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Obtaining a UV/Visible Spectrum

المؤلف:  Max M. Houck، Jay A. Siegel

المصدر:  Fundamentals of Forensic Science

الجزء والصفحة:  p101-103

2026-06-28

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Obtaining a UV/Visible Spectrum

The UV/visible spectrum of a substance is obtained by using a UV/visible spectro photometer. A simplified diagram of a spectrophotometer is shown in Figure 5.7. The source emits all of the wavelengths of UV or visible light. Most often, this is a deuterium (heavy hydrogen) lamp for UV light and an incandescent light for visible wavelengths. Next, there is a monochromator. This is a device that selects one particular wavelength (or a small packet of wavelengths) to be exposed to the sample. The monochromator is a prism or grating (see Figure 5.3) that is rotated, thus exposing the sample to steadily increasing or decreasing wavelengths of light throughout the entire spectrum. The monochromator is coupled to the detector so that the amount of transmitted light at each wavelength is known. The sample holder comes next. Most samples are run as solutions in a solvent that does not absorb light in the UV/visible range. Methyl alcohol is a popular solvent for many substances such as drugs. The liquid sample holder is a rectangular, quartz glass cuvette, usually with a 1 cm square base. Spectra of gases can also be obtained using a special gas-tight holder. Finally, there is the detector. The detector must be sensitive to changes in the intensity of UV/visible light that reaches it. The most commonly used UV/visible detector is the photocell. A photocell is a device that converts UV or visible light into an electric current. The more intense the light, the more current will be created. At a wavelength where the sample does not absorb any light, the detector will produce the maximum electric current because all of the light from the source reaches the detector. When the sample absorbs some or all of a wavelength of light, less light reaches the detector and thus, less current is created. Modern spectrophotometers are computer controlled. The computer stores the wavelength and corresponding electric current and then, when the entire spectrum has been obtained, will construct a graph of wavelength (or frequency) versus intensity of transmitted light. This is called the transmission spectrum. The computer can also convert this to the wavelength versus amount of light absorbed by the sample, the absorption spectrum. In practice, a spectrum of the analyte will be obtained and then a spectrum of just the solvent will be obtained. The latter will be subtracted from the former, resulting in the sample spectrum.

FIGURE 5.7 A diagram of an ultraviolet/visible spectrophotometer. There are two light sources; one for ultraviolet light and one for visible light. A mirror is used to select the proper light source. The diffraction grating is the monochromator that selects specific wavelengths of light. The two detectors are coupled so that reference absorptions can be subtracted from sample absorptions.