Why did one liter of nitrogen prepared by lord rayleigh appear to have a greater mass than the other liter prepared by a different method?

There were two methods that Lord Rayleigh used in his experiment. First, nitrogen was obtained from the atmosphere by passing air over hot copper. Second, air was bubbled through an ammonia mixture, then passed over hot copper. The second method would have a greater mass of nitrogen than the first, because the nitrogen from the atmosphere was contaminated by the nitrogen from the ammonia mixture.