A Comparison of the Function of the Slit in Light-Absorption Studies Using Tungsten and Laser Sources

Abstract

Recent studies have shown that the laser is an extremely useful substitute for the tungsten lamp in light absorption studies with commercial spectrophotometers. A closer conformity to the Beer-Lambert absorption law and an increase in the useful concentration range are among the advantages of the laser in this application. However, the function of the slit is different for the two types of source. With the laser the slit control acts to regulate the power incident upon the sample and detector, but due to the Gaussian distribution of intensity across the laser beam, it is not a linear relationship with slit width. For the tungsten lamp the slit determines the radiation frequency bandwidth as well as controlling the incident power. The function of the slit is described in detail for the two sources and the difference is discussed in terms of the applicability of the absorption law. Experimental data are also presented which demonstrate this difference. It is suggested that employing neutral density filters or reducing the plasma-tube excitation power may be more desirable than using a slit to attentuate the laser light incident upon a sample.