Page 92 - Inorganic Mass Spectrometry - Fundamentals and Applications

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sample vapor travels inside the membrane tube and dry, sweep gas is passed in a
tube surrounding the membrane. The removal of the solvent is driven by a concen-
tration gradient between the inside of the membrane tube, where the sample and
solvent vapor are, and the outside of the tube, where the sweep gas flows. The
advantage of this approach is that solvent vapor can be removed without the
deleterious effects of cooling with a condenser. The use of membrane separators
for solvent removal in ICP-MS was first described in 1990 [2&,29].
Two membrane types that operate on different principles have been used in
commercially available membrane separators: microporous membranes and se-
lectively permeable, nonporous polyimide or Nafion membranes. The micro-
porous Teflon PTFE membrane can be used to remove water vapor or organic
solvent vapor. Any gaseous component, inclu~i~g volatile ~~aZytes such as Hg, is
pa~ially or extensively removed. The sweep gas flow rate is typically similar to
the sample carrier gas flow rate,
by
In contrast, Nafion removes water vapor a process calledperevaporation.
Water is absorbed onto the walls of the Nafion, moves through the walls, and
evaporates into the sweep gas [30]. As a result, volatile analytes should not be lost
through the membrane. The efficiency of removal depends on digusion of water
vapor to the walls of the membrane. When the water vapor load is significantly
less than the dew point, the efficiency also improves as the temperature of the
dryer is reduced [3 l]. This membrane separator does not remove organic solvent
vapor from the Ar gas stream, but it also does not suffer from loss of volatile
analytes.
is
A nonporous aromatic polyimide membrane that selectively pemeable to
has also been used for water vapor removal before the sample
enters the ICP-MS [32]. Molecular analyte oxide ion signals were reduced ap-
proximately two orders of magnitude and 0-containing polyatomic ions, such as
Ar0+ and C10+, were reduced by one to two orders of magnitude.
Although the membrane separators are useful for solvent vapor, care must
be taken to prevent fouling the membranes with liquid solvent, as occurs when
incompletely desolvated aerosol enters the membrane separator.

irect injection Nebulizers
Direct injection nebulizers are operated without a spray chamber. Instead, the
of
concentric, pneumatic nebulizer is positioned just below the plasma and 100%
the sample is sprayed into the ICP. The nebulizer replaces the center tube of the
ICP torch. The direct injection nebulizers are particularly attractive for the anal-
ysis of small sample volumes and for elements, such as Hg, I, and
surfaces in the spray chamber and therefore have long washout times when a
is
spray chamber is used. The dead volume the direct injection nebulizers small,
of
so they have often been used to couple capillary separations with ICP-MS detec-