Atomic absorption spectroscopy

Atomic absorption spectroscopy (AAS) and atomic emission spectroscopy (AES)

is a spectroanalytical procedure for the quantitative determination of chemical
elements using the absorption of optical radiation (light) by free atoms in the
gaseous state. Atomic absorption spectroscopy is based on absorption of light by
free metallic ions.
In analytical chemistry the technique is used for determining the concentration of a
particular element (the analyte) in a sample to be analyzed. AAS can be used to
determine over 70 different elements in solution, or directly in solid samples via
electrothermal vaporization,[citation needed] and is used
in pharmacology, biophysics, archaeology and toxicology research.
Atomic emission spectroscopy was first used as an analytical technique
Atomic absorption spectrometry has many uses in different areas of chemistry such
as clinical analysis of metals in biological fluids and tissues such as whole blood,
plasma, urine, saliva, brain tissue, liver, hair, muscle tissue. Atomic absorption
spectrometry can be used in qualitative and quantitative analysis.

Principles[edit]
The technique makes use of the atomic absorption spectrum of a sample in order to assess the
concentration of specific analytes within it. It requires standards with known analyte content to
establish the relation between the measured absorbance and the analyte concentration and
relies therefore on the Beer–Lambert law.

Instrumentation[edit]

Atomic absorption spectrometer block diagram

In order to analyze a sample for its atomic constituents, it has to be atomized. The atomizers
most commonly used nowadays are flames and electrothermal (graphite tube) atomizers. The
atoms should then be irradiated by optical radiation, and the radiation source could be an
element-specific line radiation source or a continuum radiation source. The radiation then passes
through a monochromator in order to separate the element-specific radiation from any other
radiation emitted by the radiation source, which is finally measured by a detector.

Atomizers[edit]
The atomizers most commonly used nowadays are (spectroscopic) flames and electrothermal
(graphite tube) atomizers. Other atomizers, such as glow-discharge atomization, hydride
atomization, or cold-vapor atomization, might be used for special purposes.
Flame atomizers[edit]
The oldest and most commonly used atomizers in AAS are flames, principally the air-acetylene
flame with a temperature of about 2300 °C and the nitrous oxide[3] system (N2O)-acetylene flame
with a temperature of about 2700 °C. The latter flame, in addition, offers a more reducing
environment, being ideally suited for analytes with high affinity to oxygen.

A laboratory flame photometer that uses a propane operated flame atomizer

Liquid or dissolved samples are typically used with flame atomizers. The sample solution is
aspirated by a pneumatic analytical nebulizer, transformed into an aerosol, which is introduced
into a spray chamber, where it is mixed with the flame gases and conditioned in a way that only
the finest aerosol droplets (< 10 μm) enter the flame. This conditioning process reduces
interference, but only about 5% of the aerosolized solution reaches the flame because of it.
On top of the spray chamber is a burner head that produces a flame that is laterally long (usually
5–10 cm) and only a few mm deep. The radiation beam passes through this flame at its longest
axis, and the flame gas flow-rates may be adjusted to produce the highest concentration of free
atoms. The burner height may also be adjusted, so that the radiation beam passes through the
zone of highest atom cloud density in the flame, resulting in the highest sensitivity.
The processes in a flame include the stages of desolvation (drying) in which the solvent is
evaporated and the dry sample nano-particles remain, vaporization (transfer to the gaseous
phase) in which the solid particles are converted into gaseous molecule, atomization in which the
molecules are dissociated into free atoms, and ionization where (depending on the ionization
potential of the analyte atoms and the energy available in a particular flame) atoms may be in
part converted to gaseous ions.
Each of these stages includes the risk of interference in case the degree of phase transfer is
different for the analyte in the calibration standard and in the sample. Ionization is generally
undesirable, as it reduces the number of atoms that are available for measurement, i.e., the
sensitivity.
In flame AAS a steady-state signal is generated during the time period when the sample is
aspirated. This technique is typically used for determinations in the mg L−1 range, and may be
extended down to a few μg L−1 for some elements.
Electrothermal atomizers[edit]

