Lead Analysis of Soil Using Atomic Absorption Spectroscopy (article)

Procédure

1. Soil Collection and Preparation

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.
Mix the sample thoroughly by shaking for 2 min and sieve using a USS #10 sieve.
Dry the soil in a 40 °C oven for 24 h.

2. Sample Digestion

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.
In a hood, add 5 mL of water to the digestion tube.
Add 5 mL of concentrated HNO₃ to the digestion tube.
Mix the slurry with a stirring rod. Cover the digestion tube with a teardrop glass stopper.
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.
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.
Evaporate the solution to a 5 mL volume without boiling.
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.
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.
Cover the sample with the glass teardrop stoppers and heat until the volume is reduced to 5 mL without boiling.
Add 10 mL concentrated HCl to the sample and cover with the glass teardrop stopper. Heat to 95 °C and reflux for 15 min.
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

Turn on the computer and the spectrometer.
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.
Open the SpectraAA software
Open a new worksheet.
Choose “Add Method” and click on Pb to do a Lead Analysis.
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
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
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
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
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.
Exit the Edit Method menu and click on the “Labels” tab. Input information regarding sample names and number of samples.
Using the “Analysis” tab, use the “Select” button to highlight the samples to be analyzed.
Turn on the flame by pressing the ignite button on the instrument.
Zero the instrument by aspirating a blank and pressing the “Alt” and “Read” keys simultaneously.
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.
Turn off the instrument by pressing the red power off button on the instrument. Turn off all gas tanks and remove all samples.

The widespread use of paint and gasoline, along with industrial contamination, have caused elevated levels of lead in urban soil, which can lead to health problems.

Lead occurs naturally in soils, in levels ranging from 10 to 50 parts per million, or ppm. However, contaminated urban soils often have concentrated levels of lead, that are significantly greater than this background level- up to 10,000 ppm in some areas. These elevated lead levels are a concern as lead does not biodegrade, and instead remains in the soil.

Serious health risks are associated with lead poisoning, particularly in foods grown in contaminated soils and for children who come in contact with contamination. As a result, the Environmental Protection Agency has set a limit of 400 ppm in gardening and play areas, and 1,200 ppm in other areas.

The concentration of lead in soil can be determined using various elemental analysis techniques, such as atomic absorption spectroscopy. This video will introduce the principles of soil collection and the analysis of lead contamination in soil using atomic absorption spectroscopy.

Atomic absorption spectroscopy, or AAS, is an elemental analysis technique based on the absorption of discrete wavelengths of light by gas-phase atoms. For this, a hollow cathode lamp is used to emit light with a specific wavelength. The lamp consists of a hollow cathode, containing the element of interest, and an anode. When the element of interest is ionized by a high voltage, it emits light at a wavelength specific to that substance.

The sample, which as been previously digested in concentrated acid, is then introduced to the instrument in gaseous form, by way of a flame atomizer. Atoms of the element of interest absorb light emitted from the hollow cathode lamp. The energy absorbed excites the electrons in the target element to a higher energy state. The amount of light absorbed is proportional to the concentration of the element in the sample.

A standard curve, created from samples with known concentrations of the element, is used to determine the unknown concentration of the element in the sample. AAS provides quantitative information on at least 50 different elements. Concentrations as low as parts per billion can be determined for some elements, though measurement ranges of parts per million are most common for metals. This technique has many benefits in the analysis of lead in soil, as it measures the total concentration of lead, regardless of its form.

Now that the basics of lead analysis have been explained, the technique will be demonstrated in the laboratory.

To collect samples from cultivated soils such as vegetable gardens, use a soil auger. Collect the sample, and bring it back to the lab. To prepare the soil sample for digestion, mix it thoroughly by shaking for 2 min and pass it through a USS #10 sieve to remove larger chunks. Dry the sample in a 40 °C oven for 24 h.

Once dried, weigh out 1 g of the sample using an analytical balance, recording its weight to four decimal places. Place the soil in a digestion tube. In a chemical fume hood, add 5 mL of water to the digestion tube, followed by 5 mL of concentrated nitric acid. Mix the slurry using a stirring rod, and cover the tube with a teardrop stopper. Place the digestion tube in the block digester, heat it to 95 °C, and reflux for 10 min without boiling.

Remove the rack from the heat block, and allow the tube to cool. Then, add another 5 mL of concentrated nitric acid, replace the stopper, and reflux for an additional 30 min. If brown fumes are generated, repeat the acid addition and reflux.

Remove the stopper and let the solution evaporate to a volume of 5 mL, without boiling. Allow the tube to cool, then add 2 mL of distilled water and 3 mL of 30% hydrogen peroxide. Replace the stopper and heat to 95 °C until the bubbling stops, making sure the solution does not boil over. Allow the tube to cool. Repeat this heating-cooling cycle, using 1 mL of 30% hydrogen peroxide each, until the bubbling becomes minimal.

Once the tube is cooled, loosely cap the tube with the stopper and heat the solution without boiling until the volume is again reduced to 5 mL. Add 10 mL of concentrated hydrochloric acid, heat to 95 °C, and reflux for 15 min, then let the tube cool.

To remove any particulates from the solution, filter the solution using a glass fiber filter in a Büchner funnel setup. Then add distilled water to the filtrate to dilute its volume to 100 mL.

Once the sample has been prepared for analysis, turn on the AAS instrument and software. Refer to the text for details of the experimental parameters. In this demonstration, an air/acetylene flame is used with the lead protocol, with a hollow cathode lamp emitting at 217 nm.

