concentration of free atoms will be established in the flame, whilst in the latter the concentration rises to
a maximum and then falls to zero. The signal from the photomultiplier will vary in the same manner.
The direct injection method enables small samples to be analysed, compared with the 2–3 cm^3 required
by continuous aspiration. The burner consists of a metal block containing a row of circular holes or one
or more slots about 10 cm long. It is aligned along the optical axis of the instrument and just below the
beam from the lamp so as to provide a flame of long absorption path and hence maximum sensitivity.
The design of the burner coupled with the use of a separate nebulizer and spray chamber ensure the
formation of a non-turbulent or laminar flame which results in stable operation, good precision and easy
choice of optimum vaporizing conditions. The most generally useful flame is air-acetylene, with a
moderate burning velocity and a temperature of about 2500 K, although the cooler air-propane and the
hotter nitrous oxide-acetylene flames are also used. The last is particularly useful for samples
containing refractory materials. Where maximum sensitivity is sought, the use of a separated flame may
be advantageous because of reduced flame emission by species such as OH and CO resulting in a lower
level of noise in the detector circuit. This is particularly effective for the determination of elements
whose resonance lines occur below 200 nm, e.g. arsenic and selenium. A more detailed discussion of
flame characteristics is given on p. 314 et seq.
Flameless Vaporization
If the production of an atomic vapour can be achieved without using a flame, there are a number of
potential advantages:
(1) The elimination of anomalous results arising from interactions between the sample and components
of the flame.
(2) Increased sensitivity arising from a longer residence time within the beam of radiation from the
lamp. Residence times in flames are low because of strong vertical thermal currents (p. 315).
(3) Increased sensitivity because of a higher proportion of the analyte being converted to free atoms.
(The conversion may be as low as 0.1% for flame atomization.)
(4) The ability to handle very small samples such as clinical specimens. A nebulizer, spray chamber,
burner arrangement consumes several cm^3 of sample per minute, most of which runs to waste.
Two forms of flameless atomizer are in use, i.e. the graphite tube or L'Vov furnace and the carbon rod
or filament. Of these the first has proved to be the most generally effective and popular. It is widely
used in a variety of modifications. In both cases, the temperature is raised rapidly to about 2500 K by
the passage of a heavy current for a period of 1–2 minutes. Tube furnaces, which are usually 5 or 10 cm
x 3 mm, may be flushed through with argon before vaporizing the sample so as to prevent the formation
of