Using EPA 300 Method for Determination of Anions in Tap Water

Introduction

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

Sodium carbonate/bicarbonate 20x eluent concentrate

Sulfuric acid, CAS 7664-93-9

Ultrapure water, resistivity >18 MΩ·cm (25 °C)

Instruments

930 Compact IC Flex ChS

2.930.1200

IC Conductivity Detector

2.850.9010

800 Dosino

2.800.0010

919 IC Autosampler plus

2.919.0020

941 Eluent Production Module (EPM)

2.941.0010

IC Equipment: Dosino Regeneration

6.5330.190

IC Equipment: Inline Ultrafiltration

6.5330.110

MSM Rotor

6.2832.000

MSM Rotor Adapter

6.2842.020

10 μL PEEK Sample Loop

6.1825.230

MagIC Net Compact

6.6059.311

Metrosep A Supp 5 - 100/4.0

6.1006.510

Metrosep A Supp 4/5 Guard/4.0

6.1006.500

 

Solutions

Eluents

3.2 mmol/L Na2CO3 + 1.0 mmol/L NaHCO3

Regenerant

500 mmol/L H2SO4 + STREAM

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.

In ultrapure

water (mg/L)

S1

S2

S3

S4

S5

S6

 

Fluoride

0.04

0.10

0.25

0.50

1.00

2.00

Chloride

0.04

0.10

0.25

0.50

1.00

2.00

Nitrite

0.04

0.10

0.25

0.50

1.00

2.00

Bromide

0.04

0.10

0.25

0.50

1.00

2.00

Nitrate

0.04

0.10

0.25

0.50

1.00

2.00

Phosphate

0.04

0.10

0.25

0.50

1.00

2.00

Sulfate

0.04

0.10

0.25

0.50

1.00

2.00

IC parameters

Eluent flow

0.8 mL/minute

Column temperature

Ambient

Degasser

n/a

MCS

n/a

Sample loop

10 μL

MSM regenerant

500 mmol/L sulfuric acid

MSM rinsing

STREAM

MSM

Automatic Stepping

Recording time

10 minutes

Background conductivity

12-20 μS/cm

Noise

<0.15 nS/cm

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.

Analyte

Range (mg/L)

RSD (%)

Corr. Coeff.

Fluoride

0.04 – 2.0

0.210070

0.999999

Chloride

0.04 – 2.0

0.902796

0.999982

Nitrite

0.04 – 2.0

1.725342

0.999935

Bromide

0.04 – 2.0

0.442065

0.999999

Nitrate

0.04 – 2.0

0.921694

0.999981

Phosphate

0.04 – 2.0

1.266553

0.999965

Sulfate

0.04 – 2.0

0.500772

0.999994

 

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.

Tap water (n=30)

Chloride

Nitrate

Sulfate

Average (mg/L)

23.19

5.89

92.42

Standard deviation (mg/L)

0.37

0.31

6.50

RSD (%)

1.59

5.27

7.03

 

 

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

                  Day 1                Day 2                Day 3

 

Mean(mg/L)

MDL (mg/L)

LOQ (mg/L)

Mean (mg/L)

MDL (mg/L)

LOQ (mg/L)

Mean (mg/L)

MDL (mg/L)

LOQ (mg/L)

 

Fluoride

0.038

0.009

0.028

0.038

0.009

0.030

0.039

0.006

0.018

Chloride

0.037

0.006

0.018

0.038

0.004

0.014

0.046

0.005

0.016

Nitrite

0.040

0.008

0.026

0.036

0.008

0.026

0.038

0.006

0.020

Bromide

0.037

0.010

0.032

0.038

0.005

0.015

0.042

0.005

0.017

Nitrate

0.037

0.008

0.026

0.038

0.010

0.033

0.045

0.006

0.021

Phosphate

0.042

0.022

0.071

0.041

0.018

0.056

0.044

0.015

0.049

Sulfate

0.039

0.008

0.025

0.039

0.009

0.029

0.045

0.005

0.017

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 NA

Metrohm is a leading manufacturer of high-precision instruments for chemical analysis, who offer a complete line of analytical laboratory and process systems for titration, ion chromatography, spectroscopy, and electrochemistry.

Manufactured in Switzerland since 1943, Metrohm's instruments have a unique reputation for their robust design and outstanding quality. With thousands of installations worldwide, we are the established leader in titration

From the beginning, Metrohm's sales engineers guide you through the instrument purchasing process. Together with our team of application chemists they create unique solutions for your laboratory needs. From installation to routine preventative maintenance, their global team of certified service professionals ensures ongoing performance and reliability of your Metrohm instrument.

Whether it is replacing an existing instrument, developing a new analytical solution or answering your technical questions – Metrohm are here for you. Our local presence, with offices or subsidiaries in over 120 countries, makes Metrohm your neighbor. Metrohm is a familiar face and shares your professional background.


Sponsored Content Policy: News-Medical.net publishes articles and related content that may be derived from sources where we have existing commercial relationships, provided such content adds value to the core editorial ethos of News-Medical.Net which is to educate and inform site visitors interested in medical research, science, medical devices and treatments.

Last updated: Aug 9, 2017 at 5:59 AM

Other White Papers by this Supplier