Using EPA 300 Method for Determination of Anions in Tap Water

In the US, the Maximum Contamination Levels (MCL) in drinking water are established by the Environmental Protection Agency (EPA) through the Safe Drinking Water Act (SDWA). The EPA also controls industrial discharge MCL through the Clean Water Act (CWA).

The growing number of aqueous contaminants and increasingly rigorous regulatory requirements means laboratories need to process more samples in less time, without compromising on accuracy. The EPA’s established method for determining the concentration of common anions present in water samples is ion chromatography (IC). To determine the concentration of ions in each individual sample, the detection method, sample preparation and separation conditions need to be carefully selected. In environmental laboratories, automated sample preparation, including Inline Ultrafiltration, produces more accurate results, simplifies analysis and reduces the labor and total measurement time required.

This article describes how the seven common anions (fluoride, nitrite, chloride, bromide, nitrate, sulfate, and phosphate) covered in US EPA method 300 Part A ^1,2 were studied using a Metrohm ion chromatography system. The system included an automated eluent production module and the optional Inline Ultrafiltration. Using Metrosep A Supp 5 - 100/4.0, baseline separation of all seven ions is accomplished in less than 10 minutes under optimized chromatographic conditions. This article also provides a detailed description of the economic benefits of automated eluent preparation and ultrafiltration. It also presents the result of the method detection limit (MDL) study.

Reagents

Instruments

Solutions

Samples

Tampa tap water

Standards

A 100 mg/L mixed standard was used to prepare standards. A 6-point calibration was created using a 2 mg/L stock standard that was prepared manually and then diluted.

IC parameters

Calculation

Automatic integration with MagIC Net 3.1 software employing peak area for all analytes.

Results and discussion

Seven anions were separated on a Metrosep A Supp 5 - 100/4.0 column using 3.2 mmol/L sodium carbonate/1.0 mmol/L sodium bicarbonate eluent followed by chemical suppression and conductivity detection. 500 mmol/L sulfuric acid was used to regenerate the suppressor, which was then rinsed with the water from the conductivity detector. Baseline separation of the anions is achieved using the column and rapid equilibration is provided by the MSM suppressor.

A regenerant solution containing oxalic acid can be used to eliminate heavy metals such as iron that are often present in environmental water and accumulate in the suppressor. For environmental samples, this process offers consistent and stable performance. High sensitivity and a very low noise is guaranteed through complete chemical regeneration. This enables detection limits to be achieved with lower sample injection volumes, thereby increasing column life. For any environmental laboratory with heavy, medium or low sample loads, the Metrohm suppressor provides a perfect solution.

Eluent preparation

The eluent was prepared using the Metrohm 941 Eluent Production Module (EPM). For this study, commercially available 20x eluent concentrate was used. It is also possible to use customer-prepared eluent concentrates. Using the EPM, eluent concentrates can be changed without the long equilibration times and downtime usually associated with the use of proprietary eluent cartridges. The EPM guarantees more consistent retention times than the manually prepared eluent.

Sample filtration

For samples that are measured using the EPA 300 method, filtration is needed to protect the IC columns and flow paths. Filter caps are required for the common method of automated sample filtration, but the filter caps have a pore size of 20 μm and smaller particles are not removed. The longevity of the analytical column is therefore reduced as these particles are deposited on it.

Although particulates can be effectively removed using syringe filters, these are expensive and labor intensive. Commercially available 0.2 μm pore size filters are used in Metrohm Inline Ultrafiltration. A single filter provides filtration of 200 to 300 samples, with less than 0.1% carry over.

Calibration

The 2.0 mg/L stock standard was used to perform calibration ranging from 0.04 mg/L to 2.0 mg/L for all seven anions.

Shown in Figure 1 is the overlay of the six calibrations with excellent linearity, correlation coefficient (>0.9999) and relative standard deviations (RSD < 2%). The standards were also run at a flow rate of 0.7 mL/minute to show the difference in run times. Figure 2 shows an overlay of chromatograms from standard 5 (1 mg/L) at 0.7 and 0.8 mL/minute.

Figure 1. Overlay of calibration standards

Figure 2. Overlay of 1 mg/L at 0.7 mL/min & 0.8 mL/min flow rate

Sample analysis

Over a three-day period, a set of 10 tap water samples were manually diluted at a ratio of 1:100 and measured for a total of 30 analyses. Figure 3 shows an overlay of chromatograms for the three-day analysis. The results showed a sulfate level of around 92 mg/L, chloride of around 23 mg/L and nitrate near 5.9 mg/L. The following table shows the average concentration, the standard deviation and the relative standard deviation for all 30 runs.

Figure 3. Overlay of ten chromatograms for tap water

Method detection limit and limit of quantitation

By analyzing a set of seven replicates of 0.04 mg/L mixed standard over three days, a method detection limit study was carried out. By multiplying the t-test factor 3.14 and the standard deviation (n=7), the MDLs were calculated. For all the anions, the limit of quantitation (LOQ) was also calculated as 10 times the standard deviation of the 7 replicates.

Table 1 shows the results of the LOQ and MDL study. During the three-day study, only minimal variations were shown by the daily calculated MDLs. All the MDL values comply with the requirements of 40 CFR Part 136 Appendix B and the EPA methods 300.0 & 300.1.

Table 1. Summary of MDL/LOQ study Data for 7 anions

Conclusion

This article shows how the requirements for the seven anions listed under part A of EPA methods 300.0 & 300.1, are effectively met using the Metrohm IC system. The calculated MDLs comply fully with the requirements of the 40 CFR Part 136 Appendix B and EPA methods. The Inline Ultrafiltration feature used here streamlines sample preparation, improves productivity and therefore increases the productivity of any laboratory that runs the EPA 300 method. The 941 Eluent Production Module offers a simple and low-cost solution to the routine task of eluent preparation, therefore providing greater accuracy and reliability.

References

1. EPA Method 300.0 Determination of inorganic anions by ion chromatography

2. EPA Method 300.1 Determination of inorganic anions in drinking water by ion chromatography Revision 1.0

About Metrohm

At Metrohm is one of the world’s most trusted manufacturers of high-precision instruments for chemical analysis. Metrohm was founded in 1943 by engineer Bertold Suhner in Herisau, Switzerland. Today, Metrohm is represented in 120 countries by subsidiaries and exclusive distributors. The global Metrohm Group also includes the Dutch companies Metrohm Applikon and Metrohm Autolab, manufacturers of online analyzers and instruments for electrochemical research, respectively. Recently, the Metrohm Group was joined by Metrohm Raman, a leading manufacturer of handheld Raman spectrometers.

Metrohm is the global market leader in analytical instruments for titration. Instruments for ion chromatography, voltammetry, conductivity, and stability measurement make the Metrohm portfolio for ion analysis complete. Instruments for Near-infrared and Raman spectroscopy are another, strongly growing segment of the Metrohm portfolio.

Metrohm is a problem solver, both in the laboratory and within the industrial process. To this end, the company offers their customers complete solutions, including dedicated analytical instrumentation as well as comprehensive application know-how. More than 30% of the company’s employees at the Metrohm international headquarters in Herisau work in R&D.

Metrohm has been owned 100% by the non-profit Metrohm Foundation since 1982. The Metrohm Foundation, which does not exert any influence on the company’s business operations, sponsors gifted students in the natural sciences, supports charitable and philanthropic purposes and, above all, ensures the independence of the company.

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