Application of IC-MS and IC-ICP-MS in Environmental Research

LIST OF CONTRIBUTORS

Maria Balcerzak; Department of Analytical Chemistry, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland

Klaus Fischer; Faculty VI – Regional and Environmental Sciences, Department of Analytical and Ecological Chemistry, University of Trier, Behringstr. 21, 54296 Trier, Germany

Wolfgang Frenzel; Technische Universität Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany

Jay Gandhi; Metrohm USA, 4738 Ten Sleep Lane, Friendswood, TX 77546, USA

Adam Konrad Jagielski; Department of Metabolic Regulation, Faculty of Biology, Institute of Biochemistry, University of Warsaw, Miecznikowa 1, 02-096 Warsaw, Poland

Koji Kosaka; Department of Environmental Health, National Institute of Public Health, 2-3-6 Minami, Wako, Saitama 351-0197, Japan

Jürgen Mattusch; Department of Analytical Chemistry, Helmholtz Centre for Environmental Research, Permoserstr. 15, 04318 Leipzig, Germany

Rajmund Michalski; Institute of Environmental Engineering, Polish Academy of Sciences, M. Skłodowskiej-Curie 34, 41-819 Zabrze, Poland

Michal Usarek; Department of Metabolic Regulation, Faculty of Biology, Institute of Biochemistry, University of Warsaw, Miecznikowa 1, 02-096 Warsaw, Poland

PREFACE

Environmental analytical chemistry can be regarded as the study of a series of factors that affect the distribution and interaction of elements and substances present in the environment, the ways they are transported and transferred, as well as their effects on biological systems. In recent years, the importance of monitoring and controlling environmental pollutants has become apparent in all parts of the world. As a result, analysts have intensified their efforts to identify and determine toxic substances in air, water, wastewaters, food, and other sectors of our environment. The toxicological data analyses involve constant lowering of analyte detection limits to extremely low concentration levels.

Speciation analysis, understood as research into various element forms, is gaining importance in environmental protection, biochemistry, geology, medicine, pharmacy, and food quality control. It is popular because what frequently determines the toxicological properties of a compound or element is not its total content, but in many cases, it is the presence of its various forms. Elements occurring in ionic forms are generally believed to be biologically and toxicologically interactive with living organisms. Studying low analytes concentrations, particularly in complex matrix samples, requires meticulous and sophisticated analytical methods and techniques. The latest trends embrace the hyphenated methods combining different separation and detection methods. In the range of ionic compounds, the most important separation technique is ion chromatography. Since its introduction in 1975, ion chromatography has been used in most areas of analytical chemistry and has become a versatile and powerful technique for the analysis of a vast number of inorganic and organic ions present in samples with different matrices. The main advantages of ion chromatography include the short time needed for analyses, possibility of analysis of small volume samples, high sensitivity and selectivity, and a possibility of simultaneous separation and determination of a few ions or ions of the same element at different degrees of oxidation. Mass spectrometry is the most popular detection method in speciation analysis, because it offers information on the quantitative and qualitative sample composition and helps to determine analytes structure and molar masses. The access to the structural data (necessary for the identification of the already known or newly found compounds) poses a challenge for speciation analysis as higher sensitivity of detection methods contributes to the increased number of detected element forms.

Couplings of ion chromatography with MS or ICP-MS detectors belong to the most popular and useful hyphenated methods to determine different ion forms of metals and metalloids ions (e.g., Cr(III)/Cr(VI), As(III)/As(V)), as well as others ions (e.g., bromate, perchlorate). IC-MS and IC-ICP-MS create unprecedented opportunities, and their main advantages include extremely low limits of detection and quantification, high precision, and repeatability of determinations.

The intent of this book is to introduce anyone interested in the field of ion chromatography, species analysis and hyphenated methods (IC-MS and IC-ICP-MS) the theory and practice. This book should be interesting and useful for analytical chemists engaged in environmental protection and research, with backgrounds in chemistry, biology, toxicology, and analytical chemistry in general. Moreover, employees of laboratories analyzing environmental samples and carrying out species analysis might find general procedures for sample preparation, chromatographic separation, and mass spectrometric analysis.

