🔬 Selenium in Water (ICP-OES)
Determination of selenium in drinking, surface, and wastewater by inductively coupled plasma optical emission spectrometry (ICP-OES). Selenium is an essential element but toxic at higher concentrations.
Overview
Selenium (Se) is a trace element that plays an important biological role in small doses (component of selenocysteine, antioxidant enzymes), but exhibits strong toxic effects at higher concentrations. The permissible selenium concentration in drinking water is 10 µg/L according to the Ministry of Health Regulation and EU Directive 2020/2184. Exceeding this limit can lead to selenosis (hair loss, nail damage, neurological disorders).
The ICP-OES method according to PN-EN ISO 11885 allows multi-element determination of selenium along with 32 other elements (As, Cd, Pb, Hg, Cr, Cu, Zn, etc.) in one analytical cycle. Argon plasma at 6,000–10,000 K atomizes and excites selenium atoms, which emit characteristic radiation. The most commonly used Se emission line is 196.026 nm.
The challenge in selenium determination by ICP-OES is its relatively weak emission, so for low concentrations (<10 µg/L) the hydride generation technique (HG-ICP-OES) is recommended, in which Se is reduced by NaBH₄ to volatile SeH₂, increasing sensitivity 10–100-fold.
The method is used in routine monitoring of drinking water, control of industrial wastewater (metallurgy, energy, glass industry), and groundwater testing in agricultural areas (selenium fertilizers).
Method principle
A water sample acidified with nitric acid is nebulized in a nebulizer chamber and introduced into argon plasma generated by inductively coupled electromagnetic field (27.12 MHz). At 6,000–10,000 K, selenium atoms are excited and emit radiation at characteristic wavelengths (196.026 nm — main line, 203.985 nm — alternative line). The emitted radiation is separated by a polychromator (Echelle or Czerny-Turner optical system) and recorded by CCD/SCD detector. Selenium concentration is calculated from calibration curve. In HG-ICP-OES technique, sample is mixed with NaBH₄ in HCl medium, generating volatile SeH₂, which is transported to plasma by carrier gas (Ar).
Applications
- Drinking water quality monitoring — Se control (Journal of Laws 2017 item 2294)
- Groundwater testing in agricultural and industrial areas
- Industrial wastewater control (metallurgy, energy, glass industry)
- Surface water monitoring (Water Framework Directive)
- Mineral and bottled water testing
- Sewage sludge and soil analysis (after mineralization)
- Water treatment process control (selenium removal)
Key parameters
| Parameter | Value |
|---|---|
| Se emission line | 196.026 nm (main), 203.985 nm (alt.) |
| Limit of detection (ICP-OES) | ~10–50 µg/L (conventional) |
| Limit of detection (HG-ICP-OES) | 0.5–2 µg/L |
| Permissible concentration (drinking water) | 10 µg/L |
| Precision (CV) | 3–10% (depending on concentration) |
| Linear range | 1–10,000 µg/L |
Standard
- Standard number
- PN-EN ISO 11885:2009
- Title (PL)
- Jakość wody — Oznaczanie wybranych pierwiastków metodą optycznej spektrometrii emisyjnej z plazmą indukcyjnie sprzężoną (ICP-OES)
- Title (EN)
- Water quality — Determination of selected elements by inductively coupled plasma optical emission spectrometry (ICP-OES)
Step-by-step procedure
1. Sample collection and preservation
Collect sample in PP/HDPE bottle. Acidify with 1 mL conc. HNO₃ per 100 mL sample (pH < 2). Store at 4°C max. 14 days.
2. Mineralization (optional)
For samples with suspension: microwave mineralization — 5 mL HNO₃ + 1 mL H₂O₂, program: ramp to 180°C/20 min, hold 15 min.
3. Standard preparation
Prepare Se calibration series: 0, 5, 10, 25, 50, 100 µg/L in 1% HNO₃. Add internal standard (Y, Sc) if required.
4. ICP-OES startup
Turn on argon plasma. Stabilize system (15–30 min). Check plasma efficiency on Mn 257.610 nm control solution.
5. Instrument calibration
Measure calibration standards. Check linearity (R² > 0.999). Verify sensitivity — curve slope should match specification.
6. Sample measurement
Measure samples in sequence: blank → control standard → samples (max 10) → blank → control standard. Monitor Se 196.026 nm line.
7. HG technique (for low concentrations)
Connect HG module. Mix sample in 5 M HCl online with 0.5% NaBH₄. Transport volatile SeH₂ to plasma with argon. LOD ~0.5 µg/L.
8. Quality control
Analyze CRM (e.g., SPS-WW2), spike (recovery 90–110%), duplicate every 10 samples. Blank < 3× LOD.
9. Calculations and reporting
Read concentrations from calibration curve. Account for dilution and sample volume. Report result in µg/L with expanded uncertainty (k=2).
10. Shutdown and maintenance
Flush system with 1% HNO₃ (5 min), then water (5 min). Turn off plasma. Check torch and nebulizer wear.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| ICP-OES spectrometer | Thermo iCAP 7400, Agilent 5800 VDV, PerkinElmer Avio 220 Max, Spectro Arcos II | 350,000–700,000 PLN |
| Hydride generation system (HG) | PerkinElmer FIAS 400, Thermo VP100 CHGS | 40,000–80,000 PLN |
| Autosampler | Thermo ASX-560, Agilent SPS 4, CETAC ASX-520 | 25,000–60,000 PLN |
| Sample preparation system (mineralizer) | Anton Paar Multiwave 5000, CEM Mars 6, Milestone Ethos UP | 150,000–300,000 PLN |
| Argon 5.0 cylinder | Argon 99.999%, 50 L cylinder, consumption ~15 L/min | 400–800 PLN/cylinder |
| PP/PTFE digestion vessels | PTFE vessels for microwave mineralizer | 500–2,000 PLN/pc |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Selenium standard solution (Se) | 7782-49-2 | Certified CRM 1,000 mg/L Se in 2% HNO₃, e.g., Merck CertiPUR |
| Nitric acid (HNO₃) 65% suprapur | 7697-37-2 | Trace analysis grade, for sample acidification and standard preparation |
| Hydrochloric acid (HCl) 30% suprapur | 7647-01-0 | For HG technique — reaction medium with NaBH₄ |
| Sodium borohydride (NaBH₄) | 16940-66-2 | Reductant in HG technique, 0.5% solution in 0.5% NaOH |
| Sodium hydroxide (NaOH) | 1310-73-2 | Stabilizer for NaBH₄ solution |
| Argon 5.0 | 7440-37-1 | Plasma and carrier gas, purity 99.999% |
| Ultrapure water | — | Resistivity >18.2 MΩ·cm (Milli-Q), for dilutions and blanks |
Health and safety (OHS)
- Concentrated nitric acid — highly corrosive and oxidizing, work under fume hood
- NaBH₄ — flammable, reacts with acids releasing H₂, store dry
- Selenium standards — toxic, use nitrile gloves and eye protection
- Argon — asphyxiant in enclosed spaces, ensure ventilation
- ICP plasma — UV radiation, do not look directly at torch
- Collect acidified waste in inorganic waste containers