I have done DNA extraction on human saliva using Promega ReliaPrep™ Blood gDNA Miniprep System that have been expired for more than five years now. When I initially checked the DNA concentration and the absorbance ratio, it shows a decent DNA concentration (~100 ng/ul) and decent absorbance ratios (A260/A280 = ~1.8 and A260/A230 = ~2.2). When I checked the DNA concentration and absorbance ratio a second time sometimes within minutes to days from the first check, the DNA concentration and absorbance ratio either drops or increases wildly.
For every nanodrop and qubit measurements, I have vortexed the tube first. For nanodrop specifically, I have used 2 µL and make sure to eliminate air bubbles.
Why is there many fluctuations in DNA concentration and absorbance ratios on my salivary human DNA samples?
Apologies if this is the wrong place to ask.