In a statement, the OPCW said it would now evaluate and consider whether the team needs to make a second visit to Douma.
Samples will be transported back to the Netherlands and onward to the organisation's network of designated labs for analysis.
The OPCW has been investigating use of toxic chemicals in Syria's civil war since 2014. Inspectors had been trying to reach Douma for several days but were delayed after an advance security detail was fired upon on Apr 17.
The OPCW team will attempt to determine whether chemical weapons were used and if so, which. It is not mandated to conclude which side in the conflict used them.