Page 21 - Inorganic Mass Spectrometry : Fundamentals and Applications

when ions collected are compared to atoms loaded; the value of this ratio is, of
course, element- dependent.^ For many elrements, samples of 1 nanogram or less
can be routinely analyzed provided a pulse-counting detection system is em-
ployed.
A new thermal ionization source developed by Olivares and coworkers at
Los Alamos National Laboratory is much more efficient at production of ions
than the more conventional filament configurations described previously [37].
Designed originally for use in isotope separators, it has been modified for use in
isotope ratio measurements using a quadrupole mass spectrometer. Called a
thermal ionization cavity, it is designed around a tungsten tube that contains the
sample and is heated by electron bombardment. A drawing of this source is given
in Fig. 1.5. Because of the amount of heat generated the source must be water-
cooled, making it more complex than conventional designs. Ionization efficiencies
are very high in comparison to those of normal thermal ionization sources.
Efficiency for europium is 72%, for uranium 8%, and for thorium 2%, all figures
an order of magnitude or more better than single-filament results. Typical preci-
sion for isotope ratio measurements is quoted as 20.1%.

The two most common types of detection systems used in isotope ratio measure-
ments are Faraday cups and pu~se-counting electron multipliers, Multipliers are
occasionally used in cu~ent-inte~ation mode but are not as precise as Faraday
cups or as sensitive as pulse counting; there seems to be no compelling need for
this mode of operation in thermal ionization, All these collectors, of course, are
used in many different kinds of mass spectrometry. Faraday cups give better
precision and require more sample than pulse counting; choice of collector is often
dictated by the demands of the experiment (or, of course, by available equipment).
Each type of collector has its strengths and weaknesses. Constructing a Faraday
cup that is quiet and linear is not a simple task. Pulse counting, in. which each ion is
counted as it arrives at the collector, requires fast electronics that must be very
quiet; typical background counting rates in good systems are about one count a
minute, approximately the cosmic ray background. An example of a pulse-
counting detection system is shown schematically in Fig. 1.6. Multipliers are
operated at negative potential for detection of positive ions; 2-4 kV is typical in
pulse-counting applications. Each dynode is successively nearer ground than its
predecessor, so the potential gradient across the detector causes secondary elec-
trons to travel through it from the first dynode to the’last; transit times of 1 nsec or
so are typical. Ions strike the first dynode of an electron multiplier; each such
impact generates two or more electrons, resulting in amplification of the original
signal. The electron cascade passes from one dynode to the next, each stage
effecting amp~fication of the original signal, with the pulse from the last dynode