Rajmund Michalski

Zabrze, Poland

6 February 2016

CHAPTER 1

PRINCIPLES AND APPLICATIONS OF ION CHROMATOGRAPHY

Rajmund Michalski

Institute of Environmental Engineering, Polish Academy of Sciences, M. Skłodowskiej-Curie 34, 41-819, Zabrze, Poland

1.1 PRINCIPLES OF ION CHROMATOGRAPHY

1.1.1 Introduction

The history ofchromatography as a separation method began in 1903 when Mikhail Semyonovich Tsvet (a Russian biochemist working at the Department of Chemistry of the Warsaw University) separated plant dyes using adsorption in a column filled with calcium carbonate and other substances [1]. After extraction with the petroleum ether, he obtained clearly separated colorful zones. To describe this method, he used Greek words meaning color (ρωμα) and writing (γραϕω) and coined a new word, chromatography, which literally meant writing colors. At present, chromatographic methods are among the most popular instrumental methods in the analytical chemistry as they offer quick separation and determination of substances, including complex matrix samples.

Chromatographic methods are used widely on both the preparative and analytical scales. They help to separate and determine polar and nonpolar components; acidic, neutral, and alkaline compounds; organic and inorganic substances; monomers, oligomers, and polymers. It is necessary to use an appropriate chromatography type, which depends on the physicochemical properties of the examined sample and its components. Gas chromatography (GC) and liquid chromatography (LC) can be used to separate and determine approximately 20% and 80% of the known compounds, respectively. Ion chromatography (IC) is a part of high-performance liquid chromatography used to separate and determine anions and cations and also other substances after converting them into the ionic forms. In the literature, the term ion-exchange chromatography (I-EC) is found. It differs from ion chromatography even though both types are based on the widely known ion-exchange processes. Ion chromatography originates from ion-exchange chromatography. It uses high-performance analytical columns that are usually filled with homogenous particles with small diameters and most often conductometric detection. When compared to the classic ion-exchange chromatography, it is more efficient, faster, and more sensitive. It also offers very good repeatability of the obtained results. The ion-exchange chromatography term was used until 1975, when the first commercial ion chromatograph was available. At present, most analyses of ionic substances conducted with chromatographic techniques are performed with ion chromatography.

In the last 40 years, there were many state-of-the-art monographs that described the ion chromatography theory and applications in detail [2–5]. Some of these studies have already been republished. At present, there are three main separation methods in ion chromatography. They are based on different properties of substances used in the column phases and the resulting ion capacity. They include the following:

Ion chromatography (IC) and can be either suppressed or nonsuppressed

Ion exclusion chromatography (IEC)

Ion pair chromatography (IPC).

The block diagram of an ion chromatograph (cation-exchange and anion-exchange types), together with ion-exchange reactions for the most popular suppressed ion chromatography, can be seen in Figure 1.1.

Figure 1.1 Block diagram of an ion chromatograph with a conductometric detector.

The anion separation proceeds according to the following principle: analyte ions (e.g., Cl−) together with eluent ions pass through the analytical column in which the following ion-exchange reaction takes place:

The affinity of the analyte ions toward the stationary phase is diverse. Consequently, the ions are separated and leached out from the analytical column within different retention times against the background of weakly dissociated NaHCO3. Afterward, they are transported into the suppressor with high-capacity sulfonic cation exchanger. The following reaction takes place:

The NaHCO3 eluent ions are transformed into weakly dissociated carbonic acid due to the occurring reactions. The analyte ions (e.g., Cl−) react in accordance with the following formula:

Due to the reactions taking place in the analytical column and the suppressor, the analyte ions reach the detector in the form of strongly dissociated acids against the background of weakly dissociated carbonic acid. The obtained signal related to the conductivity of the analyte ions (the analyte forms a well-dissociated salt after the reactions) is high enough to use the conductometric detector to record the peaks of separated anions against the background of a weak signal related to the low eluent conductivity (forming weakly dissociated carbonic acid). Parallel reactions are observed when cations are determined. The cation-exchange column is filled with a cation exchanger with sulfonic groups. Eluent consists of water solution of, for example, hydrochloric acid. The analyte ions (e.g., Na+) together with the eluent ions pass through the analytical column in which the following ion-exchange reaction takes place:

