The Structure of a Spectrophotometer

As shown in Fig. 1, first the intensity of the measurement light beam, I₀, is measured without the sample set. Then the sample is set in the path of the measurement light beam, and the intensity of the light beam after it passes through the sample, It, is measured.

As shown in Fig. 2, a cell containing solvent is set in the path of the measurement light beam, and the intensity of the light beam after it passes through the cell, I0, is measured. Next, a cell containing a solution produced by dissolving the sample in the solvent is set in the path of the measurement light beam, and the intensity of the light beam after it passes through the cell, It, is measured. The transmittance, T, is given by equation (1), but with solution samples, it is more common to use the absorbance, Abs, which is given by equation (2).

Equation (3), which expresses the relationship between the absorbance, Abs, and the sample concentration, C, is called the “Lambert-Beer law”. There is a proportional relationship between the absorbance and concentration, and this forms the basis of quantitative analysis.

You will find from the above explanation that the indispensable elements of a spectrophotometer consist, as shown in Fig. 3, of a light source, a spectrometer, a sample compartment, and a detector. Although I said in the previous section that the sample is exposed to monochromatic light, there are instruments in which white light is passed through the sample before being passed into the spectrometer. This method is employed in high-speed photometry instruments that use array detectors. In the next sections, I will give an explanation of each element.

Fig.4 Emission Intensity Distribution of Halogen Lamp (3,000K)

The principle for light emission is the same as that for a standard incandescent bulb. Electric current is supplied to a filament, the filament becomes hot, and light is emitted. The bulb in a halogen lamp is filled with inert gas and a small amount of a halogen. While the tungsten used as the filament evaporates due to the high temperature, the halide causes the tungsten to return to the filament. This helps create a bright light source with a long service life. The emission intensity distribution of a halogen lamp can be approximated using Planck’s law of radiation. Fig. 4 shows the emission intensity distribution for a temperature of 3,000 K. A halogen lamp offers superior temporal stability, a service life of approx. 2,000 hours, and a relatively low cost. It has relatively high levels of each of the properties a) to d) mentioned above.

Fig.5 Emission Intensity Distribution of Deuterium Lamp1)

A deuterium lamp is a discharge light source in which the bulb is filled with deuterium (D2) at a pressure of several hundred pascals. Fig. 5 shows the emission intensity distribution for a deuterium lamp. Although 400 nm is, in general, an approximate usage limit at the long wavelength end, because the degree of attenuation at this end is quite low, light of wavelengths greater than 400 nm is used. In the region beyond 400 nm, there are also large numbers of bright line spectra. Among these, the bright line spectra at 486.0 nm and 656.1 nm are particularly intense, and can be used for the wavelength calibration of spectrophotometers. The usage limit at the short wavelength end is determined by the transmittance of the window material. In Fig. 5, the graphs for cases where synthetic silica and UV glass are used for the window material are given as examples.

Fig.6 Cross Section of Diffraction Grating

Spectroscopy is the technique of splitting light that consists of various wavelengths into components that correspond to those wavelengths. The element that splits this light is called a dispersive element. Prisms and diffraction gratings are typical dispersive elements. Prisms used to be commonly used as the dispersive elements in spectrometers, but recently, diffraction gratings have become the most commonly used type of dispersive element. The diffraction gratings used in spectrophotometers have from several hundred to approximately 2,000 parallel grooves per millimeter cut into them at equal intervals. An example of a cross-section is shown in Fig. 6.

Fig.10 Spectral Sensitivity Characteristics of a Photomultiplier2)

A photomultiplier is a detector that uses the fact that photoelectrons are discharged from a photoelectric surface when it is subjected to light (i.e., the external photoelectric effect). The photoelectrons emitted from the photoelectric surface repeatedly cause secondary electron emission in sequentially arranged dynodes, ultimately producing a large output for a relatively small light intensity. The most important feature of a photomultiplier is that it achieves a significantly high level of sensitivity that cannot be obtained with other optical sensors. If there is sufficient light intensity, this feature is not particularly relevant, but as the light intensity decreases, this feature becomes increasingly useful. For this reason, photomultipliers are used in high-grade instruments. The spectral sensitivity characteristics of a photomultiplier are mainly determined by the material of the photoelectric surface. Fig. 10 shows an example of the spectral sensitivity characteristics of a multi-alkali photoelectric surface, a type of surface that is often used in spectrophotometers.

Fig.11 Spectral Sensitivity Characteristics of a Silicon Photodiode3)

A silicon photodiode is a detector that uses the fact that the electrical properties of a detector change when it is exposed to light (i.e., the internal photoelectric effect). Solar cells, which have attracted much attention recently, use the same structure and principle as silicon photodiodes.
In comparison with photomultipliers, silicon photodiodes offer advantages such as low cost, little locality of sensitivity in the light-receiving surface, and the fact that a special power supply is not required. Even regarding sensitivity, if the light intensity is relatively large, they can obtain photometric data that is no inferior to that obtained with photomultipliers. Fig. 11 shows an example of the spectral sensitivity characteristics of a silicon photodiode.