Torches are typically made of materials that are transparent to the RF radiation. Therefore, they do not attenuate the field generated by the load coil/antenna. Torches can be made of materials such as ceramics or boron nitride; however, by far, most are made from quartz, which has a sufficiently high melting point to allow it to maintain its configuration when operated at temperatures commonly experienced with argon ICPs.
A simple quartz tube centered in the load coil with laminar flow argon gas passing through it at 10-20 L/min will form a simple plasma, when RF power is applied and seed electrons are provided.The plasma or "fireball" that forms will have the shape of a prolate spheroid. If an excessive amount of power is applied, the plasma will reach a sufficiently high temperature to melt the quartz tube, which confines it.
A simple quartz tube centered in the load coil with laminar flow argon gas passing through it at 10-20 L/min will form a simple plasma, when RF power is applied and seed electrons are provided.The plasma or "fireball" that forms will have the shape of a prolate spheroid. If an excessive amount of power is applied, the plasma will reach a sufficiently high temperature to melt the quartz tube, which confines it.
This configuration is not fully suitable for analytical spectrometric purposes. It is difficult to efficiently inject a sample aerosol into this type of plasma because of the "skin effect" created by the potential barrier at the surface of the plasma.
This barrier tends to deflect aerosol particles around the outside of the spheroid, rather than entrain them in the plasma. Unless appreciable sample can reach the optimal excitation region of the plasma, analytical sensitivity will be significantly limited.
This torch consists of three concentric quartz tubes.
A coolant gas (argon) is introduced into the space between the outer and center tubes at a tangential direction relative to the longitudinal axis of the torch, creating a vortical flow. This gas stream serves two purposes:
1. It isolates the plasma from the internal wall of the outer quartz tube preventing melting, and
2. It encourages the formation of a toroidal (annular)-shaped plasma.
The center tube is for the injection of sample aerosol into the plasma.
The space between the injector and the intermediate tube is used for the introduction of an auxiliary flow of argon gas to assist in the formation of the plasma and to ensure that the plasma is forced away from the tip of the injector, preventing it from melting.
This auxiliary gas flow is usually very low compared to the coolant gas flow and often is not used. Typical argon gas flow rates for the various inputs are listed below.
Typical Operating Conditions for an Argon ICP
Power-----------------------1-2 kW
Argon flow
Coolant-----------------------15 L/min
Auxiliary---------------------0-2 L/min
Nebulizer---------------------1 L/min
Sampling depth*--------------14-18 mm (* Beyond last turn of load coil).
These flow rates can vary depending on the specific dimensions and configuration of the torch being used.
Sampling depth*--------------14-18 mm (* Beyond last turn of load coil).
These flow rates can vary depending on the specific dimensions and configuration of the torch being used.
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