NITRATE ANALYSIS – PRETREATMENT & SAMPLE FILTRATION PROCEDURES

 Purpose

To define the standardized procedure for sample pretreatment and filtration prior to nitrate analysis to ensure accurate, interference-free, and reproducible results.

*. Scope

Applicable to:

• Sodium nitrate and nitrate ion analysis in water, wastewater, effluent, fertilizers, brines, industrial matrices.
• 
  
• IC, UV-Vis (220/275 nm), FIA, and other nitrate measurement techniques.

*. Responsibilities

• Analyst: Perform sample pretreatment, filtration, dilution & documentation.
• 
  
• Lab Supervisor: Verify adherence and maintain QC.
• 
  
• Quality Manager: Ensure SOP compliance with ISO 17025.

4. Equipment & Materials

4.1 Filtration Equipment

• Syringe filters: 0.45 µm (general use), 0.22 µm (IC/trace)
• 
  
• Disposable syringes (5–20 mL)
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• Vacuum filtration unit & 0.45 µm membrane filters
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• Buchner funnel & vacuum flask (for large-volume solids removal)

4.2 Pretreatment Equipment

• Tabletop centrifuge (3000–5000 rpm)
• 
  
• Ultrasonic bath (optional)
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• Magnetic stirrer
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• pH meter
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• Volumetric flasks (Class A)
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• Polypropylene or polyethylene sample vials (IC requires PP)

*Reagents

• DI water (18.2 MΩ cm)
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• Nitrate stock standard (1000 mg/L NO₃⁻)
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• C18 SPE cartridges (optional)
• 
  
• Anion-exchange SPE cartridges (optional)

Sample Collection & Preservation

• Use clean polypropylene bottles.
• 
  
• Collect at least 100 mL sample.
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• Refrigerate at 4°C immediately.
• 
  
• Maximum holding time: 48 hours.
• 
  
• For long storage (not recommended), add H₂SO₄ to pH < 2 (for NO₃⁻/NO₂⁻ combined analysis).
• 
  

*Safety Precautions

• Wear lab coat, gloves, goggles.
• 
  
• Handle industrial effluent as hazardous.
• 
  
• Dispose of filtrates/solids following chemical waste guidelines.

. Principle

Anions (including nitrate) in the sample are separated using an anion-exchange chromatographic column and detected by a conductivity detector with chemical suppression, which improves sensitivity.

• Quantification is performed by external calibration using multi-point standards

* Procedure

6.1 Sample Receipt & Homogenization

1. Check container integrity, sample volume, and chain-of-custody.
2. 
   
3. Gently invert the sample bottle several times.
4. 
   
5. Avoid aeration or frothing to prevent chemical changes.

6.2 pH Check

1. Bring sample to room temperature (20–25°C).
2. 
   
3. Measure pH and record.
4. 
   
5. For IC analysis, ensure pH 6–8.

• If needed, adjust using dilute HCl or NaOH (minimal volumes).

6.3 Removal of Coarse Particulates

Option A — Sedimentation (simple samples)

• Allow sample to stand 5–10 minutes.
• 
  
• Decant supernatant without disturbing settled solids.

Option B — Centrifugation (recommended for effluent)

1. Transfer 15–50 mL to centrifuge tubes.
2. 
   
3. Spin at 3000–5000 rpm for 10 minutes.
4. 
   
5. Transfer clear supernatant to a clean container.

6.4 Filtration (Critical Step)

A. Standard Filtration (general use)

1. Pull 5–10 mL supernatant into a syringe.
2. 
   
3. Attach 0.45 µm syringe filter (PES or Nylon).
4. 
   
5. Discard first 2 mL of filtrate (“filter conditioning”).
6. 
   
7. Collect remainder in a clean vial.

B. Fine Filtration for IC / Trace Nitrate

1. Use 0.22 µm syringe filter (PVDF/PTFE/PES).
2. 
   
3. Filter gently without over-pressurizing.
4. 
   
5. Collect filtrate in polypropylene vials (glass not recommended for IC).

C. Large-Volume Filtration (dirty wastewater, sludge extract)

1. Set up vacuum filtration unit with 0.45 µm membrane.
2. 
   
3. Pre-rinse the membrane with DI water.
4. 
   
5. Filter sample under mild vacuum to avoid forcing solids through the membrane.

* Optional Pretreatment for Complex Matrices

. Organic Removal (oil/grease, industrial wastewater)

1. Condition C18 SPE cartridge with 5 mL methanol + 10 mL DI water.
2. 
   
3. Pass sample slowly through cartridge.
4. 
   
5. Discard first 3–5 mL.
6. 
   
7. Collect effluent for nitrate analysis.

. High Salt or High Chloride Removal (if required)

Use anion-exchange SPE cartridges:

1. Condition with DI water.
2. 
   
3. Pass sample and collect nitrate-rich fraction.
4. 
   
5. Do not overload the cartridge.

 Sample Pretreatment

8.1 Homogenization

Mix sample thoroughly by gentle inversion.

8.2 pH Check

Ensure pH between 6–8 for IC. Adjust with dilute HCl or NaOH only if necessary.

8.3 Solids Removal

Option A — Centrifugation

• Spin at 3000–5000 rpm for 10 minutes.

Option B — Filtration

• Filter through 0.45 µm for general use.
• 
  
• Use 0.22 µm for low-range analysis.

Discard first 2 mL of filtrate to condition filter.

8.4 Dilution

Prepare dilution when nitrate > calibration range.

Use Class A volumetric flasks only.

1. Record dilution factor (DF).

. Colored Samples (UV-Vis only)

• Perform activated carbon treatment (0.5 g / 100 mL), mix, filter.
• 
  
• Prevents absorbance interference at 220/275 nm.

* Sample Dilution

1. Dilute high-concentration samples into the validated range.
2. 
   
3. Always use Class A glassware.
4. 
   
5. Record dilution factor (DF).

Typical dilution factors:

• Drinking water: none
• 
  
• Industrial effluent: 1:10 to 1:1000 depending on strength
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• Production brine streams: 1:100 to 1:5000

* Final Sample Handling

• Transfer filtered sample into labeled polypropylene vials.
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• Store at 4°C if not analyzed immediately.
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• Analyze within 24–48 hours (nitrate is stable but matrix may degrade).

*QC Requirements

Run before or with every batch:

• Method blank (DI water)
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• Laboratory control sample (known nitrate standard)
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• Duplicate sample
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• Matrix spike (Recovery target: 90–110%)
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• Calibration verification standard
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• Instrument check standard every 10–15 injections (IC)

* Acceptance Criteria

• Filtration must yield visually clear, particle-free sample.
• 
  
• No breakthrough clogging.
• 
  
• %Recovery of spikes: 90–110%
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• Duplicate precision: ≤5% RSD
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• No shift in retention time or absorbance baseline due to matrix.

*Documentation

Record:

• Sample ID
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• pH
• 
  
• Pretreatment steps (centrifugation rpm/time, SPE used)
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• Filter type & pore size
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• Dilution factor
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• Date/time of preparation
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• Analyst initials

* References

• APHA Standard Methods 4500-NO₃⁻
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• EPA 300.0 (IC Determination of Anions)
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• ISO 10304 (IC for Anions)
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• ISO/IEC 17025:2017 standard requirements
• 
  

Purpose

• To describe the standardized procedure for accurate, precise, and interference-free determination of nitrate (NO₃⁻) in water, wastewater, industrial effluent, manufacturing samples, and environmental matrices using ion chromatography.

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