Prepare a blank solution of nitric acid, the sample solution, and a 10-ppm lead standard sample. Turn on the flame and begin analyzing the samples. Start by inserting the pump tubing into the blank solution in order to "zero" the instrument. Continue for all samples.

The instrument automatically dilutes the lead standard to produce a calibration curve, and then automatically determines the concentration of lead in each measured sample. In this demonstration, the 100-mL sample was found to have a concentration of 6 mg/L, or 0.6 mg total. Using the mass of the initial soil sample before digestion, the concentration of lead in soil was found to be 479 ppm. This is above the EPA-recommended level for growing crops.

The analysis of lead and other elements with AAS can be used to answer a variety of questions in environmental science. The fate of other hazardous compounds that are applied to soils, such as fertilizers or pesticides, is not well understood. However, these compounds can pose hazards if they reach water sources through soil runoff. In this experiment, researchers analyzed layers of soil extracted from a pesticide treated lawn using AAS.

Results showed that the pesticide monosodium methyl arsenate leached through layers of soil to depths of 40 cm. The toxins remained within the soil for over a year, especially in soil systems with established roots from turf grass.

Another major source of heavy metal contamination in the environment is mercury, which accumulates in fish and shellfish. Various regulatory agencies have enacted guidelines or advisories to minimize human intake of mercury. Samples obtained from seafood can be analyzed with AAS to determine if their mercury levels exceed legal recommendations.

Finally, regulatory bodies, such as the US Environmental Protection Agency, or EPA, have published advisories for metals including lead, zinc, copper, nickel, cadmium, and manganese in water. AAS can be used to analyze the level of metallic elements in drinking water, which can have hazardous effects on human health. Drinking water samples are prepared for analysis by acid digestion and boiling.

Samples were then analyzed for metal contamination using AAS. The results showed that the drinking water contained less than 2 ppb of lead, well below the EPA limit of 15 ppb.

You've just watched JoVE's video on lead analysis of soil using AAS. You should now understand the principles behind this method of analysis; how to perform it; and some of its applications in environmental science. As always, thanks for watching!

Résultats

The software creates the calibration curve and automatically determines the concentration of the Pb in the samples (Figure 2).

Figure 2. The calibration curve and the concentration of the Pb in the samples automatically determined by the software.

The values given on the worksheet are mg/L of Pb in the sample solution. Additional calculations must be done to convert this number to the ppm of Pb in the soil sample.

Example:

For a soil sample that weighed 1.2523 g before digestion was measured by the AAS to have 6.0 mg/L of Pb in the 100 mL solution sample (Table 1).

Soil Lead Level (ppm)	Level of Contamination
Less than 150	None to very low
150-400	Low
400-1,000	Medium
1,000-2,000	High
Greater than 2,000	Very High

Table 1. Soil lead levels measured in ppm and the corresponding levels of contamination.

Applications and Summary

Atomic Absorption Spectrometry is a useful technique to analyze a wide range of environmental samples (e.g., water, soil, sludge, and sediment) for a large number of elements (e.g., heavy metals). This experiment highlights the use of flame AAS to determine the Pb content in soil. However, it could also be used to measure concentrations of Cu, Fe, Mn, K, Na, Mg, and Zn in soils.

Zinc is an important micronutrient and is needed for protein synthesis. Zn helps regulate the expression of genes needed to protect cells when under environmental stress conditions. Zinc deficiency is a large problem in crop and pasture plants around the world, resulting in decreased yields. It is estimated that half of all soils used for cereal production have a zinc deficiency. This leads to a zinc deficiency in the grain. As a result, zinc deficiency in humans is a serious nutritional problem worldwide, affecting 1/3 of the world’s population. A typical range of zinc in soils is 10 – 300 mg/kg with a mean of 55 mg/kg.

Iron is the fourth most abundant element on Earth. However, it is mostly found in forms not available for plants, such as in silicate minerals or iron oxides. Iron is involved in photosynthesis, chlorophyll formation, nitrogen fixation, and many enzymatic reactions in plants. Iron deficiency in soil is rare, but it can become unavailable in excessively alkaline soils. Symptoms of iron deficiency in soil include leaves turning yellow and a decrease in yield. A typical range of iron in soils is 100 – 100,000 ppm with a mean of 26,000 ppm.

Copper is an essential micronutrient for plants. Copper promotes seed production, plays a role in chlorophyll formation, and is essential for enzyme activity. Copper deficiency can be seen by light green to yellow leaves. The leaf tips die back and become twisted. If the deficiency is severe enough, growth of the grain can stop and the plants die. Available copper in soils can vary from 1 to 200 ppm. Availability of copper is related to the soil pH – as pH increases, the availability of copper decreases.

Atomic Absorption Spectrometry can also be used on non-environmental samples, including:

Water analysis (Ca, Mg, Fe, Al, Ba, Cr)

Food analysis (Cd, Pb, Al, Cu, Fe)

Additives in oils (Ba, Ca, Na, Li, Zn, Mg, V, Pb, Sb)

Fertilizers (K, B, Mo)

Clinical samples (blood, serum, plasma, urine, Ca, Mg, Li, Na, K, Fe, Cu, Zn, Au, Pb)

Cosmetics (Pb)

Mining (Au)

References

Robinson, J.W., Skelly Frame, E.M., Frame II, G.M. Undergraduate Instrumental Analysis. 6^th Ed. Marcel Dekker, New York (2005).
United States Environmental Protection Agency. “Lead based paint poisoning prevention in certain residential structures.” CFR 40 Part 745. http://www.ecfr.gov. (2015).

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