The Tip of the Iceberg: How Pipette Tips Influence Results. Part 2: Perfect Geometry Makes a Difference

The fact that a tip fits onto a pipette cone does not say anything about the performance of the pipetting system comprising the components “Pipette and Tip”. We performed a study including standard tips from 15 different manufacturers in order to investigate the tip-related influence on the pipetting result. The study results showed a dramatic influence of the tip on the pipetting accuracy. The standard ISO 8655:2002 [1] recommends using pipette and tips from the same supplier. Our study results emphasize the validity of this recommendation and the need of calibration/pipette adjustment if other tips than recommended by pipette suppliers are to be used. Introduction Within the scientific community, a rising number of published experiments cannot be reproduced by other groups. In general, plastics are not taken seriously leading to problems with analysis results caused by e.g. leachables or incorrect pipetting volumes. This may lead to results not being reproducible if performed by other groups using other consumables. Some problems with pipette tips are obvious, e.g. the need to push tips with force onto the pipette cone in order to achieve efficient tip fit. Other problems often remain unrecognized like decreased pipette accuracy when using other tips than recommended by the pipette supplier. The ISO 8655-2:2002 standard [1] defines pipette and tip as a system, which requires extra calibration for the use of other manufacturers’ tips. But why does this standard put so much focus on a product that is to be discarded after usage? This series of articles answers this question. It shows the influence of tips on the pipetting result explaining the main tip-related impact factors. Material and methods General material Eppendorf Xplorer® plus 50 –1,000 μL and 0.5 –10 μL were used. Racked 10 μL and 1,000 μL standard tips of Eppendorf and 14 other manufacturers have been tested. Exceptions: Manufacturer H did not offer racked 10 μL standard tips, manufacturers K and N offered only 1,250 μL tips for 1,000 μL pipettes. Calibration by gravimetric method The performance of the system “Pipette and Tip” was determined by calibration according to [1]. Environmental conditions were set according to requirements [1]. Calibration was performed using analytical balance Model XP26PC (MettlerToledo®) at 100 % nominal volume and 10 % nominal volume. Two series of 10 pipettings were performed. Systematic error and random error were determined for each series of 10 measurements and compared to specifications [1] and [2]. For further information please refer to [3]. Results and discussion While being perfectly within error limits with Eppendorf tips, we found the system “Pipette and Tip” to be out of specifications when using other manufacturers’ tips. As shown in Fig. 1 and 2, the allowed systematic error was exceeded with 4 manufacturers’ tips at a volume of 1,000 μL and with 5 manufacturers’ tips at 1 μL test volume. These tips with 1,000 μL test volume exceeded not only the manufacturer specifications but also the wider limits for systematic error as stated by the ISO 8655:2002 standard [1]. The random error was noticeably increased but stayed within allowed tolerances. When comparing the calibration results with the outcome of dimensional measurements, it becomes clear that with 1,000 μL the biggest impact factor is the air-cushion size: Those tips that produced error limits beyond the pipette MURIEL ART, VINCENT DUFEY, IOAN GLIGOR, EPPENDORF APPLICATION TECHNOLOGIES S.A., NAMUR, BELGIUM ULRIKE GAST, RONJA KUBASCH, EPPENDORF AG, HAMBURG, GERMANY R an d om e rr or [ % ] Systematic error [%] 10 μL