The affinity of the analyte ions toward the stationary phase is diverse. Consequently, the cations are separated and leached out from the analytical column within different retention times against the background of strongly dissociated HCl. Afterward, the ions are transported into the suppressor with high-capacity anion exchanger (e.g., with quaternary ammonium groups as functional groups). The following chemical reaction occurs:

The HCl eluent ions are transformed into water due to the reactions in the suppressor, whereas the analyte ions (Na+) react with the exchanger in the suppression column according to the following formula:

Due to the chemical reactions in the analytical column and suppressor, the analyte ions reach the detector in the form of highly dissociated hydroxides against the water background, which allows analysis in the conductometric detector.

Ion exclusion chromatography (IEC) is a comparatively old technique, which uses the Gibbs–Donnan effect. A porous ionic-exchanger functions as a semipermeable membrane separating two water phases (mobile and stationary) contained in the exchanger pores. The membrane is only permeable for nonionized or weakly ionized substances. They are separated between two water phases, whereas their migration through the column is delayed. The ionized substances do not penetrate the inside of the pores. In I-EC cation exchanger, and occasionally anion exchanger, has generally been used for I-EC separations. They are not held in the column and leave it first. IEC is mainly used for separating weak inorganic acids, organic acids, alcohols, aldehydes, amino acids, and also for the group separation of ionic and nonionic substances [5, 6].

As an alternative to conventional ion chromatography, anions and cations can be separated on a standard reversed-phase column of the type used for HPLC. Several names have been applied for this type of separation, such as the following: ion-interaction chromatography, mobile-phase ion chromatography, and mostly IPC. In ion-pair chromatography, the X substance ions react with the lipophilic L ions (constituting the eluent component) and form the XL complex. The complex can be bound to the nonpolar surface of the stationary S phase in a reversible way. The S phase makes a reversible phase, as its polarity is lower than that of the eluent. It forms the XLS complex. The separated sample ions (XL complexes) have different retention times in the column. The retention times result from different affinities that the ions have toward the nonpolar stationary-phase surface, which causes the separation. According to the alternative model, the lipophilic eluent ions are adsorbed on the stationary-phase surface and form the LS complex. As a result, an ion exchanger forms on the nonpolar stationary-phase surface. The ions of the solved X substance react with this exchanger. The hydrophobic ions (e.g., alkyl and aryl sulfonates) may penetrate the inside of the layer formed by the LS complex. Their retention time is decided by the adsorption phenomenon. More hydrophilic ions penetrate only the external zone. Their retention time in the column is decided by the ion-exchange mechanism. This chromatography type is mainly used to determine ions such as sulfates, sulfonates, alkaloids, barbiturates, fatty acid derivatives, and selected metal ion complexes [7]. Besides these three main types, reversed-phase liquid chromatography (RPLC) [8] and hydrophilic interaction liquid chromatography (HILIC) [9] can also be used to separate selected ions.

Ion determination methods used before 1975 (gravimetric, titration, spectrometric, electrolytic, and other methods) were inexpensive and easily available; however, they were also time-consuming and required large amounts of expensive and (frequently) toxic reagents. On the other hand, the chromatographic methods used at that time were mostly applied for separation and determination of organic compounds.

The chromatographic method applications for separating metal ions were intensively investigated during WWII, when the atomic bomb was constructed (Manhattan Project). Nonetheless, the real breakthrough took place in 1971 when Small and his colleagues from Dow Physical Research Laboratory (Midland, MI) examined and proposed a chromatographic method for determination of lithium, sodium, and potassium with ion exchange and conductometric detection. Generally, ion-exchange chromatography was a preparative method because determination of ions present in the sample against the mobile-phase background caused serious difficulties. The key issues were to use proper stationary phases in the columns and to elaborate the mechanism of the eluent conductivity reduction so that the separated ions could be determined with a conductometric detector.

