The determination of metal ion in a water sample was carried out with atomic emission spectroscopy and the method of standard addition. The instrument was autozeroed before measurements. Two identical portions (50 mL each) of the original sample were taken. One portion was measured directly and emission lines at 282.3 nm and 589.6 nm were detected. To the other portion, 20 μL of a 10.0 μg/mL NaCl standard was added and the resultant solution's emission intensity at 589.6 nm changed from 349 to 698, while its emission at 282.3 nm remained at 250. What was the sodium concentration (unit: μg/mL) in the original water sample? (Chlorine does not emit at 282.3 nm, nor at 589.6 nm.)