Author: Jeff Brewer and Dr. Thorsten Heinlein
Introduction
The analysis of cations in drinking and surface water is a cornerstone of water quality assessment, critical for ensuring public health, environmental protection, and compliance with diverse global standards. Cations such as lithium, sodium, ammonium, potassium, magnesium, and calcium influence water’s chemical properties, including taste, hardness, and potential health impacts. Regulatory frameworks worldwide, such as the World Health Organization (WHO) Guidelines for Drinking-Water Quality, the European Union (EU) Drinking Water Directive (2020/2184), and national standards in countries like China (GB 5749-2022), Japan (MHLW), and India (IS 10500:2012), set limits or guidelines for these cations. For example, the WHO suggests a sodium limit of 200 mg/L for taste, while the EU monitors ammonium (0.5 mg/L proposed) to prevent nitrification risks. Emerging interest in lithium, particularly in regions with high groundwater levels like South America, underscores the need for sensitive analytical methods.
Ion Chromatography (IC) is a preferred technique for cation analysis due to its precision and ability to handle complex water matrices. The Ion Chromatograph IONUS, employing non-suppressed IC, offers a streamlined, sensitive, and robust solution for quantifying cations in drinking and surface water. Its design eliminates the need for suppressor maintenance, simplifies operation, and delivers reproducible results across a wide concentration range, making it ideal for laboratories navigating varied regulatory requirements. This application note demonstrates the IONUS’s capability to perform high-precision cation analysis, complementing its established performance in anion analysis, and supports global water quality monitoring with minimal user intervention.
The Ion Chromatograph IONUS as the ideal workhorse
The Ion Chromatograph IONUS is engineered for reliable cation analysis in water quality applications. Its key features include:
- Precise Eluent Delivery: Ensures consistent flow without helium pressurization, enhancing reproducibility.
- Non-Suppressed IC: Simplifies operation by eliminating suppressor maintenance, maintaining high sensitivity for trace cation detection.
- Universal Column Compatibility: Supports standard IC columns, offering flexibility in method optimization.
- Heated Column Compartment: Maintains stable temperatures for consistent peak resolution and reproducibility.
- High-Throughput Autosampler: Handles up to 96 vials, minimizing manual effort and enabling efficient analysis.
- User-Friendly Design: Requires minimal training, with robust construction for routine laboratory use.
- Regulatory Compliance: Meets global standards, ensuring acceptance in diverse regulatory environments.
This application note presents a validated method for cation analysis, showcasing the IONUS’s performance across varied water matrices and its alignment with international regulatory frameworks.
Cation concentrations in drinking and surface water are regulated to ensure safety and aesthetic quality, with guidelines varying by region. Below is a summary of key regulatory standards for cations, emphasizing the need for a versatile analytical solution like the IONUS.
World Health Organization (WHO): Suggests a sodium limit of 200 mg/L for taste, with no health-based limits for calcium, magnesium, or potassium. Ammonium is noted at 1.5 mg/L for taste/odor, with health-based limits under review. Lithium is not currently regulated but is of interest in groundwater studies.
European Union (Directive 2020/2184): Sets a proposed ammonium limit of 0.5 mg/L to prevent nitrification. Calcium and magnesium are monitored for hardness (no specific limit), and sodium is limited to 200 mg/L for taste/health.
United States (EPA): No primary standards for these cations, but secondary standards include sodium (guidance at 30 – 60 mg/L for low-sodium diets) and calcium/magnesium for hardness (non-enforceable, based on aesthetic considerations).
Canada (Health Canada): Recommends sodium ≤ 200 mg/L (aesthetic objective) and monitors calcium/magnesium for hardness. No specific limits for ammonium or potassium.
China (GB 5749-2022): Sets sodium at 200 mg/L, with calcium/magnesium guidelines for hardness. Ammonium is limited to 0.5 mg/L.
Japan (MHLW): Aligns with WHO for sodium (200 mg/L); monitors calcium and magnesium for hardness.
India (IS 10500:2012): Specifies sodium at 200 mg/L, with calcium (75 mg/L) and magnesium (30 mg/L) limits for hardness.
South Korea (Water Environment Conservation Act): Sets sodium at 200 mg/L, ammonium at 0.5 mg/L, and monitors calcium/magnesium.
Africa and Middle East: Many countries adopt WHO or EU guidelines, with the UAE referencing EPA/EU limits for bottled water.
South America (ex. Colombia, Decree 1575 of 2007): No specific cation limits beyond general water quality parameters, but WHO guidelines are referenced.
The IONUS’s ability to quantify cations across low (sub-ppm) to high (hundreds of ppm) concentrations ensures compliance with these diverse standards, making it a vital tool for global laboratories.
