Cannula transfer  Cannula transfer or cannulation is a subset of air-free techniques used with a Schlenk line, in transferring liquid or solution samples between reaction vessels via cannulae, avoiding atmospheric contamination. While the syringes are not the same as cannulae, the techniques remain relevant. There are two methods of transfer: vacuum, and pressure. Both utilize differences in pressures between two vessels to push the fluid through. Often, the main difficulty encountered is slow transfer due to the high viscosity of the fluid. Septa (singular: septum) are rubber stoppers which seal flasks or bottles. They give an airtight seal, preventing the ingress of the atmosphere, but are able to be pierced by sharp needles or cannulae. Cannulae are hollow flexible tubes of varying bore, usually 16-22 gauge thick. They are commonly made of stainless steel or PTFE for chemical resistance. Stainless steel cannulae are usually 2–3 feet long, due to their relative inflexibility, while PTFE cannulae can be much shorter. The ends are usually sharp and non-coring, allowing them to easily pierce a rubber septum, without being clogged by rubber particles. Flat ends may be chosen on occasion, because they tend to provide more complete transfer of fluids. Stainless steel cannulae tend to collapse when cut with wire cutters. They are best cut using pipecutters of appropriate size. Other workers recommend deeply scoring the cannula with a triangular file, then sharply snapping the weakened section. Wide-bore needles of similar gauge are often used. Unlike hypodermic-type needles sometimes used in the chemistry laboratory, these needles tend to be reused due to cost. Long needles may be flexible enough to be bent in U-shapes; shorter needles often are not. Polypropylene syringes used for medical applications are least expensive. While the material is relatively solvent-resistant, though they are designed primarily for aqueous solutions, some degradation or leaching by the contents may occur. In particular, the black rubber seal may swell and cause the plunger to seize. All-glass gas-tight syringes have better solvent resistance, though they tend to leak more than plastic syringes. Greases used on the barrel may leach into the contents. Glass syringes with a teflon seal at the plunger are available as well, but they are more expensive.