Page 113 - Inorganic Mass Spectrometry : Fundamentals and Applications

I~ductively Cou~l~d Plasma Mass ~~ectromet~ 103

44Ca+ (2.1 %), suffers from a spectral overlap with CO;. The third most abundant
isotope of calcium, 4Ta+ (0.65%), suffers from a spectral overlap with ArHi. The
main isotope of iron, 56Fe+ (91.7%), is at the same mass as the intense ArO+
signal. The next most abundant isotope of iron, s4Fe+ (5.8%), suffers from a
spectral overlap with ArN+. The third most abundant isotope of iron, 57Fe+
(2.2%), is at the same nominal mass as ArOH+. Molecular ions consisting of Ar,
0, and sometimes H cause some degree of overlap with all of the isotopes of Cr,
Fe, Mn, and CO as well as the most abundant isotope of Ni, as can be seen in Table
3.3. The main isotope of sulfur, 32S+ (95.0%), occurs at the same nominal mass as
0;. Both minor isotopes of S suffer from spectral overlaps with N or 0 containing
polyatonic ions. The main isotope of silicon, 2SSi+, is at the same nominal mass as
N2+ and both minor isotopes also suffer from overlaps with molecular ions.
Phosphorus is monoisotopic at the same nominal mass as NOH+ and 15NO+. The
two most abundant isotopes of selenium suffer from overlaps with Ar2+ ions
(38A1.40Ar+ and 40Ar2+).
The choice of acids to use in sample preparation for ICP-MS can also be
affected by potential spectral overlaps. High concentrations of chloride ions in
solution, from HC1, for example, result in Cl-containing molecular ions that
produce intense signals. Arsenic, which is monoisotopic (75 daltons), suffers from
a spectral overlap with ArCl+. Analysis of V is also severely affected. The main
isotope of vanadium, (99.8%), has the same naminal mass as 3sC1160+. "he
only other isotope of vanadium, 50V+ (0.2%), suffers from a spectral overlap with
36Ar14N+ or 3sC115N+. Manganese, which is monoisotopic, suffers from an inter-
ference with 37C1180+. The main isotope of chromium, s2Cr+, is at the same mass
as ClOH+. Both Ga isotopes overlap with a Cl containing molecular ions, All of
a
the germ~um isotopes except the minor isotope 76Ge+ overlap with Cl contain-
ing molecular ion.
Use of sulfuric acid also leads to many spectral overlaps with the most
intense molecular ion signals, including SO+, SOH+, S;, S02H+, and SO2+.
Elements including Ti, V, Ni, Zn, and Ge suffer from molecular overlaps as a
result. Phosphoric acid also leads to molecular ions, including POH+, P2+,
and PO;.

Spectral Overlaps Due to Oxide and Hydroxide Species

Monoxide (MO') and hydroxide (MOH+) ions, where M can be any one of many
elements, are observed in ICP-MS [140]. Typically the molecular oxide or mo-
lecular hydroxide signals are small (<3%) relative to the elemental ion signal.
However, if one is trying to measure a small concentration of one element in the
presence of a high concentration of a second element that forms a molecular oxide
or hydroxide ion at the same mass as an analyte, the problem can be severe.
is
Furthermore, the molecular ions may overlap with an elemental ion isotope that