Page 30 - Inorganic Mass Spectrometry : Fundamentals and Applications

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There are many instances in which it is highly desirable to analyze the
smallest possible sample. This is of obvious importance when radioactive species
are involved, but it is also advantageous when analyzing smaller samples means
processing smaller amounts of material for an analysis, as is often the case in
geological applications, among others. Measurement of isotopic ratios from pico-
gram or smaller quantities of analyte has been reported for technetium [70,71],
actinide elements [72], and rare earth elements [73].
Since analysis of small samples requires pulse-counting detection systems,
most of the effort directed toward improving ionization efficiency has involved the
single-filament configuration. Most ion-optical systems are designed on the as-
sumption that there will be a point source of ions. Many ion source lenses are very
strongly focusing, and even slight deviations in sample location from the object
point of the source cause marked reduction in ion extraction efficiency. A loading
technique that serves to concentrate the sample in a very small area of the filament,
and thus to approximate a point source, is highly desirable. Two such techniques
have been developed in which this is just one of several attractive features. One is
electroplating the sample from a very small (ca. 1 pL) volume onto the rhenium
filament [ 181. The other is to load samples on the filament using single resin beads
[74]. Because each of these methods provides advantages in addition to providing
point sources (discussed later), it is difficult to quantify the benefits of this par-
ticular aspect. Overall improvement in performance with regard to reduction in
sample size is in the range of 5 to 10 for each technique.
A different issue is the challenge of improving ionization efficiency. Two
situations have been addressed. One is to combat the combination of high vola-
tility and high first ionization potential, which prevents effective production of
singly charged positive ions. The classical example here is lead, an element impor-
tant in geological age dating [a]. Its combination of first ionization potential (7.4
eV) and melting point (328°C) militates against efficient ionization. Loading lead
onto a single filament as a solution produces ionization characteristics almost
impossible to reproduce, but using silica gel slurried in phosphoric acid produces a
glass that serves as an emitter and reproducibly yields intense, long-lasting, stable
ion signals and allows analysis of nanogram-sized samples [16].
The second situation to be addressed involves improving ionization yields
from elements refractory enough that loss of sample due to volatility is not a major
problem but whose ionization potential is high enough to cause difliculty. Since
the first ionization potential of an element cannot readily be altered, it is the work
function of the surface that has drawn investigators’ attention. The work function
of polycrystalline r~enium is about 5.4 eV [36], but there are other materials with
higher values. One is platinum, whose work function is about 5.7 eV [75], but
whose melting point is too low for use with many elements. Penin and coworkers
developed a method in which a plutonium sample is electroplated directly onto a
rhenium mass spectrometer filament [ 181. This was followed by electroplating a