Ultramicro pipets

Figure 6, Self zeroing ultramicro pipets. Ay constriction at two marks By overflow type with constriction at both ends C, Lang-Levy pipet D, another type of constriction pipet Ey Speedipet of Natelson,...

Fig. 1. Ultramicro pipets, self-zeroing. A, constriction pipet to deliver from mark to mark (N2) B, overflow type showing separate holder and rubber ring to connect holder to pipet C, constriction pipet (Lang-Levy) D, capillary rise pipet (ABA-type) E, constriction pipet washout (N2).

Fig. 4. Ultramicro pipets according to Sanz (S3). A, sample pipet B, reagent pipet C, reagent pipet for corrosive liquids,...

The quality of apparatus is crucial. The quality of the pipets directly affects the reliability of the analytical information. It is necessary to use measuring pipets of high quality for reliable measures of volume, e.g., clinical, serological pipets, micropipets, syringe pipets. Syringe pipets must be used for measurement of microliter volumes. In volumetric analysis more reliable burets are necessary. For small quantities of sample, micro- and ultramicro-micrometer burets are recommended. Only well-calibrated pipets and burets will assure the accuracy of volume determination. 

While specificity of a test is nearly independent of the equipment (not completely, e.g., spectral purity of colorimeter filters ), the introduction of more or less automatic apparatus makes accuracy more and more dependent on the accuracy of these apparatus. Some authors state that ultramicro methods are more accurate than macrotechniques. In our opinion and experience this is not true, the accuracy of most ultramicro methods being the same or somewhat less. The quantity of sample generally used is 10-20 [il. The reproducibility of pipets in this range is very good. It is quite possible to perform determinations on much smaller samples [see, for instance, the outstanding work of Lowry (LI) on enzymes in single nerve cells and also the methods of Ames and Nesbett (Al)], but either the speed or the accuracy diminishes when using pipets of less than 10 [Jil. 

Place the sample to be anal) ed (1 mg or less) in a 100-jxL conical vial and add a few drops of a solvent to dissolve the sample. Use a capillary miaopipet to apply a small fraction of the solution from the vial to the plate (Fig. 5.35). (These pipets are prepared by the same technique used for constructing the capillary insert for ultramicro boiling-point determinations, see Chapter 4.) Apply the sample to the adsorbent side of the TLC plate by gently touching the tip of the filled capillary to the plate. Remove the tip from the plate before the dot of solvent grows to much more than a few millimeters in diameter. If it turns out that you need to apply more sample, let the dot of solvent evaporate and then reapply more sample to exactly the same spot. 

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