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Laser spectroscopy has become a widely used popular technique in analytical chemistry during the laser decade. Numerous studies have documented the results of the advantages of lasers (low detection limits, for example); however, most of the applications have focused on the use of relatively expensive lasers. Much less effort has been directed toward the development of analytical methods using semiconductor lasers. With the development of semiconductor lasers a new approach to laser enhanced analytical methods has become possible. Semiconductor lasers are a source of monochromatic, tunable coherent beams of light, characteristic of other types of lasers, with the additional benefit of compactness. At the present time semiconductor lasers are available only for the red and the near-infrared (NIR) region of the spectrum, a situation that required the development of new methods for giving analytically useful signals in the NIR spectral region. The shortwave NIR region of the spectrum is advantageous due to its relatively low interference. Fluorescence spectroscopy is an inherently sensitive technique; however, the analysis of complicated biological samples may require the simultaneous acquisition of absorption spectra. In this regard absorption measurement involves the use of multiple spectroscopic parameters to increase the specificity of the measurement. Laser diodes are excellent light sources for both absorption and fluorescence spectroscopy. Low detection limits may be achieved using the intracavity method for absorption measurements. Moreover, laser intracavity spectroscopy has an important advantage when implemented with laser diodes: the built-in integral monitoring photodiode (PIN) can be used as a detector. The advantages and disadvantages of different methods of laser diode control are discussed in this paper, as are several different analytical applications. Simultaneous fluorescence detection is also possible with an additional detector. Superior detection limits may be achieved using NIR laser diode excited fluorescence spectroscopy. In this paper the use of NIR laser diode spectroscopy in analytical chemistry is demonstrated.
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