GFAA method development

Graphite tube

Electrothermal AAS (ET AAS) using graphite tube atomizers was pioneered by Boris V. L’vov at
the Saint Petersburg Polytechnical Institute, Russia,[4] since the late 1950s, and investigated in
parallel by Hans Massmann at the Institute of Spectrochemistry and Applied Spectroscopy
(ISAS) in Dortmund, Germany.[5]
Although a wide variety of graphite tube designs have been used over the years, the dimensions
nowadays are typically 20–25 mm in length and 5–6 mm inner diameter. With this technique
liquid/dissolved, solid and gaseous samples may be analyzed directly. A measured volume
(typically 10–50 μL) or a weighed mass (typically around 1 mg) of a solid sample are introduced
into the graphite tube and subject to a temperature program. This typically consists of stages,
such as drying – the solvent is evaporated; pyrolysis – the majority of the matrix constituents are
removed; atomization – the analyte element is released to the gaseous phase; and cleaning –
eventual residues in the graphite tube are removed at high temperature.[6]
The graphite tubes are heated via their ohmic resistance using a low-voltage high-current power
supply; the temperature in the individual stages can be controlled very closely, and temperature
ramps between the individual stages facilitate separation of sample components. Tubes may be
heated transversely or longitudinally, where the former ones have the advantage of a more
homogeneous temperature distribution over their length. The so-called stabilized temperature
platform furnace (STPF) concept, proposed by Walter Slavin, based on research of Boris L’vov,
makes ET AAS essentially free from interference.[citation needed] The major components of this concept
are atomization of the sample from a graphite platform inserted into the graphite tube (L’vov
platform) instead of from the tube wall in order to delay atomization until the gas phase in the
atomizer has reached a stable temperature; use of a chemical modifier in order to stabilize the
analyte to a pyrolysis temperature that is sufficient to remove the majority of the matrix
components; and integration of the absorbance over the time of the transient absorption signal
instead of using peak height absorbance for quantification.
In ET AAS a transient signal is generated, the area of which is directly proportional to the mass
of analyte (not its concentration) introduced into the graphite tube. This technique has the
advantage that any kind of sample, solid, liquid or gaseous, can be analyzed directly. Its
sensitivity is 2–3 orders of magnitude higher than that of flame AAS, so that determinations in the
low μg L−1 range (for a typical sample volume of 20 μL) and ng g−1 range (for a typical sample
mass of 1 mg) can be carried out. It shows a very high degree of freedom from interferences, so
that ET AAS might be considered the most robust technique available nowadays for the
determination of trace elements in complex matrices.[citation needed]
Specialized atomization techniques[edit]
While flame and electrothermal vaporizers are the most common atomization techniques, several
other atomization methods are utilized for specialized use.[7][8]
Glow-discharge atomization[edit]
A glow-discharge device (GD) serves as a versatile source, as it can simultaneously introduce
and atomize the sample. The glow discharge occurs in a low-pressure argon gas atmosphere
between 1 and 10 torr. In this atmosphere lies a pair of electrodes applying a DC voltage of 250
to 1000 V to break down the argon gas into positively charged ions and electrons. These ions,
under the influence of the electric field, are accelerated into the cathode surface containing the
sample, bombarding the sample and causing neutral sample atom ejection through the process
known as sputtering. The atomic vapor produced by this discharge is composed of ions, ground
state atoms, and fraction of excited atoms. When the excited atoms relax back into their ground
state, a low-intensity glow is emitted, giving the technique its name.
The requirement for samples of glow discharge atomizers is that they are electrical conductors.
Consequently, atomizers are most commonly used in the analysis of metals and other
conducting samples. However, with proper modifications, it can be utilized to analyze liquid
samples as well as nonconducting materials by mixing them with a conductor (e.g. graphite).
Hydride atomization[edit]
Hydride generation techniques are specialized in solutions of specific elements. The technique
provides a means of introducing samples containing arsenic, antimony, selenium, bismuth, and
lead into an atomizer in the gas phase. With these elements, hydride atomization enhances
detection limits by a factor of 10 to 100 compared to alternative methods. Hydride generation
occurs by adding an acidified aqueous solution of the sample to a 1% aqueous solution of
sodium borohydride, all of which is contained in a glass vessel. The volatile hydride generated by
the reaction that occurs is swept into the atomization chamber by an inert gas, where it
undergoes decomposition. This process forms an atomized form of the analyte, which can then
be measured by absorption or emission spectrometry.
Cold-vapor atomization[edit]
The cold-vapor technique is an atomization method limited to only the determination of mercury,
due to it being the only metallic element to have a large enough vapor pressure at ambient
temperature.[citation needed] Because of this, it has an important use in determining organic mercury
compounds in samples and their distribution in the environment. The method initiates by
converting mercury into Hg2+ by oxidation from nitric and sulfuric acids, followed by a reduction of
Hg2+ with tin(II) chloride. The mercury, is then swept into a long-pass absorption tube by bubbling
a stream of inert gas through the reaction mixture. The concentration is determined by measuring
the absorbance of this gas at 253.7 nm. Detection limits for this technique are in the parts-per-
billion range making it an excellent mercury detection atomization method.
Two types of burners are used: total consumption burner and premix burner.
Radiation sources[edit]
We have to distinguish between line source AAS (LS AAS) and continuum source AAS (CS
AAS). In classical LS AAS, as it has been proposed by Alan Walsh,[9] the high spectral resolution
required for AAS measurements is provided by the radiation source itself that emits the spectrum
of the analyte in the form of lines that are narrower than the absorption lines. Continuum sources,
such as deuterium lamps, are only used for background correction purposes. The advantage of
this technique is that only a medium-resolution monochromator is necessary for measuring AAS;
however, it has the disadvantage that usually a separate lamp is required for each element that
has to be determined. In CS AAS, in contrast, a single lamp, emitting a continuum spectrum over
the entire spectral range of interest is used for all elements. Obviously, a high-resolution
monochromator is required for this technique, as will be discussed later.

