Lead Analysis of Soil Using Atomic Absorption Spectroscopy

1. Soil Collection and Preparation

1. In undisturbed areas, collect soil from the upper 1-2 inches of the soil. If sampling vegetable gardens, collect 6-inch deep samples. Use a soil auger to collect a 1-inch diameter soil core from sample area.
2. Mix the sample thoroughly by shaking for 2 min and sieve using a USS #10 sieve.
3. Dry the soil in a 40 °C oven for 24 h.

2. Sample Digestion

1. Using an analytical balance, weigh out 1 g of the soil sample and place in a digestion tube. Record the weight of the sample to four decimal places.
2. In a hood, add 5 mL of water to the digestion tube.
3. Add 5 mL of concentrated HNO₃ to the digestion tube.
4. Mix the slurry with a stirring rod. Cover the digestion tube with a teardrop glass stopper.
5. Put the digestion tube in the block digester and heat the sample to 95 °C and reflux for 10 min without boiling (Figure 1). Remember that this contains concentrated acid.
6. Allow the tubes to cool. Add 5 mL of concentrated HNO₃ to the digestion tube, replace the drop glass, and reflux for an additional 30 min. If brown fumes are generated, repeat this step over and over until no brown fumes are given off by the sample.
7. Evaporate the solution to a 5 mL volume without boiling.
8. Allow the tubes to cool, and then add 2 mL of distilled water and 3 mL of 30% H₂O₂. Cover with the glass stopper and heat to begin the peroxide reaction. Be sure that the solution does not boil over. Heat until the bubbling stops and allow to cool.
9. Continue to add 30% H₂O₂ in 1 mL increments, warming until the bubbling is minimal. Do not add more than a total of 10 mL of the 30% H₂O₂.
10. Cover the sample with the glass teardrop stoppers and heat until the volume is reduced to 5 mL without boiling.
11. Add 10 mL concentrated HCl to the sample and cover with the glass teardrop stopper. Heat to 95 °C and reflux for 15 min.
12. Allow the tubes to cool. If there are particulates, filter the sample using a glass fiber filter and collect filtrate in a 100-mL volumetric flask. Dilute the sample volume to 100 mL with distilled water.

Figure 1. Digestion tubes in a block digester.

3. Analyzing Samples with an Atomic Absorption Spectrometer

1. Turn on the computer and the spectrometer.
2. Set parameters on the instrument. (Parameters and procedures may vary depending on the brand of instrument used.) Set the acetylene pressure to >700 kPa (~100 psi), the acetylene valve set to 11 psi, and the air valve 45 psi.
3. Open the SpectraAA software
4. Open a new worksheet.
5. Choose “Add Method” and click on Pb to do a Lead Analysis.
6. Set Type/Mode parameters to the following:
   1. Type = Flame
   2. Element = Pb
   3. Sampling Mode = Manual
   4. Instrument Mode = Absorbance
   5. Flame Type = Air/Acetylene
   6. Air Flow = 13.5
   7. Acetylene Flow = 2.0
   8. Online Diluter Type = SIPS
7. Set the Measurements parameters to the following:
   1. Measurement Mode = PROMT
   2. Calibration Mode = Concentration
   3. Times: Measurement = 10
   4. Times: Read Delay = 10
   5. Replicates: Standard = 3
   6. Replicates: Sample = 3
   7. Precision (%): Standard = 1.0
   8. Precision (%): Sample = 1.0
8. Set the Optical parameters to the following:
   1. Lamp Position = #4
   2. Lamp Current (mA) = 10.0 mA
   3. Wavelength = 217.0 nm
   4. Slit = 1.0 nm
   5. Background = BC Off
9. Set the SIPS parameters to the following:
   1. Nebulizer Uptake Rate = 5.0 mL/min
   2. Right Pump = none
   3. Standard Additions = Unselect
   4. Calibration Mode = Auto Set Std Concentrations
   5. Dual Pump Calibration = Unselect
10. Under the Standards tab, a list of standards automatically populates for the particular test. A 1,000 ppm Pb standard for atomic absorption spectrometry purchased from a chemical supply company is used and automatically diluted by the instrument. A new calibration curve is generated each time a new set of samples is run.
11. Exit the Edit Method menu and click on the “Labels” tab. Input information regarding sample names and number of samples.
12. Using the “Analysis” tab, use the “Select” button to highlight the samples to be analyzed.
13. Turn on the flame by pressing the ignite button on the instrument.
14. Zero the instrument by aspirating a blank and pressing the “Alt” and “Read” keys simultaneously.
15. Place the pump tubing in the blank solution and press “Start.” Once the calibration has been performed, place the pump tubing in the sample and press the “Read” key. Continue for all samples.
16. Turn off the instrument by pressing the red power off button on the instrument. Turn off all gas tanks and remove all samples.