A flowcell for a sequencing instrument. The flowcell includes a fluid inlet, a fluid outlet, a flow channel formed between an at least partially transparent cover and a base and fluidly connecting the fluid inlet to the fluid outlet, and a capture substrate provided in the flow channel. The capture substrate includes microretainers configured to each receive a single microspot having a microspot diameter, and microretainer is separated from adjacent microretainers by an interstitial gap distance that is equal to or greater than the microspot diameter. A particle separator may be fluidly connected to the flowcell. The particle separator may include a microfluidic channel having an array of micropillars to transfer a plurality of the microspots to a loading buffer that may be delivered to the flowcell.