At the beginning of the ion chromatography development, eluents used to separate anions were alkaline (e.g., water solutions of NaOH/KOH, Na2CO3/NaHCO3, phenolates). The eluents used to separate cations were usually sulfuric acid or methanesulfonic acid water solutions. At first, an additional column (suppression column) was used to reduce the eluent conductivity (suppression). It was placed between the analytical column and the detector. Due to the reactions occurring in it, the eluent ions formed low-conductivity products, such as H2O (when water solutions of hydroxides were used as eluents) or H2CO3 (carbonate eluents). In the mid-1970s, the method was developed enough by Dow Chemicals to sell the conductometric suppression license to Durrum Chemical, which soon changed its name to Dionex for commercial reasons [10]. The company presented the first commercial ion chromatograph (Dionex, Model 10) during the American Chemical Society meeting in September 1975. The year 1975 is the official starting point of ion chromatography. The study by Small et al. [11] was a milestone in its development. At the turn of the 1970s and 1980s, Gjerde et al. [12] were the first to use the ion chromatography system without the suppression column and to apply the eluents with very low conductivity values. In this way, they created a new type of ion chromatography, that is, nonsuppressed ion chromatography. The most important events preceding the invention of ion chromatography and the stages of its development and popularization are given in Table 1.1.

Table 1.1 Key Events Preceding the Ion Chromatography Invention and Stages of Ion Chromatography Development and Popularization

The most important ion chromatography advantages are as follows:

1. Simultaneous determination of several ions in a short time (<10 min)

2. A small sample volume necessary for analyses (<0.5 ml)

3. Using different detectors

4. Simple sample preparation for analyses (liquid samples with simple matrices)

5. Simultaneous separation of anions and cations, or organic and inorganic ions

6. High separation selectivity (>1:10,000)

7. Determination of ions of the same element at different oxidation states (speciation analysis)

8. Safety and low exploitation costs (green chemistry).

The aforementioned advantages helped to elaborate a number of standardized methodologies soon after ion chromatography was invented. They mainly served to determine anions and cations in water and wastewater [19]. In the 1990s, the International Standard Organization (ISO) introduced numerous standards concerning determination of anions and cations in water and wastewater, as well as solid and gaseous samples. The list of standards is given in Table 1.2.

Table 1.2 ISO Standards Based on Ion Chromatography

1.1.2 Stationary Phases

The stationary phases used in ion chromatography are also known as ion exchangers. They are macromolecular solids with cross-linked space structure. They are insoluble in water or other solvents and are able to exchange ions with the solution. The stationary phases can be divided into anion exchangers (with functional groups containing cationic counterions able to exchange anions present in the solution) and cation exchangers (with functional groups containing anionic counterions able to exchange cations present in the solution). There are also amphoteric ion exchangers that can exchange anions or cations, depending on the solution pH, and bipolar/zwitterionic ion exchangers that are able to exchange both ion types.

Ion exchangers contain functional groups whose charge is defined. Appropriate counterions, located close to the functional groups, have a counter change. Together, they constitute an electrically neutral unit. The active functional groups in cation exchangers are acidic. When they dissociate with an ion part (e.g., H+), they can exchange this ion for other cations from the solution. The following functional groups are most often found in the active centers: sulfonic (−SO3H), carboxylic (−COOH), iminodiacetic (−N{CH2COOH}2), phenolic (−C6H4OH), phosphonic (−PO3H2), and phosphinic (−PO2H) ones. In the anion exchangers, the functional groups are alkaline. They normally include quaternary ammonium groups (−NR3+) and tertiary and secondary protonated amines (−NR2H+, −NRH2+) or sulfonic groups (−SR2+).

A large number of functional groups in the exchanger increases the affinity of the ion-exchange resin toward the electrolyte solution, which may cause its dissolution. To prevent it, the exchanger core is formed as linear polymers that are cross-linked spatially. Such ion-exchange resins are insoluble in water, but they may expand in the solution when they absorb the solvent. The more developed the cross-linked structure is, the lower the influence is. It also depends on the ionic strength and pH of the solution, temperature, and the exchanged ion types. Most of the ion-exchanger properties are decided by the ion-exchange resin matrix (core) construction, number and type of the functional groups, and the degree to which it is cross-linked. For the separation quality, the following parameters are the most important: exchange capacity, selectivity degree, and chromatographic column performance.

