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 DCP - Direct Current Plamsa
 

Description of Plasma Emission Spectroscopy

DCP (Direct Current Plasma) used by Rare Metals and ICP (Inductively Coupled Plasma) spectrometry, are techniques for atomic analysis of elements in liquid samples. A liquid sample is pumped into a nebulizer which converts liquid into an aerosol. The aerosol passes through a spray chamber and into a plasma. A plasma is a gas in which 2-3% of its atoms are in an ionized state, making the gas electrically conductive. They are often referred to as electrical flames and argon is most commonly used. Because argon is inert, no chemical reaction is occurring and the flame isn't actually burning. When the aerosol reaches the plasma of 5,000 - 10,000°K, thermal collisions raise the atoms to an excited electronic state. When the excited electrons return to a lower or ground electronic state, they emit photons. The emission from each element is unique and occurs at several characteristic wavelengths. It is the emitted radiation that is measurable and used for analysis. There is a direct relationship between the number of atoms emitting and the recorded intensity at each characteristic wavelength.

 

Optic System Of The DCP  - SS VI

Plasma excitation produces spectra that are extremely line rich. While this is the principle strength of plasma excitation, the potential for spectral interference is significant. This means the intensity of a nearby line can affect the intensity of the element of interest.
The function of the spectrometer, is to disperse (spread-out) emission radiation so that the characteristic wavelengths of interest may be isolated and measured. How well it performs this task is referred to as its resolution. Spectrometers used for plasma utilize a diffraction grating as the dispersing device. It is a highly reflective front surface mirror with vertical grooves ruled (cut) into it. Rulings between 300 to 3600 grooves/mm. The grooves are ruled in at an angle referred to as blaze angle. The grating is either concave or flat, depending on the intended end use. The grooves cause diffraction of light to spectra. A common example of diffraction is the rainbow colored reflection observed when light shines on a compact disk.
 Echelle Spectrometers

Rare Metals Corporation's DCP SS VI utilizes a echelle grating optic system. The name comes from the echelle grating developed by Prof. G. R. Harrison of MIT. An echelle is a coarsely ruled grating, with rulings ranging between 8 and 300 grooves/mm. It produces high dispersion spectra and retains broad free spectral ranges. The echelle produces many orders of diffraction within a small range of diffraction angles and the resulting spectrum are complex. The slits used on echelle spectrometers are very short to prevent intensity from adjacent spectral orders from interfering with the order in use.

Schematic of echelle grating and order sorting prism

The spectrum are sorted by a prism perpendicular to the echelle grating. This creates a two dimensional array of wavelengths on a 10 X 12 cm focal plane, much like lines on a typewritten page.
 

After the raw data is acquired by the spectrometer it is converted into a digital output via the use of the ADaM software utilized by the DCP SS VI. The data can be turned into a graphical representation of exactly what the DCP is interpreting for a giving output reading. The ability to see what the DCP is providing in the way of output enables us to visually see if the data is a "false positive" or really there. This is very crucial, especially when analyzing for precious metal content.

 

 

    

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