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.
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.