a Log\[Q(C$_2$)/Q(CN)\] $\geq$ -0.18 while the carbon-chain depleted comets are those below that value. This classification was confirmed later by other photometric and spectroscopic studies of large data sets [@Schleicher2008; @Fink2009; @LanglandShula2011; @Cochran2012] and must reflect some differences between the formation conditions (the pristine scenario) or a change of relative composition with time (several perihelion passages) of these comets (the evolutionary scenario). Figure \[ratios\] shows the evolution of the 21P abundance ratios of the various radicals with respect to OH (a proxy of water) and CN, as a function of heliocentric distance. It is clear that 21P abundance ratios in the 2018 return agree with the mean values of depleted comets given in [@AHearn1995]. Table \[tab:abundance\] summarizes the relative abundances in 2018 compared to 1985 and 1998 data using the same technique and the same Haser model parameters [@Schleicher2018]. Our 2018 ratios are the mean values for all the data obtained (see Table \[tab:rates\]). Like for the activity level over the past passages, the relative abundances did not change over the last 5 orbits. We note that the Af$\rho$ values derived in 1985 and in 1998 by [@Schleicher2018] were computed for the narrow band GC\[5260 Å\] filter while we used 