Hollow cathode lamp (HCL)

Hollow cathode lamps[edit]

Hollow cathode lamps (HCL) are the most common radiation source in LS AAS.[citation needed] Inside
the sealed lamp, filled with argon or neon gas at low pressure, is a cylindrical metal cathode
containing the element of interest and an anode. A high voltage is applied across the anode and
cathode, resulting in an ionization of the fill gas. The gas ions are accelerated towards the
cathode and, upon impact on the cathode, sputter cathode material that is excited in the glow
discharge to emit the radiation of the sputtered material, i.e., the element of interest. In the
majority of cases single element lamps are used, where the cathode is pressed out of
predominantly compounds of the target element. Multi-element lamps are available with
combinations of compounds of the target elements pressed in the cathode. Multi element lamps
produce slightly less sensitivity than single element lamps and the combinations of elements
have to be selected carefully to avoid spectral interferences. Most multi-element lamps combine
a handful of elements, e.g.: 2 - 8. Atomic Absorption Spectrometers can feature as few as 1-2
hollow cathode lamp positions or in automated multi-element spectrometers, a 8-12 lamp
positions may be typically available.
Electrodeless discharge lamps[edit]
Electrodeless discharge lamps (EDL) contain a small quantity of the analyte as a metal or a salt
in a quartz bulb together with an inert gas, typically argon gas, at low pressure. The bulb is
inserted into a coil that is generating an electromagnetic radio frequency field, resulting in a low-
pressure inductively coupled discharge in the lamp. The emission from an EDL is higher than
that from an HCL, and the line width is generally narrower, but EDLs need a separate power
supply and might need a longer time to stabilize.
Deuterium lamps[edit]
Deuterium HCL or even hydrogen HCL and deuterium discharge lamps are used in LS AAS for
background correction purposes.[10] The radiation intensity emitted by these lamps decreases
significantly with increasing wavelength, so that they can be only used in the wavelength range
between 190 and about 320 nm.

