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 elements 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 a 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 the “ground” state to the “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 the mid-1950s, 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 a 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 (e.g., 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 the 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 a 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. Atomic absorption spectrophotometry mainly uses the UV–visible region (about 190 to 900 nm; mostly the UV range). Elements with wavelengths of resonance lines below 190 nanometers are not ideal candidates since they are absorbed by air in the far UV region. Nonmetals (such as hydrogen, carbon, nitrogen, and oxygen) and noble gases have higher excitation energies and have absorption lines mainly in the far- and vacuum–UV region, making them unsuitable for this analytical technique. 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.

Forensic elemental results are typically produced under validated methods with documented controls (blanks, calibration checks, and reference materials) and may be performed within ISO/IEC 17025-accredited quality systems.

Use in Forensics

In a criminal investigation, atomic absorption spectrophotometry can be used to discover the presence and the concentration of the elements in a 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 spectrophotometer enables analysts to determine whether poisoning has occurred by examining the levels of toxic elements appearing in a victim’s blood, urine, and hair.

While atomic absorption spectrophotometry remains widely used for targeted elemental determinations, many forensic laboratories rely on inductively coupled plasma–optical emission spectroscopy (ICP-OES) or ICP–mass spectroscopy (ICP-MS) for multi-element trace analysis and, where needed, isotopic information. Atomic spectrometry literature emphasized that improved workflows and, increasingly, chemical speciation (the chemical form of an element) can improve toxicological interpretation.

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