Pipette Tips: Determining Stiffness with a Bend Test

Pipette Tip Bend Test

A pipette (also spelled pipet or pipettor) is used to aspirate and dispense fluids in wells for analysis and further testing.  There are many types of piston pipettes, but all work on essentially the same principle; hand or mechanical pressure on a piston or plunger working over a fixed length in a cylinder forces a pre-determined volume of liquid out of the orifice of the pipette.  Pipettes are used to hold body fluids, oily fluids, concentrated acids, nucleic acids, and volatile compounds.

An ADMET customer presented us with a problem.  Their plastic disposable pipette tips lacked sufficient stiffness to ensure the customer’s success in dispensing fluids. The objective was to find a testing machine with servo control and constant rate of crosshead movement to apply a constant bending force to the end of the pipette tip. By acquiring the flexural strength of the pipette tip, it was possible to determine whether the stiffness was at an acceptable level or if the manufacturing process needed review.

ADMET designed the fixture pictured below to hold the pipette tip by the connection end.  By loosening the hex screw on the slide base, this fixture is adjustable for different length tips.

pipette bend test fixturepipette bend test fixture

Although the system, ADMET’s eXpert 5601, is capable of up to 25 in/min, the compression bending test profile stated 2 in/min until a load limit of 6 lbs was achieved. The customer felt that if the tip bend at 6 lbs was 45 degrees or less, the tip was stiff enough and production could continue.  Each tip was put through a variety of test profiles before determining the max load that would ensure that the stiffness and angle to meet the production requirements as well as being able to compare with competitor’s similar product characteristics.  ADMET’s MTESTQuattro software enabled the Quality Manger to easily set up testing profiles, define operator and user designations, and analyze individual and group test results. By obtaining the data from the testing, management can make decisions to avoid significant revenue loss.

ADMET’s eXpert 5601 and ADMET eXpert 7601 are single column, servo controlled Universal Testing Machines. These systems can be equipped with a variety of grips and fixtures for pipette tip, tubes, microtubes, cap tubes, packaging, connectors, syringes, plungers, plastic tubing, and more.

More Information about Pipettes, their Design, Use, and Best Practice

All piston pipettes are fitted with some form of replaceable tip to minimize the risk of contamination. Several manufacturers are producing a variety of disposable plastic tips.  Pipette manufacturers recommend using the tips designated for the specific pipette in use, as these can be guaranteed to fit the pipette properly and have the correct internal volume.

Many pipette tips on the market look fine, but when studying them more carefully, it appears that the price really reflects quality. Many of the cheap tips may have flashes, protrusions, scratches, and air bubbles, or be bent or contain impurities. All these influence pipetting results. For example, if the tip is not straight this alone may result in a 10% error in pipetting accuracy (ISO 8655-2).

Moreover, if the tip orifice is not accurately centered, the dispensing of the liquid might be affected significantly.  If premium grade pure polypropylene is not used or the mold and processing are not up to the highest standards, part of the liquid stays inside the tip as a droplet, which causes an error in the pipetting result. Lastly, when the fluid is dispensed from the pipette tip, it should be at a 45 degree angle as described in the Best Practices for Pipette Testing and Equipment Maintenance article from the  National Laboratory.

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By |February 2nd, 2012|Test How To's|Comments Off on Pipette Tips: Determining Stiffness with a Bend Test