Absorbance spectroscopy is widely used in many fields and lower limits of detection are currently set by limits of sensitivity and reproducability of available instruments. An instrument with significantly improved sensitivity would extend use of many existing procedures and permit use of others not currently employed due to instrumental limitations. Wavelength modulation spectrophotometry has been used in a number of applications and has a great ability to reduce some types of noise. The proposed work will continue the development of a technique using wavelength modulation spectrophotometry to monitor kinetic assays by accurately measuring minute changes in sample absorbance. Phase I research demonstrated feasibility of such an approach by constructing and operating an experimental model of an instrument which used novel methods of eliminating spurious amplitude modulation effects that normally accompany wavelength modulation. The performance results were: noise = 6.1 x 10-6 A RMS at A = 0; max. practical sensitivity = 10-3 A full scale; drift = 0.4 x 10-6 A/min. The noise and sensitivity of this experimental instrument surpass the best reports in the literature, and obserations during operation of the instrument suggest much potential for further improvement of these parameters. Phase II research will center around two objectives: 1) reduction of instrument drift, noise, and inaccuracy and 2) evaluation of instrument performance over the full range of expected operating conditions. At successful conclusion of Phase II, we will have demonstrated instrument noise below 2x10-6 A RMS at A = 0; drift below 0.1 x 10-6 A per minute, and accuracy 5% of full scale. These goals will be achieved by investigations of noise and drift and subsequent instrument modification to improve performance.
