When analysing oilfield samples, achieving precise volumetric measurements is necessary for accurate results, with many samples having concentrations in the parts per million level. To achieve this time after time, the appropriate tools must be used. This article aims to increase understanding of the core tools for accurate small volume analysis: precision pipettes.
Types of pipettes
There are two main classes of precision pipettes: air displacement pipettes, and positive displacement pipettes. The fluid being measured dictates the type of pipette to be used.
Air displacement pipettes: strengths
Air displacement pipettes are the “traditional” pipettes, seen in many laboratories across the world. They are the recommended option for aqueous solutions. Their partial vacuum air-cushion mechanism allows for precise aspiration of the sample solution into the (single-use) pipette tip, so the sample and the piston never come into direct contact.
Air displacement pipettes: weaknesses
However, when dispensing viscous liquids, they may not be fully aspirated by an air displacement pipette, and/or stick to the inside of the tip. Volatile liquids, on the other hand, may run out of the tip before dispensing. In both cases, the volume taken and dispensed will be lower than that required.
The answer: Positive displacement pipettes
Positive displacement pipettes are ideal for viscous and volatile liquids. They use a disposable syringe-like tip, with tight-fitting integral piston, which is in direct contact with the liquid. This gives an accurate fill, whether the liquid is volatile, viscous, or neither. When the liquid is dispensed, the piston displaces all of the liquid, ensuring the accuracy of the measurement.
SUMMARY
The selection of the appropriate pipette type is important in the pursuit of accuracy within Oil & Gas analysis. Aqueous solutions are compatible with air displacement pipettes, while viscous or volatile liquids should use positive displacement pipettes.
Choosing the correct pipette type will make sample analysis easier and more accurate.