There are three factors affecting the transfer of energy from the RF
coil:
• Concentricity and alignment - these are very important in producing
a correctly shaped plasma, which consistently needs to be located
in the same position.
• Correct dimensions - being part of the RF system, the coil
dimensions are also important in the tuning of the circuit. Slight
changes in dimensions can produce small changes in resistance
or inductance. These changes can also lead to variations in the
shape of the plasma and affect energy transfer from the plasma
to the sample.
• Base metal/plating - the most efficient conductor of RF energy
is pure silver followed by annealed copper. There is very little
difference in the conductivity of these materials but there is a great
deal of difference in cost. Copper is used as the base material
of all commercially available coils but different manufactures use
different plating materials. Silver has the best conductance and
gold the best corrosion resistance.
The next step is to transfer the energy from the plasma to the aerosol
sample. There are two main factors that affect this transfer.
• Argon gas flow rates and quality. Obviously, high purity argon
is required and is generally available although it can be quite
expensive, especially is some countries. Higher nebulizer argon
flow rates will shorten aerosol residence time and can also cool the
plasma, both of which lead to lower intensities.
• The torch containing the plasma. The torch needs to be concentric
and of the highest quality quartz. If the outer tube becomes
devitrified, the transfer of energy is restricted due to losses incurred
by heating on the surface of the outer tube, thereby reducing the
available energy for the atoms and ions.
Why Choose a Glass Expansion Coil?
Glass Expansion coils are produced from the highest quality and
purity raw materials and plated using proprietary methods to obtain
maximum RF transmission efficiency. Examples of copper, gold and
silver Glass Expansion RF coils to suit the Agilent 7900 ICP-MS are
shown in Figure 2.
Application Spotlight
heating and cooling in the torch box does eventually lead to the
breaking down of the PTFE coating and exposes the silver plated
coil to the atmosphere in the torch box.
There are a couple of other important factors in the plating process
that will have major effects on the conductivity of the coil. Firstly, the
purity of the metal used for plating, and secondly how these layers
are applied.2 There are very few platers that use high purity gold and
high purity silver. Most use a brightener in the solution so the end
product has a shiny finish without the need to polish. A pure silver
or gold plated finish will be dull in appearance (there are a number
of coils now available from OEM’s that have a dull silver finish). The
coils Glass Expansion provides all use high purity gold and silver
and are all physically polished after plating to give the smoothest
finish to the outside of the coil reducing resistance.2 A side benefit
is the enhanced visual appeal of a bright and shiny finish. The more
important aspect of plating is how it is applied and what is done to
the coils during the process. The proprietary process used by Glass
Expansion ensures the plating has the highest uniformity and purity.
Figure 1.
Energy Transfer from the Coil
The RF coil is the last component in the RF system of your ICPOES
and ICP-MS that is used to generate a plasma. Therefore, the
condition and shape of the RF coil can have a major effect on the
efficiency of transferring energy to the plasma.
Being exposed to the environment of the torch box, the RF coil is
susceptible to corrosion. The greater the corrosion, the larger amount
of energy is needed to produce a plasma of the same power. This
transfers undue stress on the rest of the electrical components used
in the RF system of your ICP-OES and ICP-MS, possibly contributing
to premature failure.
Older ICP’s used a power amplifier (PA) tube as part of the RF
system. Excessive stress on this component would result in a service
call of several thousand dollars. Newer ICP’s utilize a solid state RF
system which, although more modular, would result in an even more
expensive service call to replace. Regular replacement of your RF
coil reduces the load on the RF generating system and ensures your
ICP is running optimally. The suggested rate at which you should
replace your RF coil is every 6 months for ICP-MS and every 2
years for ICP-OES. Depending on the laboratory environmental your
instrument may require more or less frequent replacement. The RF
coils pictured in Figure 1 are well past the point of replacement. Coils
in this condition increase the stress applied to other components in
the RF generating system and can shorten the lifetime of your torch,
in addition to the power amplifier or solid state generator.
Figure 2. RF Coils to suit the Agilent 7900 ICP-MS
www.geicp.com Glass Expansion Newsletter | Issue 44 2
/www.geicp.com