this maximum near 460 nm. The LED intensity is small outside of
the 420−660 nm range. By using LEDs with different emission
spectra, absorbance values could be measured at wavelengths
outside of the 420−660 nm range.
In the present configuration, the photodiode spans an
angular range of about 1.5°, corresponding to a bandwidth of
∼20 nm. A longer arm would increase the spectral resolution,
for example, doubling the length of the rotatable arm would
decrease the bandwidth to ∼10 nm. However, this significantly
decreases signal levels. Using the configuration described here,
typical voltages measured using the spectrophotometer with
blank samples in the 450−640 nm range were in the 300 mV
range. Because the voltmeter is capable of measuring to within
1 mV, measurements can be made to three significant figures.
The substantially decreased signal levels that would result from
increased arm length increase the uncertainty in each
measurement. This results in increased scatter in the measured
data. Most commonly, poor performance results from poor
alignment or the presence of excessive scattered light. After
some experimentation, the configuration described here and in
the Supporting Information provides an optimal balance
between signal levels and spectral resolution.