⚛️ Cadmium in Water — GF-AAS
Determination of trace concentrations of cadmium in water by atomic absorption spectrometry with graphite furnace (GF-AAS). Detection limit of about 0.05 µg/L.
Overview
Cadmium (Cd) is a heavy metal with strong toxic action — it accumulates in the body (half-life in kidneys is 10–30 years), causes kidney damage (cadmium nephropathy), osteoporosis (itai-itai disease), is carcinogenic (group 1 according to IARC). The permissible cadmium concentration in drinking water is 5 µg/L (EU Directive 2020/2184) — one of the lowest limits among heavy metals.
Sources of cadmium in waters are metallurgical industry (zinc, copper), Ni-Cd battery production, phosphorus fertilization (phosphates contain cadmium as contamination), electroplating, and corrosion of galvanized steel pipes. Cadmium is very mobile in the aquatic environment at low pH.
The GF-AAS technique is particularly suited for cadmium determination due to excellent sensitivity — a detection limit of about 0.05 µg/L allows reliable determination at 1/10 of the permissible standard. Cadmium atomizes at a relatively low temperature (1500–1800°C), but is sensitive to chloride interference — excess chlorides can cause premature volatilization of cadmium during pyrolysis. Therefore, use of a matrix modifier (NH₄H₂PO₄) is mandatory.
The PN-EN ISO 15586 standard provides detailed instrumental conditions for cadmium: wavelength 228.8 nm, modifier NH₄H₂PO₄ or mixture Mg(NO₃)₂ + NH₄H₂PO₄, pyrolysis temperature 500–700°C (with modifier), atomization temperature 1500–1800°C.
Method principle
A water sample (10–20 µL) is dosed into a graphite tube in the furnace. The temperature program includes: drying (110–130°C), pyrolysis (500–700°C with NH₄H₂PO₄ modifier — matrix removal without Cd losses), atomization (1500–1800°C — evaporation and atomization of cadmium), cleaning (2400°C). Free Cd atoms absorb radiation from a hollow cathode lamp at 228.8 nm. Absorbance (peak area) is proportional to the cadmium mass in the sample. Background correction (Zeeman or D₂ lamp) eliminates nonspecific absorption from matrix components.
Applications
- Drinking water quality control (Cd ≤5 µg/L)
- Industrial wastewater monitoring (metallurgy, electroplating)
- Testing of groundwater at waste disposal sites
- Control of cadmium in agricultural waters (influence of phosphorus fertilizers)
- Monitoring of surface waters (Water Framework Directive)
- Analysis of cadmium in sewage sludge (after mineralization)
- Food control — water for food production
Key parameters
| Parameter | Value |
|---|---|
| Wavelength | 228.8 nm |
| Detection limit (LOD) | ~0.05 µg/L |
| Limit of quantification (LOQ) | ~0.2 µg/L |
| Linear range | 0.2–5 µg/L |
| Atomization temperature | 1500–1800°C |
| Matrix modifier | NH₄H₂PO₄ + Mg(NO₃)₂ |
Standard
- Standard number
- PN-EN ISO 15586:2005
- Title (PL)
- Jakość wody — Oznaczanie pierwiastków śladowych z zastosowaniem atomowej spektrometrii absorpcyjnej z piecem grafitowym
- Title (EN)
- Water quality — Determination of trace elements using atomic absorption spectrometry with graphite furnace
Step-by-step procedure
1. Preparation of vessels
PP/PTFE vessels soak in 10% HNO₃ (min. 24 h), rinse three times with ultrapure water. Do not use glass — cadmium adsorbs on glass.
2. Acidification and storage of samples
Add Suprapur HNO₃ to pH <2 (1 mL conc. HNO₃ per 100 mL sample). Store in PP at 4°C. Analysis within 14 days.
3. Preparation of standards
Prepare calibration series: 0, 0.5, 1, 2, 3, 5 µg/L Cd in 1% HNO₃. Prepare standards immediately before measurement.
4. Setting temperature program
Drying: 110°C (30 s), 130°C (20 s). Pyrolysis: 600°C (20 s). Atomization: 1650°C (5 s, reading). Cleaning: 2400°C (3 s). Ar flow: 250 mL/min (stop at atomization).
5. Dosing modifier
Autosampler doses 5 µL mixture NH₄H₂PO₄ 1% + Mg(NO₃)₂ 0.1% together with 15 µL sample into graphite tube.
6. Calibration
Measure standard series. Plot calibration curve (peak area vs concentration). R² ≥0.998. Check sensitivity (slope).
7. Sample measurement
Measure samples in duplicates. Time of one measurement approx. 3 min (temperature program + cooling). Instrument works automatically.
8. Quality control
Every 10 samples: blank <LOD, control standard (recovery 90–110%), duplicate (RPD <15%). For new matrix — standard addition method.
9. Verification of graphite tubes
Monitor tube condition — replace after 100–300 atomizations. Sign of wear is sensitivity drop and repeatability deterioration.
10. Calculation and reporting
Cd concentration from calibration curve. Result in µg/L with measurement uncertainty. For results <LOQ report as "<LOQ (0.2 µg/L)".
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| AAS spectrometer with graphite furnace | Agilent 280Z AA, Analytik Jena ZEEnit 700P, Shimadzu AA-7000 | 150 000–400 000 PLN |
| Autosampler | Agilent GTA 120, Analytik Jena MPE 60 | included with instrument |
| Hollow cathode lamp (HCL) Cd | Agilent Cd HCL, Heraeus, Photron — current 4 mA | 800–2 000 PLN |
| Graphite tubes with L'vov platform | Pyrolytically coated tubes with platform — improved sensitivity and repeatability | 50–150 PLN/pc. |
| Argon 5.0 (protective gas) | 50 L cylinder, two-stage regulator, purity 99.999% | 300–600 PLN/cylinder |
| PP/PTFE vessels | Polypropylene or PTFE containers — do not use glass (Cd adsorption) | 5–30 PLN/pc. |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Cd standard solution 1000 mg/L | 7440-43-9 | Certified CRM in 2% HNO₃ (e.g., Merck CertiPUR), traceable to NIST SRM |
| Nitric acid 65% Suprapur | 7697-37-2 | Ultrapure HNO₃ for acidifying samples (pH <2) and preparing dilutions |
| Modifier — NH₄H₂PO₄ | 7722-76-1 | Ammonium dihydrogen phosphate 1% (w/v) — prevents Cd volatilization with chloride matrices |
| Modifier — Mg(NO₃)₂ | 10377-60-3 | Magnesium nitrate 0.1% (w/v) — used together with NH₄H₂PO₄ for optimal stabilization |
| Ultrapure water | — | Resistivity ≥18.2 MΩ·cm, Milli-Q system — mandatory for trace Cd analysis |
| Diluted nitric acid 1% | — | For rinsing vessels and as blank — prepared from Suprapur HNO₃ and ultrapure water |
Health and safety (OHS)
- Cadmium compounds — carcinogenic cat. 1B (H350), toxic, cumulative — absolutely gloves, goggles, work under fume hood
- Concentrated nitric acid — corrosive, fuming — work under fume hood, acid-resistant gloves
- Graphite furnace — temperatures up to 2500°C — do not touch during operation and cooling
- Cd-containing waste — collect as hazardous waste, do not pour into sewage
- Argon — in closed rooms can displace oxygen, ensure ventilation
- Regular occupational exposure testing for Cd (cadmium in urine) for laboratory workers