Measurements of HCN and HNC emission versus radius and flux. Top: integrated line intensities of HCN and HNC for seven regions throughout the Helix Nebula at varying deprojected distance from the CSPN. Computed UV flux units are shown along the upper axis. Bottom: line ratio of HNC/HCN against distance. The blue dashed line indicates a linear regression fit, with the shaded region representing 1σ uncertainty.