The Bausch and Lomb Spectronic 20 Spectrophotometer
The Bausch and Lomb Spectronic 20 spectrophotometer is a line-operated single-beam instrument. Radiation from the source (a tungsten filament lamp) is dispersed by a replica grating, and the desired wavelength is focused upon an exit slit. The emergent beam passes through the cell compartment; that portion which is not absorbed is measured by a phototube. The response of the phototube is recorded in terms of transmittance (upper scale) or of absorbance (lower scale).
The wavelength control is located on the right side of the top. Once it has been set to the desired wavelength, this control is left undisturbed. The effect of stray radiation is electronically eliminated with the dark current control, located to the left. This control is adjusted until the instrument records zero transmittance (or infinite absorbance) when the cell compartment is vacant.
The light control, located beneath the wavelength control, is adjusted until the instrument records 100 percent transmittance (or zero absorbance) with the blank in the light path.
The sample holder accommodates special test tubes (available at the stockroom). The hinged top of the compartment is raised, and the tube is gently but firmly seated inside. Care must be taken to align the etched mark on the tube with the mark on the cell compartment. The top must be closed before readings are taken.
A. Set the desired wavelength with the wavelength control.
B. Turn on the instrument by clockwise rotation of the dark current control. Allow the instrument to warm up for about 15 minutes before attempting to take readings.
C. Adjust the dark current control so that the instrument indicates infinite absorbance.
D. Place the test tube containing the blank in the cell compartment; adjust the light control so that zero absorbance is indicated.
E. Replace the blank with a tube containing the sample and read its absorbance. Use matched pairs of test tubes.
Steps B, C, and D should be repeated several times for each sample to minimize the effect of fluctuations in response.