Compact, Multipurpose, Diode-Laser-Based Photometer

A compact, rugged photometer for simultaneous collection of transmittance, fluorescence, and Rayleigh scattering signals has been designed, constructed, and characterized. This inexpensive instrument is suitable for use in the undergraduate teaching laboratory and in undergraduate research projects. A diode laser is used as the excitation source and PIN photodiodes as the detectors. Several optical arrangements, each consisting of a series of lenses and filters, have been tested; dispersive optics are not employed in the instrument. Simple DC electronics have been designed and constructed to facilitate the simultaneous collection of signals. This instrument was characterized using dyes and scattering agents. Fluorescence limits of detection (LOD) for methylene blue and Nile blue A have been calculated to be 2 ppb; use of a small photomultiplier tube as detector lowers the LOD further. The LOD for methylene blue by absorption-based measurements is five times higher than those for a commercially available spectrophotometer. The compact photometer has been applied to undergraduate student-research projects.