Page 122 - Inorganic Mass Spectrometry : Fundamentals and Applications

l12 Olesik

Load Coil Load Coil
12

1 '
0 -5' i +.l 1 '4 8 l * l
-30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 30
Height above load coil (mm) Height above load coil (mm)
1'7 The eEect of center (nebuli~er) gas flow rate plasma temperature and gas
velocity profiles. Calculations were performed by using a simulation program, HiFI 2.0.
(From Ref. 424.)

a fixed sampling depth, an increase in nebulizer gas flow rate decreases the
distance in the plasma that the sample travels before reaching the sampling orifice.
~urthermore, for pneumatic nebulizers and conventional spray chambers without
desolvation, the aerosol and water vapor transport into the plasma increases with
increasing nebulizer gas flow rate. This tends to cool the plasma and delay
completion of vaporization and ionization.
A certain amount of time is required to desolvate, vaporize, atomize, and
ionize a drop of sample. If the ions are sampled too early, vaporization and
ionization of the sample may not be complete, F~he~ore, a si~ni~cant number
of incompletely desolvated droplets may survive to the sampling orifice [ 1571.
Even a small number of incompletely desolvated droplets can affect signals
sig~ficantly because each droplet can cool a 1- to 12-m-wide region of the
plasma [l 581, After the sample is completely vaporized and converted into ions,
the ions diffuse outward in the plasma, reducing the number ions that enter the
of
mass spectrometer [159,160]. The optimal sampling location depends on a trade
off between a distance far enough so that most of the sample aerosol has been
converted into ions and a distance not so far that diffusion reduces the number of
ions sampled by the mass spectrometer.
If the plasma power is increased, the location where the plasma begins
along the center axis moves upstream (closer to the injector tube of the torch) and
the plasma temperature increases. Therefore, if the nebulizer gas flow rate were
optimized at a power of 1.0 kW and the power were increased to 1.2 kW, ions
would be produced farther from the sampling orifice. There would be more
the
extensive diffusion of the ions before they reached sampling ori
be overcome by either moving the sampling orifice closer to the load coil or
increasing the nebulizer gas flow rate.