Xenon lamp as a continuous radiation source

Continuum sources[edit]
When a continuum radiation source is used for AAS, it is necessary to use a high-resolution
monochromator, as will be discussed later. In addition, it is necessary that the lamp emits
radiation of intensity at least an order of magnitude above that of a typical HCL over the entire
wavelength range from 190 nm to 900 nm. A special high-pressure xenon short arc lamp,
operating in a hot-spot mode has been developed to fulfill these requirements.
Spectrometer[edit]
As already pointed out above, there is a difference between medium-resolution spectrometers
that are used for LS AAS and high-resolution spectrometers that are designed for CS AAS. The
spectrometer includes the spectral sorting device (monochromator) and the detector.
Spectrometers for LS AAS[edit]
In LS AAS the high resolution that is required for the measurement of atomic absorption is
provided by the narrow line emission of the radiation source, and the monochromator simply has
to resolve the analytical line from other radiation emitted by the lamp.[citation needed] This can usually be
accomplished with a band pass between 0.2 and 2 nm, i.e., a medium-resolution
monochromator. Another feature to make LS AAS element-specific is modulation of the primary
radiation and the use of a selective amplifier that is tuned to the same modulation frequency, as
already postulated by Alan Walsh. This way any (unmodulated) radiation emitted for example by
the atomizer can be excluded, which is imperative for LS AAS. Simple monochromators of the
Littrow or (better) the Czerny-Turner design are typically used for LS AAS. Photomultiplier tubes
are the most frequently used detectors in LS AAS, although solid state detectors might be
preferred because of their better signal-to-noise ratio.
Spectrometers for CS AAS[edit]
When a continuum radiation source is used for AAS measurement it is indispensable to work
with a high-resolution monochromator. The resolution has to be equal to or better than the half
width of an atomic absorption line (about 2 pm) in order to avoid losses of sensitivity and linearity
of the calibration graph. The research with high-resolution (HR) CS AAS was pioneered by the
groups of O’Haver and Harnly in the US, who also developed the (up until now) only
simultaneous multi-element spectrometer for this technique. The breakthrough, however, came
when the group of Becker-Ross in Berlin, Germany, built a spectrometer entirely designed for
HR-CS AAS. The first commercial equipment for HR-CS AAS was introduced by Analytik
Jena (Jena, Germany) at the beginning of the 21st century, based on the design proposed by
Becker-Ross and Florek. These spectrometers use a compact double monochromator with a
prism pre-monochromator and an echelle grating monochromator for high resolution. A
linear charge-coupled device (CCD) array with 200 pixels is used as the detector. The second
monochromator does not have an exit slit; hence the spectral environment at both sides of the
analytical line becomes visible at high resolution. As typically only 3–5 pixels are used to
measure the atomic absorption, the other pixels are available for correction purposes. One of
these corrections is that for lamp flicker noise, which is independent of wavelength, resulting in
measurements with very low noise level; other corrections are those for background absorption,
as will be discussed later.

Background absorption and background correction[edit]

