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LabCoda / Knowledge Base / Heavy metals in soil (ICP-OES)

🌿 Heavy metals in soil (ICP-OES)

Environmental PN-EN ISO 11885

Multi-element determination of heavy metals (Pb, Cd, Cr, Cu, Zn, Ni, As, Hg) in soil after acid digestion, by ICP-OES method. Basic test for soil contamination assessment.

Overview

Determination of heavy metals in soil is a key element of soil environment assessment. Metals such as lead, cadmium, chromium, copper, zinc, nickel, arsenic, and mercury accumulate in soil and do not biodegrade, posing a long-term threat to ecosystems, groundwater, and human health.

The 2016 Polish regulation on soil quality standards specifies permissible metal concentrations depending on soil group (I — residential areas, IV — industrial). E.g., lead: 100 mg/kg (gr. I) to 600 mg/kg (gr. IV), cadmium: 1 mg/kg (gr. I) to 15 mg/kg (gr. IV).

The method involves soil sample digestion with acid mixture (HNO₃ + HCl, i.e., aqua regia, or HNO₃ + HF + HClO₄) in microwave digester, followed by multi-element analysis of the solution by ICP-OES. In one analytical cycle, 20–30 elements are determined simultaneously.

Method principle

The soil sample is digested in closed PTFE vessels in a microwave digester with acid mixture (aqua regia: 3 vol. HCl + 1 vol. HNO₃ according to ISO 11466, or mixture with HF for complete dissolution of silicates). The digest is diluted with ultrapure water and analyzed on ICP-OES spectrometer. Argon plasma (6000–10000 K) atomizes and excites elements, which emit radiation at characteristic wavelengths. Detection on a CCD polychromator enables simultaneous measurement of many elements.

Applications

Key parameters

ParameterValue
Determined elementsPb, Cd, Cr, Cu, Zn, Ni, As, Hg, Co, Mn, Fe, V, Ba + others
Detection limit0.01–1 mg/kg d.m. (element-dependent)
Linear range3–5 orders of magnitude
Precision (CV)3–10%
DigestionAqua regia, 180°C, 30 min (microwave)

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 preparation

Dry soil at 40°C. Sieve through 2 mm sieve, then grind <2 mm fraction to <0.25 mm. Weigh 0.5 g.

⏱ Time: 24 h • 🌡 Temperature: 40°C

2. Microwave digestion

Add 9 mL HCl + 3 mL HNO₃ (aqua regia). Program: ramp to 180°C/15 min, hold 20 min.

⏱ Time: 45 min • 🌡 Temperature: 180°C

3. Filtration and dilution

After cooling, transfer quantitatively to 50 mL flask. Fill to mark with ultrapure water. Filter (0.45 µm).

⏱ Time: 15 min

4. ICP-OES calibration

Prepare standards: 0, 0.1, 0.5, 1, 5, 10, 50 mg/L in 5% HNO₃. Calibrate instrument.

⏱ Time: 30 min

5. Sample measurement

Measure samples with blank and CRM every 10 samples. Check spike recovery (90–110%).

⏱ Time: 3–5 min/sample

6. Calculations

Convert concentrations to mg/kg dry mass of soil. Account for dilution and moisture.

⏱ Time: 10 min

Required equipment and apparatus

EquipmentExampleIndicative price
ICP-OES spectrometerThermo iCAP 7400, Agilent 5800, PerkinElmer Avio 220350,000–700,000 PLN
Microwave digesterAnton Paar Multiwave 5000, CEM Mars 6, Milestone Ethos UP150,000–300,000 PLN
AutosamplerThermo ASX-560, Agilent SPS 425,000–60,000 PLN
Analytical balanceMettler Toledo ME204, 0.1 mg accuracy5,000–15,000 PLN
PTFE digestion vessels100 mL vessels, set of 6–12 pcs.500–2,000 PLN/pc.

Reagents, media and consumables

ReagentCASDetails
Nitric acid 65% suprapur7697-37-2Trace analysis grade
Hydrochloric acid 30% suprapur7647-01-0Aqua regia component
Multi-element standard solutions—ICP Multi-Element Standard IV, 1000 mg/L, certified CRM
Argon 5.07440-37-1Plasma gas, 99.999% purity
Ultrapure water—Resistivity >18.2 MΩ·cm

Health and safety (OHS)

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