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Green aspects of ion chromatography versus other methods in the analysis of common inorganic ions

https://doi.org/10.3390/separations8120235

2. Ion Chromatography

The HPLC (high-performance liquid chromatography) instrument should not be used as an ion chromatograph because of the strongly acidic and basic eluents used in IC, the conductivity detector, and the range of applications. In general, HPLC is mainly used for the analysis of organic compounds (e.g., hydrocarbons, alcohols, sugars), and IC is used for inorganic compounds (and carboxylic acids or amines). Overall, the purchase and daily operation costs of the HPLC apparatus are more expensive than an ion chromatograph. IC is a part of HPLC, used for the separation and determination of anions and cations, as well as other substances when they are converted to ionic forms [13]. Depending on the separation mechanisms used, the following can be distinguished:
  • Ion chromatography, with or without conductivity suppression;
  • Ion-exclusion chromatography (IEC);
  • Ion-pair chromatography (IPC).
Its rapid development and popularity in routine laboratories is primarily due to the following advantages: the possibility of simultaneous analysis of several ions in a short timeframe (about 5–30 min); the small amount of sample needed for analysis (<0.5 mL); the possibility of using different detectors (e.g., conductivity, UV/Vis, amperometric, mass spectrometry); the simple methods of sample preparation for water sample analysis (filtration); the possibility of simultaneous analysis of cations and anions, or organic and inorganic ions; the high selectivity of separations; the possibility of ion analysis of the same element at different oxidation levels (speciation analysis) [14,15]; as well as the safety and cost of daily operation. Compared to HPLC, which uses expensive and toxic organic solvents as mobile phases, in IC, highly dilute aqueous solutions of Na2CO3/NaHCO3 and NaOH/KOH (for anion separation), or dilute acids (for cation separation), are usually used as eluents. IC and related techniques are able to detect cations well, comparably to spectroscopic methods, and they are even more accurate for low concentrations [16]. Ammonium ions, which are not conventionally detected by spectroscopic methods, should also be kept in mind. It is often necessary to determine both anions and cations in a sample, and then IC, allowing for more information to be obtained with a single instrument, which is a more useful method. These advantages contributed to the fact that soon after the appearance of IC, a number of standardized methodologies were developed, in which it was used as a reference method for the analysis of anions and cations in various types of sample matrices [17].

