Pharmaceutical & medical
smaller possible dispenses and extremely steady flow throughout a given dispense. Accuracy and precision requirements are also
integral to fluid circuit performance. Accuracy is usually expressed as a percentage of the target volume. Precision is expressed in a CV (coefficient of variation). For applications requiring high accuracy and small coefficients of variation, it is best to size the pump such that the dispensed volume is as close to the full stroke volume as possible. Dispense speed also differentiates pumps.
Generally, the smaller the dispense resolution, the slower the dispense speed. A fine-pitched lead screw and small step angle can also lower the dispense rate. When applications require a higher dispense speed, a pump with a larger dispense volume may be necessary. However, using a pump with a faster dispense speed may have associated trade-offs, including lowering the accuracy and repeatability of each dispense. The dispense speed is also affected by system
parameters such as fluid viscosity and the tubing that connects the pump to the fluid reservoir and dispense point. Tubing with small inner diameters and long lengths can be very restrictive to flow. A pump quickly aspirating through such tubing can draw a vacuum great enough to pull entrained air out of the fluid. This issue is worsened when using fluids with high viscosities. These cases call for slower aspirations and larger tubing diameters to reduce the vacuum in the port head, preventing cavitation. Improperly sized tubing can also cause
pumps. For relatively mild fluids, acrylic (PMMA) pump housings are quite suitable. More aggressive fluids may require the use of engineering plastics such as PEEK. Some fluids may not chemically attack a pump
but will cause physical issues. When using fluids that are prone to crystallisation, the crystals can damage seals in reciprocating pumps which can lead to leakage-related failures. Other fluids that contain surfactants, which lower surface tension, tend to slip past the seals of a pump. A secondary seal may be used with the pump to mitigate these issues. Between the seals, an intermediate fluid can prevent crystallisation and dilute surfactants, greatly extending the pump’s cycle life. The below image shows a cross section of a dual seal porthead configuration.
Dispense characteristics
To select the right pump for your application, it is critical to consider a variety of dispense specifications. One example is the pump’s dispense resolution, which is the smallest volume a stepper-motor driven pump can dispense. Most reciprocating pumps use a stepper motor to turn a leadscrew, which moves the piston back and forth. Several factors determine the resolution: the piston’s diameter, the lead screw’s pitch, and the stepper motor’s angle per step. The resolution can be further modified by microstepping the motor, which subdivides the motor’s step angle. Microstepping allows for finer resolution, enabling
Instrumentation Monthly June 2021
problems. Using outlet tubing with too small a diameter may create a restriction, resulting in troublesome pressure spikes. This is because a positive displacement pump will increase system pressure to overcome downstream system resistance. If resistance is too high, it might be necessary to reduce the dispense speed or increase the tubing diameter. Otherwise, failures such as system leakage and motor stalls can happen. The above characteristics all play an important role in selecting the right pump. Overall, it is critical to determine the priorities of the pump’s specifications for any given application. An analysis of the pump’s function in an application – the dispense and aspiration volumes as well as operational sequences – should help to determine what specs are essential. For instance, some pumps intended for simple operations may only need to perform a single aspirate and dispense at a time. In cases like these, the balance between dispense speed and accuracy/repeatability should be the focus. Other pumps may be required to perform one large aspiration from a reservoir, followed by many small individual dispenses (or vice versa). As the divided dispenses get smaller, the importance of the pump’s resolution escalates. Further operations may have a pump perform many aspiration and dispense cycles to meet a large volume target. For these high-speed bulk transfers, the pump’s volume relative to its allowable size must be considered, and the accompanying tubing must be scrutinised.
ValVes
Many pumps also require the use of inlet and outlet valves. In general, valves and tubing must be selected together so they are compatible with the volume, flow rate, pressure capability, and chemical compatibility requirements of the system. In some cases, simple check valves can be used. When a system requires better performance, actively controlled solenoid valves may be a better option. Inlet and outlet valves play a critical role in the speed and accuracy of the dispense. Valves must operate consistently and quickly enough to open before the pump increases the system pressure to unsafe levels. They must also close fast enough to effectively cut off the flow when needed. It is important to select valves that are compatible with the system fluid and capable of handling the system pressure. The flow rate that the valves can offer is also critical to consider. Small valves with high restrictions will cause the same issues as improperly sized tubing, both on the inlet and outlet. Conversely, a valve that is too large generates a system volume problem. For example, a system that dispenses 1 µL through a large valve with an internal volume of 100 µL needs a large initial prime. When dispensing expensive or limited fluids, this priming can be costly.
General consiDerations
A pump’s internal volume is held within the port head or syringe and can affect a variety of system functions. The image to the left shows the internal volume of a pump’s port head in red. When priming the pump, all air bubbles must be purged out of the pump’s internal volume. Any air left in the pump can lead to erratic dispense accuracies. Pumps with larger internal volumes may require many priming cycles to ensure all air is out of the pump, which should be accounted for during instrument initialisation. Also, the internal volume of reciprocating pumps is not always fully purged with each dispense, so applications that are sensitive to cross contamination may require buffer fluids to keep the sample fluid out of the pump. Internal volume that cannot be easily flushed out is often referred to as dead volume. Even when cleaning cycles are run, a pump’s dead volume may not be completely flushed out. Pumps for medical applications come in a
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