Cell sorting produces a purified population, but what happens in the hour after the sort often determines whether the downstream experiment works. RNA degrades quickly. Proteins can be modified by collection buffer components. The decisions made at the point of collection, about buffer composition, tube coating, temperature, and timing, matter as much as the sort itself. This article covers the practical steps for the most common downstream applications.

Choosing the right collection buffer

For RNA sequencing, the collection buffer should be as simple as possible. PBS with 0.1% BSA is a reasonable default. Avoid DMEM or other culture media with high protein content, which can interfere with RNA extraction. If the sort will take longer than 90 minutes, consider collecting directly into a lysis buffer compatible with your RNA extraction kit. For proteomics, the buffer choice depends on the downstream protocol; check with your proteomics facility before the sort.

Cell count requirements for common applications

Bulk RNA sequencing typically requires 50,000 to 200,000 cells per sample, depending on the library preparation kit. Single-cell RNA sequencing (10x Genomics) requires a minimum of 500 viable cells per microlitre in the final suspension, with a target of 700-1,200 cells per microlitre. Proteomics requirements vary widely by method; label-free quantification can work with as few as 10,000 cells, while TMT-based approaches typically need more. Confirm the count requirement with your downstream facility before booking the sort.

Maintaining viability during and after the sort

Sorted cells experience shear stress from the nozzle and osmotic stress from the sheath fluid. Keeping the collection tube on ice reduces metabolic activity and slows RNA degradation. For particularly fragile cell types, adding a small volume of culture medium to the collection tube before the sort begins provides a cushion. Purity checks, which we include at no additional cost, should be performed immediately after the sort while the cells are still viable.

Timing the handoff to the downstream facility

The time between sort completion and sample handoff is a variable that researchers often underestimate. If your sequencing facility is on the same campus, a same-day handoff is straightforward. If samples need to travel, discuss the transport conditions with both facilities in advance. RNA in sorted cells degrades measurably within two to three hours at room temperature. If same-day processing is not possible, snap-freezing in liquid nitrogen immediately after the sort is the most reliable preservation method.

Documenting the sort for downstream analysis

The sort report we provide includes the gating strategy, the purity check result, the number of events collected, and the sort time. Pass this document to your downstream facility. It allows them to normalise for sort efficiency and to flag any anomalies in the data that might originate from the sort rather than the library preparation or the mass spectrometry run.

If you are planning a sort for RNA sequencing or proteomics and are not sure whether your experimental design is compatible with our facility, the panel design consultation is a good starting point. We can review the sort parameters and collection protocol before you commit to a booking.