CC Lab Pipettes

• Pipettes are used to transfer liquids and can be made of glass or plastic
• Pipettes can be reusable or disposable and are typically used for volumes of 20 mL or less
• For larger volumes, automated or jar-style pipetting apparatus are used
• Pipettes come in two main designs: To Contain (TC) and To Deliver (TD) pipettes
• In To Contain (TC) pipettes, the contained quantity of liquid corresponds to the capacity printed on the instrument
• In To Deliver (TD) pipettes, the delivered quantity of liquid corresponds to the capacity printed on the instrument
• The major difference between To Contain (TC) and To Deliver (TD) pipettes is in the amount of liquid needed to wet the interior surface of the ware and any residual liquid left in the pipet tip
• To Contain (TC) pipettes hold or contain a particular volume but do not dispense that exact volume
• To Deliver (TD) pipettes dispense the volume indicated
• Pipettes can have different drainage characteristics, such as Blow-out pipettes and Self-Draining pipettes
• Blow-out pipettes have continuous etched rings near the top to ensure the appropriate volume is transferred
• Self-Draining pipettes allow the contents to drain by gravity without markings
• Types of pipettes include Measuring or Graduated pipettes and Transfer pipettes
• Measuring or Graduated pipettes are capable of dispensing several different volumes and have uniform graduations along their length
• Transfer pipettes transfer a known volume of liquid without further subdivisions
• Examples of measuring pipettes include Serologic, Mohr, and Bacteriologic pipettes
• Examples of transfer pipettes include Volumetric, Ostwald-Folin, and Pasteur pipets
• Automatic macropipets and micropipets are also types of transfer pipettes with advantages like safety, stability, ease of use, and increased precision
• Micropipets are used for volumes less than 1 mL, while automatic macropipets are used for volumes greater than 1 mL
• Pipette tips come in disposable one-use tips for air-displacement pipets and tips for positive-displacement pipets
• Disposable pipet tips ensure a snug fit onto the pipet end and are free from deformities
• Tips for positive-displacement pipets are straight columns of glass or plastic that must fit snugly to avoid carryover and can be used repeatedly without being changed after each use
• Pipettes are used to precisely measure and transfer small volumes of liquid in experiments that require high accuracy
• Reasons for calibrating pipettes:

  • To ensure accurate dispensing of volumes
  • To prevent errors in measurements due to device failure or operator misuse
• Consequences of not calibrating pipettes:

  • Sources of error in measurements can arise from device failure or operator misuse
  • Discrepancies in dispensed volumes can affect experiment outcomes and reproducibility of results
• Frequency of pipette calibration:

  • Before newly placing pipettes into service
  • Periodically, suggested interval is quarterly or according to manufacturer guidelines
  • After cleaning
  • Anytime a pipette is dismantled and repaired
  • As a troubleshooting measure for assay problems
• Class A pipettes do not need to be recalibrated by the laboratory; automatic pipetting devices and non-Class A material require calibration
• Factors to consider before calibrating a pipette:

  • Large changes in testing environment like temperature, humidity, and atmospheric pressure can impact evaporation during calibration, leading to unreliable tests
  • Adjust pipettes to the calibration environment for over an hour before starting the calibration
• Methods of calibrating pipettes:

  I. Gravimetric method:

  • Delivering and weighing a solution of known specific gravity like distilled water
  • Use a calibrated analytic balance and at least Class 2 weights
• Methods of calibrating pipettes:

  II. Photometric method:

  • Use a spectrophotometer and potassium dichromate
  • Determine molar absorptivity of K2Cr2O7 and the change in concentration of the volume of the pipette
• Another procedure for photometric calibration:

  • Dilutions of K2Cr2O7 vs. another colored liquid using Class A volumetric labware
• Procedure for calibrating pipettes:

  1. Weigh a small weighing bottle on an analytical balance and record the weight

  2. Determine the temperature of the water in the flask and note the density of the water at that temperature

  3. Using a serologic pipette, measure a full volume of water 10 times, recording each weight and calculating the mean weight