A comparison of experimental diode array wavelength absorbance values for three different instruments to literature maxima values for scanning instruments produced complex results. However, the wavelength maxima derived by first derivative zero crossing calculations showed an improvement in relation to literature values. Experimental results are presented of repetitive testing with the diode array instrument as well as inter-laboratory comparison to demonstrate the reproducibility of using a single instrument for wavelength maximum determinations. A BASIC computer program is used for the automatic calculation of precise zero crossings in first derivative mode. It recalls stored absorbance measurements from disks and performs the necessary manipulations and calculations. Potential applications in forensic sciences include the toxicological drug analysis, street drug analysis, differentiation of ink samples, and quality control testing of standard drugs and chemicals. 4 tables, 1 figures, and 19 references (Author abstract modified)