Pipet tip quality For the most accurate and precise pipetting results, the pipet manufactur-


er’s recommended tips should be used. Achieving a proper seal between the pipet’s nose cone and the tip is critical for good performance. Some generic tips may seemingly fit on a pipet, but due to different taper angles of the nose cone and tip, a poor seal is established, resulting in errors. In our study, the generic tips fit on the pipets but still introduced errors of up to –0.6% RI and 0.8% CV, which would be additive to all other pipetting errors. If high-quality third-party tips are to be used, it should be verified that they fit well and form a tight seal with the intended pipet model.


Claimed pipet performance assumes the use of manufacturer’s tips. When calibrating a pipet, it is imperative that it be calibrated with the same tip type and under the same conditions of its use in the lab in order to avoid errors when using the pipet for analytical tests.


Pipet size Adjustable-volume pipets can be used over a large range of volumes.


Manually operated pipets usually allow the user to select volumes as low as 10% of the pipet’s nominal volume, while some electronically operated pipets offer an even wider range of selectable volumes. Best pipet perfor- mance, however, is achieved at or near the nominal volume of a pipet. For best results, it is recommended to use variable-volume pipets only down to the nominal volume of the next available, smaller denomination of pipet.


Best pipetting practices The results of this study demonstrate that even minor variation in the


operating technique of handheld air-displacement pipets can result in measurable errors in accuracy and precision. This study did not evalu- ate errors resulting from combining multiple of the discussed technique variations, although this is commonly observed in the field. Compounded errors can easily reach 12%, and are often even larger, as data from field surveys suggest.


The following steps will ensure the most accurate and precise results: • Prewet tips at least three times • Use proper pipetting mode • Work at temperature equilibrium • Immerse tips to proper depth • Aspirate with pipet in vertical position • Pause after aspirating • Do not touch vessel wall during or after aspiration • Use consistent plunger speed and pressure • Minimize heat transfer from hands • Avoid tip wiping • Examine tip prior to dispensing • Use high-quality pipet tips • Use proper pipet size.


References 1. Rodrigues, G.; Curtis, R. Instrument performance verification: micro-


pipets. Practical Approaches to Method Validation and Essential Instru- AMERICAN LABORATORY • 17 • JUNE/JULY 2014


ment Qualification; Chan, C.C., Ed.; John Wiley & Sons, Inc., New York, NY, 2010, pp 327–46.


2. Carle, A.B. Dry heat and humidity—lab environmental conditions as sources of error. Lab Business Winter 2009, 16–20; http://www. labbusinessmag.com/articles/winter09dryheat/dryheat.html.


3. ISO 8655-2:2002, ISO 8655-6:2002, ISO 8655-7:2005; Piston-operated volumetric apparatus. Part 2: piston pipets. Part 6: gravimetric meth- ods for the determination of measurement error. Part 7: non-gravi- metric methods for the assessment of equipment performance. ISO, Geneva, Switzerland; http://www.iso.org.


4. Carle, A.B. Minimizing liquid delivery risk: laboratory environmental conditions as sources of error, part 1—barometric pressure and ther- mal disequilibrium. Am. Lab. News 2008, 40(2), 8–10.


5. ASTM E1154-89(2008). Standard specification for piston or plunger operated volumetric apparatus. ASTM International, West Con- shohocken, PA; http://www.astm.org/Standards/E1154.htm.


A. Björn Carle, Ph.D., is Product Manager; Doreen Rumery is Laboratory Tech- nical Manager; and George Rodrigues, Ph.D., is Senior Scientific Manager, Artel, 25 Bradley Dr., Westbrook, ME 04092, U.S.A.; tel.: 207-854-0860; fax: 207-854-0867; e-mail: bcarle@artel-usa.com; www.artel-usa.com


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