Agilent Technologies 7500 ICP-MS Operator's Manual page 96

Agilent 7500 ICP-MS ChemStation Operator's Manual
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If the electron density is 1.47 10 cm and the electron temperature is 6,680K,
the ionization efficiency of elements will be as shown in the next figure. Elements
that have lower ionization potentials such as Li, Na, and Lanthanoids can be ionized
with nearly 100 % efficiency in the plasma. On the other hand, the elements that
have relatively higher ionization potentials such as As, Se, and Hg are ionized about
30 to 50 %. Ar can be ionized only about 0.009%. So, it is assumed that polyatomic
ions exist in the Ar plasma under normal plasma conditions, and they are detected
even when the ShieldTorch system is used.
Ionization Efficiency of Elements in an Ar Plasma
However, the polyatomic ions are observed under normal plasma conditions with
the ShieldTorch system: 1200W RF power and about 1.2 l/min carrier and blend gas
flow. To reduce the level of polyatomic ions the plasma temperature must be
reduced.
When reducing the RF power cools the plasma temperature or increasing the carrier
gas flow, the ionization efficiency of elements changes as shown in the next figure.
Upon decreasing the electron temperature (by decreasing the plasma temperature),
lower ionization potential elements such as Li, Mg and Fe are still ionized at nearly
100%. On the other hand, the ionization efficiencies of elements with higher
ionization potentials such as Cl and Ar are drastically reduced. So, the ionization
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