General principles for use of micropipettes (pipettes)

In immunoassays and other biomedical studies, the use of a pipettor is inseparable from disposable plastic tips. Although the use of disposable plastic tips increases the cost of the experiment, it reduces the exposure of experimental technicians to infectious pathogens and harmful The possibility of experimental materials such as radionuclides, and avoids the cleaning process and the use of corrosive detergents necessary for multiple reuse of the tip. In addition, in some applications, such as PCR assays, the tip must be disposable to avoid the potential for potential contamination.



Samplers are also constantly evolving according to the requirements of use. Such as the emergence of the whole autoclave sampler. The use of a UV-stabilized plastic pipettor is very important for the application of the pipettor in many aspects, so that in actual work, there is no need to worry about the placement of the sampler in the ultraviolet irradiation sterilization of the clean bench or the extractor. Due to the effect of ultraviolet rays, its production materials will be damaged, which will affect the sample adding function.



According to the physical principle and structure of the sample adder, its application characteristics are different. The piston is moving the sampler without any correction, and it can be used to draw liquids with different chemical compositions and characteristics (density and viscosity); on the contrary, the use of the air pad sampler is more limited. Liquids with high vapor pressure, such as chloroform, do not generally achieve the same accuracy and precision as aspiration of sampled distilled water. Due to the partial evaporation during the liquid suction, the sample volume is reduced. The accuracy of the addition can be improved by moistening the tip several times with the liquid in advance so that the vapor phase is saturated with the liquid. In addition to this, in order to prevent the liquid from leaking out of the tip due to the high vapor pressure, a tip with a flap at the bottom can be used, which opens only when it comes into contact with the wall of the tube. With the air cushion sampler, the expansion of the volume of air above the liquid varies with the density of the added liquid. When sucking liquids with a higher density than water, the volume of the suction head is too low. For example, for a highly concentrated liquid with a density of 1.1, the amount of error will reach o. 2%. However, this error in drawing a relatively dilute aqueous solution is negligible. Therefore, when drawing high-density liquids, the volume of the sampler must be calibrated accordingly to ensure that the correct volume is obtained. However, in the actual work of the laboratory, the users of the pipettes rarely encounter situations in which liquids with high densities are to be accurately taken in. Therefore, it is often difficult to meet the limitations on the use of the samplers due to the density of the liquid.



In general, in order to prevent errors in the suction volume, there are some basic operating principles that must be observed. There are three main factors affecting the volumetric error: 1 hydrostatic pressure; 2 wetting of the tip; 3 fluid dynamics. The relationship of the physical force changes when the sample volume is reduced from the milliliter range to the microliter range. For the sample, it means the force effect of the liquid surface and the force effect of its volume or mass (for example, gravity). The ratio has increased, so the sampler manufacturer must carefully consider this situation in the design and construction of pipettes and tips, and must also pay attention when using.



Hydrostatic pressure: When the liquid is sucked, the applicator tip can only be immersed in a few milliliters of liquid to ensure the same hydrostatic pressure conditions as when the liquid is discharged. Therefore, the pipettor must add liquid in an almost vertical manner because of the inclination This will reduce the height of the liquid column, resulting in too much liquid being drawn. If the pipettor draws the liquid vertically at 30°C, it can draw up to 0.15% more liquid.



Tip wetting: When the tip is emptied, there will still be some residual liquid in the form of a film 6f remaining on the side of the tip, the amount of which depends on the interaction of the liquid and the tip surface, so it is a constant, However, it varies depending on the liquid material. For aqueous solutions, this wetting effect should be taken into account when building a sampler. For liquids with high viscosity, such as protein solutions, it is recommended to inject the liquid several times before adding the sample to ensure the consistency of the sample.



Hydrodynamics: The third effect on volumetric uptake is the release of liquid from the outer wall of the pipette tip. During this process, the tip geometry plays a key role. To ensure stable conditions during loading, the pipette tip should rest against the wall of the tube so that the liquid can flow out of the tube wall without droplets, and droplet formation can be prevented by the surface tension. Liquid is released from the tip. If the tip is not mounted correctly or if an unmatched tip is used, the relative error will be more than 0.4%.



Although the use of the air cushion sampler has certain limitations, its advantages are also obvious. It covers a large volume range and is easy to use. This type of pipettor is very light and can be easily applied in various situations.

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