Page 92 - Tandem Techniques

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sample support took the form of a suitable sample transport system then the Raman spectrometer could,
indeed, be associated with a liquid chromatograph as a tandem instrument.

Atomic Spectra

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. These bands, or lines of emitted
light, represent energy changes that occur when electrons orbiting the nucleus of the respective atom
change energy levels. 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 they are fairly easy to incorporate with a chromatograph in the form of a
tandem system. The older types of spectrometer were a little cumbersome, and not very sensitive, but
with the advent of simple and inexpensive ways of producing inert gas plasma the situation has changed
radically. The system is now very sensitive and, in conjunction with the gas chromatograph, has been
used successfully for a number of years as a tandem combination for specific element detection.

The Atomic Emission Spectrometer

The atomic emission spectrometer is extremely versatile, with a very high sensitivity and selectivity.
The model described here will be that 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 2.18. 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