Experimental
Sample Collection
Samples were collected in pre-cleaned polypropylene bottles. Because of our sample collection process we have found the pre-cleaned polypropylene bottles to be more durable, less weight, cost-effective, and the polypropylene has low reactivity or leaching of inorganic ions. Each bottle was again rinsed multiple times onsite using the water source, and filled to the top of the bottle. Caps were immediately tightened, and the samples were placed in a cooler on ice. Once back at the laboratory, the samples were transferred to a refrigerator with a temperature held at 4∞C.
Samples were collected from drinking water treatment facilities (labeled North and South) and four lakes within a 50-mile radius, representing diverse water sources with runoff from rural, urban, suburban, and golf course areas. Pre-cleaned polypropylene bottles were used due to their durability, low cost, and minimal ion leaching. Bottles were rinsed onsite with source water, filled to the brim, capped tightly, and stored in a cooler with ice during transport. In the laboratory, samples were refrigerated at 4°C until analysis.
Operating Conditions of the Method
| IC System | Ion Chromatograph IONUS |
| Column | Repromer CAT (SS, 4 mm x 250 mm) |
| Column Thermostat | 40 °C |
| Eluent | 3.5 mM Nitric Acid, isocratic |
| Flow rate | 1.0 mL/min |
| Injection | 20 µL |
| Sample | Filtration (0.45 µm), Dilution as applicable, 1:5 to 1:20 |
| Injection | 20 µL |
| Water Source | Aquinity² P10 Analytical (0.055 µS/cm, Type I) |
Reagents and Standards
Reagents: Ultra-pure water (18.2 MΩ·cm, membraPURE Aquinity² P10), analytical-grade nitric acid (Millipore-Sigma Chemicals, United States).
Standards: A primary stock mixed cation standard (lithium, sodium, ammonium, potassium, magnesium, calcium) was prepared from 1000 mg/L certified standards (Inorganic Ventures, Christiansburg, VA, USA) at 0.005 – 36.683 mg/L, variable by analyte. Each primary stock standard includes a Certificate of Analysis.
Standard Preparation
During instrument equilibration, nine levels of mixed working standards were prepared by weight from the primary stock, tailored to optimized ranges: Lithium (0.005 – 1.888 mg/L), Sodium (0.103 – 36.66 mg/L), Ammonium (0.021 – 7.398 mg/L), Potassium (0.053 – 18.856 mg/L), Magnesium (0.103 – 36.683 mg/L), Calcium (0.205 – 73.052 mg/L). Standards were recorded and entered into the Clarity 10 software.
Following a linearity test, the range of calibration was optimized to Lithium (0.005 – 0.501 mg/L), Sodium (0.103 – 9.728 mg/L), Ammonium (0.021 – 1.963 mg/L), Potassium (0.053 – 5.003 mg/L), Magnesium (0.103 – 9.734 mg/L), Calcium (0.205 – 19.384 mg/L).
Sample Preparation
Sampes were filtered through 0.20 µm syringe filters acquired from Macherey-Nagel. The filters are Chromafil Xtra H-PTFE with a pore size of 0.02 µm. Drinking water samples were diluted 1:5 to 1:10, and surface water samples 1:10 to 1:20, using ultra-pure water, based on expected cation concentrations. No further pretreatment was performed. Blanks, standards, quality control samples, and prepared samples were loaded into the IONUS’s chilled (20°C) 96-position autosampler, controlled by Clarity 10 software.
Calibration Discussion:
The calibration ranges were selected to align with typical sample concentrations and global regulatory requirements, focusing on low to mid-range concentrations (0.005 – 10 mg/L) to enhance linearity and accuracy. These ranges cover diluted sample concentrations (0 – 10 mg/L) and quality control standards (0.057 – 2.196 mg/L), as shown in Appendix Table 2. The IONUS’s flexibility allows laboratories to adjust ranges to meet specific regional standards.
The Context of the Correlation Coefficient
The coefficient of determination (R²) measures the linearity between cation concentration and detector response (area counts). Values ≥ 0.997 indicate excellent linearity, with ≥ 0.999 being ideal for regulatory compliance. The IONUS achieves R² > 0.999 for most cations across the optimized ranges (Table 1), demonstrating robust performance. This linearity supports accurate quantification across diverse water matrices, reducing the need for complex quadratic models or extensive sample dilution.
A value of 0.999 is generally considered a highly acceptable standard in the industry for most applications, indicating a strong linear relationship.
A value of 0.997 is also very common and well-regarded, especially when analyzing a wide concentration range or with analytes that can show minor non-ideal behavior at the lower or higher ends of the curve.
The results presented in Table 1 present the exceptional linearity of the membraPure IONUS across a wide dynamic range, a critical requirement for water quality analysis. The coefficient of determination (R2) values for all analytes consistently exceed 0.997 and in most cases, are above 0.999, demonstrating a durable, repeatable, and reliable linear relationship between concentration and detector response.
