High-Precision TNb Analysis in Water Using ISO 20236 Standards

Total bound nitrogen (TN_b) is an accepted quantity in water analysis. It is used to test and monitor the quality of many different types of water, with wastewater and surface water analysis being the most common applications.

DIN EN ISO 20236¹ is an international standard that outlines a catalytic high-temperature method for determining total organic carbon (TOC), TN_b, dissolved organic carbon (DOC), and dissolved bound nitrogen (DN_b).

This new standard will replace the previous DIN EN 12260² for TN_b determination in Europe. The DIN EN ISO 20236 technique for determining TN_b involves catalytically oxidizing water samples at 720 °C or above in an oxygen-rich environment.

The NO produced in this process is then detected using a chemiluminescence detector (CLD) or an alternate detector, such as the electrochemical detector described in annex C of the standard, short form: chemodetector (ChD).

Before measuring the TN_b in an unknown water sample, the analyzer must be calibrated with nitrogen standards at various values. According to the standard, nitrogen-mixed standard solutions of potassium nitrate and ammonium sulfate are employed.

The first step is to make two stock solutions of KNO₃ and (NH₄)₂SO₄, each with 1000 mg/L N. A mixed standard stock solution is created by combining equal volumes (1:1) of the two distinct N stock solutions.

The combined standard stock solution is diluted with ultrapure water to prepare the calibration solutions. TN_b analyzers are typically calibrated between 0 and 20 mg/L N; however, the calibration range can sometimes be increased to 50 mg/L.

DIN EN ISO 20236 requires a daily system test. This is completed using at least two nicotinic acid standards, and the N concentrations should be within the working range.

The nicotinic acid standards for the system test are also created by diluting a stock solution containing 1000 mg/L N with ultrapure water. The system test is regarded as passed when the following conditions are met:

1. The measured value should not differ from the theoretical value by more than ± 5 % or ± 1 mg/L (whichever is higher).
2. The repeatability coefficient of variation for at least two injections of nicotinic acid standards should not exceed 5 % or ± 1 mg/L (whichever is higher).
3. For concentrations less than 10 mg/L, individual readings cannot differ by more than 1 mg/L.

DIN EN ISO 20236 also discusses interferences that can negatively affect the detection of TN_b.

Materials and Methods

The TN_b determinations were made using the multi N/C 3300 TOC/TN_b analyzer. The focus was on TN_b determination; however, a combined non-purgeable organic carbon (NPOC)/TN_b approach was chosen because it is employed in most normal laboratory measurements to determine organic carbon and TN_b simultaneously.

The acidification required for this form of TOC/TN_b determination can be completed manually in advance (typically while sampling) or automatically using an autosampler just before the measurement. The acidified samples are then automatically purged with a portion of the carrier gas utilized.

This method removes the total inorganic carbon (TIC) in the form of carbonates or hydrogen carbonates from the samples. The completeness of TIC removal can be automatically verified by activating the TIC control measurement in an NPOC approach.

After TIC removal, the sample is delivered straight into the analyzer’s combustion tube containing the catalyst. The nitrogen and organic carbon components in the sample are oxidized at high temperatures.

The NO produced in this process is fed into a CLD or ChD, whereas the carbon dioxide produced is fed into a Focus Radiation Non-Dispersive Infrared Detector. For the automated TN_b/NPOC determination, the AS vario autosampler was used with a tray with 72 spots for 40 mL vials.

The following influences have been named:

1. High TOC/DOC levels in samples can limit nitrogen recovery. This can be mitigated by measuring TN_b in various sample dilutions or using the standard addition technique.
2. Nitrogen compounds with double or triple bonds may not fully oxidize to NO.
3. Calibration using a mixed standard solution of potassium nitrate and ammonium sulfate may result in a positive bias for nitrate-N determinations (e.g., in a KNO₃ solution) and a negative bias for ammonium-N determinations (e.g., in a [NH₄]₂SO₄ solution).

This article demonstrates how the multi N/C 3300 TOC/TN_b analyzer meets the DIN EN ISO 20236 criteria for nitrogen determination and impresses with excellent performance characteristics. For this objective, both standards and samples with varying compositions were examined.

Samples and Reagents

• Calibration solutions with nitrogen concentrations ranging from 1 mg/L to 50 mg/L, derived from KNO₃ and (NH₄)₂SO₄.
• System test using nicotinic acid solutions at concentrations of 5 mg/L, 10 mg/L, 20 mg/L, and 50 mg/L N.
• Standard solutions of potassium nitrate and ammonium sulfate, with NO₃-N to NH₄-N ratios of 1:1 and concentrations of 5 mg/L, 10 mg/L, 20 mg/L, and 50 mg/L N.
• Control standard solutions of potassium nitrate (10 mg/L and 50 mg/L N).
• Control standard solutions of ammonium sulfate at 10 mg/L and 50 mg/L N.
• Use 2 mol/L HCl to acidify samples and standard solutions.
• Collect five water samples: one surface water, two process water, and two municipal wastewater. Samples: A to E

Sample Preparation

All samples were acidified with 2 mol/L HCl (0.5 mL per 100 mL sample) and kept in the refrigerator at around 4 °C until measured. After warming to room temperature, the samples were transferred to 40 mL sample vials and placed on the autosampler tray.

On the day of measurement, all calibration and control standard solutions and nicotinic acid solutions for the system test were made fresh from their respective stock solutions.

Calibration

The calibration was carried out with two distinct device configurations:

• multi N/C 3300 with CLD
• multi N/C 3300 with ChD

In both arrangements, a multi-point calibration was performed with concentrations ranging from 1 to 50 mg/L N. This was accomplished using mixed standard calibration solutions (potassium nitrate and ammonium sulfate in ultrapure water, with equal concentrations of nitrate-N and ammonium-N).

