Page 93 - Inorganic Mass Spectrometry - Fundamentals and Applications

Inducti~ely Coupled Plasma Mass Spectro~et~ 83

tion [33] and to increase sample throughput rates [34]. One of the disadvantages of
the direct injection nebulizers is that molecular oxide ion/elemental ion signal
ratios are higher by about a factor of 3 [35]. The sample liquid uptake rate to the
nebulizer is limited to 120 p,L/rnin or less to prevent solvent overloa~ng of the
ICP.
Two different kinds of direct injection nebulizers are available commer-
al. [36] for
cially. The total consumption nebulizer was developed by Greenfield et
ICP optical emission spectrometry. The concept for the Cetac direct injection
nebulizer (DIN) was developed by Fassel, Houk, and coworkers [35,37]. It has a
narrow sample-carrying capillary [30-50 pm inner diameter (id.), 0.5 to 1 m
long] that extends slightly past the nebulizer gas tube. A second, auxiliary or
makeup, nebulizer gas is introduced through another concentric tube outside the
nebulizer gas tube. A gas displacement pump (up to 1500 psi) or HPLC pump is
used to deliver the sample to the nebulizer through the long, narrow capillary.
The second type of direct injection nebulizer, called the direct injectio~
(DIHEN), is a specific type
~ig~-e~ciency ne~~lizer of the 81
that is inserted into the ICP torch in place of the center, inj in
advantage of the DIHEN compared to the Cetac DIN is that a high-pressure pump
is not needed to deliver sample to the nebulizer. An unusually low nebulizer gas
n) and high ICP power (1.5 kW) were found to provide
itivity when ~IHEN is used [38],

eneration Sample ~nt~o~u~tion
Several elements (including As, Bi, Ge, Pb, Sb, Se, Sn, and Te) form volatile
hydrides when reacted with sodium borohydride at room temperature.
ducing the analyte as a volatile hydride, high-transport efficiencies, and therefore
improved detection limits, can be achieved. Often as importantly, much of the
sample matrix is not introduced into the ICP because those species do not form
volatile compounds. Commercial hydride generation sample introduction systems
are available.
Continuous, batch, and flow injection modes of hydride generation have
been used successfully [39-411. In the commonly used continuous mode the
sample and sodium borohydride solutions are pumped by using a dual-channel
peristaltic pump into a mixing chamber. The volatile hydride gas and hydrogen are
carried into the plasma with a flowing argon gas and the excess liquid is directed
to the drain.
for
Key experimental and chemical considerations are necessary successful
use of hydride generation [39,40]. The reaction to form the volatile hydride may
be highly species- as well as element-dependent. Therefore, the analyst must be
aware of the chemistry and realize that the response may be species-dependent.
The signal may not be directly related to the total elemental concentration in the