Page 90 - Inorganic Mass Spectrometry : Fundamentals and Applications

80 Olesik

flows across the transducer surface. This has three important implications: No
turbulent gas is needed to produce the aerosol, so turbulence induced aerosol
losses and droplet-droplet coagulation in the spray chamber should be reduced
compared to those of pneumatic nebulizers. The flow rate of the gas used to carry
the aerosol to the ICP can be optimized independently of the aerosol generation
process. Finally, there are no small orifices to become clogged.
Most ultrasonic nebulizers use a somewhat larger sample uptake rate (2-3
~/min) than pneumatic nebulizers. Typically the spray chamber and/or a tube
following the spray chamber is heated to evaporate water partially from the
aerosol, Because the aerosol transport efficiency is higher when an ultrasonic
nebulizer is used, particularly with a heated spray chamber, a system to remove
is
solvent (typically a condenser and/or membrane separator) essential to prevent
deleterious cooling of the ICP by excess water.
The combination of the ultrasonic nebulizer, heated spray chamber and
condenser/desolvator leads to improvements in detection limits by a factor of
about 10 compared to that of a pneumatic nebulizer without a desolvation system.
This is the main reason ultrasonic nebulizers are used despite their higher cost
(approximately U.S. $15,000 in 1998).
There are several drawbacks to ultrasonic nebulizer/desolvation systems.
Precision is typically somewhat poorer (1% to 3% relative standard deviation)
than for pneumatic nebulizers (0.5% to 1.0% relative standard deviation) and
washout times are often longer (60 to 90 sec compared to 20 to 30 sec for a
pneumatic nebulizer/spray chamber without desolvation). Furthermore, chemical
matrix egects are dependent on the amount of concomitant species that enter the
ICP per second. Therefore, use of any sample in~oduction device that increases
the amount of sample entering the plasma per second also naturally leads to more
severe matrix effects when the sample contains high concentrations of concomi-
tant species.
~esoiv~tio~ Systems

Desolvation systems can provide three potential advantages for ICP-MS: higher
analyte transport efficiencies, reduced molecular oxide ion signals, and reduced
solvent loading of the plasma. Two different approaches have been used for
desolvation in ICP-MS. The heated spray chamber/condenser combination has
been discussed; it is the most commonly used system. The extent of evaporation of
the solvent from the aerosol and cooling to reduce vapor loading varies from
system to system. The second approach is the use of a membrane separator to
remove solvent vapor before it enters the ICP.

~eate~ Spray C~a~~ers. The use of a heated spray chamber to evaporate
the aerosol partially leads to reduction in drop size and therefore higher analyte
transport efficiencies. Often the drying of the aerosol droplets is incomplete.