Applications
Quantitative determination of metals in solution, especially alkali, and alkaline earth metals in clinical
samples. Relative precision 1–4%.
Disadvantages
Intensity of emission is very sensitive to changes in flame temperature. Spectral interferences and self-
absorption are common problems.
Flame emission spectrometry closely resembles other techniques of optical emission spectrometry in
principle and instrumentation. The difference lies in the substitution of a chemical flame for the high-
temperature electric discharges or plasmas. The general characteristics of flames, and the analytically
relevant processes occurring within them are discussed later. It is sufficient to note here the relatively
lower temperature of flames, typically 2000–3000 K, and the consequently lower energy available to
induce electronic excitation. It follows that the flame-induced emission spectrum of an element will be
much less complex than the corresponding spectrum resulting from an electric discharge or plasma.
Flame emission spectrometry is a particularly useful technique for the determination of volatile
elements with low excitation energies such as the alkali and alkaline earth metals. This is in part due to
a high sensitivity resulting from the relatively low degree of ionization induced by cool flames, and in
part to longer residence times for the analyte atoms in the flame as compared to an arc, spark or plasma.
Instrumentation
A typical instrumental arrangement for flame emission measurements is shown in Figure 8.21(a). The
sample, in the form of a solution, is drawn into a nebulizer where it is converted into a fine mist or
aerosol. From there it passes into the flame along with air or oxygen and a fuel gas. Following thermal
excitation, the radiation emitted as excited atoms relax is viewed by a photocell or photomultiplier. The
current generated in the detector circuit may be read directly or, more conveniently, converted to a
meter or digital readout in analyte concentration. It should be noted that the relatively stable emission
resulting from flame excitation, facilitates the almost instantaneous measurement of line intensities and
this should be contrasted with the need to measure time-integrated intensities where arc/spark excitation
is employed. Plasma emission signals are, of course, also measured in short time periods. Low-
temperature flames produce spectra that are sufficiently simple to allow the use of narrow bandpass
filters to isolate the required emission lines for quantitative measurement. Such an instrument is termed
a flame photometer. If better resolution is needed to isolate lines in more complex spectra, or to
minimize interference from background emission, a flame atomic emission spectrometer incorporating
prism