Sd. Tanner et al., GAS AND ION DYNAMICS OF A 3-APERTURE VACUUM INTERFACE FOR INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY, Applied spectroscopy, 48(11), 1994, pp. 1373-1378
The equations describing the pressure, density, and temperature charac
teristics of isentropic flow and of the formation of a shock structure
due to the sudden termination of the directed motion of a flowing pla
sma are reviewed. The results are applied to describe the flow charact
eristics of a novel ICP-MS vacuum interface which consists of three ap
ertures: a conventional sampler and skimmer and a third aperture conta
ined in a blunt support which is normal to (or nearly normal to) the a
xis of the primary expansion through the sampler and skimmer. The flow
through the interface apertures is characterized as continuum, effusi
ve, or transitional, and the impact of these forms of expansion on the
ion dynamics (kinetic energies and plasma neutrality) is examined. A
shock wave may form in front of the third aperture. The effect of this
flow disturbance on the gas and ion dynamics in the vicinity of the a
perture is discussed. Experimental neutral and ion flow results are co
mpared to the theoretical predictions. It is concluded that the plasma
retains its charge neutrality as it flows through the sampler and ski
mmer and, under the conditions studied, also through the shock and sub
sequent expansion through the third aperture. The gas behind the shock
flows across the surface of the blunt tip of the third aperture, and
the aperture itself may be offset from the axis of the original expans
ion to eliminate clogging of the aperture by unvaporized particles and
condensed salts from the plasma and to prevent source plasma photons
from contributing to the background signal continuum. The reduction in
the ion current introduced into the ion optics region of the mass spe
ctrometer reduces the magnitude of the space charge field and results
in a gain in ion transmission efficiency which offsets the reduction o
f the ion flow.