Page 109 - Inorganic Mass Spectrometry : Fundamentals and Applications

Inductively ~oupled Plasma Mass Spect~o~etry 99

sample and solvent or plasma species. The vast majority of ions observed in ICP-
MS are singly charged. A few elements, such as Ba, that have a relatively low
second ionization potential, are observed as +2 ions, but their signal is small
(typically less than 2%) compared to that of the singly charged ion. Several
elements (including Na, P, Al, Sc, Mn, CO, As, U, Nb, Rh, Ag, and Au) have only
one isotope. Most elements have a few to several isotopes. Sn has l0 isotopes.

Spectral Overlaps Due to El~mental Ions

If the spectra consisted only of elemental ions, every element except In would
have at least one isotope that was free of spectral overlaps from other elemental
ions (Table 3.2). In most cases, the isobaric overlaps are easily predictable. The
contribution of the interferent ion to the signal measured at the mass of the analyte
a
by
elemental ion can be subtracted measuring the interferent ion signal at second
mass. The known isotopic distribution of the interferent element can be used to
deternine the contribution of the interferent ion at the analyte ion mass. Alter-
natively, a solution contai~ng only the interferent element can be measured in
at
order to determine the interferent signal the analyte mass and at another isotope
of the interferent element.
Spectral Overlaps Due to olecular ions from Plasma and
Solvent Species
a
Molecular ions present a more complex problem in ICP-MS. With combination
of molecular ion interferences and isobaric interferences, all the isotopes of the
of
analyte ion of interest may suffer from a spectral overlap. The molecular ion
signals can also be strongly dependent on the sample composition and experimen-
tal parameters. It is often more difficult to identify and correct for molecular ion
spectral overlaps than for isobaric overlaps. Because the resolution of the com-
mercial quad~pole mass spectrometers is 0.5 dalton or less, isotopic patterns,
rather than exact mass, must be used in an attempt to identify the interfering
molecular ion.
The molecular ion overlaps due to plasma and solvent species are most
severe below masslcharge 82, A high-resolution, double-focusing mass spec-
3.3) [3J.
trometer was used to identify molecular ions observed in ICP-MS (Table
Common molecular ions that produce intense signals from plasma and solvent
species include ArO+, ArOH+, ArH+, ArN+, Ar;, Ar2H+, O+, N,+, NO+, and 0;.
Other molecular ions become a problem at lower analyte concentrations. These
include CO;, C02H+, NO;, ArO+ ions with minor isotopes of Ar or 0, Arc+,
ArN;, and minor isotopes of Ar as Ar;.
Ca, Fe, S, Si, P, and Se are most severely affected by spectral overlaps in
ICP-MS. The main isotope of calcium, 40Ca+ (96.9%), occurs at the same mass as
the main isotope of argon, 40Ar+. The second most abundant isotope of calcium,