Both the height of the ion-exchange resin and the column length are among the most important factors deciding on the peak resolution. If large amounts of ions should be separated and determined in a quantitative way, it is better to use a shorter column rather than increase the eluent concentration. A highly concentrated eluent can quickly overload the suppressor. Moreover, when the column is shorter, the pressure at the column outlet is lower. Consequently, a higher flow rate can be applied. For highly polarized ions that elute late in the form of broad peaks, it may be necessary to replace the used column with a shorter one. An opposite situation is encountered for the components that elute very quickly (e.g., low-molecular-weight organic acids). In such a case, good results are obtained when longer columns (sometimes, two columns connected in series) are used. Due to hydration, anions and cations with a large ionic radius can be eluted more quickly from the column filled with an ion-exchange resin cross-linked to a lower degree.

The order in which the selected anions (separated on the typically highly alkaline anion exchangers) are leached out is as follows: OH− < F− < ClO2− < BrO3− < HCOO− < IO3− < CH3COO− < H2PO4− < HCO3− < Cl− < CN− < NO2− < Br−< NO3− < HPO4²− < SO3²− < SO4²− < C2O4²− < CrO4²− < MoO4²− < WO4²− < S2O3²− < I− < SCN− < ClO4− < salicylates < citrates. The order for the selected cations separated on the highly acidic cation exchangers is as follows: Li+ < H+ < Na+< NH4+ < K+ < Rb+ < Cs+ < Ag+ < Tl+ ≪ UO2²+ < Mg²+ < Zn²+ < Co²+ < Cu²+< Cd²+ < Ni²+ < Ca²+ < Sr²+ < Pb²+ < Ba²+ ≪ Al³+ < Sc³+ < Y³+ < Eu³+ < Pr³+< Ce³+ < La³+ ≪ Pu⁴+. The presented correlations help to conclude that trivalent ions are more strongly bound to the ion-exchange resin than divalent or monovalent ones. The ion-exchange resins hold ions whose valence is the same but the radius is larger in a stronger way than the ions with a small radius.

The ion-exchanger applications in the chemical analysis were overviewed in the studies by Walton and Rocklin [20] and Quershi and Varshney [21]. The stationary phases used in ion chromatography were described in the paper of Weiss and Jensen [22]. The anion exchangers can be divided into the following categories: organic polymers (including polystyrene/divinylbenzene (PS/DVB) copolymers, ethylenedivinylbenzene and divinylbenzene (EVB/DVB) copolymers, and polymethacrylate and polyvinyl polymers); exchangers in the agglomerate form; exchangers based on silica; and other types (e.g., crown ethers, cryptand phases).

Organic copolymers (i.e., polystyrene/divinylbenzene (PS/DVB) copolymers, ethylenedivinylbenzene and divinylbenzene (EVB/DVB) copolymers) and polymethacrylate and polyvinyl copolymers are the most often used exchangers in ion chromatography. There are many commercially available anion-exchange columns filled with exchangers based on the PS/DVB copolymer with quaternary ammonium groups on the surface (as functional groups). One of the disadvantages that such anion exchangers had was the difficulty with the appropriate separation of fluoride ions against the background of the so-called water peak and the micromolecular anions of carboxylic acids. Dionex researched this problem. As a result, the company used anion exchangers with ethylenedivinylbenzene instead of polystyrene. Such anion exchangers were obtained due to the so-called grafting technology, which was applied for the first time by Schomburg [23] in the 1980s.

The agglomerate ion exchangers contain particles built of the spherical core that is surrounded by small molecules of the polymer containing functional groups. This particular type of ion exchanger is known as the latex-agglomerated anion exchanger. They are built from the PS/DVB core. On the surface, it is covered with sulfonic groups and completely aminated particles (diameters of 5–25 µm) made of polyvinyl chloride or polymethacrylate (generally known as latex). The latex particle diameters are approximately 0.1 µm. They are kept on the exchanger surface due to the electrostatic reactions and van der Waals forces. The properties of the exchanger can be modified as needed through changing the number of the latex particles bound to the basic exchanger and the degree to which they are cross-linked. In 1987, the gradient elution was introduced in ion chromatography. Since then, there have been commercially available latex exchangers adapted for the hydroxide-selective latex-based anion exchangers [24]. Latex columns with agglomerate exchangers have high mechanical and chemical resistance, high exchange capacity, and very good separation parameters. The exchanger core is completely sulfonized on the surface, which prevents the diffusion of inorganic ions into the inside of the stationary phase in the Donnan dialysis process. The diffusion processes mainly take place in the latex layer. The size and number of latex groups decide on their properties.

