Accuracy micropipette

In order to indicate the accuracy that can be obtained from a capillary pipette, one may refer to the so called Sahli pipette, which is approximately 80 mm from mark to tip. This pipette is used as a washout pipette and can measure samples which are readily reproducible to 1 part in 80, since it is a relatively simple thing for the eye to see 1 mm. On the other hand, if one places a bulb in the tube, and cuts the bore down at the mark by 30%, then this would decrease the error by a factor of 4. It is thus practicable to sample from a micropipette, which is used as a washout pipette, with greater accuracy than one can sample from a conventional macro pipette, which delivers 1 ml by blowout, or to deliver. Figures5 and 6 illustrate various designs of micropipets (13,14). 

Fig. 2. Schematic illustration of a newly designed friction apparatus. The valve A is opened immediately after the valve B is closed when the measurement begins. The velocity of water flow in the micropipette is measured by a microscope which is set on a mechanical taranslational stage with micrometers with an accuracy of 0.001 mm. After the measurement, the value A is dosed and the valve B is opened to equalize the pressures of the either sides of the sample gel. The valve C is used to change the hydrostatic pressure...

The tips used with the micropipette contribute greatly to the accuracy of the analysis and different types are better suited to certain uses and volumes. In general, the dead air space between the sample and the plunger seal should be kept to a minimum since this can seriously affect both accuracy and precision, due to the air expanding in response to even very small... 

For injection of the water sample into the atomiser, micropipettes are used these are now commercially available and commonly specified to a 1% accuracy. Pipette tips are known to be contaminated with Fe, Zn and Cd, thus they should be soaked in 10% nitric acid and then washed in distilled-deionised water and sample prior to use. Accurate, precise pipetting and the correct adjustment of the drying, ashing and atomisation programme are essential factors required for a successful flameless atomic absorption analysis. When pipetting the sample, the water droplet must be positioned reproducibly on the filament or in the furnace and it should be of an optimum size such that it does not run or spit during heating. If this happens, irreproducible absorption peaks may result. 

Before measuring a volume it is important to choose the most appropriate equipment in order to achieve the greatest accuracy. The volume and level of accuracy will help determine which piece of equipment should be used. The most common equipment includes various pieces of glassware (e.g. volumetric flask, measuring cylinder, burette, pipette), mechanical micropipette (or pipettor) and syringes (Figure 2.3). 

A line is drawn witii a pencil p allel to, and 2 cm from, the bottom of the plate. The s ples e spotted on to this line, called the origin, starting 2 cm from die side of the plate and at least 1 cm fi-om each odier. The sample, normally 1 to 10 pg of material depending on the thickness of the plate, is applied in as small a volume of solvent as possible (usually 1 to 10 jU). It may be applied by a micropipette (commercially available products deliver a known volume with an accuracy of 2%), by a capillary tube drawn out to a fine point, or by a calibrated micro-syringe. Whichever way it is applied, it is vital diat the spot is no more thm 4 mm in diameter or resolution will be lost. The plate surface must not be cut or gouged by the applicator. The solvent used to apply the spot should be volatile and have low polarity so that the spot does not difruse too much. The solvent may be applied to the plate in aliquots, and dried off naturally or by use of a hot air blower. It is essential that the spot is dry at the end of application, especially if the solution contains water. Even a small amoimt of a polar solvent adsorbed on the plate can drastically alter chromatographic properties. 

Figure 16.16 presents the 02 response profiles for the hybrid class II xerogels prepared by hand using micropipettes (A), and the ALHS (B). Inspection of these results demonstrates again how critical accuracy and precision are when attempting to characterize formulations in which the molar ratio of the precursors is varied by a small... 

To increase the mass transfer rate, Tokuda et al. [7] carried out normal and differential pulse voltammetry at micropipettes and extracted the rate constant values within the range from 0.009 to 0.2 cm/s for facilitated transfers of Li+, Na+, Ca2+, Sr2+, and Ba2+ to nitrobenzene (NB) with two different crown ethers (DB18C6 and DB24C8). The assumption of a = 0.5 for all IT reactions and the use of TR-drop compensation may have affected the accuracy of those results. The upper limit for the measurable rate constant was about 0.5 cm/s, too slow to probe facilitated transfer of potassium ions. 

P) Micro bulb pipettes exist in many forms. The microcaps (Firm 51) are intended for a single use. These are precision capillaries of capacity 1 — 100 [xl. They may be used with or without a filling device (Fig. 20b). Accuracy of better than 1 % is claimed. The self filling and adjusting Lambda pipette (Fig. 20 d) exists in the normal form with mark (Fig. 20e) and in several other forms from 1 to 1000 d (Firm 117). The automatic micropipette is based on the same principle (Fig. 20 c) (Firm 33). 

The accuracy of the IT kinetic measurements performed with micropipette tips may have been affected by their small RG, that is, the ratio of the outer glass radius to the aperture radius, which is normally <2 [68]. In earlier SECM literature, the theory was developed for RG= 10 that is typical for metal-in-glass tips. Because of this issue, no satisfactory theoretical fit could be obtained for approach curves of facilitated potassium transfer either for the diffusion-controlled positive feedback or pure negative feedback [65]. Both conductive and insulting curves were fitted later to the theory developed for small RG values [69] (see Chapter 5). However, the SECM theory for finite substrate kinetics and small RG was developed much later [70], and to our knowledge has not yet been used in any study of IT at the ITIES. 

---