Chromatography
The Importance of Understanding Pump Flow Carlo Dessy, Testa Analytical Solutions eK
Pumps are one of the most common tools enabling liquids to be moved, delivered, pushed through a separation column, supplied as a dose or transferred from one container to another. Several different pumping technologies are available to choose from, depending on your application or problem. As you might expect - a pump designed to pump crude oil through a pipeline has very different requirements compared to a pump designed to deliver a constant stream of medication into a patient’s vein. As such, the term ‘fl ow’ can be interpreted very differently, depending on the context.
This article aims to shed some light onto fl ow of liquid supplied by pumps most used in a laboratory for analytical purposes such as chromatography, and for lab-level fl ow chemistry applications.
The Context
Most modern analytical labs are today equipped with Liquid Chromatography (HPLC, UHPLC, GPC, SEC, Ion-Chromatography) systems. The fl exibility of this technology makes it applicable for an almost endless number of analytical problems. Independently of the particular setup or application, all liquid chromatography systems share the necessity of pushing the carrier solvent (eluent) through the separation column for an undefi ned period of time with the same fl ow rate (defi ned as volume of Eluent / minute) with high accuracy, low fl uctuations and, in most cases, overcoming the resistance given by all connected components including the column, which becomes what we generally defi ne backpressure of a system. This pressure can easily exceed 1200 bar (c. 17,000 PSI), representing therefore a huge challenge for what we would like to be a ‘constant’ fl ow.
Today, reciprocating piston pumps (see Figure 1) have become the gold standard for delivering liquids in liquid chromatography systems. This type of pump is proven to be reliable and accurate, though is sensitive to problems associated with dissolved gases and air bubbles in particular. Reciprocating piston pumps are also widely used when dosage of chemicals in a high-pressure system is needed, for example to produce nanoparticles.
Peristaltic pumps are widely acknowledged to be very easy to use and maintain and are relatively inexpensive. While they cannot achieve high pressure, they can achieve higher fl ow rates than reciprocating piston pumps.
Flow, Flow Rate and Mysteries
Figure 2: Schematic of a peristaltic pump.
Whatever technology the pump in use is based upon, you would trust that the delivered fl ow was constant, and this is where troubles begin. When fl ow is constant, this
simplifi es a series of further considerations, like quantities delivered within any given time, fl uctuation of pressure and an expectation of reproducible elution times. As such, a pump delivering a constant fl ow simplifi es our lab life considerably. Now, the trouble of ‘constant fl ow’ is its intrinsic relation to a timeframe. If we were to measure the volume delivered by a pump over a period of 1 hour, and we repeat this experiment fi nding exactly the very same volume after each hour, then we could indeed defi ne the fl ow to be constant. Moreover, within a given experiment in which 1 hour is a small timeframe compared to the total length of the experiment itself, this defi nition of constant fl ow may very well be accurate and correct. This investigation, however, will tell us nothing about the properties of the fl ow within any one minute of that hour. More accurately, we should therefore examine whether the fl ow rate is constant, therefore we should examine the amount of liquid delivered within a set time.
Let us consider a 15-minute long HPLC application run at 1 ml/min, here 1 minute is the defi ned time period. In this experiment the fl ow rate should be considered constant only if within 1 minute the fl uctuation is zero or extremely small. To prove this, real-time measurement of the fl ow is necessary, suffi ciently fast to supply a relevant number of measurement points within 1 minute so to allow statistical evaluation of the obtained data. The frequency of data collection will have an impact on the obtained fl ow rate, average fl ow rate and extreme values. It must be therefore selected carefully in regard to the time period and set fl ow rate of the application.
Figure 1: Schematic of a reciprocating piston pump.
A second type of pump, commonly used in laboratories for fl ow chemistry, are peristaltic pumps (see Figure 2). The principle of these pumps is extremely simple, a wheel with two or more runners rotates having a fl exible tube fi xed on the perimeter of the wheel. The runners therefore squeeze the tubing during rotation creating a moving closure which in turn forces the liquid to move in the direction of rotation.
Figure 3: The impact on HPLC fl ow data acquired at different frequencies.
INTERNATIONAL LABMATE - JULY 2022
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