Atomic Absorption Spectroscopy
The components of an atomic absorption spectrometer are a radiation
source, an atomization cell, a sample introduction system, a method of
wavelength selection and a detector (Fig. 27.2).
The main radiation source for AAS is the hollow-cathode lamp (HCL). The
HCL (Fig. 27.3) emits radiation characteristic of a particular element. The
choice of HCL for AAS is simple. For example, if you are analysing for lead,
you will need a lead-coated HCL. It is normal to pre-warm the HCL for
about 10min prior to use. This can be done either by using a separate pre-heater unit, capable of warming up several HCLs simultaneously, or by
inserting the HCL in the AAS instrument and switching on the current. The
lamp is typically operated at an electric current between 2 and 30 mA.
|Fig.27.2 Components of an atomic absorption spectrometer.
|Fig. 27.3 Components of a hollow-cathode lamp (HCl).
Several types of atomization cell are available: flame, graphite furnace,
hydride generation and cold vapour. Flame is the most common. In the premixed
laminar flame, the fuel and oxidant gases are mixed before they enter
the burner (the ignition site) in an expansion chamber. The more commonly
used flame in FAAS is the air-acetylene flame (temperature, 2500 K), while
the nitrous oxide-acetylene flame (temperature, 3150 K) is used for refractory
elements, e.g. AI. Both are formed in a slot burner positioned in the light
path of the HCL (Fig. 27.4).
In the graphite furnace atomizer, a small volume of sample (5-100,&micrp;L) is
introduced onto the inner surface of a graphite tube (or onto a platform
placed within the tube) through a small opening (Fig. 27.5). The graphite
tube is arranged so that light from the HCL passes directly through the
centre. Passing an electric current through the tube allows the operator to
program a heating cycle, with several stages (Fig. 27.6) including the
elimination of water from the sample (drying), removal of the sample matrix
(ashing), atomization of the analyte (analysis), and removal of extraneous
material (cleaning). An internal gas flow of inert gas (N2
or Ar) during the
drying and ashing stages removes any extraneous material.
|Fig.27.4 Components of a slot burner for FAAP.
Hydride generation is a sample introduction technique exclusively for
elements that form volatile hydrides (e.g. As, Se, Sn). An acidified sample
solution is reacted with sodium borohydride solution, liberating the gaseous
hydride in a gas-liquid separator. The generated hydride is then transported to an atomization cell using a carrier gas. The atomization cell is normally an
electrically heated or flame-heated quartz tube. Using arsenic as an example it is
possible to write the following equation for the generation of arsine (AsH3
|⇒ Equation [27.1]
||3BH4− + 3H+ + 4H3AsO3 → 3H3BO3 + 4AsH3 + 3H2O
Cold vapour generation is the term exclusively reserved for mercury.
Mercury in a sample is reduced to elemental mercury by tin (II) chloride
|⇒ Equation [27.2]
||SN2+ + Hg2+ → SN4+ + Hg0
and the mercury vapour produced is transported to an atomization cell by a
carrier gas. The atomization cell consists of a long-path glass absorption cell
located in the path of the HCL. Mercury is monitored at a wavelength of
Sample introduction into the flame
|Fig. 27.5 Schematic diagram of a graphite furnace atomizer.
|Fig. 27.6 Heating cycle for a graphite furnace atomizer. 1. drying; 2. ashing; 3. analysis; 4. cleaning and 5. cooling
Samples are almost exclusively introduced into flames as liquids. Solid
samples need to be converted to aqueous solutions using methods such as
decomposition. Once in the aqueous form the sample is
introduced into the flame using a nebulizer/expansion chamber.
The pneumatic concentric nebulizer consists of a stainless
steel tube through which a Pt/Ir capillary tube is located. The aqueous sample is drawn up through the capillary tube by the action of the oxidant
gas (air) escaping through the exit orifice that exists between the outside of
the capillary tube and the inside of the stainless steel tube. The action of the
escaping air and aqueous sample is sufficient to form a coarse aerosol in a
process termed the Venturi effect. The typical uptake rate of the nebulizer is
between 3 and 6 mL min−1
|Fig.27.7 Schematic diagram of a nebulizerexpansion chamber for FAAS.