This high degree of linearity, maintained across multiple, partially overlapping concentration ranges, is a testament to the instrument’s stability and precision. It confirms that the IONUS can accurately quantify anions from low-level drinking water standards to higher concentrations found in surface water samples, ensuring a single instrument can address the diverse analytical needs of laboratories globally.
Demonstration of Extended Linear Dynamic Range: IONUS
While many regulatory methods specify a narrower calibration range for optimal accuracy at low detection limits, the membraPure IONUS is engineered to deliver exceptional performance across a wider dynamic range. To demonstrate the system’s stability and linearity, an additional calibration curve spanning all eight standard levels was analyzed.
The results, as shown in Table 1 and graphically displayed in Fig. 2 & Fig. 3 (see appendix) , confirm that the IONUS maintains a linear relationship between analyte concentration and peak area throughout this extensive range.
This high degree of linearity across such a broad concentration range (two orders of magnitude for Fluoride) is a testament to the stability of the IONUS pumping, injection, and detection systems. This capability is particularly beneficial for laboratories that handle a diverse range of samples, from low-level drinking water to more concentrated industrial effluents or surface waters, as it can reduce the use of multiple calibration curves or extensive sample dilution. It provides confidence that even unforeseen high-concentration samples will be accurately quantified within a single run.
Table 1: Results of the determination of the Correlation Coefficient for the different cations and 9 levels of concentration.
| Analyte | Linear fit | Quadratic fit |
| Lithium | 0.9997 | 0.9999 |
| Sodium | 0.9998 | 0.9999 |
| Ammonium | 0.9998 | 0.9999 |
| Potassium | 0.9997 | 0.9999 |
| Magnesium | 0.9997 | 0.9999 |
| Calcium | 0.9999 | 0.9999 |
Results and Discussion
This application note aims to demonstrate: (1) the IONUS’s ability to deliver high-quality, reproducible cation analysis over optimized calibration ranges, meeting global regulatory requirements; (2) consistent performance across low, mid, and high concentration ranges, with minimal sample preparation; and (3) the reliability of its non-suppressed IC technology for routine laboratory use without performance degradation.
Samples were collected from drinking water facilities (McKinney North and South) and four lakes with varied runoff sources (rural, urban, golf course, suburban), as detailed in Table 4. A chromatogram of the sample „Mc Kinney North drinking water“ is shown as an example in Appendix Fig. 5. Results show cation concentrations compliant with regulatory guidelines (Ex: sodium < 200 mg/L, ammonium < 0.5 mg/L where applicable), with surface water reflecting higher levels due to environmental inputs (Ex: calcium from geological runoff). Lithium levels vary, and upon further research, lithium is known to be in the surface water in this region.
Quality control checks (Appendix Tables Table 5–8) before and after sample analysis confirm recoveries of 94–98% for most cations, validating system stability and precision. A chromatogram of the standard is shown as an example in Appendix Fig. 5. The non-suppressed IC method simplifies operation, eliminating suppressor-related failures, while the autosampler and Clarity software ensure minimal user intervention. The optimized calibration ranges improve linearity (R² > 0.999) compared to broader ranges, reducing bias at low concentrations and aligning with sample and QC ranges (0 – 10 mg/L).
The IONUS’s performance across these matrices, as shown in calibration plots (Fig. 2 & Fig. 3) confirms its suitability for global laboratories. Its ability to handle diverse regulatory needs with simple sample preparation (filtration and dilution) underscores its versatility.
The corresponding chromatograms of Level 1- 8 are displayed as overlay in Appendix Fig. 4.
Conclusion
A Global Solution for Water Quality Analysis
The Ion Chromatograph IONUS delivers a reliable, sensitive, and robust solution for cation analysis in drinking and surface water. Its non-suppressed IC technology simplifies operation, achieving high linearity (R² > 0.999) and recoveries (94 – 98%) across optimized calibration ranges tailored to sample concentrations (Table 3 & Table 4). This performance meets stringent global standards, from WHO’s sodium guidelines to EU’s ammonium limits, ensuring compliance across diverse regions.
Drinking water samples showed cation levels within safe limits (Ex: sodium 8.5–68 mg/L), while surface water reflected environmental influences (Ex: higher calcium in lakes with geological runoff). The IONUS’s ability to quantify cations from sub-ppm to tens of ppm, with minimal sample manipulation, eliminates the need for multiple methods or extensive dilutions. Its durable design and automation make it an ideal choice for laboratories worldwide, complementing its proven anion analysis capabilities and offering a unified solution for water quality monitoring.
Fig. 1: Ion Chromatograph IONUS