2.1. Ion Chromatography Advances for Green Analytical Chemistry

Recently, IC has achieved a very high technological level. An overview of the technological advances in IC was recently presented by Wouters et al. [18]. These advances are mainly related to the introduction of more selective stationary phases, suppressor technologies, detection methods, and capillary [19] and multidimensional IC [20].
In 1983, Rokushika et al. described the theoretical basis of capillary IC, which has been commercially available since 2010 [21,22]. Great credit for the development and popularization of capillary IC belongs to the Japanese scientist Takeuchi, who, with his team, in the late 1980s, began his research related to this subject, which concerned both new stationary phases, detection methods, including a non-contact conductometric detector, and applications [23]. The implementation of capillary IC into laboratory practice has brought many benefits. These include the following: higher laboratory productivity (faster achievement of system stability); possible isocratic and gradient elution; higher determination sensitivity with smaller sample volume; 100-fold increase in absolute sensitivity compared to traditional columns (4 mm); particularly important in the context of green analytical chemistry, lower eluent consumption and minimization of waste generation. A comparison of selected parameters of conventional and capillary IC is given in Table 1.
Table 1. Comparison of selected parameters of conventional and capillary IC [24].
These features make miniaturized systems inherently “green” [25,26]. Multi-dimensional separations, in which the output of one chromatographic separation is interfaced to a second chromatographic separation, have increased the separation power of analytes in complex sample matrices. In the case of IC, there is significant potential to implement this approach by coupling different ion-exchange separations together, or even by combining ion exchange with some related IC methods (e.g., ion-exclusion chromatography). This approach has led to profound improvements in the ability of IC to handle very complex samples, especially those containing mixtures of inorganic and organic ions. Again, further developments in this area can be anticipated. There is a growing number of applications on multidimensional IC × IC techniques that use columns with significantly different selectivity. The first step involves the initial separation of analytes (or groups of analytes), followed by further, more selective, separation of the individual fractions. Although this procedure is complex and expensive, it provides very good results, especially for samples with complex matrices. As with other analytical methods and techniques, the most important problem is still the sample preparation for analysis, especially for those samples with complex matrices. Sample preparation for analysis by IC requires different steps, and their proper selection is determined by the physical sample state, its composition, and the availability of suitable apparatus. The correct ways of collecting, storing, and preparing the sample for analysis are key elements that affect the reliability of the analyses and the validity of inclusion of the sample in the green analytical chemistry method [27].
The main trends of green analytical chemistry focus on the effective reduction/elimination of organic solvents and other toxic reagents used, as well as the miniaturization and automation of applied methods. Moreover, the minimization of energy consumption, reduction in waste, and reuse of solvents and materials are very important. Compared to the status quo in the 1990s [28], current sample preparation methods tend to be faster, more efficient, more user and environmentally friendly, and easier to automate and miniaturize [29].
Let us assume that we are dealing with a routine analytical laboratory performing analyses of water and wastewater. It has unlimited access to a variety of methods and techniques, both manual and instrumental, with no economical limits. Its only criteria are the use of standardized methodologies and the demonstration of green aspects of applied methods. The comparison of both the financial and environmental costs of determining the same substances using different analytical methods is difficult, and is subject to considerable risks and errors. In order to assess the “environmental friendliness” of a given method more fully, it would be necessary to take into account the costs of energy, labor, analysis time, and the possibility of disposing the waste. In order to prove the thesis that IC has aspects of green analytical chemistry, an attempt has been made below to make such an estimation related to the determination of selected inorganic anions using the IC method. The total costs of single analyses of 1000 samples for the content of major inorganic anions (F, Cl, NO2, NO3, SO42−) and cations (Na+, K+, NH4+, Mg2+, Ca2+), taking into account the prices in Poland at the end of 2021, are summarized in Table 2. These vary depending on the type of sample matrix (clean water and wastewater), and represent approximate costs that may vary depending on the ion chromatograph manufacturer and the consumables and reagents used.
Table 2. Estimated averaged costs (euro) for determination of common inorganic anions (F, Cl, NO2, NO3, PO43−, SO42−) and cations (Na+, K+, NH4+, Mg2+, Ca2+) in 1000 water and wastewater samples by isocratic IC.
Costs Water Wastewater
Type of analysis Anions Cations Anions Cations
Eluent 2 5 2 5
Suppressor operating 10 10
Analytical column 2000 2000 2000 2000
Filtration 7 7 20 20
Total costs 2019 2012 2032 2025

2.2. Comparison of Ecological Aspects of Ion Chromatography and Other Standard Methods Case Study

Among the many factors affecting the assessment of applied methods as environmentally friendly, the type and amount of reagents used, their toxicity, and the amount and type of waste generated are very important. The choice of compared methods was based on their common use in laboratories accredited for routine water and wastewater analyses. The amount of reagents needed was calculated per 1000 water samples. The summaries for Cl, NO2, NO3, and NH4+ are given in Table 3Table 4Table 5 and Table 6. When using the IC method (according to the ISO 10304-1 standard) for Cl determination, the only reagents needed are sodium carbonate and sodium bicarbonate, which are safe and used for eluent preparations. In turn, the determination of Cl in 1000 water samples by the Mohr method, according to the ISO 9297 standard, requires the consumption of 100 g of K2CrO4 and up to 193 g of AgNO3. This undoubtedly puts this method at a disadvantage compared to IC. It is even worse when the flow methods FIA (flow injection analysis) or CFA (continuous flow analysis) (according to the ISO 15682 standard) are used for the same purpose. In this case, it may be necessary (depending on the variant adopted) to use the highly toxic Hg(SCN)2. Moreover, the ISO 10304-1 standard allows simultaneous determination of not only Cl, but also NO2, NO3, PO43−, and SO42− anions. In the case of NO2, NO3 determination by using the FIA method (ISO 13395), depending on the option (FIA or CFA), it is necessary to use small, but toxic, amounts of organic compounds (e.g., 4-aminobenzenesulfonamide and N-(1-Naphthyl) ethylenediamine dihydrochloride).

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