The ICP light source generated in the ICP spectrometer torch tube has three observation methods: radial observation，axial observation  and DUO observation . Let's have an analysis about it.

1. Radial observation

 Radial observation uses a radially placed ICP spectrometer torch tube, and the "flame" airflow direction is perpendicular to the direction of the light-taking path in this way all signals in the entire analysis area can be received from the spectrometer.

No torch tube adjustment is required for different elements, which is a common observation method for analytical testing. It has less matrix effects and interferences, especially for organic samples. It also has good detection limits for complex matrices. It can measure  solution of any matrix with lower argon consumption, such as the determination of high salt samples, the analysis of complex samples, organic matter and relatively less severe carbon deposition. The side-viewing torch is a radial torch, and the heat and analytical exhaust gases  flow upwards into the exhaust system.

2. Axial observation

Axial observation of ICP spectrometer uses a horizontally placed ICP spectrometer torch tube, and the direction of the "flame" airflow and the direction of the light-taking path are horizontally coincident, which enable every single part of light from the whole flame comes through the slit.

The advantages of the axial observation method are: the light from all parts of the entire "flame" can be collected, resulting in high sensitivity and better detection limit for simple samples. Its disadvantages are as following: large matrix effect and ionization interference, small linear range, and the contamination of the torch tube solution caused by carbon deposits and salt deposits. The torch tube needs to be cleaned and maintained in time. The RF power setting cannot generally exceed 1350W. It is used in the water quality analysis of the spectrometer.

3.DUO observation

DUO observation is introduced on the basis of axial observation, and the purpose is to make up for the defects of ionization interference, decreased linear range and large matrix effect in axial observation. The design idea is to add a radial observation light path on the basis of axial observation.

The torch tube of DUO observation has a small hole or a small groove on the side of the outer tube, so that the light signal can be collected from the side. When the reflecting mirror M cuts out the optical path, it is a axial observation, and it is a standard axial observation instrument at this time. When the reflecting mirror M cuts into the optical path, its back side blocks the light in the axial direction, and the optical signal on the side of the plasma is reflected three times by M1, M2, and M into the main optical path, which becomes a radial observation. Obviously, the DUO observation effectively widens the linear range.

However, it should be noted that the DUO observation is actually an additional radial observation based on the axial observation. The radial observation at this time is not as good as a true radial observation whether it is in adjustment of the optimal observation area or in terms of optical path, optical focusing and incident slits etc.  The defects in the axial observation, such as large matrix effect, high background noise, easy contamination of the rectangular tube, troublesome cleaning and unsuitable high power , also exist in the DUO observation. Therefore, the  application scope of DUO observation is still similar to that of axial observation, and it is also not suitable for the field of high matrix.

Fourth, the advantages of radial observation

Radial observation is the classic observation method of ICP. It collects light from the side of the plasma and measures the signal in the ion reflection area (the optimal observation area). Obviously, the signal in this area is not as strong as the signal in the entire analysis channel when observed horizontally(axial observation). Therefore, the sensitivity is not as high as that of axial observation, but the signal in this area can provide the best signal-to-background ratio(SBR), especially in the complex matrix, the signal-to-background ratio(SBR) is more prominent. At the same time,radial observation does not collect optical signals in the atomized region and atomic reflection region in the plasma, so it has no easy ionization interference at all, has a very good linear range, very small matrix effect and has very low background. Besides,it is flexible and convenient. The torch tube for radial observation is very short.It not easy to be contaminated, and not easy to generate heat under high power. Furthermore, it is more suitable for direct sample analysis of high matrix samples, oil samples and organic samples.

The ICP-6800 and 6810 radial observation instruments launched by MACY Company keep not only all the advantages radial observation, but also have ideal external light path lighting focusing system, unique back reflector signal enhancement technology and excellent sample injection system which make the the instrument has unparalleled sensitivity and detection limit. They are widely used in metallurgy, petrochemical, geological and mineral commodity inspection and other fields with complex sample matrix.