Overview of GB/T 17141-1997 Standard
GB/T 17141-1997 is a Chinese national standard that specifies the method for the determination of lead (Pb) and cadmium (Cd) in soil quality—graphite furnace atomic absorption spectrophotometry. This standard is applicable to environmental monitoring, agricultural soil surveys, and contaminated site assessments.
Graphite furnace atomic absorption spectrometry has advantages such as high sensitivity, small sample volume, and low detection limit, making it especially suitable for trace heavy metal analysis. This article provides a systematic interpretation from principles, reagents, procedures, parameters, and precautions.
Method Principle and Scope of Application
Basic 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 and atomized at high temperature to form ground-state atomic vapor. This atomic vapor absorbs the characteristic spectral lines emitted by lead and cadmium hollow cathode lamps, and the absorbance is proportional to the element concentration, conforming to the Lambert-Beer law.
By measuring the absorbance of standard series and plotting a calibration curve, the content of lead and cadmium in soil can be quantitatively calculated.
Scope of Application and Detection Limit
This standard is applicable to the determination of lead and cadmium in various types of soil. When the sample amount is 0.5 g and the constant volume is 25 mL, the detection limit for lead is 0.1 mg/kg, and for cadmium is 0.01 mg/kg. In actual testing, it can be adjusted according to instrument performance.
Reagents and Instruments
Main Reagents
- Nitric acid (HNO₃): Guaranteed reagent, used for sample digestion.
- Hydrochloric acid (HCl): Guaranteed reagent, used for preparing aqua regia or dilution.
- Hydrofluoric acid (HF): Guaranteed reagent, used for decomposing silicates.
- Perchloric acid (HClO₄): Guaranteed reagent, used for oxidizing organic matter.
- Lead and cadmium standard stock solutions: Concentration typically 1.000 g/L, stored in nitric acid medium.
- Ammonium dihydrogen phosphate: Matrix modifier, eliminating matrix interference.
Main Instruments
- Graphite furnace atomic absorption spectrophotometer: Equipped with lead and cadmium hollow cathode lamps and graphite tubes.
- Analytical balance: Sensitivity 0.0001 g.
- Electric heating plate or microwave digestion system: Used for sample digestion.
- Volumetric flasks, pipettes: Soaked and cleaned with nitric acid.
Analysis Procedure and Operation Process
Sample Pretreatment
Air-dry, grind, and sieve the soil sample (usually through a 100-mesh nylon sieve), mix well and set aside. Weigh 0.5 g (accurate to 0.0001 g) of sample into a polytetrafluoroethylene crucible.
Digestion Procedure
- Add a small amount of water to moisten the sample, add 10 mL nitric acid, 5 mL hydrochloric acid, 5 mL hydrofluoric acid, and 2 mL perchloric acid.
- Place on an electric heating plate and heat at low temperature. After violent reaction stops, raise the temperature to about 200°C and evaporate to near dryness.
- Add a small amount of nitric acid, continue heating until white fumes are exhausted, dissolve the residue with 1% nitric acid and dilute to 25 mL.
- Prepare a blank solution simultaneously.
Instrument Measurement Conditions
The following are typical measurement parameters; in practice, they should be optimized according to the instrument manual:
| Parameter | Lead (Pb) | Cadmium (Cd) |
|---|---|---|
| Wavelength | 283.3 nm | 228.8 nm |
| Slit width | 0.7 nm | 0.7 nm |
| Lamp current | 5-10 mA | 5-10 mA |
| Drying temperature | 80-120°C | 80-120°C |
| Ashing temperature | 500-600°C | 300-400°C |
| Atomization temperature | 1800-2000°C | 1500-1700°C |
| Cleaning temperature | 2200°C | 2000°C |
Calibration Curve Preparation
Dilute the lead and cadmium standard stock solutions stepwise with 1% nitric acid to prepare a standard series with at least 5 concentration points. The recommended range for lead is 0-100 μg/L, and for cadmium is 0-10 μg/L. After adding matrix modifier, measure the absorbance sequentially and plot the calibration curve.
Result Calculation and Quality Control
Result Calculation
The content of lead or cadmium in soil (mg/kg) is calculated according to the following formula:
w = (ρ × V) / (m × 1000)
Where: ρ—concentration obtained from the calibration curve (μg/L); V—constant volume (mL); m—sample mass (g).
Quality Control Points
- At least two blanks should be prepared for each batch of samples, and the blank value should be below the detection limit.
- Use certified reference materials for accuracy control; the recovery rate should be between 90%-110%.
- Insert one parallel sample for every 10 samples; the relative deviation should be less than 10%.
- Regularly check the condition of the graphite tube to avoid memory effects.
Common Problems and Precautions
Matrix Interference and Elimination
Soil digestion solutions often contain large amounts of matrix elements such as iron, aluminum, and calcium, which can easily cause background absorption. It is recommended to use ammonium dihydrogen phosphate as a matrix modifier and enable Zeeman effect or deuterium lamp background correction.
Graphite Furnace Operation Points
- Ensure the graphite tube is dry and free of residue before injection.
- The ashing temperature should not be too high to prevent loss of lead and cadmium.
- During the atomization stage, gas should be stopped to improve sensitivity.
- Regularly clean the graphite cone and quartz window.
Safety Tips
The digestion process uses highly corrosive reagents such as hydrofluoric acid and perchloric acid, and must be operated in a fume hood with protective gloves and goggles. Perchloric acid can explode when in contact with organic matter, so the amount and temperature must be strictly controlled.
Standard Text and Download
The full text of the GB/T 17141-1997 standard is provided below for online viewing or download: