Precautions for Using a Total Nitrogen Analyzer

When determining total nitrogen (TN) in water samples, correct operation of the analyzer is crucial for accurate and reproducible results. Below are key precautions to follow during use.

1. Sample Pretreatment
Ensure samples are homogeneous and free of large particles. Filter or homogenize if necessary. Store samples at 4°C and analyze within 24 hours to prevent nitrogen loss or conversion.

2. Reagent Quality and Storage
Use only high-purity reagents (e.g., low‑blank potassium persulfate, NaOH). Check reagent expiry dates. Prepare alkaline persulfate solution fresh and keep it away from light and ammonia vapor to avoid contamination.

3. Digestion Process
Complete digestion is essential for converting all nitrogen forms (organic, ammonia, nitrate, nitrite) to nitrate. Control temperature (120–124°C) and time (30–40 min) strictly. Incomplete digestion leads to low results; over‑digestion may cause volatile loss.

4. Calibration and Blanks
Run a calibration curve with certified standards daily or before each batch. Always include a reagent blank to subtract background nitrogen. The blank absorbance should be low and stable; high blanks indicate contaminated reagents or glassware.

5. Cuvette Handling
Use matched quartz cuvettes (UV detection at 220 nm and 275 nm). Keep cuvettes scrupulously clean – rinse with deionized water, then with a portion of the sample. Wipe fingerprints and droplets with lint‑free cloth. Air bubbles in the cuvette must be eliminated.

6. Interference Control
Chloride, bromide, and dissolved organic matter can interfere. For high‑chloride samples (e.g., seawater), use a correction formula or pre‑treatment (ion exchange). Check turbidity at 275 nm for organic interference correction.

7. Instrument Maintenance
Regularly inspect the UV lamp – deteriorated lamps reduce sensitivity and accuracy. Clean the flow cell and tubing weekly with dilute acid and deionized water to prevent biofilm buildup. Recalibrate after lamp replacement or major maintenance.

8. Safety
Alkaline persulfate digestion at high temperature is hazardous. Wear gloves, goggles, and a lab coat. Perform digestion in a fume hood to avoid exposure to ammonia or acid fumes.