Chromatography
Comparing techniques for fl ow rate measurement in Liquid Chromatography
Carlo Dessy, Testa Analytical eK
Accurate determination of fl ow rate is one of the most common, and often overlooked, means of assessing or validating the performance of a pump serving a HPLC, UPLC, Ion-Chromatography or GPC/SEC system. Using the right technique, it is nowadays possible to use fl ow rate monitoring as a powerful diagnostic tool, allowing fast detection of leaks, faulty check- valves and worn seals.
Several different technologies might be considered for liquid chromatographic fl ow measurement. It can be expected that each technique will have its own pros and cons. Consequently, prior knowledge of the limitations of each particular technique is of great help in better understanding fl ow measurement results and their positive or negative consequences.
This short paper provides an introductory background to different fl ow rate monitoring techniques and seeks to explain, for both beginner and expert chromatographers, the value of measurements achievable with each method. With this information our aim has been to provide an independent, informed interpretation of the fl ow rate monitoring results and how this might lead to higher confi dence in performance validation of the liquid chromatography system under test.
Necessity for an accurate determination of fl ow rate
Modern Liquid Chromatography pumps are incredible instruments, capable of delivery of a constant stream of solvent at very high pressure and with fl ow rates ranging from a few µL/min to, in the case of preparative systems, several liters per minute. In many liquid chromatography systems, multiple reciprocating pistons are used to achieve the desired pump performance. The value of the fl ow rate is given, in principle, by the mechanical dimensions of the pump piston and chamber assembly plus the linear velocity of the piston in the chamber.
The concept of relating fl ow rate only to mechanical dimensions and fl uid velocity is perfectly adequate in a perfect world. However, In the real world a number of parameters negatively infl uence the real fl ow rate, pressure being one of the most obvious. Pump manufacturers have implemented different ways to correct the real fl ow rate in order to obtain the desired stream of solvent. This correction, however, is infl uenced by the general status of the pump itself and on the particular solvent which is delivered. Pump maintenance, which often includes regular replacement of seal rings,
pistons, and check valves, might mitigate the possibility of error, but a difference in real fl ow rate from the setpoint cannot be excluded completely. Therefore, the performance of your pump must be periodically validated to ensure reliable and accurate fl ow rate data. In other words, the real fl ow rate at a specifi c setpoint must be measured on a regular basis to reveal deviations.
Traditionally, fl ow rate determination involved manually timed measurement of the volume or weight of the solvent delivered by the pump using a stopwatch, a scale or a graduated cylinder. The necessity for very accurate timing in combination with volume or weight measurement techniques makes both methods impractical for use in modern lab protocols. Using these traditional techniques, it has been found that human error can have a huge impact on the results. As a consequence, using automatic fl owmeters, capable of measurement of fl ow rate without direct human intervention, are nowadays, the well-established fl ow monitoring instruments of choice. One further advantage of these modern instruments is the much better documentation of the process done and results obtained, which is particularly important in quality sensitive environments in regulated industries.
In this paper, we defi ne a fl ow meter as a device capable of determining the fl ow rate (usually as volume unit per time unit) delivered by a pump within the context of an HPLC, UPLC, GPC/SEC or Ion-Chromatography context, without the necessity of human intervention.
Ultrasonic Flowmeters
Ultrasonic fl owmeters are not commonly used in analytical liquid chromatography systems. However, they are used with some preparative scale systems if the dimension and material of the tubing allows. The fundamental principle of ultrasonic fl owmeters is based on the measurement of time required by an ultrasound wave to reach two detectors, one placed upstream of the emitters, and the second downstream of the emitter. The measured travel time difference is a function of the fl ow rate. A similar method of determination is based on the Doppler effect, thus frequency shift.
Employing a ‘clamp-on’ principle, ultrasonic fl owmeters are typically relatively small and usefully provide non-invasive measurement. However, limited sensitivity at low fl ow rates and even more limited resolution, make ultrasonic fl owmeters useful as monitoring devices in process applications but much less adequate for the accurate (and therefore high resolution) determination of low fl ow rates as is required in analytical tasks.
LAB ASIA - FEBRUARY 2024
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