Without the presence of ions and their transfer, many natural and industrial processes would not exist; plants could not perform photosynthesis, mammals wouldn’t maintain a stable blood pH and there would be no batteries for electronic devices. However, there are many situations where the presence of ions is unwelcome. Even at trace concentrations, ionic impurities can cause problems. For example, disinfection by-products in drinking water can have carcinogenic effects at µg/L, while ambient air pollutants impact health at very low concentrations (µg/m3).
Therefore, highly sensitive analytical methods are needed to detect and quantify the presence of ionic impurities. While gas chromatography (GC)-based methods are commonly used for analyzing compounds that can be easily vaporized, GC analysis of ionic impurities can be challenging. Baseline movement (e.g., noise and drift) is a frequently encountered issue, often caused by contamination from a previously injected sample. Troubleshooting this problem can be time-consuming and labor-intensive.
However, determining low levels of ionic impurities no longer needs to be slow and complicated, as modern and optimized instruments are available to simplify and expedite analysis. In this article, we use real-life examples to showcase ion chromatography (IC) as an efficient and sensitive method that helps to overcome analytical challenges.
Run time boosted by eliminating the need for derivatization
Characterization of ionic and polar compounds can be achieved using a range of techniques, including GC, which is often used as the established method despite its limitations. Not all compounds are amenable to analysis by GC in their natural state. For example, some highly polar compounds with low volatility may not elute from the GC column, precluding detection. Such compounds must be modified into stable and volatile derivatives suitable for GC elution and separation. This process is known as “derivatization”, and can involve a range of reactions to improve sensitivity, selectivity or specificity for a particular separation in GC combined with mass spectrometry (MS).
While derivatization is a useful tool, it has been described as the bottleneck of GC-MS due to its labor-intensive and time-consuming nature. The United States Environmental Protection Agency (EPA) Method 552.3 describes a tedious multi-step sample preparation process for the detection of haloacetic acids (HAA) and dalapon in drinking water, which involves liquid-liquid microextraction, derivatization and GC with electron capture detection executed over a number of days.
HAAs arise from a reaction between chlorine-containing disinfectants and other compounds and are a focus of modern water analysis. In common GC methods, derivatization of HAAs is achieved using diazomethane or methanol. This poses health risks to those involved in analysis. So much so that in Sweden, diazomethane is listed as a carcinogenic air pollutant in the workplace and has
Advances in IC technology remove the need for such extensive sample pre-treatment, without compromising speed and sensitivity. A modular IC system, using a newly developed column, when coupled to a triple quadrupole mass spectrometer (IC-MS/MS), can divert common interfering anions to waste and allow the direct detection of HAAs in water samples in 35 minutes. Compared with a standard GC method for water quality analysis, this simplified method enabled determination of key HAAs without sample acidification, extraction and derivatization.
Safer manufacturing made simple with automated eluent generation
The remarkable reduction in run time for water analysis is a consequence of new IC technologies, which provide benefits across a range of industries. One example can be found in manufacturing processes, where sulfuric acid is commonly used during production. As trace ionic impurities directly impact yield and reliability of wafer cleaning (a vital stage in electronics production, involving the use of sulfuric acid), determining their presence is critical. However, detecting anionic impurities in concentrated acids, such as sulfuric acid, poses many analytical challenges related to:
● Cumbersome and tedious titration methods
● The use of hazardous materials during titration
● Insufficient sensitivity: there are limits to the amount of concentrated acid that can be injected onto an anion-exchange column
● Long retention times for phosphate, rendering the determination of phosphate contaminants in sulfuric acid difficult
Progress in column technology has increased capacity and improved the resolution of inorganic ions, providing an attractive alternative to titration. Column selectivity has also been modified to facilitate ionic determinations, as exemplified by one developed recently that can be used to determine phosphate contamination in sulfuric acid reliably and accurately due to a modification in the order of anion elution.
Advanced IC columns are also paired with instrument technology, such as an eluent generator, to simplify the workflow. Like other types of chromatography, eluent for IC can be prepared manually by diluting an eluent concentrate or with stock chemicals. With the need for dilutions and preparation from dry chemicals, there is a strong potential for lot-to-lot variation, which can affect IC peak retention and resolution, and there is an additional safety concern relating to exposure to hazardous chemicals. Removing manual elements of a process can reduce the opportunity for error, thereby, improving reproducibility, while providing a high degree of automation. By just adding water, eluent can be automatically generated online by software control, allowing reliable and consistent eluent creation.
Robust ion chromatography key to air quality expedition in the Arctic
A simple operation combined with high performance is a dream combination for any environmental analysis, especially in challenging research conditions. Monitoring air quality in polar regions is necessary for the development of mitigation strategies for reducing air pollution. In particular, the measurement of atmospheric amines is critical to forecasting climate impacts. Between sample collection and analysis, however, amines tend to decompose, creating logistical challenges for analysis.
To overcome this, onboard analysis was trialed on an arctic voyage spanning several months. The “Snow Dragon,” a 25-year old research vessel, carried an ambient ion monitoring (AIM) system for IC analysis of ambient air. The operation was successful, with 27 inorganic anions and cations, carboxylic acids and amines evaluated every hour for the entire trip, producing accurate and high-resolution data. The low maintenance requirements of the AIM system made for a smoother voyage; only water needed to be added for monitoring to continue.
There are also more direct and obvious benefits to advances in air quality monitoring for human health, as newer IC methods also enable more sensitive determination of inorganic anions and carboxylic acids in fine urban ambient particles.
Overcoming sensitivity limitations to protect human health
Advanced, and yet easy to use, IC systems have found their place across a range of settings, from food and pharmaceutical industries to municipal water treatment and bottled water quality control. Anion-exchange chromatography can be used to separate mixtures of organic acids and inorganic ions, regardless of whether the goal is to ensure flavor, safety or biotherapeutic quality. Successful analysis is dependent on the method having sufficient sensitivity, and this is particularly important for the analysis of drinking water.
In the US, thyroid dysfunction has been linked to perchlorate exposure, an oxidizer that has been used in rocket propellant, explosives, fireworks and road flares since the 1950s. Consequently, several states have set standards and guidelines for perchlorate levels in drinking water (2 and 6 µg/L for Massachusetts and California, respectively) creating the need for reliable detection methods. A high-pressure IC system can be coupled with a single quadrupole MS to provide higher sensitivity, along with mass confirmation, compared to using only conductivity detection. This platform can be integrated with user-friendly software to ensure the technology is suitable for even novice users.
Ion chromatography tools are providing solutions across industries
Advanced IC technology has simplified and fast-tracked the detection of ionic impurities across a wide range of applications, from sulfuric acid quality control to assessing air quality. Fortunately, we can leave many chromatography challenges in the past, including labor-intensive derivatization and manual eluent preparation. Modern IC platforms are important tools for manufacturing, research, and environmental analysis, where the determination of ionic impurities is critical for quality control and our health.