Chromatography
Comparing volumetric and thermal fl owmeters for assessing and validating liquid chromatography performance
Carlo Dessy, Testa Analytical Solutions eK
Assessing and validating the performance of a pump serving HPLC, UHPLC, Ion Chromatography or GPC/SEC system is a crucial task in many laboratories nowadays. For this purpose, accurate determination of fl ow rate is a powerful tool. Using a real time fl owmeter, fl ow rate monitoring of liquid chromatography systems has been demonstrated to be a powerful diagnostic tool, allowing fast detection of leaks, faulty check-valves and worn seals. Traditionally, fl ow rate determination involves 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 laboratories short on time and trained personnel.
These traditional techniques are also prone to human error, which may have a signifi cant impact on the results. Consequently, using automatic fl owmeters, capable of real time measurement of fl ow rate without direct human intervention, is increasingly becoming the technique of choice. An additional advantage of this new generation of fl ow measurement devices is the much better documentation of the process undertaken and results obtained, which is particularly important in quality sensitive environments in regulated industries.
Commercial volumetric fl owmeters
A volumetric fl owmeter is the automated version of the classical ‘Stopwatch and graduated cylinder’ method. A fl owmeter of this type consists of a tube and two optical level sensors placed at a known distance from each other. The time the solvent front takes to travel from one light trap to the next is a function of the fl ow rate and the volume of the tube, which is constant. After each measurement Is completed, the tube must be automatically emptied so that the measurement can be repeated. As such, a volumetric fl owmeter is not capable of continuous measurements.
Volumetric fl owmeters are commonly used in conjunction with HPLC systems as validation tools. It should however be noted that because they are based on the measurement of time required to fi ll a known constant volume, that the accuracy of results decreases markedly at higher fl ow rates.
A new generation of thermal fl owmeters
Thermal fl owmeters were one of the fi rst instruments used for the measurement of fl ow rate in liquid chromatography. The method these devices use is based on the measurement of the difference in temperature between two temperature sensors, one placed upstream of a heating element and the other placed downstream of it.
Figure 2: Scheme of Testa thermal fl owmeter. Figure 1: Schematic of operation of a typical volumetric fl owmeter.
For analytical purposes, volumetric fl owmeters must be positioned perfectly vertically and at the end of a liquid chromatography system. As a result, their use is limited to confi rmation of performance of the pump, no real diagnostics are possible as each value reported is not related to the previous one.
Furthermore, as the fl ow rate is averaged over the measurement volume, you cannot determine pulsations or variations in fl ow with a duty-cycle shorter than the time required to fi ll the volumetric tube. However, this technique has been proven to be very reliable in determining the fl ow rate of aqueous HPLC separations. Unfortunately, the different surface tensions of other solvents do not allow volumetric tubes to be voided with the same effi cacy as with water.
For applications with a fl ow rate of a few microliters per minute up to several millilitres per minute, thermal fl owmeters typically employ a quartz fl ow measurement tube. Introduced in 2021, the new generation of thermal fl owmeters from Testa Analytical have been demonstrated to work with virtually any solvent. Benefi ting from advanced design and modern microelectronics, all components necessary for fl ow measurement using these devices are located on the outer wall of the quartz tube and have no contact with the liquid. Measurement using these devices is therefore completely non-invasive. These fl owmeters come with fi ve solvent calibration factors: water, methanol, THF, acetonitrile- water 40:60 and ethanol water 10:90. Other common solvents have been tested for compatibility and the fl owmeters can be calibrated accordingly upon request.
Being non-invasive, the thermal fl owmeter ensures the unperturbed operation of the whole liquid chromatography system. Measurement with thermal fl owmeters is continuous, allowing use of these devices for real-time monitoring of pump performance.
Unlike volumetric fl owmeters which only offer a fi xed data acquisition frequency for any given fl ow rate, frequency of data acquisition using a thermal fl owmeter is independent on the fl owrate and can be selected allowing a high resolution of up to 12 data points per second.
LAB ASIA - FEBRUARY 2025
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44
