Final answer:
The tungsten light source is not necessary for obtaining the spectrum from 190 to 500 nm because it is better suited for wavelengths higher than those in the UV range. A deuterium light source is needed for the UV part of the spectrum, and tungsten light does not efficiently cover this region.
Step-by-step explanation:
The item that is NOT necessary for obtaining the spectrum of a compound from 190 to 500 nm is e. Tungsten light source. This wavelength range includes the ultraviolet (UV) and visible regions of the spectrum. A deuterium light source is typically used for the UV range (190-380 nm), while a tungsten light source is more appropriate for the visible to near-infrared range (above 380 nm). Since the UV part of the spectrum is of interest here, deuterium light is needed, and tungsten is not.
A photomultiplier is helpful for detecting faint light across this range, double-beam spectrophotometer technology can improve accuracy by correcting for changes in the light path or sample absorbance over time, and quartz cuvettes are necessary because they do not absorb UV light and are transparent across the entire 190-500 nm range.