2.2  Components of a Flame Atomic Absorption/Emission Spectrometer System

2.2.2 Optical Radiation Sources

For FAAS and FAES, the wavelengths of interest are in the UV and visible range. There are three basic types of radiation sources that are utilized in these instruments: continuous sources, line sources, and laser sources. A continuous source, also referred to as a broadband source, emits radiation containing a broad range of wavelengths. A plot of intensity on the y axis and wavelength on the x axis is shaped like a broad Gaussian distribution with a few small peaks and shallow valleys. The emission wavelengths of a continuous source can range over hundreds of nanometers. Examples of lamps considered to be continuous sources are deuterium, mercury, xenon, and tungsten lamps. These various lamps are used as background correction lamps (signal to noise correction devices) in AAS and AES instruments and not as source lamps for analyte detection.

Line sources are lamps that emit very narrow bands of radiation, but this source of radiation is not as pure as radiation from a laser. The most common line source radiation generator used in AAS is the hollow cathode lamp (HCL). A schematic of a Calcium HCL is shown in Animation 2.1 below. These lamps are encased in a cylinder made out of glass walls and a quartz end cap. Glass end caps can be used for visible wavelength emitting materials while quartz must be used for UV emitting HCLs. These cylinders are filled with a noble gas (Ne or Ar) to sub-atmospheric pressures of 1 to 5 torr. HCLs also contain a tungsten anode, a cathode composed of the metal of interest, and various insulators (usually made out of mica). Lamps containing more than one element in their cathode are also available but most FAAS and FAES instruments can only measure one element at a time.

A HCL works by placing a 300 V potential across the electrodes that generates a current of 5 to 15 milliamps (mA). As the electrons migrate toward the cathode, they collide with the noble gas atoms and ionize them. These charged noble gas atoms collide with the cathode. If the kinetic energy of the noble gas cations is high enough, some of the metal atoms on the cathode will be dislodged, producing an atomic cloud of metal in the gaseous phase. This process is referred to as sputtering. A portion of the collision energy will cause the gaseous phase metal atoms to enter into an excited electronic state. These excited gaseous-phase atoms relax from the excited state to the ground state through resonance fluorescence that emits a specific wavelength of UV or visible radiation (Section 1.2.2). This emission usually corresponds to only one or a few energy gaps that results in the generation of a relatively pure source of light. Higher currents can be used to generate more photons but too much current results in self absorption of the photons and Doppler broadening of the bandwidth (a degradation of the spectral purity of the signal (Section 2.2.6)). A proper balance of current and generated radiation intensity allows for maximum sensitivity for absorption spectrometry.

While the use of a HCL is most common in AAS systems, other line source generators are available including lasers and electrode-less discharge lamps (EDLs) such as microwave EDLs and radiofrequency EDLs. Lasers produce the most pure form of radiation but are of little to no use in AAS or AES and thus, will not be discussed here because they are not tunable to the range of wavelengths needed in AAS. Lasers are utilized for sample introduction for inductively coupled plasma systems; this laser ablation technique is discussed in Chapter 4. EDLs are also relatively rare in AAS instruments and are only used for a few selected elements. In these lamps, the metal atoms are excited using microwave or radiofrequency generators. EDLs produce higher intensity radiation than an HCL but are generally not as reliable or commercially available for all elements of interests. As a result, most analytical technicians use HCLs in their FAAS systems.

Please view the animation of the hollow cathode lamp below.


Animation 2.1 Animation of a Hollow Cathode Lamp.

Frank's Homepage

©Dunnivant & Ginsbach, 2008