Background: The sample injection systems of flow cytometers employ either a pressure differential between the sample vial and the sheath fluid reservoir or volumetric injection of the sample from a syringe. The pressure differential method facilitates rapid and efficient flushing to eliminate carryover between samples, but does not allow accurate determination of the rate of sample flow and cell concentration. Volumetric injection, which comprises a valve for switching the sample flow, facilitates highly accurate measurement of the cell concentration, but requires a less efficient and more time-consuming flushing procedure. Methods: Applying a removable syringe, which connects to the inlet of the sample tubing via a tight sealing, we eliminate the valve and obtain efficient flushing while maintaining the advantage of volumetric sample injection. Results: This device gives highly constant sample flow rates strictly proportional to syringe velocity over the range 0.2-50 mul/min with a settling time of about 2 sec. Conclusion: This device has the same precision as the conventional sample injection system, whereas the speed and efficiency of flushing are improved greatly. (C) 2002 Wiley-Liss, Inc.
