To determine the N, P, and K content in your liquid nanofertilizers, use inductively coupled plasma (ICP) spectrometry for simultaneous analysis, Dumas combustion for nitrogen, or traditional gravimetric and photometric methods. Prepare your samples appropriately to ensure accurate measurements.

Additionally, you could employ the techniques of atomic absorption spectroscopy or atomic emission spectroscopy. All of these very efficient for determination of atom composition. And these techniques can be conducted for samples in the liquid state.