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High Performance Liquid Chromatography - Lough W.J.

Lough W.J. High Performance Liquid Chromatography - Blackie academic, 1977. - 282 p.
Download (direct link): highperoranceliquidchromatographi1977.pdf
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Figure 5.3 An arrangement to overcome the problem of pulsation using two reciprocating pistons, operated by the same motor but with their pumping cycles 180 out of phase.
the use of pulse dampening systems, and electronic control of piston speed.
Figure 5.3 shows an arrangement in which two reciprocating pistons are operated by the same motor, but due to the arrangement of their cams, their pumping cycles are 180 out of phase. Thus, when one piston is driving, the other piston is refilling, and vice versa. With clever cam design, the crossover between the driving phases of each piston can be extremely smooth, making the flow output from pumps of this type extremely stable. Dual piston pumps of this general type form the bulk of those in current analytical HPLC use. There was a brief trend towards pumps with three pistons during the late 1970s. Each of the three pistons was operated 120 out of phase, which resulted in an almost completely pulse-free delivery. However, the additional piston, seals and check-valves meant additional maintenance problems. HPLC pump technology in the meantime advanced to the point where equivalent performance could easily be obtained from a dual piston system, largely consigning the triple headed monster to the shelf of chromatographic history.
5.2.2.2 Syringe pumps. One of the first types of HPLC pump to be specifically built for the purpose was the syringe type. Such pumps are essentially very simple in design, employing a stepper motor to drive what is effectively the plunger of a large syringe to push mobile phase at a constant flow rate through the column. The capacity of the syringe can be virtually anything up to 500 ml. The flow rate from syringe pumps is extremely constant, and can be accurately and precisely controlled by altering the speed of the stepper motor.
Syringe pumps are not widely used, due to the tedious need with some models to dismantle the syringe in order to refill it with mobile phase. Other designs of syringe pump do not need to be dismantled for refilling, but use a check-valve arrangement to allow the reverse stroke of the plunger to automatically refill the syringe chamber. However, this process itself is very slow.
A further problem associated with syringe pumps arises from the fact that most of the fluids used as mobile phases in HPLC can be compressed, to some degree. This is especially the case with the purely organic solvents used in normal phase HPLC, some of which are reduced in volume by up to 0.015% for each bar of pressure that is applied. So for example, 200 ml of a mobile phase having a compressibility of 0.01% per bar, being driven at 100 bar to give a flow rate of 1 cm3 min-1 will require at least 10 minutes to reach its nominal flow rate. The initial displacement of the plunger only serves to compress the mobile phase.
Syringe pumps are very well suited to very small-bore HPLC. Such separations require the very low flow rates and excellent precision provided by syringe pumps. Due to the very small amounts of mobile phase consumed, the syringe has to be refilled only infrequently, making that aspect of the use of syringe pumps much less of a disadvantage. With the growing interest in small, and micro-bore, LC it is likely that the use of syringe pumps will increase once more.
5.3 Approaches to the reduction of flow pulsation
As stated above, it is highly desirable that the flow generated by a pump should be practically free from pulsations. Pulsations in flowrate can reduce the sensitivity of a method, and in extreme cases, damage the column. The most commonly used class of analytical HPLC pump, the reciprocating piston type, inherently provides relatively constant flow -particularly when multiple pistons are used. However, the current drive towards the detection of smaller and smaller quantities means that levels of pulsation that were perfectly acceptable only a few years ago are now regarded as unsuitable for many applications. The minimisation or eradication of pump pulsation may be achieved by mechanical or electronic means, or by a combination of the two.
5.3.1 Mechanical or physical pulse damping
A number of physical devices have been developed to minimise flow pulsations. The most simple examples employ a reservoir of fluid between the pump and the injector, which absorbs some of the excess energy of the pulse. At its simplest, this may comprise a simple length of tubing placed in-line after the pump. Alternatively, air-filled snubbers may be used. These are made up of a length of tubing which is sealed at one end. On the driving stroke of the piston, the air in the tube is compressed. On the reverse stroke, the air expands again, the overall effect being a smoothing of the flow profile. A further system employs a diaphragm which divides the mobile phase from a chamber filled with another fluid. Pulses in the
mobile phase are transferred to the fluid on the other side of the diaphragm, resulting in a smoother flow reaching the column.
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