Friday, 29 May 2015

ATOMIC ABSORPTION SPECTROMETRY

INTRODUCTION

Atomic Absorption Spectrometry (AAS) is a technique which is used for the analysis of quantities of elements present in a sample by measuring the absorbed radiation by the chemical element of interest.
This is done by measuring the spectra produced when the sample is excited by radiation. The atoms absorb ultraviolet or visible light and get excited to higher energy levels. Atomic absorption technique measures the amount of energy in the form of photons of light that are absorbed by the sample.
A detector measures the wavelengths of light transmitted by the sample, and compares them to the wavelengths which originally passed through the sample. A signal processor then integrates the changes in wavelength absorbed, which appear in the readout as peaks of energy absorption at discrete wavelengths.

The energy required for an electron to leave an atom is known as ionization energy and is specific to each and every element. When an electron moves from one energy level to another within the atom, a photon is emitted with energy E. Atoms of an element emit a characteristic spectral line. Every atom has its own distinct pattern of wavelengths at which it will absorb energy, due to the unique configuration of electrons in its outer shell.
This enables the qualitative analysis of a sample. The concentration is calculated based on the Beer-Lambert law. Absorbance is directly proportional to the concentration of the analyte absorbed for the existing set of conditions. The concentration is usually determined from a calibration curve, obtained using standards of known concentration or certified reference materials (CRMs). However, applying the Beer-Lambert law directly in AAS is difficult due to:
·       variations in atomization efficiency from the sample matrix non-uniformity of concentration and path length of analyte atoms (in graphite furnace AA).

The chemical methods used are based on matter interactions, i.e. chemical reactions. For a long period of time these methods were essentially empirical, involving, in most cases, great experimental skills. In analytical chemistry, AAS is a technique used mostly for determining the concentration of a particular metal element within a sample. AAS can be used to analyse the concentration of over 62 different metals in a solution. Typically, the technique makes use of a flame to atomize the sample, but other atomizers, such as a graphite furnace, are also used. Three steps are involved in turning a liquid sample into an atomic gas:

1. Desolvation – the liquid solvent is evaporated, and the dry sample remains;
2. Vaporization – the solid sample vaporizes to a gas; and
3. Volatilization – the compounds that compose the sample are broken into free atoms.

To measure how much of a given element is present in a sample, first of all , we must establish a basis for comparison using certified reference materials or known quantities of that element to produce a calibration curve.
To generate this curve, a specific wavelength is selected, and the detector (Usually Photomultiplier tube detectors are used) is set to measure only the energy transmitted at that wavelength. As the concentration of the target atom in the sample increases, the absorption will also increase proportionally.

A series of samples containing known concentrations of the element to be measured are analysed, and the corresponding absorbance, which is the inverse percentage of light transmitted, is recorded.

The measured absorption at each concentration is then plotted, so that a straight line can then be drawn between the resulting points. From this line, the concentration of the substance under investigation is extrapolated from the substance’s absorbance. The use of special light sources and the selection of specific wavelengths allow for the quantitative determination of individual components in a multi-element mixture.

BASIC PRINCIPLE

The selectivity in AAS is very important, since each element has a different set of energy levels and gives rise to very narrow absorption lines. Hence, the selection of the monochromator is vital to obtain a linear calibration curve (Beers' Law), the bandwidth of the absorbing species must be broader than that of the light source; which is difficult to achieve with ordinary monochromators. The monochromator is a very important part of an AA spectrometer because it is used to separate the thousands of lines generated by all of the elements in a sample.

Without a good monochromator, detection limits are severely compromised. A monochromator is used to select the specific wavelength of light that is absorbed by the sample and to exclude other wavelengths. The selection of the specific wavelength of light allows for the determination of the specific element of interest when it is in the presence of other elements. The light selected by the monochromator is directed onto a detector,typically a photomultiplier tube, whose function is to convert the light signal into an electrical signal proportional to the light intensity. The challenge of requiring the bandwidth of the absorbing species to be broader than that of the light source is solved with radiation sources with very narrow lines.

The study of trace metals in wet and dry precipitation has increased in recent decades because trace metals have adverse environmental and human health effects. Some metals, such as Pb, Cd and Hg, accumulate in the biosphere and can be toxic to living systems.
Anthropogenic activities have substantially increased trace metal concentrations in the atmosphere. In addition, acid precipitation promotes the dissolution of many trace metals, which enhances their bioavailability. In recent decades, heavy metal concentrations have increased not only in the atmosphere but also in pluvial precipitation. Metals, such as Pb, Cd, As, and Hg, are known to accumulate in the biosphere and to be dangerous for living organisms, even at very low levels. Many human activities play a major role in global and regional trace element budgets. Additionally, when present above certain concentration levels, trace metals are potentially toxic to marine and terrestrial life. Thus, biogeochemical
perturbations are a matter of crucial interest in science.

