Gerry, a spectrophotometer beams light of a very narrow wavelength
through the object and meansures the transmission (or reflectance). A
scanning spectrophotometer is just one that does a number of usually
closely spaced wavelengths, in succession. They may do the entire
spectrum, or just a certain portion of it (such as ultraviolet).
I do not know for sure, but I would imagine that the synthetic quartz
is purer, and would not contain some of the natural contaminants you
would probably find in natural quartz. These contaminants would cause
it to show a different transmission in the particular wavelengths that
are characteristic of the contaminants.