The calibration curves were analyzed by linear regression. Figures 1 and 2 show the calibration curves (for CLD and ChD) and the obtained correlation coefficients.

Figure 1. Calibration curve 1‒50 mg/L TN_b, R₂ = 0.99981, determined with chemiluminescence detector (CLD). Image Credit: Analytik Jena US

Figure 2. Calibration curve 1‒50 mg/L TN_b, R₂ = 0.99996, determined with chemodetector (ChD). Image Credit: Analytik Jena US

Table 1. Device and method settings for the standard and sample measurements. Source: Analytik Jena US

Results and Discussion

A predefined measurement sequence was run in parallel with two alternative device configurations. Samples, control standards, and nicotinic acid standards were all measured alternatively.

A total of 68 test vials were filled, with four each for the sample, system test, and control standard. Each vial was used to administer at least three injections.

Table 2 summarizes the findings. The measurement order does not conform to the table’s presentation but does provide a summary of the control standards and samples measured during the series.

In addition to the sample names, the NPOC values of the samples were recorded and published in brackets for instructive purposes.

Table 2. Measurement results. Source: Analytik Jena US

Fig. 3. TN_b measuring curve sample A (surface water) with CLD. Image Credit: Analytik Jena US

Fig. 4. TN_b measuring curve sample B (process water) with ChD. Image Credit: Analytik Jena US

Fig. 5. TN_b measuring curve nicotinic acid standard 50 mg/L N with CLD. Image Credit: Analytik Jena US

Fig. 6. TN_b measuring curve KNO3 standard 10 mg/L N with ChD. Image Credit: Analytik Jena US

Looking specifically at the data for the nicotinic acid solutions with nitrogen concentrations of 5 mg/L, 10 mg/L, 20 mg/L, and 50 mg/L, which were assessed as part of the system test, it is obvious that the test is met without fail across the full concentration range.

On one side, the measured values meet the criteria that the divergence from the theoretical value should not exceed ± 5 %. This is supported by the recovery rates, which range between 97 % and 104 %. On the other hand, the excellent consistency of nitrogen measurement values in system test solutions demonstrates the stability of the analytical system.

Standard deviations ranging from 0.02 to 0.5 mg/L (or 0.4 % to 3.2 % relative standard deviation) were attained. These were determined using at least 12 sample injections (individual measured values) of a standard, with the standards dispersed throughout the measurement sequence. This more than meets DIN EN ISO 20236’s criteria for the coefficient of variation for nicotinic acid solutions.

The deviation of the individual values in the < 10 mg/L N is also less than 1 mg/L for all system test samples.

Extremely reproducible sample findings were obtained. Samples were distributed in blocks throughout the measurement series to expose the combustion tube to a specific matrix load, similar to system test solutions.

Control standards of potassium nitrate and ammonium sulfate were evaluated throughout the tests to ensure calibration stability. Recovery rates from 96 % to 102 % demonstrate the analyzer and calibration’s reliability and stability.

The TN_b content was measured in control solutions containing nitrate or ammonium nitrogen. DIN EN ISO 20236 states that calibration using a NO₃/NH₄ mixed standard can result in an overestimation of nitrate-N determinations and an underestimation of ammonium-N determinations. The examined nitrate standard solutions with 10 and 50 mg/L N were identified with a high recovery rate ranging from 97 % to 104 %.

The same is true for the studied ammonium standards at 10 and 50 mg/L N. Recovery rates ranged from 99 % to 101 %; this means that no over- or under-detections were recorded for the specific nitrate and ammonium standards, demonstrating the analytical instruments’ outstanding performance.

In conclusion, the measurement data collected with two alternative device configurations demonstrate no significant changes in the quality of the observed values based on the detection technology utilized (CLD or ChD). Both detection methods are fully compliant with DIN EN ISO 20236 for nitrogen determination.

Summary

The systems in the multi N/C x300 series have good performance qualities for determining TN_b in line with DIN EN ISO 20236.

For this reason, two equivalent detection technologies can be utilized: chemiluminescence or a maintenance-free electrochemical method, as used in the ChD.

In addition to the multi N/C 3300 basic device with loop injection technology, the multi N/C 2300 analyzer with direct injection technology is also suitable for this application.

The multi N/C 2300 can also be used with either of the two detectors (CLD or ChD) to achieve comparable nitrogen determination results.

Numerous experiments were conducted using the multi N/C x300 series systems to determine the total organic carbon in water of various origins in line with DIN EN ISO 20236, and compatibility with standard techniques was proven without restriction.

The analyzers are distinguished by their long-term stability, excellent particle management, and low consumable wear. The devices operate intuitively with contemporary software.

Figure 7. Multi N/C 3300 with AS vario (left) and multi N/C 2300 with AS 60 (right). Image Credit: Analytik Jena US

The TOC/TN_b analyzers in the multi N/C x300 series provide consistent and cost-effective routine analysis for determining both parameters in water samples in line with DIN EN ISO 20236 at all times.

Recommended Device Configuration

Table 3. Overview of devices, accessories, and consumables. Source: Analytik Jena US

References

1. DIN EN ISO 20236:2023-04 „Water quality - Determination of total organic carbon (TOC), dissolved organic carbon (DOC), bound nitrogen (TN_b) and dissolved bound nitrogen (DN_b) after catalytic oxidative high-temperature combustion (ISO 20236:2018)“
2. DIN EN 12260:2003 “Water quality - Determination of nitrogen - Determination of bound nitrogen (TN_b), following oxidation to nitrogen oxides”

This information has been sourced, reviewed, and adapted from materials provided by Analytik Jena US.

For more information on this source, please visit Analytik Jena US.