The relatively small number of atomic absorption lines (compared to atomic emission lines) and
their narrow width (a few pm) make spectral overlap rare; there are only few examples known
that an absorption line from one element will overlap with another.[citation needed] Molecular absorption,
in contrast, is much broader, so that it is more likely that some molecular absorption band will
overlap with an atomic line. This kind of absorption might be caused by un-dissociated molecules
of concomitant elements of the sample or by flame gases. We have to distinguish between the
spectra of di-atomic molecules, which exhibit a pronounced fine structure, and those of larger
(usually tri-atomic) molecules that don't show such fine structure. Another source of background
absorption, particularly in ET AAS, is scattering of the primary radiation at particles that are
generated in the atomization stage, when the matrix could not be removed sufficiently in the
pyrolysis stage.
All these phenomena, molecular absorption and radiation scattering, can result in artificially high
absorption and an improperly high (erroneous) calculation for the concentration or mass of the
analyte in the sample. There are several techniques available to correct for background
absorption, and they are significantly different for LS AAS and HR-CS AAS.
Background correction techniques in LS AAS[edit]
In LS AAS background absorption can only be corrected using instrumental techniques, and all
of them are based on two sequential measurements:[11] firstly, total absorption (atomic plus
background), secondly, background absorption only. The difference of the two measurements
gives the net atomic absorption. Because of this, and because of the use of additional devices in
the spectrometer, the signal-to-noise ratio of background-corrected signals is always significantly
inferior compared to uncorrected signals. It should also be pointed out that in LS AAS there is no
way to correct for (the rare case of) a direct overlap of two atomic lines. In essence there are
three techniques used for background correction in LS AAS:
Deuterium background correction[edit]
This is the oldest and still most commonly used technique, particularly for flame AAS. In this
case, a separate source (a deuterium lamp) with broad emission is used to measure the
background absorption over the entire width of the exit slit of the spectrometer. The use of a
separate lamp makes this technique the least accurate one, as it cannot correct for any
structured background. It also cannot be used at wavelengths above about 320 nm, as the
emission intensity of the deuterium lamp becomes very weak. The use of deuterium HCL is
preferable compared to an arc lamp due to the better fit of the image of the former lamp with that
of the analyte HCL.
Smith-Hieftje background correction[edit]
This technique (named after their inventors) is based on the line-broadening and self-reversal of
emission lines from HCL when high current is applied. Total absorption is measured with normal
lamp current, i.e., with a narrow emission line, and background absorption after application of a
high-current pulse with the profile of the self-reversed line, which has little emission at the original
wavelength, but strong emission on both sides of the analytical line. The advantage of this
technique is that only one radiation source is used; among the disadvantages are that the high-
current pulses reduce lamp lifetime, and that the technique can only be used for relatively volatile
elements, as only those exhibit sufficient self-reversal to avoid dramatic loss of sensitivity.
Another problem is that background is not measured at the same wavelength as total absorption,
making the technique unsuitable for correcting structured background.
Zeeman-effect background correction[edit]
Main article: Zeeman effect

An alternating magnetic field is applied at the atomizer (graphite furnace) to split the absorption
line into three components, the π component, which remains at the same position as the original
absorption line, and two σ components, which are moved to higher and lower wavelengths,
respectively.[citation needed] Total absorption is measured without magnetic field and background
absorption with the magnetic field on. The π component has to be removed in this case, e.g.
using a polarizer, and the σ components do not overlap with the emission profile of the lamp, so
that only the background absorption is measured. The advantages of this technique are that total
and background absorption are measured with the same emission profile of the same lamp, so
that any kind of background, including background with fine structure can be corrected
accurately, unless the molecule responsible for the background is also affected by the magnetic
field and using a chopper as a polariser reduces the signal to noise ratio. While the
disadvantages are the increased complexity of the spectrometer and power supply needed for
running the powerful magnet needed to split the absorption line.

Background correction techniques in HR-CS AAS[edit]

In HR-CS AAS background correction is carried out mathematically in the software using
information from detector pixels that are not used for measuring atomic absorption; hence, in
contrast to LS AAS, no additional components are required for background correction.
Background correction using correction pixels[edit]
It has already been mentioned that in HR-CS AAS lamp flicker noise is eliminated using
correction pixels. In fact, any increase or decrease in radiation intensity that is observed to the
same extent at all pixels chosen for correction is eliminated by the correction algorithm.[citation
needed]
 This obviously also includes a reduction of the measured intensity due to radiation scattering
or molecular absorption, which is corrected in the same way. As measurement of total and
background absorption, and correction for the latter, are strictly simultaneous (in contrast to LS
AAS), even the fastest changes of background absorption, as they may be observed in ET AAS,
do not cause any problem. In addition, as the same algorithm is used for background correction
and elimination of lamp noise, the background corrected signals show a much better signal-to-
noise ratio compared to the uncorrected signals, which is also in contrast to LS AAS.
Background correction using a least-squares algorithm [edit]
The above technique can obviously not correct for a background with fine structure, as in this
case the absorbance will be different at each of the correction pixels. In this case HR-CS AAS is
offering the possibility to measure correction spectra of the molecule(s) that is (are) responsible
for the background and store them in the computer. These spectra are then multiplied with a
factor to match the intensity of the sample spectrum and subtracted pixel by pixel and spectrum
by spectrum from the sample spectrum using a least-squares algorithm. This might sound
complex, but first of all the number of di-atomic molecules that can exist at the temperatures of
the atomizers used in AAS is relatively small, and second, the correction is performed by the
computer within a few seconds. The same algorithm can actually also be used to correct for
direct line overlap of two atomic absorption lines, making HR-CS AAS the only AAS technique
that can correct for this kind of spectral interference.

