Differences between [C ii] emissivity and [¹³C ii] emissivity multiplied by the standard elemental isotopic ratio of 67 showing conditions where a increased C⁺/¹³C⁺ fractionation ratio turns into an observable increased [C ii]/[¹³C ii] intensity ratio. The upper plot uses a radiation field of 1000χ₀ and varies the surface gas density: 10³ cm^-3 (black), 10⁴ cm^-3 (grey), 10⁵ cm^-3 (blue), 10⁶ cm^-3 (red). The lower plot uses a gas density of n_surf = 10⁴ cm^-3 and varies the radiation field: 1χ₀ (black), 10χ₀ (grey), 100χ₀ (blue), 10³χ₀ (cyan), 10⁴χ₀ (green), 10⁵χ₀ (red), 10⁶χ₀ (orange). The dashed line at 10   K km s^-1 (cm^-3 pc)^-1 gives a rough indication what differences might be observable with current technology in a deep integration.