Page 116 - Inorganic Mass Spectrometry : Fundamentals and Applications

106 Olesik

being used for an isobaric interference correction. The molecular ion overlap in
this case would result in an overcorrection for an isobaric overlap.
Among the commonly observed spectral overlap problems due molecular
to
oxide and molecular hydroxide ions are those due to TIO+ (with 5 isotopes of TI
from mass 46 to 50) that result in overlaps with a minor isotope of nickel, (j2Ni+;
both isotopes of copper, (j3Cu+ and (j5Cu +; and the two major isotopes of zinc,
and 66Zn+. Calcium oxide and hydroxide ions overlap with all five isotopes
of nickel, both isotopes of zinc, and three of the four isotopes of iron. The analysis
of rare earth elements is particularly complicated by molecular oxide and hydrox-
ide ion spectral overlaps [ 14 1,1421.
There are several sources of information that can aid in identifying potential
spectral overlaps. Instrument manufacturers typically include spectral overlap
of
information in the instrument software. An atlas elemental ion spectra as well as
many of the molecular ions is available in a very convenient software package,
and
MS Interview, that was publis~ed in ~p~ct~ochimica Acta ~lect~oni~a is
at http://www.elsevier.nl~80/inc~omepage/sa~
available in the Program Library
sab (download file 47/1621/92 for the Apple MacIntosh version, file 48/1063/93
for the PC version). This program also allows users to add additional ions to the
spectral database.
~nfo~unately, the severity of the molecular ion overlaps can be more dif-
ficult to predict because the molecular ion signal magnitude can be very dependent
on the sample composition (including solvent and acid), the extent of solvent
loading, and inst~ment parameters. Drift in the polyatonic ion signals is often
more severe than for elemental ions.
Ion
educing the Severity of ~ole~ular Spectral ~verlaps
There are several ways to reduce the signals due to molecular ions in ICP-~~.
by
Oxide and hydroxide molecular ion signals can be reduced greatly decreasing
the amount of water that enters the ICP or by using mixed gas plasmas. “Cold7’ or
COO^" plasma conditions can be used but with some potentially severe trade-offs
of
including chemical matrix effects and loss sensitivity for analyte ions with high
ionization energies. Recently, reaction cells to remove the polyatomic ions that
cause spectral overlaps by reaction with a reagent gas in a several-torr cell be-
tween the skimmer and mass spectrometer have been described. Alternatively,
high-resolution instrum~nts can be used to separate many of the molecular ion
signals from elemental ion signals.

e
~ i Gas ~PZasmas. ~Water loading can be reduced by a desolvation
system (condenser or membrane separator) only if the vast majority of the water
can be removed. One way to eliminate the introduction of water into the plasma
during measurement of the analyte signals is with electrothermal vaporization,
laser ablation, or other direct solid sampling techniques. Mixed gas plasmas,