The Spectronic 20
The Bausch and Lomb Spectronic 20™ is the instrument most commonly used for
spectroscopy in the general chemistry laboratory. It works in the wavelength region of
380 to 620 nanometers (nm), which is the visible range of the electromagnetic spectrum.
To Use the Spec 20
1. Switch on the machine. Let it warm up for at least 10 minutes. You will need two
solutions--a blank solution, containing only solvent, to correct for the light absorbed
by the solvent and a sample of the solution to be measured.
2. Select the desired wavelength by using the wavelength control, the knob on the
top of the instrument. (The wavelength controller adjusts the position of the
diffraction grating so that different colors of light are selected by the slit.)
Figure 1
A Spectronic 20
3. Set the percent transmittance (top scale on the meter) to zero, using the control
on the left of the front panel. (The zero control adjusts the electronics. With the
sample compartment cover down and no sample, the control should be adjusted until
the meter reads 0 for transmittance. This control adjusts for the so-called dark
current, the current transmitted by the photocell in the dark.)
4. Place a cuvette, (a test tube shaped container used in Spec 20s, note they are much
more expensive than a test tube so treat with care) filled with the appropriate blank in
the sample holder. Close the cover.
5. Use the 100 percent T control on the front right to set the needle to 100 percent
transmittance. This adjusts for the transmittance of the solvent, test tube, and so
forth. (The 100 percent T control controls the amount of light output. A mechanical
linkage is used to drive a wedge into the light beam. This should be adjusted so that
with a test tube full of solvent in the sample holder, the transmittance is 100 percent.
This allows the operator to correct for any stray absorbance resulting from the
solvent.)
6. Remove the blank, and close the cover over the sample holder. The needle should
swing back to 0 percent transmittance. If it does not, repeat the procedure all over
again.
The instrument is now correctly calibrated, but for one particular wavelength only. To
take a reading, place a cuvette containing your sample in the sample holder, close the
cover, and record the transmittance after the needle has settled down.
The cuvette must be free of fingerprints, bubbles, and other dirt. When you are using the
Spectronic 20, keep the cuvette clean; avoid leaving fingerprints on the bottom part
where the light passes through. There should be no air bubbles on the inside. Suspended
material in solutions will result in strange readings.
The cuvette has a mark on it that should be lined up with the line on the sample holder. If
there is no mark, make a small one at the top of the tube using a wax pencil.
The calibration with the blank must be redone at each new wavelength and should
be repeated every few minutes, even at the same wavelength, to compensate for any
drift in the electronics.
Remove the cuvettes in order to fill them. Water and electricity do not mix!