Orange points show measured flux ratio between the H¹³CO⁺ and HC¹⁸O⁺ (2−1) lines, in those cores where both transitions are detected (labelled on the y-axis). Error bars show the 3σ level. The vertical dashed orange line shows the weighted average of the sample. The expected value of 557/68 = 8.2 is shown with the vertical green line, and the shaded green area shows a variation of a factor of 2 around it.