Advantages and disadvantages of syringe-type pumps

Syringe-type pumps deliver a pulseless flow, thus offering better baseline stability and a lower detection limit than pulsating pumps. They are generally silent, reliable and easy to operate. 

However, these pumps tend to be expensive. With such pumps, direct pumping recycle chromatography is impossible. Another disadvantage is the limited volume of solvent held in the cylinder (although sometimes two pumps can be used in tandem). This volume is 250-500 ml for most of these pumps, whereas the flow--rate through LC columns is usually 0,2 to a few ml/min, so that this disadvantage is not serious in analytical separations, but remains important in preparative--scale chromatography. 

It seems generally accepted that these pumps offer a very constant flow-rate, independent of the value of the downstream back-pressure and of its possible variations. In fact, it has been shown that, because of the compressibility of liquids, the time necessary to achieve a steady-state flow-rate can be long, typically 15-60 min. During this time, the flow-rate increases continuously. 

Because the retention volumes remain unchanged when the flow-rate varies, however, the use of a volume-based recorder is recommended for comparing retention data. 

This problem has been solved by Varian through the use of a high piston-speed for fast compression and a check valve which opens only when a pre-set pressure is achieved. Then the piston speed is reduced to the corresponding value. If correctly used this new system permits reproducible measurements in routine analysis. 

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