I would recommend purifying the sample first, there could be a lot of contaminants that could intefere with the reading. Also, when using nanodrop, it is important to take into account the light-related properties. It could happend that due to the opacity of saliva itself, the readings done could be misleading or even impossible for nanodrop to perform.

Good luck

I would guess that saliva has more protein than RNA. If you try to measure RNA in saliva on the Nandrop without purifying first, the protein content will alter your RNA absorbance peak and give false RNA concentrations. I am unclear on your goal and why you want to know this information. But, if it is to have a certain amount of RNA in your samples, it has to be determined experimentally (i.e. purifying different amounts or saliva, trying different purification methods, and tweaking whatever protocol you decide on will be required.)

NO, you can't. Nice idea but not practical.

There are many things which will not allow this measurement.