• Sample introduction system (nebulizer).

  • Torch assembly.

  • High-frequency generator.

  • Transfer optics and spectrometer.

  • Detector(s).

  • Computer interface.

For analysis, samples generally are dissolved to form an aqueous solution of known weight and dilution. The solution is aspirated into the nebulizer, which transforms it into an aerosol. The aerosol then proceeds into the plasma, it is transformed into atoms and ions in the discharge, and the atoms (elements) are excited and emit light at characteristic wavelengths. The intensity of the light at the wavelengths associated with each element is proportional to that element’s concentration.

The ICP-OES torch consists of three concentric tubes—known as the outer, middle, and inner tubes—usually made of fused silica. The torch is positioned in a coil of a radio-frequency generator. The support gas that flows through the middle annulus, argon, is seeded with free electrons that gain energy from the radio-frequency field. The energized electrons collide with the argon gas and form Ar+ ions. Continued interaction of the electrons and ions with the radio-frequency field increases the energy of the particles and forms and sustains a plasma, a gas in which some fraction of the atoms are present in an ionized state. At the same time, the sample is swept through the inner loop by the carrier gas, also argon, and is introduced into the plasma, allowing the sample to become ionized and subsequently emit light.

Temperatures in the plasma are typically 6,000–10,000 K.9 To prevent a possible short circuit and meltdown, the plasma must be insulated from the rest of the instrument. Insulation is achieved by the flow of the outer gas, typically argon or nitrogen, through the outer annulus of the torch. The outer gas sustains the plasma and stabilizes the plasma position.

Each element emits several specific wavelengths of light in the ultravioletvisible spectrum that can be used for analysis. The selection of the optimal wavelength for a sample depends on a number of factors, such as the other elements present in the sample matrix. The light emitted by the atoms of an element must be converted to an electric signal that can be measured quantitatively. That is achieved by resolving the light with a diffraction grating and then using a solid-state diode array or other photoelectric detector to measure wavelength-specific intensity for each element emission line. The concentration of the elements in the sample is determined by comparing the intensity of the emission signals from the sample with that from a solution of a known concentration of the element (standard).

9  

Willard, H. H.; Merritt, Jr., L. L.; Dean, J. A.; Settle, Jr., F. A. Instrumental Methods of Analysis, Seventh Ed.; Wadsworth Publishing: Belmont, CA, 1988.



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