Page 104 - Inorganic Mass Spectrometry - Fundamentals and Applications

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tage. Dramatic [l 191 reduction, up to nine orders of magnitude, of Ar+ and
be
molecular ion signals was observed. Analyte ion sensitivities could maintained
or increased while “continuum” background count levels were decreased to less
than 1 countfsec. Iron detection limits of 0.3 ppt were reported. The background
as
preventing lower detection limits appeared to be iron in the blank water supply
the background spectrum showed an isotope pattern matching that of Fe. Other
reported detection limits included 1 ppt As, 1 ppt Ca, 1 ppt K, and 10 ppt Se.
Several important concepts for successful operation of the dynamic reaction
cell were described [ 1 181. The contribution of the rf field of the quadrupole to ion
of
energy must be considered together with the thermodynamics the desired (and
undesired) ion-molecule reactions. This is because the rf field can impart kinetic
energy to the ions in the cell. Endothermic reactions could proceed at significant
rates if sufficient ion energy were available the ion-molecule reaction. The cell
for
pressure and rf field must be properly controlled in order to obtain an appropriate
number of reactive collisions in the cell. A balance between promotion of desir-
of
able reactions at sufficient rates and unwanted scattering losses analyte ions or
loss of ion transmission due to nearly complete thermalization (so that ions no
longer have a significant velocity along the beam axis) must be struck. The
production of ions from single-step reactions and sequences of reactions must be
considered.
Discri~nation against undesirable reaction product ions is important. The
qua~pole cell can be used in a bandpass mode to remove the product ions of
inte~ediate reactions and thus intercept the series of sequential reactions that
might otherwise introduce new spectral interferences. The selection of the appro-
priate reaction gas and cell operating conditions must be assessed, depending on
the analyte ion of interest and the spectral overlap ions to be removed.

uadrupole mass spectrometers were used in both the early ICP-MS ins~ment
development research and the first commercial instruments.
To date, quadrupole-
based ICP-~S inst~ments continue to be predominant, During the last several
years, ICP ion sources have been coupled with mass spectrometers of several
different designs, including double-sector, single-sector, time-of-flight, ion trap,
and Fourier transform ion cyclotron resonance.
is
The operation of the quadrupole mass spectrometer only briefly described
here. Excellent descriptions are available in the literature including an introduc-
tion to quadrupole mass Spectrometers [ 12 1,1221, analysis of the mathematics
associated with quadrupole [ 1231, and extensive treatises [ 124,1251.
The quadrupole mass filter (Fig, 3.13a) passes ions with a particular range
of
mass-to-charge ratios. Four precisely made rods are mounted with stringent
(10-pm) tolerances. Opposin~ pairs of rods are connected to a radio frequency