Determination of Lead in Soil by Graphite Furnace Atomic Absorption Spectrometry

1. Method Overview

The method of complete digestion with hydrochloric acid-nitric acid-hydrofluoric acid-perchloric acid is adopted to completely destroy the mineral lattice of the soil, so that the elements to be measured in the sample all enter the test solution. Then, the test solution is injected into the graphite furnace. Through the preset drying, ashing, atomization and other heating programs, the coexisting matrix components are evaporated and removed. At the same time, under the high temperature in the atomization stage, the lead compounds are dissociated into atomic vapor, which produces selective absorption of the characteristic spectral lines emitted by the hollow cathode lamp. Under the selected optimal determination conditions, the absorbance of lead in the test solution is determined by background correction.

2. 2. Instruments and Reagents

2.1 Instruments and Equipment

2.1.1 Detection Instruments

Graphite Furnace Atomic Absorption Spectrophotometer(Lead Hollow Cathode Lamp)

Argon(Purity≥ 99.99%)

Cooling Circulating Water(Pump rated flow: 3.5 L/min)

2.2 Reagents

2.2.1 Reagents

Hydrochloric Acid(Ultra-Pure Grade)

Nitric Acid(MOS Grade)

Hydrofluoric Acid(Ultra-Pure Grade)

Perchloric Acid(Ultra-Pure Grade)

Diammonium Hydrogen Phosphate(Ultra-Pure Grade)

3. Operation Procedures

3.1 Sample Treatment

3.1.1 Preparation of Test Solution

Wet Digestion: Accurately weigh 0.1-0.3 g (accurate to 0.0002 g) of the sample into a 50 mL polytetrafluoroethylene crucible. Moisten with water, add 5 mL of hydrochloric acid, and heat at a low temperature on an electric hot plate in a fume hood to preliminarily decompose the sample. When the volume is evaporated to about 2-3 mL, take it down and cool slightly. Then add 5 mL of nitric acid, 4 mL of hydrofluoric acid, and 2 mL of perchloric acid. Cover and heat at medium temperature on an electric hot plate for about 1 hour. Then uncover and continue heating to remove silicon. To achieve good silicon removal effect, shake the crucible frequently. When heating to produce thick white smoke of perchloric acid, cover to fully decompose the black organic carbides. After the black organic matter on the crucible disappears, uncover to drive away the white smoke and evaporate until the content is viscous. Depending on the digestion situation, 2 mL of nitric acid, 2 mL of hydrofluoric acid, and 1 mL of perchloric acid can be added again, and the above digestion process is repeated. When the white smoke is basically exhausted again and the content is viscous, take it down and cool slightly. Rinse the crucible cover and inner wall with water, and add 1 mL of nitric acid solution to warm and dissolve the residue. Then transfer the solution to a 25 mL volumetric flask, add 3 mL of diammonium hydrogen phosphate solution, cool, dilute to volume, and shake well for testing.

Due to the variety of soil types and the large difference in organic matter content, during digestion, pay attention to observation, and the amount of various acids can be increased or decreased as appropriate according to the digestion situation. The soil digestion solution should be white or light yellow (soil with high iron content), and there is no obvious precipitate.

3.1.2 Preparation of Standard Solution

1.Preparation of Lead Standard Intermediate Solution

Lead standard solution: (1.0 μg/mL). Accurately pipette 0.1 mL of lead standard solution (1000 μg/mL) into a 100 mL volumetric flask, dilute to the mark with (1+99) nitric acid solution, and shake well.

2.Preparation of Lead Standard Series

Accurately pipette 0.0, 0.5, 1.5, 1.0, 2.0, 3.0 mL of lead standard solution (1.0 μg/mL) into 100 mL volumetric flasks respectively, and dilute to the mark with (1+99) nitric acid solution to obtain lead standard series solutions of 0.0, 5.0, 10, 20.0, 30.0 ng/mL.

3.2 Sample Testing

In the order of low to high mass concentration, inject 10 μL of the lead standard series solution and 5 μL of the diammonium hydrogen phosphate solution into the graphite furnace respectively. After atomization, measure their absorbance values. Take the mass concentration as the abscissa and the absorbance value as the ordinate to make a standard curve, read the data, input the sample weight, and view and record the results.

4. Result

CalculationThe content of lead in the soil sample W(Pb), (mg/kg) is calculated according to formula:

Where:

C – The absorbance of the sample minus the absorbance of the blank test, and then the lead content (μg/L) is found on the calibration curve;

V – The constant volume of the sample, in milliliters (mL);

m – The weight of the sample taken, in grams (g);

f – The moisture content of the sample, (%).