Relative gas phase abundance of C¹⁸O against that of N₂H⁺. At X(C¹⁸O)≈ 2 × 10^-8 the correlations turn into an anti-correlation. While at high X(C¹⁸O), the N₂H⁺ molecules are destroyed by reactions with CO, at low X(C¹⁸O) (and high densities) freezeout of nitrogen is the dominant reason for depletion of N₂H⁺.