Yes you can use direct absorption @ 280nM for DNA & RNA and 260nM for proteins
In fact it is routine to measure DNA and RNA in particular that was by spec or nanodrop
However, other molecules absorb at those wavelengths, particularly organics
Thus, for example, if you have extracted DNA or RNA with Trizol or phenol and chloroform, any contaminating residue will contribute to the reading
In addition, @ 260nM nucleotides absorb strongly as well as the polymerised version or nucleic acid
Colorometric assays rely on reagents that will only absorb if they bind specifically to DNA - say sybr dyes - or protein - say Bradford reagent: Thus the reading you obtain is more accurate than simple absorption @ 260nM or 280nM
When the molar extinction coefficient is known, absorption spectroscopy is the best method to determine concentration. Even using the theoretical extinction coefficient for a protein is better than Lowry/Bradford assay. However, for a protein that does not have trp/tyr, you will not be able to use absorption spectroscopy directly.
Yes you can use direct absorption @ 280nM for DNA & RNA and 260nM for proteins
In fact it is routine to measure DNA and RNA in particular that was by spec or nanodrop
However, other molecules absorb at those wavelengths, particularly organics
Thus, for example, if you have extracted DNA or RNA with Trizol or phenol and chloroform, any contaminating residue will contribute to the reading
In addition, @ 260nM nucleotides absorb strongly as well as the polymerised version or nucleic acid
Colorometric assays rely on reagents that will only absorb if they bind specifically to DNA - say sybr dyes - or protein - say Bradford reagent: Thus the reading you obtain is more accurate than simple absorption @ 260nM or 280nM