Page 283 - Inorganic Mass Spectrometry - Fundamentals and Applications

of
~na~ys~s Noncon~uctive Sample Types 269

where subsequent ionization occurs. Although there are many volatilization-re-
lated matrix effects in the laser ablation of nonconductors, this too would appear
to be a general method for at least the qualitative analysis of nonconductive mate-
rials.

One of the common methods of performing secondary ion mass spectrometry
(SIMS) analysis of insulating samples involves the placement of a metallic elec-
that
the
trode (grid) in contact with the sample such built-up charge can be removed
to
[48,49]. Milton and Hutton extended this approach GD-MS in an ~angement
they termed a secondary cathode [21]. In short, a thin (0.25-mm) metal foil with
a circular aperture is placed across the face of the flat sample such that when the
discharge potential is applied, the secondary cathode is sputtered and the released
atoms redeposit onto the exposed portion of the insulating sample surface. West-
are rede-
wood [so] showed many years ago that as much 50% of sputtered atoms
posited in low-pressure (150-mtorr) glow discharges, with higher pressures pro-
7.3
ducing larger percentages. Figure is a simple diagram of the discharge cell with

L
EkCaiCll Smndary
Chmct Cathode ~ ~ ~ C ~ ~ o d e
Secondary Insulator
Cathode
Figure 3 Schematic representation (not to scale) the source assembly used in the sec-
of
ondary cathode approach to glow discharge mass spectrometry (GD-MS) analysis of non-
conductive samples. (From Ref. 21.)