The inspectors, from the Organisation for the Prohibition of Chemical Weapons (OPCW), arrived in the town a week after they first set foot in Syria.
The OPCW said in a statement on Saturday that it will evaluate the situation and consider future steps including another possible visit to Douma.
It added the samples would be analysed in its designated labs.
The organisation’s fact-finding team was delayed for several days in its attempts to reach the town.
The attack being investigated killed at least 70 people on 7 April.
Images showed collapsed bodies in crowded rooms, some foaming from their noses and mouths.
However, journalists who were escorted to the site on 16 April by Syrian government authorities filed differing accounts of what happened in the alleged attack.
Some reported having spoken to locals who said that the chemical attack had been staged, claiming there had been a conventional attack which produced a huge amount of dust.
This is what happens during a chemical weapons inspection
At the time of the chemical attack, Douma had been one of the last Syrian areas still under rebel control but it was facing a massive air and ground assault from Syrian government forces.
Rebels left the town days after the alleged attack.
Both Syria’s President Bashar al Assad and his ally Russia have denied responsibility, while France has warned that the delay in inspections has made it highly likely some evidence has disappeared.
Russia’s foreign ministry said on Saturday Such delays in such a resonant case… we consider unacceptable.
We expect from the inspectors of the organisation the most impartial investigation of all the circumstances of what happened in the city of Douma and the speedy submission of an objective report.
At the same time, the OPCW experts’ attempts to limit the places of visitation and the circle of respondents connected with the prospective chemical suspect can not but cause the most serious concern.
This is followed by a reluctance to shed light on another staged provocation with the use of toxic chemicals, which has already served as an occasion for a missile strike by the Western ‘troika’ of the permanent members of the UN Security Council.
The statement was referring to airstrikes by the US, UK and France on three sites in Syria believed to have been linked to the country’s chemical weapons programme.
The OPCW said on Wednesday that a UN security group had visited and assessed the site ahead of the inspection team.
In a statement, the OPCW said On arrival at Site One, a large crowd gathered and the advice provided by the UNDSS (United Nations Department of Safety and Security) was that the reconnaissance team should withdraw.
At Site Two, the team came under small arms fire and an explosive was detonated. The reconnaissance team returned to Damascus.
At that time they said the UNDSS would continue to work with the Syrian National Authority, the local councils in Douma, and the Russian Military Police to review the security situation.
News Source SkyNews