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Agilent 7500 ICP-MS ChemStation Operator’s Manual
Tuning
The ShieldTorch system (Cool plasma)
Advantage
• Analysis of elements which suffer interference from polyatomic ions due to Ar
and C.
Ar --- Ca (Ar), K (ArH), Cr (ArC), and Fe (ArO)
C --- Mg (C
2
), Cr (ArC)
• Analysis of some lighter elements, such as Li, Na, Mg, and Al. Under cool
plasma conditions, the sensitivity of lighter masses increases and the background
due to the sampling cone is minimized due to the elimination of secondary
discharge.
Disadvantage
• Matrix effects are more severe.
• Difficult to analyze the elements, which have higher boiling points, higher
ionization potentials or higher bond energies with oxygen. The cool plasma
conditions prevent decomposition of sample in the plasma. Some elements,
which easily form oxides are not decomposed well. Bond energy is a good
indicator of whether oxide formation will occur, if the bond energy with oxygen
is high and the ionization potential of the oxide is low, you will see the oxide ions,
e.g. bond energies of REEs with oxygen are higher than 150 kcal/mol. Si-O bond
energy is 193 kcal/mol, so it is also difficult to decompose. Bond energy for Ba-O
is 130 kcal/mol and the BaO and BaOH ions are commonly found even at lower
concentrations. For higher boiling point elements such as W, Ta and Mo,
ionization might be difficult under cool plasma conditions. The bond energy of
these elements with oxygen is also quite high.
• Generation of polyatomic ions. Although the polyatomic ions, which have higher
ionization potentials, can be decreased by the ShieldTorch system with cool
plasma conditions, the polyatomic ions, which have lower ionization potentials,
are drastically increased. In particular, the polyatomic ions due to Ca are
generated more than without the ShieldTorch, and these interfere with many
isotopes for transition metals such as Fe and Ni. Therefore, the ShieldTorch
should not be used for environmental samples, which normally contain a lot of
Ca.
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