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A review of possible uses of ion chromatography (IC) in combination with mass spectrometry (MS) detection for environmental research
Since its introduction in 1975, ion chromatography (IC) 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 ions present in the environment. Although conductivity detection is still the most popular detection method, other types of detection can be applied for different analytes. These include the following methods: electrochemical (for example, amperometric and potentiometric), photometric (UV–vis and chemiluminescence), and spectrometric (used mainly in hyphenated techniques). The most versatile and powerful detection method is mass spectrometry (MS). The main advantages of IC–MS are extremely low detection and quantification limits, insignificant interference influence, and high precision and repeatability of the determinations. This article is a review of possible uses of IC in combination with MS detection for environmental research.
Speciation, as a word borrowed from biology, is a term describing the existence of various chemical and physical forms of a particular element, and speciation analytics denotes the determination of those forms (1). The notion of speciation is used in chemistry to determine the occurrence of diverse forms of a given element (for example, elements at various oxidation states or bound with different ligands) in the analyzed sample. The forms might differ in physical and chemical characteristics as well as in the influence they exert on living organisms. In the last several decades, speciation analytics has become one of the most central issues in analytical chemistry. Even though its cost is significant, speciation analytics has become more important when it comes to solving problems that concern not only the determination of total element contents, but also taking into account various forms of occurrence. It plays an exceptional role in the examination of biochemical cycles of selected chemical compounds, determination of toxicity and ecotoxicity of selected elements, food and pharmaceutical product quality control, and technological process control as well as health risk assessment and clinical analytics (2).
It is reasonable to differentiate between chemical and physical speciation. In chemical speciation, it is possible to distinguish between screening speciation, which searches for and determines selected chemical forms, and distribution speciation, which searches for and determines selected chemical individuals in specific elements of the examined sample. Another division within chemical speciation concerns group speciation, which is defined as searching for and determining specific groups or classes of chemical forms, and individual speciation, which is searching for and determining all chemical individuals present in the sample. When it comes to liquid sample analyses, the most common technique is the one developed by Florence and Batley (3). According to this method, a water or wastewater sample filtered through a 0.45-μm filter is divided into a solid phase and a mobile phase, in which the determinations of total metal contents and the metal labile and bound forms are carried out. The division suggested by Tessier and colleagues (4) is recommended in the research concerning the speciation of heavy metals in bottom sediments. They distinguished and defined five fractions: exchangeable metals, carbonate-bound metals, iron and manganese oxides-bound metals, organic matter-bound metals, and other mineral-bound metals. Nonetheless, this method of speciation does not differentiate between oxidation states of elements, which may be of great importance when considering their toxicity.
Lowering the detection limits of analytes to extremely low concentration levels resulted in methods that did not always meet the necessary requirements. For that reason, there has been a tendency to combine various methods and techniques. These combinations are known as hyphenated techniques. A suitable hyphenated technique should be selective toward determined analytes, should be sensitive within a wide range of concentrations, and should enable the best possible identification of the determined substances.
In speciation analytics, chromatographic methods are largely used for separation whereas spectroscopic ones are used for detection (5).
The application of hyphenated techniques entails a perfect understanding of analytical methodologies and detailed knowledge of instrumentation. These are expensive systems used in scientific research rather than in routine analyses. The earliest hyphenated techniques were developed by coupling gas chromatography (GC) with various detectors. The following systems were then developed: gas chromatography–atomic absorption spectrometry (GC–AAS), gas chromatography–atomic emission spectrometry (GC–AES), gas chromatography–mass spectrometry (GC–MS), and gas chromatography–inductively coupled plasma–time-of-flight mass spectrometry (GC–ICP-MS-TOF). Because of technological reasons, systems using liquid chromatography (LC) methods for the separation of analyzed substances, such as high performance liquid chromatography (HPLC)–ICP-MS, appeared in the market slightly later.
There are couplings of various LC types among the most popular hyphenated techniques used to determine different ionic forms of metals and metalloids. These include HPLC, ion chromatography (IC), ion-exclusion chromatography, and size-exclusion chromatography (SEC) with ICP-MS or electrospray ionization (ESI) MS. The most popular hyphenated techniques using ion chromatography are IC–ICP-MS and IC-MS (6).
