Dear Naresh, thank for posting this interesting technical question on RG. As a synthetic inorganic chemist I'm not a specialist in this field of research, but I think that the following relevant literature reference could help you in your analysis:
Determination of concentration of total sodium and potassium in surface and ground water using a flame photometer
https://d-nb.info/1213972795/34
This useful article is freely available on the internet as public full text (please see attached pdf file). Good luck with your work and best wishes!
Yes, for Na and K, not necessary to preserve. By any means, if not for long duration ( more than 1 month). Flame photo meter/ AAS are commonly used for analysis.