because they are enclosed, but instead use a motor-driven syringe to aspirate fluid containing the event of interest. The decisions as to which "drops" to sort are governed by the same sort logic procedures as on a stream-in-air sorter. However, they are comparatively slow (500 events/s) and are able to sort only one defined population at a time. However, because the system is enclosed, there is reduced contamination risk, they are alignment-free, and they are easier to set up so no dedicated operator is required.

As seen in Table 1, stream-in-air sorters are able to sort at speeds of up to 30,000/s but this is still slow in relation to bulk separation methods such as cell filtration or cell affinity methods (39,40), fractionation (41), or centrifugal elutriation (42). Another way to sort cells is to use magnetic beads (43), it is possible to positively select cells of interest by adding magnetically labeled antibodies to specifically select the population of interest or by negatively selecting a population by adding antibodies, coupled to magnetic beads, specific for cells other than those of interest. Cells are then passed through a column between a strong magnetic field to either elute or retard the population of interest. However, a flow sorter should give a higher purity as well as be able to separate populations on the basis of multiple parameters, the expression of fluorescent proteins, nucleic acid content, and the level of fluorescence expression.

Stream-in-air sorters are also more versatile, being able to be modified, especially in terms of the excitation light sources and the emission optics used. In addition, up to four specifically identified subpopulations may be isolated simultaneously. However, they are more expensive to purchase, maintain, and run and, for optimal efficiency, need a dedicated operator.

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