can be made. The maximum value of the absorption coefficient kmax is directly proportional to the
oscillator strength f and indirectly proportional to the Doppler linewidth, ∆λ (equation (8.4)), hence the
most sensitive lines are those which combine a narrow linewidth with a large oscillator strength. A wide
range of linear response is attained if the width of the emission line is less than one-fifth that of the
absorption line. Guides to the appropriate choice of emission line are given in specialist texts on atomic
absorption spectrometry.
Hollow-cathode lamps are currently available for over sixty elements. Several multi-element lamps
have been constructed and are useful for routine determinations, but they have proved to be of doubtful
performance up to now. More successful with regard to multi-element analysis have been computer
controlled automated systems, which enable a 'programme' of sequential measurements to be made with
instrumental parameters being adjusted to the optimum for each element to be measured.
Electrodeless Discharge Tubes
Radiation is derived from a sealed quartz tube containing a few milligrams of an element or a volatile
compound and neon or argon at low pressure. The discharge is produced by a microwave source via a
waveguide cavity or using RF induction. The emission spectrum of the element concerned contains
only the most prominent resonance lines and with intensities up to one hundred times those derived
from a hollow-cathode lamp. However, the reliability of such sources has been questioned and the only
ones which are currently considered successful are those for arsenic, antimony, bismuth, selenium and
tellurium using RF excitation. Fortunately, these are the elements for which hollow-cathode lamps are
the least successful.
Sample Vaporization
The production of an homogeneous atomic vapour from a sample is achieved by aspirating a solution
into a flame or evaporating small volumes in an electrically heated tube furnace or from the surface of a
carbon rod. In all cases, the thermal energy supplied must (a) evaporate the solvent and (b) dissociate
the remaining solids into their constituent atoms without causing appreciable ionization.
Flame Vaporization
The sample solution is drawn first into a nebulizer by the flow of support gas where it forms a mist or
aerosol. Fuel gas is introduced and the mixture passed to a spray chamber where large droplets
condense and run to waste. Alternatively a small volume of solution (10– 100 μl) may be injected
directly into the orifice of the nebulizer using a micropipette.
The resulting homogeneous mixture of sample droplets and gases passes to the burner for combustion.
With the former method an equilibrium