Fluorescence-Activated Cell Sorting

Fluorescence-Activated Cell Sorting (FACS) is a flow cytometry method that physically sorts individual cells into separate collection vessels based on their fluorescence profiles ².

How It Works

FACS extends standard flow cytometry by adding a sorting mechanism. After laser interrogation, the fluid stream is broken into droplets, each containing at most one cell. Based on the measured fluorescence signals, an electrical charge is applied to droplets containing cells of interest. Charged droplets are then deflected by electrostatic plates into collection tubes, while unwanted cells pass to waste.

Modern sorters achieve speeds exceeding 70,000 events per second with sort purities above 99%. Multiple populations can be sorted simultaneously into two or four collection vessels. In synthetic biology, FACS is used to isolate cells expressing specific circuit phenotypes, enrich libraries for desired expression levels, and select rare variants from combinatorial libraries.

The technique requires careful optimization of laser power, detector voltages, drop delay calibration, and gating thresholds. Sample preparation, including appropriate fluorescent labeling and viability staining, directly impacts sort quality and downstream experimental success.

Computational Considerations

Automated gating algorithms reduce operator bias in defining sort boundaries, while computational tools predict optimal sort strategies that balance purity, yield, and speed ¹. Machine learning approaches can classify cells in real time across high-dimensional parameter spaces, enabling more sophisticated sorting criteria than manual two-dimensional gates. Post-sort analysis pipelines verify enrichment efficiency and assess population purity.

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