Glossary
Gauge
Needle gauge is an inverse scale: the bigger the number, the finer the needle. What 27G, 29G and 31G mean for peptide handling and why gauge influences shear.
Gauge (G) is the conventional measure of a needle’s outside diameter, and it works in reverse — a larger number stands for a finer needle. A 31G needle measures about 0.26 mm, a 29G around 0.34 mm, a 27G around 0.41 mm and an 18G around 1.27 mm. Wall thickness is not fixed either, so a thin-wall needle has a larger bore, and therefore faster flow, than a standard-wall needle of the same gauge.
Gauge and peptide handling
Resistance to flow rises with the fourth power of the inner radius, so a small step in gauge changes considerably how much force is needed to move a liquid. That is the key handling point: viscous or concentrated solutions pass slowly through fine needles, and pushing them through creates shear that may promote aggregation of delicate sequences. On the other hand, wide needles inserted repeatedly through a vial stopper are more prone to coring it and releasing rubber particles into the liquid.
Labs usually keep the two tasks apart. A larger bore is used to draw up diluent and move liquid between vials when reconstituting or preparing aliquots; a finer needle serves for measured withdrawal with a U-100 syringe. See 31G syringes, luer-lock syringes and the gauge and volume guide.
Related terms
luer-lock · syringes and vials