Sample Preservation for Total Nitrogen Determination in Water: Technical Guidelines, Holding Times,

Proper sample preservation is essential for accurate total nitrogen (TN) determination in water quality analysis, because microbial activity and chemical transformations can change nitrogen species between sample collection and laboratory testing. This technical guide explains accepted preservation protocols, including container selection, chemical preservation, refrigerated storage, and maximum holding times, aligned with standards such as HJ 636-2012, ISO 11905-1, ISO 5667-3, and ASTM D3590. Follow these practices to ensure defensible, compliant TN data for environmental monitoring, discharge permitting, and laboratory accreditation.

Technical Overview

Total nitrogen (TN) is defined as the sum of all nitrogen species present in a water sample, including nitrate nitrogen (NO3-N), nitrite nitrogen (NO2-N), ammonia nitrogen (NH3-N), and dissolved and particulate organic nitrogen. It is one of the principal eutrophication indicators used worldwide and is a regulated parameter in surface water quality standards such as GB 3838-2002 and municipal wastewater discharge standards such as GB 18918-2002.

Unlike conservative parameters, nitrogen species in water are biologically and chemically reactive. Once a sample is removed from its source, its nitrogen composition begins to change immediately unless preservation measures are applied. Accurate total nitrogen determination therefore depends as much on correct sample preservation as on the analytical measurement itself.

The principal instability mechanisms that preservation must control include:

• Microbial nitrification, which converts ammonia nitrogen to nitrite and then nitrate, altering the distribution of nitrogen fractions.
• Denitrification under anoxic storage conditions, which converts nitrate to nitrogen gas (N2) and causes an irreversible loss of total nitrogen mass from the sample.
• Ammonia volatilization from samples with elevated pH, particularly wastewater with high free ammonia content.
• Mineralization of organic nitrogen and oxidation of nitrite during storage.
• Adsorption of nitrogen species onto container walls or suspended sediments.
• Photochemical reactions affecting nitrate and nitrite when samples are exposed to light.

The objectives of a valid preservation program are to arrest microbial metabolism, suppress chemical transformation, prevent physical losses, and deliver the sample to the analytical instrument with its in-situ total nitrogen content intact.

Testing Samples

Surface Water

Rivers, lakes, and reservoirs typically contain total nitrogen in the range of 0.1 to 4 mg/L. Samples should be collected at representative locations and depths, commonly 0.5 m below the surface and away from riverbanks or discharge points. Because algal activity in nutrient-rich waters is intense, immediate cooling and dark storage are particularly important for lake and reservoir samples.

Groundwater

Groundwater often has low total nitrogen concentrations and frequently anoxic conditions. Monitoring wells should be purged of three to five well volumes until pH, conductivity, and temperature stabilize before sampling. Anoxic groundwater samples are highly susceptible to denitrification losses, so preservation must begin in the field without delay.

Drinking Water and Source Water

Low-level nitrogen concentrations demand rigorous contamination control. Sampling personnel must avoid ammonia-based cleaning agents near the sampling point, and all containers must be acid-washed and rinsed with reagent water. High-sensitivity determinations benefit from field blanks carried through the entire sampling and preservation sequence.

Municipal and Industrial Wastewater

Wastewater samples may contain 10 to 100 mg/L or more of total nitrogen and frequently require dilution before analysis. Samples containing residual chlorine must be dechlorinated at the time of collection with an appropriate quenching agent before further preservation. Twenty-four-hour composite samples must be continuously refrigerated during the collection period, with the holding time counted from the end of the compositing cycle.

Seawater and Estuarine Water

High chloride, bromide, and iodide concentrations in saline waters can interfere with the ultraviolet spectrophotometric detection step of the persulfate digestion method. Estuarine surveys should record salinity with each sample so that laboratories can apply appropriate corrections or select alternative digestion-detection schemes.

Sample Preservation Requirements

Container Selection and Preparation

Total nitrogen samples shall be collected in chemically resistant borosilicate glass or polyethylene/polypropylene bottles with a minimum volume of 250 mL, preferably 500 mL to allow for reanalysis and quality control. Containers must be washed with laboratory detergent, rinsed with water, soaked in dilute hydrochloric or nitric acid, and finally rinsed with reagent water. Bottles that previously held nitrogen-containing reagents must not be reused. For grab samples, a final rinse with the sample water is recommended unless the container already contains preservative.

Chemical Preservation Options

Two preservation schemes are widely accepted:

• Cold preservation without acidification: In accordance with HJ 636-2012, samples for alkaline potassium persulfate digestion are stored at 1 to 5 degrees Celsius in the dark and analyzed within 7 days. This approach avoids the neutralization step required before alkaline digestion.
• Sulfuric acid preservation: Many internationally recognized procedures for total Kjeldahl nitrogen and digestion-based total nitrogen permit acidification with concentrated sulfuric acid to pH below 2, typically about 2 mL per liter of sample, combined with refrigerated storage and a holding time of up to 28 days.

Acid-preserved aliquots must be neutralized with sodium hydroxide before alkaline potassium persulfate digestion, because residual acid consumes the oxidant and suppresses the oxidation efficiency, causing systematically low results. Unapproved preservatives, including heavy-metal biocides, shall be avoided. Chlorinated samples must be dechlorinated before acidification.

Temperature Control and Storage

Samples shall be placed on ice or refrigerant packs immediately after collection and maintained at 1 to 5 degrees Celsius, protected from light, until analysis. Field transport should use insulated coolers with temperature verification upon receipt at the laboratory. Freezing is not a standard preservation technique for total nitrogen unless validated by the laboratory for the specific matrix.

