Cosmological Microwave Background (CMB) observations in past 50 years have provided wealth of information about Cosmological Parameters. Planck CMB observations favour
Universe dominated by Dark Energy (69%) and amount of Baryonic matter is about 2%. It turns out that using Primordial abundances of light nuclei some of the Cosmological Parameters can be constrained. Deuterium, that undergoes very less astration, is the best ‘Baryometer’ and is used to constrain cosmological Baryonic Density Parameter. In this talk we will focus on how Primodial Nucleosynthesis can be used as a tool to constrain Baryonic parameter using Quasar
absorption line studies. In this context our recent result based on analysis to measure Deuterium
abundance, along QSO J1558-0031 observed with HIRES/Keck, will also be presented.