Ion chromatography is the most popular method used to separate and determine organic and inorganic ionic substances (7). Ion chromatography as a kind of liquid chromatography is primarily applicable to (in terms of the hyphenated techniques) the determination of inorganic ions. Determination and separation of ions (which until recently have been thought of as difficult or even impossible to separate especially in the case of complex matrix samples) has become more effective with the implementation of new highly selective stationary phases in ion-exchange columns and new modes of detection.
Using an IC–MS system allows users to not only obtain information about the quality and quantity composition of the sample, but also to define the structure of analytes and their molar weights. It is necessary to maintain extremely low pressure in the spectrometer, and the separated ions of the analyte leave the chromatographic column under relatively high pressure. Research into such applications started in the 1980s, but it was only recently introduced on a commercial scale.
The ion source is used for transferring the eluent into the spectrometric detector. It consists of the conversion of the liquid eluent containing the analyte into its gas phase under atmospheric pressure. The analyte must be thermally stable and the eluent should vaporize without leaving salts behind. Organic solvents such as methanol and acetonitrile are sometimes added to the eluent to assist its vaporization.
The application of MS detection allows users to obtain information about the qualitative and quantitative content of the sample and also to determine the structure and molar masses of the analytes. The main difficulties in using an MS detector coupled with chromatographic methods result from the fact that it is necessary to maintain very low pressure in the spectrometer while separated analyte ions leave the chromatographic column under comparatively high pressure. Another combination is ion chromatography coupled with negative thermal ionization isotope dilution mass spectrometry (IC–NTI-IDMS) analysis (9) and ion chromatography coupled with electrospray ion tandem mass spectrometry (IC–MS-MS) (10). The application of atmospheric pressure ionization-mass spectrometry (API-MS) coupled with ion chromatography demonstrated performance comparable to that of IC–MS-MS and IC–ICP-MS (11). Compared with IC–API-MS, IC–ICP-MS can tolerate a higher salt concentration in the eluent, which allows for the use of high-capacity columns and larger sample volumes. These capabilities lower the detection limits by one order of magnitude for ICP-MS detection compared to API-MS.
Various sources of ionization may be used in HPLC-MS systems (12), including ESI, atmospheric pressure chemical ionization (APCI), and atmospheric pressure photochemical ionization (APPI). The scope of these applications depends on the polarity and molar mass of analytes, as well as the eluent flow rate. Only the first variant of the above-mentioned ionization types is used in IC–MS systems. ESI is the so-called soft ionization method. In contrast to other ionization methods, it is able to convert multivalent ions into the gas phase. The MS detection can be conducted in two different modes: selected ion monitoring (SIM) or scan mode. In SIM mode, the information on the analyte molar mass is obtained and the method is usually applied for quantitative analyses. In scan mode, the retention time, mass spectra, and mass distribution information is obtained. This mode is primarily used in qualitative analyses. Identification is relatively simple when it pertains to analytes with low molar mass. The identification problems concerning large molecules are primarily related to the higher number of possibilities in terms of obtaining spectra with the same molar mass-to-charge ratios.
Coupling ion chromatography with ICP-MS is a powerful tool to determine unambiguously different organic and inorganic compounds in a single run. IC–ICP-MS is a suitable technique for complex speciation because the conditions of the mobile phase can be manipulated accordingly to provide optimal separation. Liquid sample introduction is a standard in ICP-MS. Therefore, the simplest form of LC and ICP-MS coupling is the connection of the column outlet with the nebulizer of the sample introduction system via transfer tubing. It should come as no surprise that the hyphenated system that results from the coupling of LC and ICP-MS is the system most often used for speciation analysis related to ICP-MS detection.
Despite its advantages, this hyphenated technique, like other methods, has some drawbacks. For example, one of the major limitations of As or Cr speciation with ICP-MS is the formation of 75As or 52Cr isotopically equivalent species such as 40Ar35Cl+ and 40Ar12C+ in the plasma, because of the presence of chlorides or carbon in the matrix that interfere with the accurate determination of As at m/z 75 or Cr at m/z 52. There are two main approaches to address this problem. The first approach is to use ion chromatography to separate interferences, such as chloride, from arsenic before the introduction of the sample into the plasma. It usually can be accomplished during IC by a simple manipulation of the mobile phase. The other approach developed in parallel to the ion chromatography solution is the reduction or elimination of the 40Ar35Cl+ and 40Ar12C+ interferences after the sample introduction into the plasma by using collision–reaction cell techniques (13). Some instruments use a collision mechanism to dissociate polyatomic interferences, whereas others use gas-phase reaction chemistry to specifically induce dissociation or formation of secondary species that can be rejected by the mass analyzer. The isobaric overlaps are generally not an issue when using a double focusing sector field instrument that offers the higher resolution that may be required for the interference-free determination of sulfur, arsenic, or chromium. However, an increase in the resolution inevitably leads to a dramatic decrease in sensitivity. It should also be noted that the sensitivity of the latest generation of quadrupole instruments is only a factor of 2–3 lower than that of high-resolution ICP-MS operated in low-resolution mode. A good tradeoff between sensitivity, freedom from isobaric interferences, and price is offered by ICP-MS instruments equipped with a collision cell.
