A pipette is one of the most commonly used precision liquid transfer tools in the laboratory, mainly used to accurately transfer a certain volume of liquid. Correctly reading the scale and operating the pipette are key to ensuring experimental accuracy. This article details the types of pipettes, scale reading methods, usage steps, and precautions.
The scale reading method of a pipette varies by type: drain-out pipettes have scales with "0" at the top and the maximum scale at the bottom; after the liquid drains naturally, do not blow out the residual liquid in the tip. Blow-out pipettes are marked "Blow out" on the barrel, and the residual liquid in the tip must be blown out.
1. Types and Selection of Pipettes
According to structure and purpose, pipettes are mainly divided into the following categories:
1.1 Classification by Structure
- Graduated Pipette: The barrel has continuous scales and can transfer any volume (within the range). Suitable for scenarios requiring flexible adjustment of transfer volume.
- Volumetric Pipette / Transfer Pipette: Has only one calibration line and can only transfer a fixed volume. Higher precision, suitable for standard solution transfer.
- Blow-out Pipette: The scale goes to the tip, and the residual liquid in the tip needs to be blown out. The barrel is usually marked "Blow out" or "Blow-out".
- Drain-out Pipette: After the liquid drains naturally, the amount of residual liquid in the tip is included in the calibration, and must not be blown out.
1.2 Classification by Scale Orientation
| Type | Scale Orientation | Reading Method | Blow out? |
|---|---|---|---|
| Drain-out (Top scale) | 0 at top, max scale at bottom | Lowest point of meniscus at scale | ❌ Do not blow |
| Blow-out (Bottom scale) | 0 at bottom, max scale at top | Lowest point of meniscus at scale | ✅ Blow out |
| Volumetric Pipette | Only one calibration line | Meniscus tangent to calibration line | Depends on marking |
1.3 Common Specifications
Common pipette specifications in the laboratory include: 0.1mL, 0.2mL, 0.5mL, 1mL, 2mL, 5mL, 10mL, 25mL, 50mL, etc. For experiments with high precision requirements, it is recommended to choose a volumetric pipette; for experiments requiring flexible adjustment of transfer volume, choose a graduated pipette.
2. How to Read a Pipette Scale? Key Techniques
Correctly reading the pipette scale is the basis for accurate liquid transfer. The following are detailed steps and precautions:
When reading the scale, the pipette must be kept vertical; it must not be tilted, otherwise the liquid level will shift, causing reading errors.
Your line of sight must be horizontal with the liquid surface. Looking down from above will make the reading larger; looking up from below will make the reading smaller.
The liquid forms a concave meniscus inside the pipette. For transparent liquids (such as water, transparent solutions), read the scale corresponding to the lowest point of the concave meniscus. For dark or opaque liquids, read the highest point of the liquid surface.
To observe the meniscus more clearly, you can place a white paper behind the pipette as a background, so that the meniscus will be seen more clearly.
1. Eye not horizontal: Looking down makes the reading larger; looking up makes the reading smaller. The error can reach 2%~5%.
2. Reading the highest point of the meniscus: For transparent liquids, read the lowest point. Reading the highest point will make the transferred volume larger.
3. Pipette tilted: Tilting will cause the liquid level to shift, resulting in inaccurate readings.
3. Correct Pipette Usage Steps
Step 1: Choose the right pipette
According to the volume of liquid to be transferred, select the pipette with the closest range. For example, if you need to transfer 9.5mL of liquid, you should choose a 10mL pipette, not a 25mL pipette, because the closer the range, the higher the precision.
Step 2: Rinse the pipette
Use the liquid to be transferred to rinse 2~3 times. The method is: suck in a small amount of liquid, slowly tilt and rotate the pipette to let the liquid wet the inner wall, then discharge from the tip. This prevents concentration changes caused by adsorption on the tube wall.
Step 3: Suck in the liquid
Insert the pipette into the liquid (depth about 1cm), use a rubber bulb (must not suck directly with the mouth!) to slowly suck the liquid, so that the liquid level rises to about 1~2cm above the scale line.
Step 4: Adjust the liquid level to the scale
Quickly use the index finger (or thumb) to block the upper end of the pipette, then slowly relax the finger, so that the liquid level slowly drops until the lowest point of the meniscus is tangent to the scale line. At this point, stop relaxing the finger and keep the liquid level stable.
Step 5: Discharge the liquid
Rest the pipette tip against the inner wall of the receiving container (tilted about 45°), and discharge the liquid naturally. For drain-out pipettes, after the liquid stops flowing, wait 3~5 seconds to let the residual liquid in the tip flow out naturally (or along the wall). Do not blow out. For blow-out pipettes, you need to blow out the residual liquid in the tip.
When discharging liquid from a pipette, the tip should rest against the container wall, not suspended in the air, otherwise the liquid will flow down the outer wall, causing inaccurate transfer volume. At the same time, the container should be tilted about 45° to facilitate liquid inflow.
4. Precautions for Pipette Use
- Never suck liquid directly with the mouth: Toxic and corrosive liquids can cause injury. Use a rubber bulb or pipettor.
- Keep the pipette clean: Before use, clean with distilled water, then rinse with the liquid to be transferred.
- Distinguish between drain-out and blow-out types: Confirm the pipette type before use to avoid operational errors.
- Wait for the liquid to completely drain: After draining, wait 3~5 seconds for drain-out pipettes to ensure the residual liquid in the tip is completely drained.
- Avoid temperature effects: The pipette calibration temperature is usually 20°C. If the liquid temperature is too high or too low, it will affect volume accuracy.
- Calibrate regularly: Pipettes should be sent to a measuring agency for calibration regularly (usually once a year) to ensure accuracy.
- Do not use damaged pipettes: A damaged tip will cause inaccurate outflow volume and should be replaced immediately.
5. Choosing Between Pipette and Pipettor
In modern laboratories, pipettors are becoming increasingly popular. When to use a pipette and when to use a pipettor?
| Comparison Item | Pipette | Pipettor |
|---|---|---|
| Precision | High (Class A: ±0.2%~0.8%) | Relatively high (±0.5%~3%) |
| Volume Range | 0.1mL~50mL (larger volumes need graduated cylinder) | 0.5μL~10mL |
| Ease of Operation | Requires rubber bulb, more cumbersome to operate | One-handed operation, very convenient |
| Suitable Scenarios | Large volume transfer, standard solution transfer | Small volume transfer, PCR, cell experiments |
| Cost | Low (glass pipettes can be reused) | High (pipettor price is higher) |
Summary: When the transfer volume is > 1mL and the precision requirement is high, priority is given to the pipette; when the transfer volume is < 1mL or high-throughput operation is required, priority is given to the pipettor.