High-resolution mass spectroscopy:

 The molecular weight is calculated by mass spectroscopy and is generally measured like a whole number with no decimal places. Though, it is possible to compute the molecular weight more precisely (high resolution mass spectroscopy) to four decimal places and establish the molecular formula. Refer the molecules CO, N₂, CH₂N and C₂H₄. All of these molecules have similar molecular weight of 28 and in a common mass spectrum would produce a molecular ion of that value. In a high-resolution mass spectrum, the molecular ion is calculated to four decimal places and thus we have to consider the precise atomic masses of the component atoms. The correct mass values for the ions are as follows:

CO⁺ = ¹²C¹⁶O⁺            Accurate mass = 12.0000 + 15.9949 = 27.9949

N₂⁺ = 14N₂⁺               Accurate mass = 28.0061

CH₂N⁺ = ¹²C¹H₂ ¹⁴N⁺  Accurate mass = 12.0000 + 2.0156 + 14.0031 = 28.0187

C₂H₄⁺ = ¹²C₂¹H₄⁺       Accurate mass = 24.0000 + 4.0313 = 28.0313

If the calculated mass of the molecular ion is 28.0076, this would be in line with the theoretical accurate mass for nitrogen (that is 28.0062).