Page 63 - Inorganic Mass Spectrometry - Fundamentals and Applications

Glow Discharge Mass Spectrometry 53

surface soon becomes conducting and begins to attract argon ions. Because the
energy of the impinging ions is sufficient to penetrate the thin metallic film, atoms
from the underlying insulator are sputtered. An equilibrium is soon established
between the redeposition of metal atoms and the sputtering of insulator atoms,
resulting in a steady-state discharge with ions representative of the insulating
material and the cathode metal. Several investigations have focused on the surro-
gate cathode material and geometry [94-961. The results obtained have been on
par with radio frequency analysis for similar samples, with one group demonstrat-
ing precision of 6% relative standard deviation (RSD) at the part-per-~llion
(ppm) level and detection limits in the part-per-billion range [94]. Interferences
from the mask are a concern, however, because of the relative sputtered atom
of
densities (i.e., the atom density the mask material is orders of magnitude greater
than that of the sample).

Unlike in optical methods, in GDMS it is necessary to transfer species out of
the plasma and into the detection system. Although the pressure differential alone
is sufficient to transfer a mixture of atoms and ions from the plasma, an extraction
voltage applied just beyond the exit orifice of the cell enhances the fraction of ions
in the final beam. Depending on the type of mass analyzer used, an additional
degree of energy filtering may be necessary to reduce the energy spread of the
ions. Two commonly employed energy filters are Bessel boxes and electrostatic
analyzers (see Fig. 2.1 1). Five types of mass analyzers are used extensively with
glow discharge plasmas. Two of these, magnetic sectors and quadrupole, are
illustrated schematically in Fig, 2.12; ion traps and time-of-flight analyzers are
discussed in later chapters. For a magnetic sector, fields generated by spinning
charged particles (ions) interact with a magnetic field imposed on their flight. The
particles follow a curved path with a radius propo~ional to their mass-to-charge
ratios, separating ions of different mass in space. A series of mechanical slits
define the beam shape and hence the resolution. Electrostatic energy analyzers
(ESAs) are often employed in combination with magnetic sector devices. In the
case of the commercially available VG9000 [20,97] glow discharge mass spec-
trometer, the ESA is positioned after the magnetic sector (in the so-called reverse
Nier-Jo~son geometry). The ESA deflects the ion beam 90" select and transmit
to
a nominal ion energy, and the unwanted ions deviate from this path and are
absorbed by the walls (Fig. 2.11).
A second commonly used mass analyzer is the quadrupole. A quadrupole
ratio
acts as a mass filter, allowingta certain mass-to-charge of ions to be transrnit-
ted while filtering out others. The quadrupole consists of a set of four electrodes
all
positioned in an array. Superimposed radio frequency and direct current electric
fields can be mutually tuned to allow transmission of ions of the selected mass-to-