A portable colorimeter using light-emitting diodes and photodiodes with fast-Fourier-transformation signal processing

A portable colorimeter equipped with light-emitting diodes (LEDs) and two photodiodes (PDs) as light sources and detectors, respectively, was developed. LEDs were electronically modulated with square waves. Light from an LED was divided by optical fibers and conducted to a sample and reference cell. The sample or reference light was detected by each PD. Then detected signals were fed to a notebook computer through a dual-channel analog-to-digital converter. Neither monochromator nor bandpass filter was necessary, because an LED's spectral bandwidth is relatively narrow. A signal from an LED in the time domain was converted into a frequency-domain signal by fast Fourier transformation on a computer. Absorbance was calculated from intensities of sample and reference signals. This colorimeter was successfully applied to the determination of cobalt in a synthetic water sample and to the on-site analysis of nitrite ions in river water. © 1998 John Wiley & Sons, Inc. Field Analyt Chem Technol 2: 173–178, 1998