Page 80 - Inorganic Mass Spectrometry : Fundamentals and Applications

70 Olesik

diffuse outward as they travel though the plasma toward the sampling orifice
of
the
mass spectrometer.
Analyte ions generated in the plasma pass through the sampling orifice (Fig.
3.2) and then the gas expands. A fraction of the gas is sampled through the skim-
mer. A positive ion beam is formed as the electrons di&se and charge separation
occurs. A series of ion optics and typically a radio frequency- (rf-) only quadru-
pole lens focus positive ions into the mass spectrometer. Ions a particular mass-
of
to-charge ratio exit the mass spectrometer and are detected. Many the comrner-
of
cia1 ins~ments use dual-mode (pulse counting and analogue) detection in order
to provide an extremely wide dynamic range.

is a
The inductively coupled plasma (ICP) flowing, partially ionized gas (typically
Ar). The ICP is sustained in a quartz torch that consists of three tubes (Fig. 3.3).

Plasma

Ar + sample
Typical quartz plasma torch positioned within a (induction) load coil. The
center (injector) tube can be made of quartz or inert materials (alumina, platinum, or
sapphire) to allow corrosive samples (including those containing hydrofluoric acid) to be
introduced into the plasma. One-piece quartz torches, torches with demountable injector
tubes, or completely demountable tubes are used.