Fig. 20: Cut-out of a torch for ICP-OES with the various gases used.
The torch is made of 3 concentric tubes. Its aim is to allow plasma creation by giving the required speed of gas and also to contain the plasma and avoid its expansion. These 3 tubes are called the outer tube, inner tube and injector. Gas flowing between outer and inner tubes is called plasma gas, and the gas flowing between the inner tube and injector is called auxiliary gas.
The sample is introduced into the plasma through the injector. The plasma gas is used to create the plasma and its flow depends on the nature of the sample (content of dissolved solids, volatility of sample). Typical values are in the range 12-18 L/min. Auxiliary gas is used only for high total dissolved solids and volatile solvents. It helps in increasing lifetime of the inner tube to avoid diffusion of volatile solvents before reaching the center of the plasma.
Typical flows are in the range 0-0.8 L/min. Torch tubes are usually made of glass but ceramic tubes may be used to increase lifetime with organics, or for low level Ca, B analysis with HF matrices.
The injector is the central tube of the torch. The sample pass through the injector to be introduced into the plasma. The injector can be made of glass or of ceramic to be HF compatible. HORIBA Scientific uses a 3 mm i.d. Diameter injector made of ceramic. This is the largest diameter used on the market. The speed of the sample into the injector is controlled by the sum of the nebulizer and sheath gas flows along with injector diameter.
Typical nebulization flow for aqueous samples is 0.8 L/min. For volatile organic solvents, it can be reduced to 0.5 or 0.3 L/min according to the volatility of the sample. The residence time of the sample into the plasma and then the efficiency of energy transfer will be directly linked to this speed of the sample.
The inner diameter of the injector has a strong effect on the residence time of the sample into the plasma. Residence time increases with the diameter of the injector, and when residence time increases, detection limits are improved and the robustness of the ICP-OES is increased. It has also been proven recently in scientific papers that the use of a 3 mm i.d. injector drastically reduces matrix effects, and that the use of a reduced RF power is sufficient to ensure robustness compared to instruments with a lower injector diameter.
To generate the electrical field, a RF generator and an induction coil are used. Authorized frequencies for generators are 27.12 MHz and 40.68 MHz. Generators can use the power tube technology or the solid state technology. A power tube RF generator will require increased maintenance as the tube should be regularly changed over time. A solid state RF generator will be more reliable and requires less maintenance.
As the generation of the electrical field generates an increase of temperature of the coil and of the generator, cooling is necessary. This cooling can be done with air or water. Water cooling systems are more reliable, as temperature and flow of cooling are perfectly managed.
The inner diameter of the injector has a strong effect on the residence time of the sample into the plasma. Residence time increases with the diameter of the injector, and when residence time increases, detection limits are improved and the robustness of the ICP-OES is increased. It has also been proven recently in scientific papers that the use of a 3 mm i.d. injector drastically reduces matrix effects, and that the use of a reduced RF power is sufficient to ensure robustness compared to instruments with a lower injector diameter.
Fig. 21: Schematic of the plasma and identification of its various zones.
The energy of the plasma comes from the induction coil. Its maximum energy is on the outside part of the plasma and spreads to the inner part. This lower viscosity zone in the center of the plasma is called the “central channel.” Low viscosity means facilitated sample introduction and facilitated containment of the sample.
Several zones are identified in the central channel: The Pre-heat Zone where desolvation of the sample occurs (this zone is surrounded by the Induction Zone), the Initial Radiation Zone where atoms and ions are excited, the Normal Analytical Zone where photons are emitted, and the Recombination Zone where low temperatures are observed and recombination between atoms are observed.
Available RF generator frequencies used for ICP-OES are 27.12 and 40.68 MHz. The use of 40.68 MHz is preferred because it creates a wider central channel into the plasma, facilitating the introduction of the sample. Thus, the use of a larger inner diameter for the injector is possible, leading to improved performance.
Fig. 22: Radial viewing with vertical torch.
There are two ways to observe photons emitted into the plasma. The first way is to observe the plasma radially. In this case, the torch is usually vertical and the optics faces the plasma from its side. Axial viewing is also possible. It was designed to improve sensitivity of ICP-OES instruments.
Plasma is observed axially and the torch is usually horizontal. Such observation modes require a specific interface between the torch and the optics to avoid damage due to the high temperature of the plasma.
Fig. 23: Axial viewing with horizontal torch.
As axial viewing mode exhibits many limitations for difficult matrices (see Chapter Performances), some systems using both axial and radial viewing mode were developed; For such systems using a horizontal torch, it did not solve all issues.
Other ways to improve sensitivity while maintaining ease of use of radial viewing were explored and HORIBA Scientific introduced the Total Plasma View, allowing the measurement of the full Normal Analytical Zone where all atoms and ions emit their photons. The development of this unique viewing mode was only possible thanks to the high quality optics and the size of the optical components used.
Fig. 24: Radial viewing with vertical torch – Yellow bars show the zone measured with Total Plasma View.
