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.
