Page 91 - Inorganic Mass Spectrometry - Fundamentals and Applications

I~ductivel~ Coupled Plasma Mass Spectrometry 81

Typically, either the walls of the spray chamber have been heated or, in the case
of the Mistral system from VG Elemental, infrared radiation heating is used [25].
Microwave heating of the aerosol seems to be inefficient. It is unlikely that the
aerosol is completely dried in these systems.
Using a microcentric nebulizer at low flow rates (typically about 50
pLlmin), a heated spray chamber, and a heated rnicroporous membrane desolva-
tor, the Cetac MCN-6000 system can provide analyte transport efficiencies of
50% to 90%. This system is made completely of HF resistance materials.
Recently, Legere [26] presented some very interesting results using a spe-
cially designed spray chamber to heat a secondary gas before it enters the spray
chamber and then efficiently mix that
gas with the aerosol produced by the nebu-
lizer. Because the aerosol “jet” typically entrains large volumes of gas, the addi-
tional, heated gas can be very efficiently mixed with the gas from the nebulizer in
the
of
order to promote rapid heating and evaporation sample aerosol. The goals of
this design include virtually complete vaporization of liquid aerosol, prevention of
aerosol from striking the walls of the spray chamber, and 100% analyte transport
efficiency. It appears that the liquid water is almost completely removed from the
aerosol for sample uptake rates up to 300 pL/rnin (then the heated Ar becomes
saturated with water vapor and the dew point is reached). A Nafion membrane
desolvator is used to remove most of the water vapor before it enters the ICP. As
the sample uptake rate is increased from 25 to 250 pL/min, the ICP-MS signal
increases by a factor of 10 (in stark contrast to the data shown in Fig. 3.8). This is
strong evidence that the analyte transport efficiency is constant and virtually
loo%, even at a sample uptake rate of 250 pLlmin. As a result, sensitivities are
enhanced by about a factor of 10 compared to those of a conventional pneumatic
nebulizerlScott spray chamber used with an uptake rate of 1 mL/min. Washout
times are also very fast.
Condensers to Remove Solvent Vapoz The use of a condenser to cool the
vapor and remove by condensation on the walls of the condenser is one means to
it
reduce solvent vapor loading of the ICP. However, this approach entails a prob-
lem: the solvent recondenses on the desolvated particles as well. In an attempt
to
get around this problem and to remove
as much solvent vapor as possible, Houk et
al. E271 have used a three-stage cryogenic desolvation system. “he sample aerosol
passes through a heated spray chamber, a condenser at 0°C to - 10°C, and a
cryogenically cooled condenser, and then goes through three cycles of heating and
cryogenic cooling before entering the plasma. Although this is an elaborate and
inconvenient system, signals from polyatomic ions including ArO+, ClO+, and
ArCl+ are reduced by a few orders of magnitude. This reduction significantly
enhances detection limits for Fe, V, and As [2’7].

~em~ra~e Separators. Solvent vapor can be removed from the sample
carrying gas by establishing a concentration gradient across a membrane. The