Page 259 - Inorganic Mass Spectrometry - Fundamentals and Applications

The mission Ions 245
of

focusing. Improperly designed deflection electrodes that deflect the ions a sub-
stantial distance can severely defocus a beam, however. A voltage spread across
the face of the emitter adds an element of asymetry at the worst possible loca-
tion in the entire lens, causing random defocusing of the ion trajectories. Conven-
2-V
tional thermal ionization sources have up to a drop along the length of the fil-
ament facing the lens from resistance heating. An experiment conducted in the
of
author’s laboratory, but never published, showed significant defocusing a pla-
nar beam from this eBect. In a lens with cylindrical geometry there is considerable
Thus consideration must
defocusing, as is discussed in the section on ion imaging.
be given to const~cting an ion emitter with little or no voltage spread across the
face.

ounting Samples
%o types of miniaturized sample mounting procedures have been developed. In
of
the first, the sample is mounted on a miniature version a single-fila~ent design
l and mentioned previously. This design is shown
of the type described in Chapter
schematically in Fig. 6.1. The distinguishing features of this design are that the

ront
ure 1 Illustration of the “mini~lament” used to minimize voltage spread in the sam-
ple region of the ion source and to concentrate the sample closer to the central axis of the
ion optics.