The experiment tells you! Which pipette or pipette is more accurate!
Which pipette or PK pipette is more accurate? In this article, we will discuss the structure of pipettes and pipettes and the effect of liquid aspiration principles on the accuracy of actual use with examples. I believe that after reading this article, my friends will have a deeper understanding of the accuracy comparison between pipettes and pipettes.
The same calibration method
Before that, we must first know that the method of calibration (calibration) of pipettes and pipettes in accordance with their respective measurement standards (JJG 196 and JJG 646) is exactly the same, that is, the method of weighing is used to determine the volume of the pipette. Perform calibration.
Calibration steps:
1. Tare the weighing container on the analytical balance in a constant temperature and humidity environment
2. Suction the distilled water of the nominal range (ie the maximum range), pour it into the weighing container and weigh to obtain the weight of the discharged distilled water Δm
3. Convert the weight Δm of distilled water to volume at the corresponding water temperature using the Kt value of water, that is, V=Kt*Δm. The formula of Kt value can check the corresponding measurement standard. From the formula, it can be known that the main influencing factor of the Kt value is the density of water
The difference between structure and lotion principle
Secondly, from the point of view of the measurement method, the method of calibrating the volume of the pipette and the pipette is almost exactly the same. If the nominal accuracy is the same, will the accuracy performance be exactly the same when operating liquids other than distilled water? the answer is negative.
In order to understand this, we need to know the difference between the structure of pipettes and pipettes and the principle of liquid aspiration.
Pipette:
Simple physical structure, generally made of glass, with a scale to determine the volume. When calibrating the corresponding scale of the corresponding volume, the method adopted is the above-mentioned calibration method. The corresponding mass of the corresponding volume of distilled water is constant, then if the quality of the distilled water discharged after liquid absorption is "correct", it means that the distilled water is discharged after liquid absorption. The volume is "correct (within the allowable error range of the nominal value)", then the suction volume represented by the lowest point of the concave surface of the distilled water when aspirating liquid is "correct", and this point is the corresponding volume Where the scale is. Once this point is determined, the volume inside the pipette is physically determined. This volume will hardly change with external factors, such as liquid density, air pressure and other factors.
Pipette:
The pipette in the usual sense is an air piston pipette. When the pipetting button is moved, the position of the internal piston changes, which in turn generates negative pressure, thereby sucking the liquid into the tip. The suction volume is determined by the internal and external pressure difference and a series of other factors, but the main determining factor is the internal and external pressure difference. When calibrating, if the mass of the absorbed distilled water after discharge is "correct" (weighing method), it means the volume of distilled water discharged after suction is "correct (within the allowable error range of the nominal value)" , Then it means that the aspirated volume is "correct". Therefore, when using distilled water to determine the volume of a pipette, what is actually determined is that the combined effect of the pressure difference between the inside and outside of the pipette and a series of other factors can just absorb the correct volume of distilled water.
At this time, it is well understood that the performance of the pipette will be significantly different when sucking different types of liquids and under different environmental conditions.
For example, when the environmental conditions have not changed, that is, the pressure difference is constant, if the density of the liquid is different from distilled water, that is, when the mass of the same volume of liquid is different from distilled water, in order to balance the internal and external pressures, the volume of the liquid must be different.
Let's take a less rigorous chestnut to verify this. See the result?
Experimental verification
Instruments and reagents
instrument:
Use a 10 mL graduated pipette
One 10 mL pipette
One HandStep Touch electronic continuous dispenser
Pipette 75% ethanol to the same 10 mL Class A volumetric flask
From left to right are BLAUBRAND graduated pipettes (left one), Transferpette S pipettes (left two), HandStep Touch electronic continuous dispenser (right two), BLAUBRAND volumetric flasks (right one)
Reagents:
75% ethanol
Experimental steps
Step 1: In order to reduce the influence of ethanol evaporation on the experiment, create a suitable experimental environment.
We covered the entire experimental environment with a plastic tablecloth, as shown in Figure 2, and poured 75% ethanol into the beaker and placed it in the test environment for more than 2 hours to saturate the partial pressure of ethanol in the environment as much as possible and reduce the ethanol during operation. Evaporation rate.
lab environment
Step 2: Compare the pipette (left) and the pipette (right) to draw 10 mL of 75% ethanol into the same volumetric flask, as shown in Figure 3.
It can be seen from the figure below that under the same conditions, pipettes are significantly more than pipettes if 10 mL of 75% ethanol is drawn into the same volumetric flask. We know that the density of ethanol is lower than that of distilled water, that is, ethanol is lighter than water. Therefore, when the pressure difference between the inside and outside of the pipette is constant, the volume of ethanol drawn will be more than the volume of water drawn. When the pipette is calibrated, it is the volume that is directly marked. Therefore, the volume after the pipette absorbs and discharges ethanol is very small compared to water. The results after discharging into the volumetric flask verify this. Of course, due to the high volatility of ethanol and the existence of other factors that affect pipette aspiration, the difference in density between ethanol and water is not fully reflected in the difference in pipetting volume.
Comparison of the results of 10 ml of ethanol drawn with a pipette (left) and a pipette (right)
From the above analysis and examples, the principle of the pipette to determine the volume is more straightforward, so in general, under the same nominal accuracy, the pipette will have a better actual pipetting accuracy performance than the pipette. Of course, there are many factors that affect the accuracy of pipettes and pipettes. We cannot generalize that in any case, pipettes with the same nominal accuracy are better than pipettes for any liquid. Device.
The pipette we compared above is a common air piston pipette in the laboratory. However, there is another category of pipettes-external piston pipettes. From the principle of liquid suction and discharge, the external piston pipette performs better than the air piston pipette when pipetting high volatile and high viscosity liquids. When we conducted this comparison experiment, we combined the experimental results of the pipette and the air piston pipette, and also used the external piston principle of the HandyStep Touch pipetting mode to make a comparison.
Step 3: HandyStep Touch with 10 mL PD tip draws 10 mL of 75% ethanol into a 10 mL grade A volumetric flask, the result is shown in the figure.
Use HandyStep Touch with 10 mL PD tip to pipette 10 mL ethanol result
The results show that the accuracy performance is better than that of the air piston pipette of the same range. It is true that the measuring range of the pipette is smaller, the use of the pipette is more convenient, and the pipetting method of the pipette is easier to master, but it cannot be ignored whether the pipette is defined in accuracy or in actual work performance. , Can not be completely compared with the pipette.
Of course, there are many ways to improve the accuracy of pipettes, such as using low-adsorption tips, such as recalibrating the pipette for different liquids, such as using an external piston pipette.
However, in general, the advantages of pipettes are more reflected in convenience and a smaller measuring range.
In actual work, you should decide whether to use a pipette or a pipette for liquid measurement according to actual work needs. If it is an experiment that cannot be compromised in accuracy, I am afraid that pipettes, especially pipettes for fat belly, are the best The choice, on the contrary, if the experimental speed and efficiency are the priority factors, then the pipette will do your part.
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