Page 19 - Inorganic Mass Spectrometry : Fundamentals and Applications

The~al Ionizatio~ Mass Spectrometry 9

face in question and ranges from about 5.0 eV to about 5.8 eV, with 5.4 eV the
average for the polycrystalline material [36]. Platinum has a higher work function
(5.7 eV) than rhenium, but its melting point is too low (1772OC) for it to be useful
as filament material in many applications. In addition to its high work function,
rhenium has mechanical properties that make it attractive as filament material. For
example, unlike tungsten, it retains its ductility after heating or spot welding.
Another advantage important in many applications is that rhenium metal is
available in high purity (>99.999%). The advent of zone refining led to large
improvements in purity; prior to that, it was extremely difficult to analyze thorium
using rhenium filaments because thoria-lined furnaces were used in annealing the
metal. Aside from rhenium, the two most commonly used metals for filaments are
tungsten and tantalum. The work functions for these elements are about 1 eV
lower than rhenium's, so in general ion emission is lower. Tantalum has seen
widespread use as an evaporator filament in multifilament con~gurations; work
function is irrelevant, it is less costly than rhenium, and the relatively low
temperatures required of evaporators make demands on purity less stringent.
There are also special applications in which use of rhenium is inappropriate; an
obvious instance is an application in which rhenium is itself the analyte element.
There are two filament configurations used in thermal ionization: single-
filament and multifilament; multifilament configurations can have two or three
individual filaments. All these configurations are illustrated in Fig. 1.4. The choice

Single Filament Double Filament Triple Filament
Ionizer
A Evaporator I I ionizer Ci Evaporators
t J I 1 1
Case Plate Case Plate Case Plate

Case Plate
-;=;. Focus Plates
raw Out Plate

First Collimating Slit Plate

Z Deflection Plates
Exit Slit Plate

Figure 4 Ion source con~guratio~s used in thermal ionization mass spectrometry.