Atomic emission spectroscopy (AES) is a method of chemical analysis that uses the intensity

of light emitted from a flame, plasma, arc, or spark at a particular wavelength to determine the
quantity of an element in a sample. The wavelength of the atomic spectral line in the emission
spectrum gives the identity of the element while the intensity of the emitted light is proportional to
the number of atoms of the element. The sample may be excited by various methods.

Flame[edit]

A flame during the assessment of calcium ions in a flame photometer

A sample of a material (analyte) is brought into the flame as a gas, sprayed solution, or directly
inserted into the flame by use of a small loop of wire, usually platinum. The heat from the flame
evaporates the solvent and breaks intramolecular bonds to create free atoms. The thermal
energy also excites the atoms into excited electronic states that subsequently emit light when
they return to the ground electronic state. Each element emits light at a characteristic
wavelength, which is dispersed by a grating or prism and detected in the spectrometer.
Sodium atomic ions emitting light in a flame displays a brilliantly bright yellow emission at 588.9950 and
589.5924 nanometers wavelength.

A frequent application of the emission measurement with the flame is the regulation of alkali
metals for pharmaceutical analytics.[1]

Inductively coupled plasma[edit]

Main article: Inductively coupled plasma atomic emission spectroscopy

Inductively coupled plasma atomic emission source

Inductively coupled plasma atomic emission spectroscopy (ICP-AES) uses an inductively

coupled plasma to produce excited atoms and ions that emit electromagnetic radiation at
wavelengths characteristic of a particular element.[2][3]
Advantages of ICP-AES are the excellent limit of detection and linear dynamic range, multi-
element capability, low chemical interference and a stable and reproducible signal.
Disadvantages are spectral interferences (many emission lines), cost and operating expense and
the fact that samples typically must be in a liquid solution. Inductively Coupled Plasma (ICP)
source of the emission consists of an induction coil and plasma. An induction coil is a coil of wire
that has an alternating current flowing through it. This current induces a magnetic field inside the
coil, coupling a great deal of energy to plasma contained in a quartz tube inside the coil. Plasma
is a collection of charged particles (cations and electrons) capable, by virtue of their charge, of
interacting with a magnetic field.The plasmas used inatomic emissions are formed by ionizing a
flowing stream of argon gas. Plasma’s high-temperature results from resistive heating as the
charged particles move through the gas. Because plasmas operate at much higher temperatures
than flames, they provide better atomization and a higher population of excited states. The
predominant form of sample matrix in ICP-AES today is a liquid sample: acidified water or solids
digested into aqueous forms. Liquid samples are pumped into the nebulizer and sample chamber
via a peristaltic pump. Then the samples pass through a nebulizer that creates a fine mist of
liquid particles. Larger water droplets condense on the sides of the spray chamber and are
removed via the drain, while finer water droplets move with the argon flow and enter the plasma.
With plasma emission, it is possible to analyze solid samples directly. These procedures include
incorporating electrothermal vaporization, laser and spark ablation, and glow-discharge
vaporization.

Spark and arc[edit]

Spark or arc atomic emission spectroscopy is used for the analysis of metallic elements in solid
samples. For non-conductive materials, the sample is ground with graphite powder to make
it conductive. In traditional arc spectroscopy methods, a sample of the solid was commonly
ground up and destroyed during analysis. An electric arc or spark is passed through the sample,
heating it to a high temperature to excite the atoms within it. The excited analyte atoms emit light
at characteristic wavelengths that can be dispersed with a monochromator and detected. In the
past, the spark or arc conditions were typically not well controlled, the analysis for the elements
in the sample were qualitative. However, modern spark sources with controlled discharges can
be considered quantitative. Both qualitative and quantitative spark analysis are widely used for
production quality control in foundry and metal casting facilities.