A microfluidic device includes a platform with a microstructure. The microstructure include a primary channel and a plurality of chambers that open to the primary channel to enable a sample fluid that is loaded into the device via the primary channel to flow into the chambers. Each chamber has a volume that is less than tens of nanoliters and is connected by a vent to a secondary channel of the microstructure. A width of the vent is configured to enable a gas to escape from the chamber to the secondary channel while inhibiting flow of the sample fluid from the chamber into the secondary channel.