In dispensing or pipetting applications in laboratory automation, it is important to obtain reliable results, which are analysed by calculating the accuracy and precision. To achieve these results, several influencing factors have to be taken into account, from the measuring equipment to external and internal influences.
Liquid handling results depend first and foremost on reliable measurement results. If, for example, there is a large discrepancy between the delivered and the measured volume because of poor measurements, this will lead to misleading performance metrics. It is therefore crucial to use appropriate measuring devices. ISO 8655 and ISO 23783 describe different measurement methods and conditions, such as a the maximum systematic and random measurement errors that are allowed for various volume ranges. However, a basic requirement is to keep the ambient conditions as constant as possible and to exclude both external and internal influences.
Even if the measuring devices are functioning correctly, unsatisfactory results may occur that are not caused by the dispensing or pipetting head. Some typical mistakes and countermeasures include:
As already mentioned, external influences can drastically affect the results. Nevertheless, even small changes to a particular dispensing and pipetting head can improve results, especially for small target volumes. Festo uses a pressure-controlled dispensing and pipetting approach. In this case, it is absolutely crucial to use a valve with good repeatability and reproducibility without pause time effects. When it comes to dispensing applications, choosing the right needle is also very important.
Let's check this behaviour using an example with the Festo dispense head VTOE. The dosing results with different needle diameters reveal two main findings:
Looking at the corresponding channel CV values (also called intra-run or intra-assay CV) as a function of the dosing results of a needle, two further important findings are immediately noticeable:
The reason for higher CVs with shorter pulse times is simple: there are many parameters and properties that can vary slightly when opening and closing the valves (e.g. time resolution of the control unit, switching behaviour of the valve, liquid flow, etc.). If the pulse time is low, the time portion of switching on and off is higher and therefore has a greater influence. This leads to a greater dispersion of the dosing results and poorer CVs.
To avoid this problem, the findings from the first diagram are significant. By using a needle with a lower flow rate (e.g. smaller inner diameter), the pulse time can be increased to achieve a certain amount of volume. This improves the CV values again while maintaining the same target volume.