首先,液体搬运结果取决于测量结果的可靠性。若测量不当,造成实际搬运量和测量量之间存在很大误差,那么会得出错误的结果数据。因此,选择合适的测量设备至关重要。ISO 8655 和 ISO 23783 对各种测量方法及测量条件做出了描述,包括不同体积范围内允许的最大系统及随机测量偏差。然而,需要满足这样的基本要求,周围环境条件要尽可能保持恒定,以便排除外部和内部影响。
即使测量设备没有问题,实际结果也可能不尽人意,而这些结果并非由分液头或移液头引起。典型的错误及其应对措施如下:
如前所述,外部因素会极大影响搬运结果。尽管如此,即使对特定的分液头和移液头进行微小改动也能改善结果,当移液目标体积较小时尤其如此。Festo 采用压控式分液和移液方法。这种方式必须要使用重复性和再现性良好、且不受停顿时间影响的阀门。在实际分液的过程中,选用合适的滴定头也是非常重要的。
我们可以用 Festo 的滴定头 VTOE 来对此进行验证。通过观察使用不同直径滴定头的分液结果,我们可以得出这两条结论:
Measurement specifications
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:
Measurement specifications
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.
In summary, it is obvious that a good liquid handling result depends on several parameters. First of all, suitable measuring equipment is required. In addition, it is very important to avoid external influences by designing suitable fluid channels, creating a clean working atmosphere and keeping the ambient conditions as constant as possible. And finally, internal influences can be eliminated by using the right components for each application.