these ions will impact on the surface of the cathode leading to vaporization and excitation of atoms
from its surface. This process, known as sputtering, is also the basis of the hollow cathode lamp used in
atomic absorption spectrometry (vide infra). If the cathode comprises an electrically conducting sample,
as in metallurgical analysis, its surface atoms will be volatilized and excited, producing on relaxation an
emission spectrum characteristic of the composition of the sample surface. Furthermore, as
volatilization depends only on the sputtering process all elements will be volatilized at more or less the
same rate. Hence many of the complex matrix problems experienced by arc/spark methods will be
avoided. As a result, calibration curves will be linear over a wider range with much better precision for
the analysis (Figure 8.7). Additionally, as the sputtering process steadily erodes the surface (2– 5 μm
min–^1 ) changes in composition in the surface layers may also be studied.
Figure 8.7
Typical GDL and spark emission calibration curves
contrasting range and linearity (with permission
from Jobin–Yvon).Instrumentation
The overall instrumentation, with the exception of the GDL itself, resembles that for other forms of
atomic optical emission spectrometry. The construction of the GDL is shown schematically in Figure
8.8. Initially the lamp is flushed with argon, evacuated and then filled with argon to low pressure for the
sputtering and glow discharge to operate. After analysis the system is flushed with argon at a high flow
rate, the sample replaced and subsequent analysis carried out by the same sequence. The surface of the
sample needs to be smooth.