Methacrylate polymers are another type of exchangers used in ion chromatography. They are not as hard and firm as the PS/DVB copolymers. That is why they cannot be used in the columns working under high pressures. On the other hand, they are less hydrophobic and are better to separate large particles. The stationary phases based on silica, commonly used in high-performance liquid chromatography, are applied less often in ion chromatography as eluents with different pH are used in it. Silica itself can be used within a limited scope (pH = 2–8). When this pH range is exceeded, the silane chains are easily destroyed and the exchanger mechanical stability quickly decreases [25]. Nonetheless, the silica exchangers have better mechanical resistance than the organic ones. Similarly to silica, Al2O3 (common in HPLC) does not have any significant applications in ion chromatography due to the low exchange capacity and insufficient resistance to strong acids and bases. Organic and inorganic ions can be also separated on the cross-linked organic copolymers modified with the crown ethers or polycyclic cryptands. Lamb et al. [18] carried out the first studies on this topic. The most important advantage of the exchangers used in the gradient elution is the simultaneous elution of the polarized and nonpolarized ions.

Columns filled with traditional ion exchangers have relatively high resistances of the mobile-phase flow, which requires applying high flow rates. The maximum flow rates used in ion chromatography do not exceed 5 ml min−1 and the maximum pressure is 35 MPa. In the monolithic columns, the obtained flow rate resistances are much lower. Their phases are made of one homogenous porous rod. The commercially available monolithic columns have diameters of a few millimeters and are filled with the PS/DVB copolymers, polymethacrylate, or silica [26]. The eluent flow rates in the monolithic columns can be up to 10 ml min−1. Consequently, the analyte ions are separated within approximately 1 min. Under classic conditions, separation takes approximately 20 min. The monolithic columns are used in many ion chromatography varieties, such as low- and medium-pressure ion chromatography, ultrafast ion chromatography, capillary ion chromatography, gradient ion chromatography, or multidimensional ion chromatography [27].

Similarly to anion exchangers, the following stationary phases are used to separate organic and inorganic cations: polystyrene/divinylbenzene (PS/DVB) copolymers; ethylenedivinylbenzene and divinylbenzene (EVB/DVB) copolymers; polymethacrylate and polyvinyl polymers; cation exchangers in the agglomerate form; cation exchangers based on silica; and other cation exchangers.

Cation exchangers based on the polystyrene/divinylbenzene (PS/DVB) copolymers are commonly used in cation-exchange chromatography as the basic material to produce modified stationary phases. They are completely sulfonated on the surface, and the number of the sulfonate groups decides on their exchange capacity. The main disadvantage of the sulfonated cation exchangers based on PS/DVB is that they do not offer a possibility for the simultaneous separation of the alkaline metal and alkaline earth metal ions. The process is possible only after carboxylic groups are introduced as functional groups into cation exchangers. For nonsuppressed ion chromatography, ethylenediamine with tartaric acid (also constituting the eluent components) can be used as the complexation agent for divalent metal ions. The other option is to use a water solution of HCl and 2,3-diaminopropionic acid. In the exchangers based on EVB/DVB, a thin layer of the ion-exchange copolymer with carboxylic functional groups is bound to the copolymer particles. The main advantage is the possibility of using water acid solutions (e.g., H2SO4 or methanesulfonic acid – MSA) for simultaneous separation and determination of alkaline metals and alkaline earth metals.

The agglomerate cation exchangers were introduced commercially 10 years later than the latex anion exchangers, that is, in the mid-1980s. Such a delay was caused by the search for appropriate ion-exchange resins that could function as the base substrate. They contain a weakly sulfonated PS/DVB copolymer and completely aminated latex particles bound to the layer surface, on which they are held due to the electrostatic reactions and van der Waals forces. The discussed layer is covered with another completely sulfonated latex layer.

The zwitterionic ion exchangers [28] also offer interesting possibilities for ion chromatography. The presence of the negatively and positively charged groups in the exchangers results