Applications of IC–MS and IC–ICP-MS systems include, among other things, the determination of fluoroacetate (14), endothal (14), ascorbic acid (15), perchlorate (16,17), chlorophenols (18), phosphorous oxyanions (19), chelating agents (20), amines (21), polychlorinated biphenyls (22), epichlorohydrin (23), and metal–EDTA complexes (24).
The main applications of ion chromatography in speciation analytics can be divided into three areas:
Determination of nitrogen and sulfur ion forms has been performed since the beginning of ion chromatography and is usually carried out with the classic ion chromatograph equipped with an appropriate anion- or cation-exchange column and suppressed conductivity detection. From an environmental and toxicological point of view, the most important ion determinations are halides and metals or metalloids.
Protecting people against health-hazardous microorganisms present in drinking water requires disinfecting the water by various methods. Water chlorination is a well-known and effective technology that has been used for many years; however, it can cause the formation of dangerous by-products such as trihalomethanes. Because of that drawback, there has been a search for other water disinfection methods, among which ozonation has become the most popular one. Even though modern water disinfection methods have their undeniable advantages, they also have certain negative aspects and limitations. These limitations mainly involve the formation of inorganic oxyhalide by-products such as bromate, chlorite, and chlorate.
Bromate can form in raw water containing bromides that are subjected to the ozonation process. The International Agency for Research Cancer (IARC) classified bromate as a potential carcinogen (B2 group), whereas the World Health Organization (WHO) and the United States Environmental Protection Agency (US EPA) initially established 0.8 μg/dm3 as the bromate level that is safe for human consumption. Because there is no simple analytical method to make the determination of such a low concentration of bromate, the provisional permissible content in drinking water was increased to a level of 25 μg/dm3. Nowadays, in most highly industrialized countries the permissible bromate content in drinking water is 10 μg/dm3. The methods to determine bromate, chlorite, and chlorate in water with ion chromatography can be categorized into three groups depending on the detection mode (30):
The direct methods rely on the selective BrO3- ion separation in the presence of other anions in the sample and its detection with suppressed conductivity detection. These methods are relatively simple and inexpensive but their main flaw is difficulty with the appropriate separation of BrO3- and Cl- ions, whose concentrations in real samples differ significantly. The derivatization methods belong to the indirect methods category and involve converting the determined substance (after its separation in the analytical column) into its derivatives that can be then detected with the UV–vis detector. The third category encompasses the hyphenated techniques such as IC–ICP-MS and IC–MS.
The simultaneous separation and determination of metals and metalloid ions at different oxidation states belongs to another important area of ion chromatography: applications in speciation analytics (31).
Because of a strong environmental impact, metals or metalloids and selected halide ions determination and speciation have received significant attention in the past few years. Ion chromatography has become one of the main powerful analytical tools for the analysis of complex matrices and speciation studies in that field of analysis.
The hyphenated techniques IC–ICP-MS and IC–MS create completely new and immense possibilities in speciation analysis. The main advantages of those techniques are extremely low detection and quantification limits, insignificant interference influence, and high precision and repeatability of the determinations.
Like all other methods, the hyphenated techniques have their shortcomings. The limitations include issues such as the high price of the apparatus and their complexity, which causes their practical limited availability and usage in laboratories. Using hyphenated techniques requires an in-depth understanding of the analytical methodologies and instrumentation. The systems discussed are expensive, which has limited their use to scientific research rather than routine analyses. Nevertheless, the development of hyphenated techniques is becoming more and more important and the growing number of works concerning this subject seems to corroborate that (32).
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Rajmund Michalski is with the Institute of Environmental Engineering of Polish Academy of Science in Zabrze, Poland. Direct correspondence to: email@example.com.