Hardware manual

Technical Information 6
Agilent 7500 Series ICP-MS Hardware Manual 6-13
Now, consider the inside of the plasma. Since the plasma temperature is high,
about 7,000 K under the normal operating conditions, positive ions and
electrons exist in the plasma. The number of positive ions and electrons is the
same, so, the plasma is electrically neutral. Since the interface is cooled by
water, the plasma temperature decreases drastically when the plasma comes
close to the interface. So, positive ions and electrons cannot exist any more
and the neutral Ar atom is dominant (Figure 130) and it creates a sheath
between the interface and the plasma.
The sheath acts as a condenser and the plasma potential is grounded to the
interface and vacuum chamber through the sheath. Here the so- called
secondary discharge occurs, because the plasma potential is hard to ground.
The plasma potential is divided by two capacitances: one is between the
plasma and the RF coil and the second is between the plasma and the
interface. Behind the sampling cone, the ions and the electrons are oscillating,
and the movement of the electrons is much faster than that of the ions, so that
the RF potential is rectified.
The thicker the sheath, the smaller the capacitance. If the capacitance
between the plasma and the RF coil is constant, the plasma potential is
increased when the sheath becomes thick. The plasma potential can be
considered as the initial potential energy of the ions.
The kinetic energy of ions is expressed as (mv
2
)/2, where m is the mass of the
ion and v is the velocity of the ion. During the expansion process behind the
sampling cone, the speed and direction of all ions tend to become the same
since neutral Ar dominates them. When the ion speed is the same for all ions,
the kinetic energy of lighter masses is smaller than that of heavier masses.
Therefore the distribution of ion energy can be expressed as in Figure 131;
actual distribution will be narrower.
Since the Q- pole accepts a wide range of ion directions, the optimal lens
setting has a band range. When the ion lens is optimized for Y, other ions such
as Li and Tl can also be accepted by the Q- pole. When the plasma temperature
is reduced by reducing the RF power or by increasing the carrier gas flow, the
thickness of the sheath increases, which increases the plasma potential and
the potential energy of ions. Consequently the distribution of the ion energy of
each element shifts toward higher values. Then, the values of the optimal lens
setting for middle or higher masses become those for lighter masses
(Figure 131). And even lighter masses have enough ion energy to go through
the ion lenses, although many ions might diffuse, due to low ion energy, before
the lens. As a result, the sensitivity of lighter masses increases rather than
those of middle and higher masses. Actually, there might be other parameters,