Calibration of Air Displacement Pipets Piston-activated (a'ir-displacement) pipets have been used in our biochemistry laboratory courses for a number of years, and with the incorporation of microscale experiments into our organic chemistry course many more students are being introduced to these devices for the first time. Although these pipets allow the easy and convenient delivery of small (0.01-1.0 mL) volumes, it is too often assumed by students that the nominal volume is exactly delivered. In order for students to gain a better appreciation of the use of these devices, an early experiment in our biochemistry lab includes the calibration of a number of air-displacement pipets. In this experiment it is emphasized that the volume delivered depends not onlyupon the proper use ofthe pipet hythe experimenter but other factorsas well, suchas the working condition of the pipet and the nature and temperature of the liquid. Pipets that deliver at least 0.02 mL are calibrated gravimetrically by pipetting water consecutively 10-20 times into a preweighed beaker, and the weight of the beaker plus the water is determined after each delivery (the pipet tip is not changed between deliveries). The volume delivered ia calculated from the known density of water a t the measured temperature, and a mean volume (accuracy) plus a standard deviation (precision) for each pipet are calculated from the data. During the prelab lecture, pipetting technique is reviewed and demonstrated by the instructor, and each student is asked to calibrate several pipets having a range of volumes from 0.02 to 1.0 mL. This calibration exercise can be used as a means to detect students who need to improve their pipetting skills. Precisions of 0.1-0.8% can be expected for delivery of the volumes indicated above'.2. and so students who submit data havine errors ereater than 0.8% can be given further mrtrurtion cimr~rningproper pipetting rrchnique. Orhrr student.+ xho pipct repnrduclhle, hut inaccurate, volumes can alw he d~recredby having three or four atudrnts ralihrute the mne pipct and then &atistirally comparing the average volumer strained by each nxen~brroi thr group. A n impwprrlg pipetted vulume uill he auhxtant~all)dtr'frrent t'rwm volumes that were delivered correctly. T o show how pipetting procedure may affect the results, each student is asked to recalibrate a pipet using the reversemode pipetting method.' Chou e t al. compared the precisions of conventional mode and reversemode pipetting methods. They reported that the reverse mode appeared to be more p ~ e c i s eThe . ~ data obtained in our laboratory generally seems to SUDDOrt .. this conclusion. Finallv. .. as ademonstration of how the conditionof the liouid to he samoled can affect the volume oioetted. the students recalibrate an air-disolacement oioet "sine methanol as solvent. This exoeriment is oarticularlv ~~~~~~, in~tructivesinwthe vulumeufmethand udl hrsuhrtnnriallygr~nr~rthnn thrvolumro! u,atrrdrlivrred by thesame pipct. Assured ahow. via thisexperiment several aspects uf ptprttmg techniqupnre pointed out to the students. I n additiun. a list of references is made available to them in which these as well as other parameters that affect piston-activated pipet performance are discussed. (This bibliography is available upon request from the authors.)
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Liodd Measurement:. Rainin Instrument Co.: Woburn., MA.. 1987. ~
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Inrtrurtion manual for usr of the Kppendorf 1)iyitol I'iprttr, Hrinktnan I~wtrumenrr,Inc, U'rsthury, NY. 11590. T h o u , P. P.;.lanes, P. K.; King, H. I'.. Chapman, E.; Ha~ley.J. I.. (?tmrol C'hum. 1984,30,958. Edward L. Ferronl and Katherlne M. Lelang Illinois Benedictine College Lisle, IL 60532
596
Journal
of Chemical Education
