Sampling for Nitrate-N Determination: Key Precautions

Accurate nitrate‑nitrogen (NO₃⁻‑N) analysis begins at the very moment a water sample is taken. Improper collection, handling, or storage can introduce errors far larger than any laboratory instrument’s analytical uncertainty. This note outlines the essential requirements for obtaining a representative and stable sample for nitrate‑N testing.

1. Container selection

Use only clean, high‑density polyethylene (HDPE) or glass bottles with screw‑caps. Avoid soft plastics or rubber seals that may leach nitrogen‑containing compounds. All containers must be pre‑rinsed three times with the water to be sampled, except when sampling for trace levels, where acid‑washed and deionised‑water‑rinsed vessels are mandatory.

2. Sampling point and procedure

Collect samples from well‑mixed zones, away from stagnant surfaces, bottom sediments, or floating debris. For surface waters, submerge the bottle below the surface to avoid surface film contamination. For groundwater or tap water, let the water run for at least 2–3 minutes before filling to flush the stagnant portion of the system. Fill the bottle completely to the brim, leaving no headspace, and cap immediately to minimise atmospheric oxygen or ammonia contamination.

3. Preservation and storage

Nitrate is biologically labile – microbial activity can reduce it to nitrite or assimilate it into biomass. Therefore, refrigerate samples at 1–4 °C immediately after collection. If analysis cannot be performed within 24 hours, add a chemical preservative: either 2 mL of concentrated sulphuric acid per litre (lowering pH to <2) or 40 mg/L of mercuric chloride, depending on the analytical method’s compatibility. Acidified samples must be neutralised before analysis.

4. Filtration – when and why

For samples containing suspended particles or algae, filter through a 0.45 µm membrane filter on‑site or immediately upon return to the laboratory. Delayed filtration allows particulate nitrogen to leach into the dissolved phase, falsely elevating nitrate readings. However, if the method measures total nitrate (including particulate forms), filtration should be omitted – specify this in the sampling protocol.

5. Holding time

Under optimal preservation (acidification + 4 °C), samples remain stable for up to 7 days. Without preservation, analyse within 48 hours, but ideally within 24 hours. Always record the exact collection time and preservation steps on the sample label.