Comparison of Pipets: A Laboratory Experiment
The variety of pipetsavailable today makes it imperative that the student he acquainted with the advantages and disadvantages of each type.'l'he intnrduction of the "pushbutton" pipertor with plastic tips has made the transferof liquid* more rapid and easier than with t h e c k i c a l t)'pesr,ipippts IlamMa. Mohr, serological, and volumetric). Whether these new pipets are more precise andlor aceurate is a consideration for the user as they become commonly used to the exclusion of the classical pipets. This experiment was devised to teach the student to evaluate the precision and accuracy of the volumes de- livered by the various types of pipets. In addition, it demonstrates the direct application of a statistical method for the comparison of and calibration of the pipets. One of the companies that manufactures plastic tips for the pipettor has provided the procedure that can be usedfor thecomparison ofthepre- cision of vnrious tins.' .~ ~~'~
'l'he measurement of thedelivery volume is made by weighing theamount ofdistilled water delivered hy the pipet. This is done in a heaker to which ten deliveriesof volumeare made. l'he beaker ra weighed at the start of the lirst delivery and after eachdelivery. The lemprratureof the water is recorded su that the absolute value of each delivered volume can be calculated.
The average and standard deviation of the volumes for each pipet is calculated using standard techniques. Comparison between types of pipets of the same volume are made using a student's t - test.
We have used the Oxford Samoler Micronioettine Instrumentzas an examole of the oushbutton ~ ~~ -~ ~ ~~~~ ~ .~ . ~ ~~ ~ .~~~~ ~.~ pipettors. T h i ~ pipettcn provides three volumes-2W. 500, and IMX) ul. For comparison with the firer two volumes, we use Lang-I.e~y m~erupipeu and the conventional I -ml vnlumetric pipet for the highest volume. The choice of pipets for comparison is often dictated by what pipets are readily available. Other types of pipets could be included, and the selection and number will depend upon the time available to the student to perform the comparisons.
The student is required to report the accuracy of the various pipets in addition to the precisian. The results of a t-test between airs of ~ i o e t s are also reported. For ease of calculation. the t -test can be performed with the university's combtiter, thus provihing the student with some experience in the use of computers. The student is also requested to evaluate the pipets with regard to ease ofuse, accuracy, reliability, etc.
"Procedure for comparing Precision of Pipet Tips," Bio-Rad Laboratories, Richmond, CA 94804 Catalog No. 8M9, Oxford Laboratories, Foster City, CA 94404
T h e Catholic University of America Leopold May Washington. DC 20064
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