Page 98 - Inorganic Mass Spectrometry - Fundamentals and Applications

88 Olesik

sample is carried as small particles. These particles can be formed by self-
of
nucleation or by addition of physical carriers, such as high concentrations NaCl.
Several groups have investigated the use of chemical modifiers and their effect
on analyte transport [76-781.
Detection limits are typically less than picograms per milliliter (pg/mL,)
[79,80] with absolute detection limits as low as 2 attograms [81]. Often standard
addition calibration and a good knowledge of the chemistry of the sample in the
vaporizer are required for successful use of ETV-ICP-MS.
Direct SampZe Insertion. In direct sample insertion (DSI) [82], the sample
is placed on a rod, metal loop, or cup on a rod. After desolvation (by inductive
is
heating of the rod or use of a heat gun), the sample inserted into the plasma. The
advantages of the DSI system include nearly 100% sample transport efficiency
into the ICP and use of a single power source. The most exciting capability of DSI
of
is preconcentration using aerosol deposition that can provide two orders magni-
tude of improvement in ICP-MS detection limits [83]. Detection limits as low as
0.06 parts per trillion were obtained.

Other Solid Sample Introd~cti~n Systems. Arc discharges (continuous
to
of
discharges between two electrodes, one which is the sample) have been used
generate dry aerosols from conducting samples [84] and powders mixed with a
conducting matrix. Spark discharges (short, pulsed discharges) have also been
used for solid sampling into ICP-MS [85,86]. A unique system to introduce
powders quantitatively directly into the ICP at a controlled rate has also been
described [%',SS].

The interface used today between the atmospheric-pressure plasma and the low-
pressure mass spectrometer is based on a differentially pumped two-stage inter-
face similar to those used for molecular beam techniques [89-9 I]. The key to
successful development of ICP-MS inst~ments was the use of a relatively large
(~1-mm-diameter) sampling orifice so that continuum flow was attained with an
unrestricted expansion of the plasma to form a free jet, When small orifices were
used, a cold boundary layer formed front of the orifice, resulting in substantial
in
cooling of the plasma, including extensive ion-electron recombination and mo-
lecular oxide formation. The smaller orifices were also susceptible to clogging.
Plasma gas (overall neutr~) flows through the sampling orifice and expands
of
in the first stage of the mass spectrometer (at a pressure a few torr). Assuming
ideal, neutral gas flow, approximately 2 atm Umin [at standard temperature and
pressure (STP)] of gas flows through the sampling orifice [92]. Therefore, most
of
the analyte ions from the center channel of the ICP pass through the sampling
orifice.