Micropipet

Wang and Taha described an interesting application of potentiometry called batch injection. As shown in the following figure, an ion-selective electrode is placed in an inverted position in a large-volume tank, and a fixed volume of a sample or standard solution is injected toward the electrode s surface using a micropipet. 

In one biosensor design, the chem preceptive nerve fibers of the anteimules of the blue crab Callinectes sapidus are coimected to a micropipet electrode. This assembly has been termed a receptrode (2). The receptrode created from Callinectes sapidus responds to the presence of amino acids (1) (qv) in concentrations as low as 10 M. 

Hi) Using a 5- iL micropipet, carefully blow 1 p,L of the sample solution onto the surface of the matrix. 

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). 

It is advantageous to attach the micropipet to a sampler diluter. In this manner, the sample is aspirated by means of a plunger. The technician is not required to adjust the volume nor to make a judgement, because this is being done by a plunger, and accuracy is a function of the sampler diluter. If then one wipes off the tip and then ejects a diluting fluid, one could then proceed to measure out very small samples limited only on the nature of the tip. This can be seen in the sequence of events in Figure 7. 

Fig. 6. Double-barrel micropipet ISE for measuring transient intracellular ion activities, (From 11, with permission)...

Frog heart ventricle Cr liquid membrane micropipet (Corning exchanger 477915) Cl" 17.6 + 0.57 mM 136)... 

Neutrons of the molluse, Aplusia californica Ca " liquid membrane micropipet (ion-exchanger type with PVC added) Ca + 540nA/ 139)... 

Vacnoles of plant cells (A cer pseudoplatanus) H liquid membrane micropipet (tridodecylamine as neutral ligand) pH 6-6.5 140)... 

Renal tubules of rats K liquid membrane micropipet — double-barrel design (valinomycin) K (proximal tubule) 54.4 + 2.5 mM 141 ... 

Muscle fiber of frog satorius liquid membrane micropipet —double-barrel design (valinomycin as carrier) K+ 105 mM 14S)... 

Chironomus parvae salivary eland cells Na liquid membrane micropipet [ion-exchanger based on tetra(-p-chloro-phenyl)borate] Na 16 mM 149 ... 

Y. Yuan and S. Amemiya, Facilitated protamine transfer at polarized water/l,2-dichloroethane interfaces studied by cyclic voltammetry and chronoamperometry at micropipet electrodes. Anal. Chem. 76, 6877— 6886 (2004). 

Ca2+ is necessary for transmission at the neuromuscular junction and other synapses and plays a special role in exocytosis. In most cases in the CNS and PNS, chemical transmission does not occur unless Ca2+ is present in the extracellular fluid. Katz and Miledi [16] elegantly demonstrated the critical role of Ca2+ in neurotransmitter release. The frog NMJ was perfused with salt solution containing Mg2+ but deficient in Ca2+. A twin-barrel micropipet, with each barrel filled with 1.0mmol/l of either CaCl2 or NaCl, was placed immediately adjacent to the terminal. The sodium barrel was used to depolarize the nerve terminal electrically and the calcium barrel to apply Ca2+ ionotophoretically. Depolarization without Ca2+ failed to elicit an EPP (Fig. 10-6A). If Ca2+ was applied just before the depolarization, EPPs were evoked (Fig. 10-6B). In contrast, EPPs could not be elicited if the Ca2+ pulse immediately followed the depolarization (Fig. 10-6C). EPPs occurred when a Ca2+ pulse as short as 1 ms preceded the start of the depolarizing pulse by as little as 50-100 (xs. The experiments demonstrated that Ca2+ must be present when a nerve terminal is depolarized in order for neurotransmitter to be released. 

Using a micropipettor, add 10 pL of the reaction buffer to a small reaction vial or microcentrifuge tube. Add 15 pL of DNA. Then, with a fresh micropipet tip, add 15 pL of one enzyme. 

The milliliters of 1000 ppm needed is pipetted into separate 50-mL volumetric flasks and water is added to each to the 50-mL mark. Each flask is then shaken to make the solutions homogeneous. The pipet needed would be a small serological pipet, perhaps 0.50 mL in capacity. Alternatively, a micropipet, such as described in Chapter 4, can be used. 

The analyte solution is placed (injected) into the furnace with a micropipet or auto-sampler. Following this, a temperature program is initiated in which the furnace heats rapidly to 1) evaporate the solvent, 2) char the solid residue, and finally 3) atomize the analyte, creating the atomic vapor. 

Lee. S.. Kim, D.H., and Needham, D. Equilibrium and dynamic interfacial tension measm-ements of microscopic interfaces using a micropipet technique. 1. A new method for determination of interfacial tension, Lang/nurr. 17(18) 5537-5543,2001. 

---