Methods Micropipettes

According to Walker s method, micropipettes with 0.5-1 pm size tips are made from borosilicate glass tubes previously cleaned in hot ethanol vapor. After pulling, the pipette tip is dipped in a 1% fresh solution of Siliclad (siloxane polymer. Clay Adams) in 1-chloronapthalene. The solution is allowed to rise about 200 pm inside the shank. The pipette, held vertically with the tip up, is baked... 

This method relies on the simple principle that the flow of ions into an electrolyte-filled micropipette as it nears a surface is dependent on the distance between the sample and the mouth of the pipette [211] (figure B 1.19.40). The probe height can then be used to maintain a constant current flow (of ions) into the micropipette, and the technique fiinctions as a non-contact imaging method. Alternatively, the height can be held constant and the measured ion current used to generate the image. This latter approach has, for example, been used to probe ion flows tlirough chaimels in membranes. The lateral resolution obtainable by this method depends on the diameter of the micropipette. Values of 200 nm have been reported. 

However, the droplet method has its own drawbacks, such as the degradation of information about the analyte s localization at a spot where the matrix droplet spreads. In general, a dispensed matrix droplet makes a spot of more than 1 mm in diameter on a tissue surface because of the lower limit of a pipetting volume of 500 nL with an ordinary micropipette. For such a large spot, it is insufficient to perform a precise high-resolution IMS. Therefore, technical improvements are needed to dispense the smallest droplets possible. 

Micropipes, silicon carbide, 22 532 Micropipette solution deposition fabrication method for inorganic materials, 7 415t... 

The amphiphilic material to be studied is dissolved at a known concentration in a volatile solvent which is not miscible with water and a known quantity is spread at the water surface using a micropipette. In order to study the physical properties of the film thus formed, one needs to be able to confine the film to a definite area and to be able to vary this area at will. It might appear that it would be equally possible to maintain a constant area and vary the amount of material which is spread. For the majority of materials this latter procedure is not satisfactory as equilibrium is not arrived at in a reasonable period of time and this method would not allow one to take the material through successive cycles of compression and expansion. We thus turn to a discussion of the various ways in which a film can be confined and its area varied in a systematic manner. Leaving aside methods which are really only of historical interest, for which reference should be made to the book by... 

UMEs used in our laboratory were constructed by sealing of carbon fibre into low viscosity epoxy resin (see Fig. 32.4) [118]. This method is simple, rapid and no specialised instrumentation is required. Firstly, the fibres are cleaned with this aim. They are immersed in dilute nitric acid (10%), rinsed with distilled water, soaked in acetone, rinsed again with distilled water and dried in an oven at 70°C. A single fibre is then inserted into a 100- iL standard micropipette tip to a distance of 2 cm. A small drop of low-viscosity epoxy resin (A. R. Spurr, California) is carefully applied to the tip of the micropipette. Capillary action pulls the epoxy resin, producing an adequate sealing. The assembly is placed horizontally in a rack and cured at 70°C for 8h to ensure complete polymerization of the resin. After that, the electric contact between the carbon fibre and a metallic wire or rod is made by back-filling the pipette with mercury or conductive epoxy resin. Finally, the micropipette tip is totally filled with epoxy resin to avoid the mobility of the external connection. Then, the carbon fibre UME is ready. An optional protective sheath can be incorporated to prevent electrode damage. 

The mechanical properties of polyelectrolyte multilayer capsules have been subject of several studies using different methods. Baumler and co-workers [7] have used the micropipette technique and found that PMCs are not conserving their volume if pressure differences are applied between inside and outside of the shell. This is expected, since the shells can only be formed in first place because the membrane is permeable to low molecular weight species, the core dissolution products. They found no deformation up to a critical pressure followed by an irreversible collapse, showing that shells deform not elastically but plastically for large deformations. First quantitative estimates of the Young s modulus of the shell material were obtained by Gao and coworkers, using osmotic pressure differences between inside and outside of the shell [8,9], These authors monitored the onset of the buck-... 

Figure 5. The four-patch clamp recording methods. In each example, the cell is shown to the left, the micropipette to the right. For a discussion of the methods see text. Reproduced with permission from Hille, B., "Ionic channels of excitable membranes" 1 st ed., Sinauer, Sunderland, MA (1984) p. 217.

The increased sensitivity which is the main feature of electrothermal atomisation methods introduces a number of difficulties connected with the handling and preparation of samples. Some practical guidance on the avoidance of errors through contamination and on the choice and use of micropipettes is set forth in the following subsections. The analyst must appreciate, however, that we are dealing with a technique of ultramicroanalysis, and any advice or experience that he can make use of on that subject will be entirely relevant here. 

Unless a special type of autosampler can be used, micropipettes are an essential part of electrothermal atomisation techniques, as they give one of the most reliable methods of introducing small volumes of liquid samples into the graphite atomizer. 

A more recently developed force measurement technique, coined the liquid siu-face force apparatus (LSFA), brings a drop made from a micropipette close to a flat liquid/liquid interface [29-32]. A piezo electric drive is used to change the position of the micropipette while the deflection of the pipette and the radius of the drop are recorded with piezo motion. The drop radius and thus the film thickness between the two liquid/liquid interfaces are recorded using interferometry. The method requires a calibration of the interferometer, where the drop must come into contact with the other liquid interface. The distance resolution of the film is about 1 nm at a 50-nm separation and 5 nm at a separation of 10 nm. This is a very robust technique where the authors have proposed attaching a particle to the end of the pipette instead of a drop [29]. In comparing this method to AFM, the only drawback of the LSFA is the weaker distance resolution. It is important point out that both methods required a contact point for distance calibration. 

Passive deformability. Two methods were devised to measure cell properties that are related to their deformability. The micropipette aspiration method (Sato and Suzuki, 1976) consists in measuring the negative pressure required for a cell or a portion thereof to enter a pipet the opening of which is smaller than the diameter of the cell. This method is laborious and requires a skilled operator. Cells have to be measured one at a time, and since cell and capillary diameters must have a constant ratio, the sampling of a population is time consuming and requires a set of different pipettes. As a consequence, this method is not widely used, and will not be further described. 

The force that optical tweezers can exert is modest, namely, several tens of piconewtons. Much larger forces (hundreds of piconewtons) can be exerted using a micropipette, which, as the name implies, generates a suction force through a pipette tip that is only a few microns in diameter (Evans et al. 1995, 1996 Smith et al. 1996). Yet another micromanipulation method uses atomic force microscopy (AFM), in which a sharp tip mounted on the end of... 

This is achieved by gravimetrical means with a microbalance or with volumetrical ones with a micropipette. For precise measurements, the former method is preferable, in which a differential weighing with a pycnometer is essential to avoid the appreciable amount of evaporation of solution in the course of the weighing (0.5%). 

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