A hollow cathode lamp is used to emit light with the specific frequency. Atoms of different elements absorb characteristic wavelengths of light. The energy absorbed excites the electrons in the target element from their ground state to a higher energy state. The amount of light absorbed is proportional to the concentration of the element in the sample. Using a standard curve, the concentration of the element in the sample can then be determined.
Figure 1. Digestion tubes in a block digester. 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, 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 ppm in gardening and play areas, and 1, 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. 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.
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. 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. 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. Then add distilled water to the filtrate to dilute its volume to mL. Once the sample has been prepared for analysis, turn on the AAS instrument and software.
Uses – Atomic Absorption Spectroscopy Learning Module
Refer to the text for details of the experimental parameters. Prepare a blank solution of nitric acid, the sample solution, and a 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. Using the mass of the initial soil sample before digestion, the concentration of lead in soil was found to be 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.
Applications of atomic spectroscopy to environmental analysis
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 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! 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. Additional calculations must be done to convert this number to the ppm of Pb in the soil sample.
For a soil sample that weighed 1. Table 1. Soil lead levels measured in ppm and the corresponding levels of contamination. Atomic Absorption Spectrometry is a useful technique to analyze a wide range of environmental samples e. This experiment highlights the use of flame AAS to determine the Pb content in soil. Zinc is an important micronutrient and is needed for protein synthesis.
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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. 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 — , ppm with a mean of 26, 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. In this particular experiment, soft water, acidic water, and chlorinated water were all analyzed. The sample preparation consisted of exposing the various water samples to copper plates with solder for various intervals of time. The samples were then analyzed for copper and zinc with air-acetylene flame AAS. A deuterium lamp was used. Geological analysis encompasses both mineral reserves and environmental research. When prospecting mineral reserves, the method of AAS used needs to be cheap, fast, and versatile because the majority of prospects end up being of no economic use.
When studying rocks, preparation can include acid digestions or leaching.
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If the sample needs to have silicon content analyzed, acid digestion is not a suitable preparation method. An example is the analysis of lake and river sediment for lead and cadmium. Because this experiment involves a solid sample, more preparation is needed than for the other examples. Standards of lead and cadmium were prepared. The standards and samples were then analyzed with electrothermal AAS. In order for the sample to be analyzed, it must first be atomized.
This is an extremely important step in AAS because it determines the sensitivity of the reading. The most effective atomizers create a large number of homogenous free atoms. There are many types of atomizers, but only two are commonly used: Flame atomizers accept an aerosol from a nebulizer into a flame that has enough energy to both volatilize and atomize the sample. When this happens, the sample is dried, vaporized, atomized, and ionized. Within this category of atomizers, there are many subcategories determined by the chemical composition of the flame. The composition of the flame is often determined based on the sample being analyzed.
The flame itself should meet several requirements including sufficient energy, a long length, non-turbulent, and safe. Although electrothermal atomizers were developed before flame atomizers, they did not become popular until more recently due to improvements made to the detection level. They employ graphite tubes that increase temperature in a stepwise manner. Electrothermal atomization first dries the sample and evaporates much of the solvent and impurities, then atomizes the sample, and then rises it to an extremely high temperature to clean the graphite tube.
Some requirements for this form of atomization are the ability to maintain a constant temperature during atomization, have rapid atomization, hold a large volume of solution, and emit minimal radiation. Electrothermal atomization is much less harsh than the method of flame atomization. The radiation source then irradiates the atomized sample. The sample absorbs some of the radiation, and the rest passes through the spectrometer to a detector. Radiation sources can be separated into two broad categories: Line sources excite the analyte and thus emit its own line spectrum. Hollow cathode lamps and electrodeless discharge lamps are the most commonly used examples of line sources.
On the other hand, continuum sources have radiation that spreads out over a wider range of wavelengths. These sources are typically only used for background correction. Deuterium lamps and halogen lamps are often used for this purpose. Spectrometers are used to separate the different wavelengths of light before they pass to the detector. The spectrometer used in AAS can be either single-beam or double-beam. Insert diagrams The single-beam spectrometers have less optical components and therefore suffer less radiation loss. Double-beam monochromators have more optical components, but they are also more stable over time because they can compensate for changes more readily.
Sample preparation is extremely varied because of the range of samples that can be analyzed. Regardless of the type of sample, certain considerations should be made. These include the laboratory environment, the vessel holding the sample, storage of the sample, and pretreatment of the sample. Sample preparation begins with having a clean environment to work in. AAS is often used to measure trace elements, in which case contamination can lead to severe error. Possible equipment includes laminar flow hoods, clean rooms, and closed, clean vessels for transportation of the sample.