The atmospheric input of metals exhibits strong temporal and spatial variability due to short atmospheric residence times and meteorological factors. As in oceanic chemistry, the impact of trace metals in atmospheric deposition cannot be determined from a simple consideration of global mass balance; rather, accurate data on net air or sea fluxes for specific regions are needed.

Particles in urban areas represent one of the most significant atmospheric pollution problems, and are responsible for decreased visibility and other effects on public health, particularly when their aerodynamic diameters are smaller than 10 μm, because these small particles can penetrate deep into the human respiratory tract. There have been many studies measuring concentrations of toxic metals such as Ag, As, Cd, Cr, Cu, Hg, Ni, Pb in rainwater and their deposition into surface waters and on soils. Natural sources of aerosols include terrestrial dust, marine aerosols, volcanic emissions and forest fires. Anthropogenic particles, on the other hand, are created by industrial processes, fossil fuel combustion, automobile mufflers, worn engine parts, and corrosion of metallic parts. The presence of metals in atmospheric particles are directly associated with health risks of these metals. Anthropogenic sources have substantially increased trace metal concentrations in atmospheric deposition.

The instrument used for atomic absorption spectrometry can have either of two atomizers. One attachment is a flame burner, which uses acetylene and air fuels. The second attachment consists of a graphite furnace that is used for trace metal analysis. Figure 1 depicts a diagram of an atomic absorption spectrometer.



Fig. 1. The spectral, or wavelength, range captures the dispersion of the grating across the linear array.
           
 Flame and furnace spectroscopy has been used for years for the analysis of metals. Today these procedures are used more than ever in materials and environmental applications. This is due to the need for lower detection limits and for trace analysis in a wide range ofsamples. Because of the scientific advances of Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), have left Atomic Absorption (AA) behind. This technique, however, is excellent and has a larger specificity that ICP does not have.

Figure 2 shows a diagram of an atomic absorption spectrometer with a graphite furnace.


 AAS is a reliable chemical technique to analyze almost any type of material. This post describes the basic principles of atomic absorption spectroscopy in the analysis of trace metals, such as Ag, As, Cd, Cr, Cu, and Hg, in environmental samples. For example, the study of trace metals in wet and dry precipitation has increased in recent decades because trace metals have adverse environmental and human health effects. Anthropogenic activities have substantially increased trace metal concentrations in the atmosphere. In recent decades, heavy metal concentrations have increased not only in the atmosphere but also in pluvial precipitation.
Many human activities play a major role in global and regional trace element budgets. Additionally, when present above certain concentration levels, trace metals are potentially toxic to marine and terrestrial life. Thus, biogeochemical perturbations are a matter of crucial interest in science.
The atmospheric input of metals exhibits strong temporal and spatial variability due to short atmospheric residence times and meteorological factors. As in oceanic chemistry, the impact of trace metals in atmospheric deposition cannot be determined from a simple consideration of global mass balance; rather, accurate data on net air or sea fluxes for specific regions are needed.

Particles in urban areas represent one of the most significant atmospheric pollution
problems, and are responsible for decreased visibility and other effects on public health, particularly when their aerodynamic diameters are smaller than 10 μm, because these small particles can penetrate deep into the human respiratory tract. There have been many studies measuring concentrations of toxic metals such as Ag, As, Cd, Cr, Cu, Hg, Ni, Pb in rainwater and their deposition into surface waters and on soils. Natural sources of aerosols include terrestrial dust, marine aerosols, volcanic emissions and forest fires. Anthropogenic particles, on the other hand, are created by industrial processes, fossil fuel combustion, automobile mufflers, worn engine parts, and corrosion of metallic parts. The presence of metals in atmospheric particles and the associated health risks of these metals.

Anthropogenic sources have substantially increased trace metal concentrations in
atmospheric deposition. In addition, acid precipitation favors the dissolution of many trace metals, which enhances their bioavailability. Trace metals from the atmosphere are deposited by rain, snow and dry fallout. The predominant processes of deposition by rain are rainout and washout (scavenging). Generally, in over 80 % of wet precipitation, heavy metals are dissolved in rainwater and can thus reach and be taken up by the vegetation blanket and soils. Light of a specific wavelength, selected appropriately for the element being analyzed, is given off when the metal is ionized in the flame; the absorption of this light by the element of interest is proportional to the concentration of that element.

Quantification is achieved by preparing standards of the element.
  • AAS intrinsically more sensitive than Atomic Emission Spectrometry (AES)
  • Similar atomization techniques to AES
  • Addition of radiation source
  • High temperature for atomization necessary
  • Flame and electrothermal atomization
  • Very high temperature for excitation not necessary; generally no plasma/arc/spark in AAS

We will discuss the Flame AAS technique and AAS with Graphite Furnace (GFAA) in the upcoming posts.

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