The goal is to measure the primordial abundance ratios of the light nuclei made in BBN. We normally measure the ratios of the abundances of two nuclei in the same gas, one of which is typically H, because it is the easiest to measure.
The two main difficulties are the accuracy of the measurement and departures from primordial abundances. The state of the art today (1 ) is about 3% for Yp , 10% for D/H and 8% for 7Li , for each object observed. These are random errors. The systematic errors are hard to estimate, usually unreliable, and potentially much larger.
By the earliest time at which we can observe objects, redshifts z 6, we find heavy elements from stars in most gas. Although we expect that large volumes of the intergalactic medium (IGM) remain primordial today , we do not know how to obtain accurate abundances in this gas. Hence we must consider possible modifications of abundances. This is best done in gas with the lowest abundances of heavy elements, since this gas should have the least deviations caused by stars.
The nuclei D, 3He, 6Li and 7Li are all fragile and readily burned inside stars at relatively low temperatures of a few 106 K. They may appear depleted in the atmosphere of a star because the gas in the star has been above the critical temperature, and they will be depleted in the gas returned to the interstellar medium (ISM). Nuclei 3He , 7Li and especially 4He are also made in stars.