What I would suggest is to   grind  the  leaves & freeze the  paste in liq.nitrogen & then freeze dry under vacuum  . I dont know how much quantity you  are processing  but  even then this process can be  conveniently carried out     on  around a Kg scale.  Other wise  , if the quantity to be handled is too high , then air drying in an vacuum oven at  about 60 -80  degree C   can be done   & then later can be processed as desired. 

Thank you for your response Abid. I am aware that some compounds will be lost from air-drying, but I am wondering if air-drying samples that are frozen will be any worse than air-drying fresh leaf samples. 

I am concerned that drying previously frozen leaves may destroy some secondary metabolites, as freezing-induced plant cell lysis may cause cell contents to leak out and be exposed to higher rates of oxidation during the drying process. I cannot find literature to confirm this though. If there will be significantly higher losses, then I will have to freeze-dry my plant samples. 

Thanks

Hi Pinelope :  Pl note that the  during the freeze drying , u will be  freezing the samples in  either dry ice (  frozen CO2  gas ) or Liq. Nitrogen , & the subjecting them to the vacuum drying .  This is still the best , most reliable, safe with respect to the stability of the  metabolite samples.  You can further   blanket the running  conditions under the  nitrogen gas , that is that  do all the previous steps under this gas. All others will  can never be that reliable , as per so far my knowledge.  So use the methods as per your sample size  & condition.