Page 99 - Inorganic Mass Spectrometry : Fundamentals and Applications

I~ductively Coupled Plasma Mass Spectrometry 89

The free jet foms downstream of the sampling orifice with a barrel shock
and Mach disk [93]. A skimer (also with a diameter of -1 mm) is placed
approximately two thirds of the distance between the sampling orifice and the
Mach disk for optimal sampling. Flow along the center line travels through the
skimmer to fom a directed beam into the second stage of the ICP-MS (0.1 to 1
mtorr). Although most analyte ions likely pass through the sampling orifice, only
ions from a narrow spatial region of the plasma (approximately 0.2- to 0.3-mm
wide) appear to reach the detector of the mass spectrometer [94,95].
It appears that the sampling process is mainly determined by neutral atoms.
of
This is reasonable because only a small percentage the plasma is ionized (Ar is
about 0.1% to 0.2% ionized). Therefore, there are orders of magnitude more Ar
atoms than any atoms of other species, including sample ions, in the plasma.
Moreover, it has been concluded that ion-molecule reactions are not a major
source of molecular ions observed in ICP-MS [92]. This conclusion is consistent
with theoretical calculations of collision rates [95]. Recently, Houk has reported
that theoretical calculations of the relative abundance of molecular ions in the ICP
itself are consistent with ICP-MS experimental observations [96].
If the flow of gas through the sampling and skimmer orifices is predomi-
nantly determined by neutral gas, then the analyte ions should be traveling at the
same velocity, regardless of mass. As a result, the ion kinetic energy increases
with the mass of the ion [97,98]. Ion kinetic energies vary with ICP power,
nebulizer gas flow rate, and MS interface pressure [99,100]. Ion kinetic energies
typically range from about to l0 eV for ions of mass 7 to 250. The spread of ion
3
kinetic energies for midmass ions is typically about 3.8 eV (full width half
maxim~m).

ss

Because of the higher mobility of electrons compared to much heavier elemental
ions and the electrostatic field produced by the ion optics designed to focus
positive ions, charge separation occurs so the overall neutral plasma gas beam
becomes a positive ion beam. This most likely occurs in the region near or just
after the skimmer [99,101- 1031. If the number of ions in the positive ion beam is
The
large enough, space charge defocusing occurs. ion current passing through the
[ 1041; therefore, space charge
skimer has been estimated to be as high as 1.5 mA
defocusing is significant.
Ion Optic Designs

The function of the ion optics is to maximize trans~ssion of ions from the
to
skimmer into the mass spectrometer while minimizing background due photons
of
to
or fast neutrals. The ion optics need focus ions with a relatively wide spread