Not only must the sample be kept clean, it also needs to be conserved in terms of pH, constituents, and any other properties that could alter the contents. When trace elements are stored, the material of the vessel walls can adsorb some of the analyte leading to poor results. To correct for this, perfluoroalkoxy polymers PFA , silica, glassy carbon, and other materials with inert surfaces are often used as the storage material. Acidifying the solution with hydrochloric or nitric acid can also help prevent ions from adhering to the walls of the vessel by competing for the space.
The vessels should also contain a minimal surface area in order to minimize possible adsorption sites. Pretreatment of the sample is dependent upon the nature of the sample. In order to determine the concentration of the analyte in the solution, calibration curves can be employed. Using standards, a plot of concentration versus absorbance can be created. Three common methods used to make calibration curves are the standard calibration technique, the bracketing technique, and the analyte addition technique. This technique is the both the simplest and the most commonly used. The concentration of the sample is found by comparing its absorbance or integrated absorbance to a curve of the concentration of the standards versus the absorbances or integrated absorbances of the standards.
In order for this method to be applied the following conditions must be met:. This ensures that the fit is acceptable. A least means squares calculation is used to linearly fit the line. In most cases, the curve is linear only up to absorbance values of 0. The absorbance values of the standards should have the absorbance value of a blank subtracted. The bracketing technique is a variation of the standard calibration technique.
They bracket the approximate value of the sample concentration very closely. This method is very useful when the concentration of the analyte in the sample is outside of the linear portion of the calibration curve because the bracket is so small that the portion of the curve being used can be portrayed as linear.
Although this method can be used accurately for nonlinear curves, the further the curve is from linear the greater the error will be. To help reduce this error, the standards should bracket the sample very closely. The analyte addition technique is often used when the concomitants in the sample are expected to create many interferences and the composition of the sample is unknown. The previous two techniques both require that the standards have a similar matrix to that of the sample, but that is not possible when the matrix is unknown.
To compensate for this, the analyte addition technique uses an aliquot of the sample itself as the matrix. The aliquots are then spiked with various amounts of the analyte.
This technique must be used only within the linear range of the absorbances. Interference is caused by contaminants within the sample that absorb at the same wavelength as the analyte, and thus can cause inaccurate measurements. Corrections can be made through a variety of methods such as background correction, addition of chemical additives, or addition of analyte. Brief overview of atomic absorption spectroscopy History of atomic absorption spectroscopy The earliest spectroscopy was first described by Marcus Marci von Kronland in by analyzing sunlight as is passed through water droplets and thus creating a rainbow.
Journal of Spectroscopy
German chemist Robert Bunsen - German physicist Gustav Robert Kirchhoff - British physicist Sir Alan Walsh - Theory of atomic absorption spectroscopy In order to understand how atomic absorption spectroscopy works, some background information is necessary. Danish physicist Niels Henrik David Bohr - French physicist and a Nobel laureate Louis de Broglie - Image used with permission public domain.
Austrian physicist Wolfgang Pauli - Biological analysis Biological samples can include both human tissue samples and food samples. Environmental and marine analysis Environmental and marine analysis typically refers to water analysis of various types. Geological analysis Geological analysis encompasses both mineral reserves and environmental research.
Instrumentation Atomizer In order for the sample to be analyzed, it must first be atomized. A schematic diagram of a flame atomizer shoing the oxidizer inlet 1 and fuel inlet 2. Electrothermal atomizer Although electrothermal atomizers were developed before flame atomizers, they did not become popular until more recently due to improvements made to the detection level. Schematic diagram of an electrothermal atomizer showing the external gas flow inlet 1 , the external gas flow outlet 2 , the internal gas flow outlet 3 , the internal gas flow inlet 4 , and the light beam 5.
Radiation source The radiation source then irradiates the atomized sample. Spectrometer Spectrometers are used to separate the different wavelengths of light before they pass to the detector. Obtaining Measurements Sample preparation Sample preparation is extremely varied because of the range of samples that can be analyzed.
Sample Examples Pretreatment method Aqueous solutions Water, beverages, urine, blood Digestion if interference causing substituents are present Suspensions Water, beverages, urine, blood Solid matter must either be removed by filtration, centrifugation or digestion, and then the methods for aqueous solutions can be followed Organic liquids Fuels, oils Either direct measurement with AAS or diltion with organic material followed by measurement with AAS, standards must contain the analyte in the same form as the sample Solids Foodstuffs, rocks Digestion followed by electrothermal AAS.
Calibration curve In order to determine the concentration of the analyte in the solution, calibration curves can be employed. Standard calibration technique This technique is the both the simplest and the most commonly used. In order for this method to be applied the following conditions must be met: