Atomic absorption spectrophotometry (AAS)
Atomic absorption spectrophotometry (AAS) is an analytical technique used to measure the concentrations of metal elements in a sample by observing the absorption of light energy by gaseous atoms. This method is especially significant in forensic science, where it aids in analyzing evidence collected from crime scenes. By identifying the elemental composition of samples, forensic scientists can potentially link evidence to suspects or other crime scenes. AAS operates based on the principle that each element absorbs light at specific wavelengths, a phenomenon first documented through the study of absorption lines in the sun's spectrum.
The technique utilizes an atomic absorption spectrophotometer, which comprises several essential components, including a light source, absorption cell, monochromator, detector, and readout system. The light source emits wavelengths specific to the element being analyzed, and the absorption cell contains the sample, where atoms are excited and absorb the light. Various methods, such as flame or graphite furnace atomization, are employed to introduce samples in either liquid or solid forms.
In forensic investigations, AAS is instrumental in detecting residues from explosives or toxic substances, confirming the presence of harmful elements in biological samples, and thereby assessing whether poisoning has occurred. However, it is important to note that AAS is limited in its ability to analyze certain elements, particularly those with wavelengths below 190 nanometers or those forming strong refractory compounds.
Atomic absorption spectrophotometry (AAS)
DEFINITION: Technique used to determine the concentrations of metal elements in a sample based on the absorption of light energy by atoms.
SIGNIFICANCE: By using atomic absorption spectrophotometry, forensic scientists can determine the concentrations of the elements that are present in evidence samples collected at crime scenes. Using this information, they may be able to match evidence samples with materials linked to suspects or found at other crime scenes.
The phenomenon of atomic absorption was discovered as the result of the observation of the dark absorption lines in the spectrum of the sun, which are caused by the absorption of light by elements existing as gaseous atoms being promoted from “ground” state to “excited” state in the sun’s atmosphere. These dark lines were first observed by William Hyde Wollaston in 1802, then rediscovered by Joseph von Fraunhofer in 1814; they are now known as Fraunhofer lines. In 1953, Alan Walsh developed the first chemical analysis using atomic absorption.
Atom-Light Relationship
Atoms absorb light energy based on electrons surrounding the atomic nuclei. Every atom of a specific element has a specific number of electrons in orbital positions. The most stable orbital configuration for an atom, called the “ground” state, possesses the lowest energy. The light energy resonates, or travels in space, like waves with specific wavelength. If light energy strikes an atom, the light is absorbed by the atom, and the electron in the outer orbital position is promoted to an unstable higher energy configuration, called the “excited” state. The excitation from ground to excited state is called atomic absorption; this absorption can be measured by the instrument known as the atomic absorption spectrophotometer. Because of the instability of the excited state, the electron decays and returns to the ground state; in doing so, it emits energy equivalent to the energy absorbed during the excitation process. The energy emitted during the decay process is not measured by the instrument.
Instrumentation and Sample Analysis
The atomic absorption spectrophotometer has five basic features: a light source that emits a spectrum specific to the element of interest, an absorption cell in which gaseous atoms are produced during excitation, a monochromator that disperses light, a detector that measures absorption, and a readout system (printer or computer) that shows the results of the analysis. The spectrum emitted by the light source (for example, a hollow cathode lamp) is focused through the absorption cell leading to the monochromator. The lamp contains a specific metal element that emits a specific wavelength of light for the same element to be determined in the sample. For example, to determine the concentration of iron in the sample, the lamp used must contain iron. The light source must be modulated, or chopped, so that it is possible to distinguish between the emission from the lamp and the emission from the absorption cell. The monochromator disperses the modulated signal emitted from the lamp (not from the absorption cell) and isolates the specific wavelength of light that passes to the detector, which processes the light absorbed by the atoms. The absorption, which is proportional to the concentration of the element in a sample, is then displayed in the readout system.
A sample is introduced and atomized in the absorption cell in liquid or solid form (depending on atomizers) to accomplish the excitation process. If a liquid solution is required, elements are extracted by liquid reagents from solid materials. The liquid sample in the absorption cell is atomized, with thermal means (flame or graphite furnace) or chemical means (hydride or mercury vapor generator) used to excite the atoms. Flame produced by an air-acetylene mixture (2,100-2,400 degrees Celsius) is used for most metal elements (such as calcium or zinc) that do not form refractory compounds, which cannot be ionized or atomized at this temperature range. A hotter nitrous oxide-acetylene flame (2,600-2,800 degrees Celsius) is used for elements (such as silicon or aluminum) forming refractory compounds (silicon dioxide or aluminum dioxide). A graphite furnace atomizer provides a wider range of temperatures (2,100-2,900 degrees Celsius) and handles liquid or solid samples.
A hydride or mercury vapor generator converts certain elements into gas. Elements that chemically react with sodium tetrahydridoborate (such as arsenic and selenium) are reduced to form hydride vapor. Mercury is reduced to mercury vapor. The vapors in the absorption cell are excited to absorb energy from the light source.
Not all elements are detected by atomic absorption. Elements with wavelengths of resonance lines below 190 nanometers, nonmetals (such as hydrogen, carbon, nitrogen, and oxygen), and noble gases are volatile, easily absorbed by air, and immediately lost at temperatures greater than 2,100 degrees Celsius they disappear before excitation. Elements that form very strong refractory compounds have extremely high melting points (greater than 2,900 degrees Celsius). Because these compounds cannot be atomized by flame or furnace, no atoms can be excited.
Use in Forensics
In a criminal investigation, atomic absorption spectrophotometry can be used to discover the presence and the concentration of the elements in sample evidence. When explosives or poisons are used to kill, for example, they leave evidence that can be examined through chemical analysis. Some explosives contain platinum; others contain nickel, silver, cadmium, or mercury. The elements that are found in the residues of signature explosive products can be used to find the sources, manufacturers, and buyers of such explosives.
Atomic absorption spectrophotometry can also be used to detect concentrations of elements from poisons found in human victims. Poisoning is confirmed to have occurred (whether accidentally or intentionally) if concentrations of toxic elements—such as arsenic or mercury—exceed safe levels in the body. The atomic absorption spectrophometer enables analysts to determine whether poisoning has occurred by examining the levels of toxic elements appearing in a victim’s blood, urine, and hair.
Bibliography
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