Maximum Holding Times

Under HJ 636-2012, the maximum holding time is 7 days from collection to analysis for samples stored at 1 to 5 degrees Celsius. Sulfuric-acid-preserved samples analyzed by Kjeldahl-based or digestion-based international methods may be held for up to 28 days. The collection date and time must be recorded on every container, and any result generated beyond the validated holding time must be flagged in the analytical report.

Field Handling and Quality Control

Each sampling event should include field blanks, duplicate samples, and a complete chain-of-custody record identifying the sample ID, location, time, preservative, and requested determinations. For total nitrogen, the sample must remain unfiltered whole water; if dissolved total nitrogen is required, a separate aliquot shall be filtered through a 0.45 micrometer membrane in the field and then preserved identically. General sampling and preservation practice should conform to the guidance of ISO 5667-1 and ISO 5667-3.

Testing Items

The core testing item is total nitrogen in whole water, reported in mg/L as N. laboratories accredited for nutrient analysis typically offer the following related determinations from parallel, appropriately preserved samples:

• Total nitrogen (TN), unfiltered whole water.
• Dissolved total nitrogen, 0.45 micrometer field-filtered aliquot.
• Total Kjeldahl nitrogen (TKN), the sum of organic nitrogen and ammonia nitrogen.
• Ammonia nitrogen (NH3-N), nitrate nitrogen (NO3-N), and nitrite nitrogen (NO2-N) as supporting fractionation parameters.
• Particulate organic nitrogen, derived from the difference between total and dissolved fractions.

Because different nitrogen fractions require different preservation conditions, sampling plans should specify separate containers where necessary. In particular, nitrate and nitrite determinations generally require unpreserved bottles analyzed within 24 to 48 hours, while acidified bottles serve TKN and digestion-based total nitrogen.

Testing Methods

Alkaline Potassium Persulfate Digestion UV Spectrophotometry (HJ 636-2012)

This is the standard method for total nitrogen in surface water, groundwater, and wastewater in China, replacing the former GB 11894-89. An aliquot of sample is mixed with alkaline potassium persulfate in a stoppered digestion tube and heated in an autoclave at 120 to 124 degrees Celsius for 30 minutes. All nitrogen species are oxidized to nitrate, and the nitrate is measured spectrophotometrically at 220 nm with correction at 275 nm to compensate for dissolved organic interference. The calibrated working range is approximately 0.05 to 4 mg/L, with higher concentrations determined after dilution. Known interferences include elevated concentrations of certain metals, high carbonate alkalinity, and iodide and bromide in saline samples.

International Reference Methods

ISO 11905-1 specifies the determination of nitrogen by oxidative digestion with peroxodisulfate, following the same chemical principle of converting all nitrogen forms to nitrate prior to spectrophotometric detection. ASTM D3590 covers total Kjeldahl nitrogen by block digestion followed by colorimetric or titrimetric finishing, and total nitrogen can be obtained by combining TKN with separately measured nitrate plus nitrite. Accredited third-party laboratories select the method appropriate to the matrix, expected concentration range, and regulatory reporting requirement, and verify performance through certified reference materials, matrix spikes, and duplicate digestions.

Applications

Reliable total nitrogen data underpin a wide range of environmental and industrial programs, provided samples are preserved correctly:

• Regulatory compliance monitoring against GB 3838-2002, in which Class III surface water is limited to 1.0 mg/L total nitrogen, and against GB 18918-2002 discharge limits of 15 mg/L for Class 1A and 20 mg/L for Class 1B municipal effluent.
• Process control at wastewater treatment plants, where nitrification and denitrification performance in anoxic-oxic systems is evaluated from total nitrogen and its fractions.
• Eutrophication assessment and nutrient loading studies for lakes, reservoirs, and coastal receiving waters.
• Discharge permit verification, pollution source apportionment, and environmental impact assessment.
• Academic and applied research on nitrogen cycling, and method validation work within accredited third-party laboratories.

FAQ

How should water samples for total nitrogen determination be preserved?

Collect the sample in an acid-washed glass or polyethylene bottle. Following HJ 636-2012, store it at 1 to 5 degrees Celsius in the dark and complete the analysis within 7 days. Alternatively, acidify with concentrated sulfuric acid to pH below 2 and refrigerate, which extends the holding period to 28 days under internationally recognized digestion-based methods; neutralize the aliquot before alkaline persulfate digestion.

Why must acid-preserved samples be neutralized before persulfate digestion?

The persulfate digestion requires an alkaline medium. Residual sulfuric acid consumes the oxidant and lowers the digestion temperature chemistry, resulting in incomplete oxidation of ammonia and organic nitrogen and systematically low total nitrogen values. Adjust the aliquot to the required alkaline pH with sodium hydroxide and account for any dilution.

What is the maximum holding time for total nitrogen samples?

Seven days at 1 to 5 degrees Celsius under HJ 636-2012, or twenty-eight days for sulfuric-acid-preserved samples analyzed by Kjeldahl-based or digestion-based international procedures. The holding clock starts at the moment of collection, not at laboratory receipt.

Should total nitrogen samples be filtered?

No. Total nitrogen is determined on unfiltered whole water so that particulate organic nitrogen is included. Only the dissolved total nitrogen fraction requires field filtration through a 0.45 micrometer membrane, followed by the same preservation steps.

Why do total nitrogen results sometimes decrease during storage?

Declining values are usually caused by denitrification converting nitrate to nitrogen gas in anoxic samples, volatilization of free ammonia from alkaline wastewater, or microbial immobilization. Prompt cooling, dark storage, correct acidification where permitted, and analysis within the holding time prevent these losses.

What should be done if the sample contains residual chlorine?

Residual chlorine chemically transforms nitrogen species during storage. Quench it immediately at the time of collection with a stoichiometric amount of sodium thiosulfate or ascorbic acid, then apply the appropriate preservation scheme and note the treatment on the chain-of-custody record.