Overview of GB/T 17141-1997 Standard
GB/T 17141-1997 "Soil Quality - Determination of Lead and Cadmium - Graphite Furnace Atomic Absorption Spectrophotometry" is an important foundational standard for heavy metal detection in soil in China. This standard specifies the method principles, reagents, instruments, analytical procedures, and result calculations for the determination of lead (Pb) and cadmium (Cd) in soil using graphite furnace atomic absorption spectrophotometry. It is applicable for routine testing in environmental monitoring, agricultural soil surveys, and scientific research laboratories.
Graphite furnace atomic absorption spectrometry has advantages such as high sensitivity, small sample consumption, and low detection limits, making it particularly suitable for quantitative analysis of trace lead and cadmium. The standard clarifies key technical requirements including sample pretreatment, use of matrix modifiers, and background correction.
Method Principle and Scope of Application
Atomic Absorption Principle
After acid digestion of soil samples, lead and cadmium enter the solution in ionic form. The test solution is injected into a graphite tube, where it is atomized at high temperature to form ground-state atomic vapor. The ground-state atoms absorb the characteristic spectral lines emitted by lead and cadmium hollow cathode lamps, and the absorbance is proportional to the atomic concentration. By measuring the absorbance, the content of lead and cadmium in the soil can be quantitatively calculated.
Scope of Application and Detection Limits
This standard is applicable to the determination of lead and cadmium in various soils and sediments. The method detection limits are affected by instrument performance and matrix interference, typically 0.1 mg/kg for lead and 0.01 mg/kg for cadmium (based on a sample weight of 0.5 g). In actual testing, verification should be performed according to instrument conditions.
Main Instruments and Reagents
Instruments and Equipment
- Atomic absorption spectrophotometer: equipped with a graphite furnace atomizer and background correction device (such as Zeeman effect or deuterium lamp).
- Lead and cadmium hollow cathode lamps: as sharp-line light sources.
- Graphite tubes: pyrolytically coated graphite tubes are recommended to improve sensitivity and lifespan.
- Micropipettes: 10–100 µL, for precise injection.
- Common laboratory glassware: must be soaked and cleaned with nitric acid to avoid contamination.
Reagents and Standard Solutions
- Guaranteed reagent grade nitric acid, hydrochloric acid, hydrofluoric acid, perchloric acid: for sample digestion.
- Lead and cadmium standard stock solutions: 1 mg/mL, commercially available certified reference materials.
- Matrix modifiers: such as ammonium dihydrogen phosphate and magnesium nitrate, used to eliminate matrix interference.
- Deionized water: resistivity ≥18 MΩ·cm.
Analytical Procedures
Sample Pretreatment
- Air-dry and grind the soil sample, pass through a 100-mesh (0.15 mm) nylon sieve, and mix well.
- Weigh 0.2–0.5 g (accurate to 0.0001 g) of sample into a polytetrafluoroethylene crucible.
- Add nitric acid-hydrofluoric acid-perchloric acid (e.g., 5:5:1) mixed acid, cover and soak overnight.
- Heat and digest on a hot plate until white fumes are exhausted, then dissolve the residue with 1% nitric acid and dilute to volume.
- Prepare blank solutions simultaneously; at least 2 blanks per batch of samples.
Instrument Condition Settings
The following are typical instrument parameters; actual optimization should be based on the instrument model:
| Parameter | Lead (Pb) | Cadmium (Cd) |
|---|---|---|
| Measurement wavelength | 283.3 nm | 228.8 nm |
| Lamp current | 5–10 mA | 3–8 mA |
| Slit width | 0.7 nm | 0.7 nm |
| Drying temperature/time | 100–120°C / 30 s | 100–120°C / 30 s |
| Ashing temperature/time | 500–600°C / 20 s | 300–400°C / 20 s |
| Atomization temperature/time | 1800–2000°C / 5 s | 1500–1800°C / 5 s |
| Cleaning temperature/time | 2200°C / 3 s | 2000°C / 3 s |
Measurement Operation
- Establish the calibration curve according to the instrument manual; the concentration ranges for lead and cadmium are 0–50 µg/L and 0–5 µg/L, respectively.
- Inject the prepared sample solution into the graphite tube and record the absorbance.
- If the sample concentration exceeds the curve range, dilute and re-measure.
- Insert an intermediate concentration standard point every 10 samples to check for drift.
Interference and Elimination
Common Types of Interference
- Matrix interference: large amounts of silicon, aluminum, iron, etc. in soil affect atomization efficiency.
- Background absorption: smoke and molecular absorption produce false signals.
- Chemical interference: lead and cadmium form refractory compounds with phosphates and silicates.
Elimination Methods
- Use matrix modifiers (e.g., ammonium dihydrogen phosphate + magnesium nitrate) to increase ashing temperature and eliminate matrix.
- Enable Zeeman effect background correction or deuterium lamp correction.
- Use the standard addition method to compensate for matrix effects.
- Optimize the graphite furnace temperature program to ensure complete atomization.
Result Calculation and Quality Control
Calculation Formula
The content of lead or cadmium in soil (mg/kg) is calculated as follows:
ω = (ρ × V × f) / (m × 1000)
Where: ρ—concentration obtained from the calibration curve (µg/L); V—final volume (mL); f—dilution factor; m—sample weight (g).
Quality Control Requirements
- At least 2 blanks per batch of samples; blank values should be below the detection limit.
- Use certified reference materials (e.g., GBW07401) to verify accuracy; recovery should be 90%–110%.
- Relative deviation of parallel samples ≤10%.
- Calibration curve correlation coefficient r≥0.999.
Standard Original Text and PDF Download
The following is the full-text PDF of the standard, available for online viewing or download:
Frequently Asked Questions (FAQ)
1. Why add matrix modifiers when measuring lead and cadmium by graphite furnace atomic absorption?
Matrix modifiers can increase the ashing temperature, allowing matrix components to volatilize before atomization, while inhibiting the formation of refractory compounds of lead and cadmium, thereby improving sensitivity and reducing interference.
2. Why use hydrofluoric acid during soil digestion?
Hydrofluoric acid can destroy the silicate crystal lattice in soil, allowing complete release of lead and cadmium. Without hydrofluoric acid, results may be low.
3. How to determine when the graphite tube needs replacement?
When sensitivity significantly decreases, peak shape broadens, or double peaks appear, the graphite tube should be inspected or replaced. Typically, pyrolytically coated graphite tubes can be used 200–500 times.
4. What are the measurement wavelengths for lead and cadmium in the standard?
Lead is 283.3 nm and cadmium is 228.8 nm. Selecting a less sensitive line can expand the linear range, but sensitivity decreases.
Summary
GB/T 17141-1997 provides a standardized graphite furnace atomic absorption spectrophotometry method for the determination of lead and cadmium in soil. Mastering sample digestion, matrix modification, instrument condition optimization, and quality control is key to obtaining accurate results. Laboratories should strictly follow the standard operating procedures and conduct regular intermediate checks to ensure reliable data.