there are relatively few articles concerning SIDMS of chromium in water samples [64,91,94]. The first work to consider SIDMS in application to chromium species determination was published in 1998 [91]. The authors described the first standard method for environmental measurements and outlined the general procedures for SIDMS application in an analysis of drinking water, river water and deionized water samples. Simultaneously, they indicated the potential that SIDMS could offer as a diagnostic tool for the validation of other methods of speciation analysis. The second group took on similar research determined chromium species in ground and surface waters in the vicinity of chromite ore processing residue disposal sites [94]. The results obtained by those authors showed that Cr was present predominantly (490%) in a hexavalent form as CrO4 2 . In turn, Ma and Tanner developed an IDMS method for determining Cr(III) and Cr(VI) in natural waters (local nullah spiked water samples) [64]. They described an approach that, allowed chromium species to be determined without any need for a separation instrument connected to an ICP-MS spectrometer. In this work DRC, with ammonia as a reaction gas, was involved in the elimination of interferences. The applications of SIDMS for determining chromium species in water matrices are presented in Table 4 together with details concerning isotopic spikes, their preparation, analytical techniques used,

optimization of ICP-MS parameters for ID measurements, determined concentration levels and figures of merit. At this point the significance of the adequate preparation of Cr isotopic spike solutions must be emphasized. It should be remembered that a thorough preparation process is the most important step in the analytical procedure. Different approaches to isotopic spike solution preparation were widely commented by Novotnik et al. [95]. The authors pointed out the risk, which in the previously reported procedures may arise depending on the excess of H2 O2 remains. The pe-pH relationships of CrO4 2 =CrðOHÞ3 and O2 =H2 O2 couples show that H2 O2 is an oxidant of Cr(III) at pH47.5; while, at lower pH values, it becomes a reductant and in this condition its strength as a reductant strongly increases [96]. Hence, H2 O2 is used as an oxidizing and/or reducing agent in the preparation process of chromium spike solutions. However, if H2 O2 residues are still present in the solution it can cause artefacts in chromium speciation [95]. In order to be sure that no excess of the reducing oxidizing agents remain in the prepared solution, Novotnik et al. developed a new procedure involving alkaline melting of 50Cr(III) enriched oxide for the preparation of pure 50Cr(VI) spike solution. No other oxidizing agents except air oxygen were used for quantitative oxidation of Cr(III)–Cr(VI). Cr(III) was quantitatively oxidized to Cr(VI) with air oxygen without applying any other oxidizing agents. Furthermore, the authors suggested that a microwave assisted digestion procedure of 53Cr enriched oxide could be used in order to prepare 53Cr(III) spike solution without any need for reducing agents. FPLC coupled with ICP-MS was used to examine the purity of 50Cr(VI) and 53Cr(III) spike solutions [95]. Therefore, in spite of difficulties with spectral interferences affecting the measured Cr isotopes as well as those connected with the preparation of isotopic spike solutions, SIDMS offers the highest accuracy and precision, hence it can be considered as a reference or the highly qualified definitive method for element speciation analysis [86].

7. Conclusion How is the issue of chromium speciation presented in the literature published since 2000? Trace speciation of chromium is a topic of considerable importance due to great differences in the biochemical behavior of its single chemical forms. Chromium is an excellent example of an element whose species present opposite effects: Cr(III) is essential for living organisms, while Cr(VI) is considered toxic. To date researchers have described many analytical aspects concerning the speciation analysis of chromium in water matrices by HPLC/ICP-MS, especially in terms of the development of new procedures, improving existing ones as well as dealing with the problem of eliminating the interferences accompanying chromium species determination. What are the outlooks? Currently, we have to provide mathematically proven competent results of chromium species determination. Such results are essential in health protection and doping control, radiochemistry, trace analysis of toxic elements in the environment, food and materials, as well as in verification of the purity of products. We have no other alternative than to introduce metrology rules into chemical measurements, thus solving problems related to insufficient knowledge of the three basic metrological principles: validation of measurement procedure, establishing traceability and uncertainty of measurement results. Nowadays the online HPLC/ICP-MS system has become commercially available and its purchase should be perceived as an investment. Additionally, using this hyphenated technique in combination with SIDMS offers a huge potential to establish the highest quality of metrological traceability. Such an attitude to scientific research on


speciation analysis, especially in chromium species determination in water, will broaden our knowledge, alter our points of view and show a new direction. Most of all, it will prove efficient in practice.

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Acknowledgment

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This work was financially supported by the Research Project of the Polish Ministry of Science and Higher Education, MNiSzW N N204 140439.

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