The expansion chamber (Fig. 27.7) has two functions. The first IS
concerned with aerosol generation the objectives of which are:
- to convert the aqueous sample solution into a coarse aerosol using the
- to disperse the coarse aerosol further into a fine aerosol, by interaction
with baffles located within the chamber;
- to condense any residual aerosol particles, which then go to waste.
The second function involves the safe pre-mixing of the oxidant and fuel
gases before they are introduced into the laminar flow burner.
Wavelength selection and detection
As AAS is used to monitor one metal at a time, the spectrometer used is
termed a monochromator. Two optical arrangements are possible; single and
double beam. The latter is preferred as it corrects for fluctuations in the HCL
caused by warm-up, drift and source noise, thus leading to improved
precision in the absorbance measurement. A schematic diagram of the optical
arrangement is shown in Fig. 27.8. The attenuation of the HCL radiation by
the atomic vapour is detected by a photomultiplier tube (PMT), a device for
proportionally converting photons of light to electric current.
Background correction methods
|Fig. 27.8 Schematic diagram of the optical
arrangement for AAS.
One of the main practical problems with the use of AAS is the occurrence of
molecular species that coincide with the atomic signal. One approach to
remove this molecular absorbance is by the use of background correction
methods. Several approaches are possible, but the most common is based on
the use of a continuum source, D2
. In the atomization cell (e.g. flame)
absorption is possible from both atomic species and from molecular species
(unwanted interference). By measuring the absorption that occurs from the
radiation source (HCL) and comparing it with the absorbance that occurs
from the continuum source (D2
) a corrected absorption signal can be
obtained. This is because the atomic species of interest absorb the specific
radiation associated with the HCL source, whereas the absorption of
radiation by the continuum source for the same atomic species will be
Interferences in the flame
Interferences in the flame can be classified into four categories: chemical,
ionization, physical and spectral.
Chemical interferences occur when the analyte forms a thermally stable
compound with a molecular or ionic species present in the sample solution.
Examples include the suppression of alkaline earth metals due to the presence
of phosphate, silicate or aluminate in the sample solution in the air-acetylene
flame. The most well-known example of this is the absorption signal
suppression that occurs for Ca at 422.7nm owing to increasing amounts of
phosphate. This signal suppression is due to the formation of calcium
pyrophosphate, a thermally stable compound in the flame.
Ionization interferences occur most commonly for alkali and alkaline earth
metals. The low ionization potential of these metals can lead to their
ionization in the relatively hot environment of the flame. If this occurs, no
absorption signal is detected, since FAAS is a technique for measuring atoms
not ions. This process can be prevented by the addition of an ionization
suppressor or 'buffer', e.g. an alkali metal such as Cs. Addition of excess Cs leads to its ionization in the flame in preference to the metal of interest, e.g.
Na. This process is termed the 'mass action' effect.
Physical interferences are due to the effects of the sample solution on
aerosol formation within the spray chamber. The formation of an aerosol is
dependent upon the surface tension, density and viscosity of the sample
solution. This type of interference can be controlled by the matrix matching
of sample and standard solutions, i.e. add the same sample components to
the standard solution, but without the metal of interest. If this is not possible,
it is then necessary to use the method of standard additions (Box 27.3).
Spectral interferences are uncommon in AAS owing to the selectivity of
the technique. However, some interferences may occur, e.g. the resonance
line of Cu occurs at 324.754 nm and has a line coincidence from Eu at
324.753 nm. Unless the Eu is 1000 times in excess, however, it is unlikely
to cause any problems for Cu determination. In addition to atomic
spectral overlap, molecular band absorption can cause problems, e.g.
calcium hydroxide has an absorption band on the Ba wavelength of
553.55 nm while Pb at 217.0 nm has molecular absorption from NaCl.
Molecular band absorption can be corrected for using background
correction techniques. The operation of a flame atomic
absorption spectrometer is described in Box 27.6.