Page 117 - Inorganic Mass Spectrometry : Fundamentals and Applications

Inductively Coupled Plasma Mass Spectrometry l07

particularly the addition of 5% or less N, to the plasma, can also reduce the
of a
molecular ion signals. A combination of reduced water loading and use mixed
gas plasma can be particularly effective [ 1431. The addition of CH,, CHF,, and
C,H, has also been reported to reduce the ArCl+, Ar2+, ArO+, ClO+, and MO+ ion
signals dramatically [ 144,1451.
Cool or Cold Plasma. COO^^^ or “cold” plasma conditions can be used to
reduce the magnitude of the Ar containing molecular ions whereas the sensitivities
for many elements with ionization potentials less than 6 eV are not significantly
affected [ 10,1461. By using low powers (600 W) and high nebulizer gas flow rates
(1.0 L/min or greater) signals due to several ions that are dominant under normal
plasma conditions (Fig. 3.1%) can be reduced. The Ar+ signal can be reduced by
six orders of magnitude or more, ArH+ can be reduced by four orders of magni-
tude, and ArO+ can be reduced by three orders of magnitude or more [ 1471. As a
result, detection limits for K, Ca, and Fe can be dramatically improved. The
signals from other ions, including H30+, NO+, and 02+, increase dramatically, to
as shown in Fig. 3.15b. To attain
become the dominant ions in the mass spectrum,
“cold” plasma conditions that are analytically useful, the plasma potential must
be low enough to prevent discharge formation. This can done either by using an
be
instrument with a balanced or interlaced load coil or by using a grounded electrical
shield between the load coil and the torch to reduce capacitive rf coupling.
Use of “cold” plasma conditions is not recommended for elements with
high ionization potentials, elements that form refractive oxides, or samples with
total dissolved solid concentrations greater than about 50 ppm, Sensitivities for
elements with ionization potentials between 6 and
8 eV are up to 100 times lower
under “cold” plasma conditions than under normal plasma conditions, and ele-
ments with ionization energies greater than 8 eV exhibit sensitivities that are
several orders of magnitude lower under “cold” plasma conditions. Analyte
oxides are much more readily formed under “cold” plasma conditions. For
example, the formation of scandium oxide results in a Sc+ ion signal that is several
orders of magnitude lower under “cold” plasma conditions than under normal
conditions [ 1471, In some cases it is possible to attain better detection limits for the
analyte oxide ion, MO+, than by measuring the elemental analyte ion, M+. Finally,
ICP, and are therefore similar for light
chemical matrix effects that originate in the
and heavy mass analytes, are severe under “cold” plasma conditions. Changes in
the extent of ionization due to the addition of efficiently ionized elements occur
because the plasma temperature and electron number density much lower than
are
under normal plasma conditions. Under “cold” conditions the addition of large
concentrations of efficiently ionized elements affects the electron number density
in the plasma and results in significant decreases in the number of analyte ions
produced. As a result, “cold” plasma conditions are useful mainly for very clean
smples, such as acids used in the semiconductor industry.