it looks like a typo error.

"...purified by column chromatography on silica gel using pet- roleum dichloromethane–methanol (10/1) as an eluent..." 

my guess is there shouldn't be the word petroleum in there, it should be DCM:MeOH 10:1 ratio as the eluent. You can easily check the TLC of the mixture and should always be done before doing the column. You can also email the corresponding author to verify.