Gas Chromatography - Tandem Techniques - The Characteristics of Atomic Absorption > The Atomic Emission Spectrometer > Page 32

The Characteristics of Atomic Absorption


When a solid is heated to incandescence, it emits a more or less continuous spectrum over a wide range of wavelengths. However, when gases or vapors are heated under the same conditions, spectroscopic examination of the light emitted discloses a series of lines, often very complicated in structure, at those specific wavelengths that are characteristic of the elements present. As discussed, these bands, or lines of emitted light, represent transition energy changes that occur when electrons orbiting the nucleus of the respective atom change from one energy level to another. Atomic emission spectroscopy is commonly used to identify the presence of certain elements in a sample; the procedure can be very sensitive and, at the same time, provide completely unambiguous identification. Atomic emission spectrometers are in common use in many analytical laboratories, and, with appropriately designed interfaces, are fairly easy to incorporate with a chromatograph in the form of a combined system. The older types of spectrometer were a little cumbersome, and, relatively, not very sensitive, but with the advent of simple and inexpensive ways of producing inert gas plasma the situation has changed radically. The atomic spectrometer is now very sensitive and, in conjunction with the gas chromatograph, has been used successfully for a number of years as a combination system for specific element detection.


The Atomic Emission Spectrometer


The atomic emission spectrometer is an extremely versatile device, with a very high sensitivity and excellent selectivity. The model described here was originally designed and manufactured by the Hewlett-Packard Corporation. Basically, atomic emission is achieved by means of a helium plasma, and the light emitted is analyzed by a diode array spectrometer. A diagram showing the basic principles of the helium plasma atomic emission spectrometer is shown in figure 21. The plasma is microwave induced into a helium stream employing a water-cooled transducer. The sample, mixed with the pure helium make-up gas, enters the plasma and the elements present in the solute emit light, the wavelength of which is characteristic for each element. The sample residue subsequently passes to waste. The light emitted is transmitted through a quartz window, and is then focused by a quartz lens and spherical mirror onto a diffraction grating.