Cannula transfer

A rubber septum. The top folds over the neck of a flask to give an airtight seal

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.[1] 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.[1]

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. They tend to be used for microsyringes (usually containing less than 100 μL). Luer fittings are preferred, as needles are locked in even under higher pressure, e.g. when transferring viscous liquids.[2]

Cannulae and needles should be quickly flushed out with an appropriate solvent to prevent undetectable corrosion damage to the stainless steel. Since they are usually used for air-sensitive work, they are commonly kept in a hot oven, to reduce the adsorption of water molecules. Before use, they are usually subjected to three vacuum-refill cycles to remove any traces of air.

The two ends of the cannula are inserted through the septa covering donating and receiving flasks. The cannula extends below the surface of the fluid to be transferred. A vacuum is applied to the receiving flask, and the low pressure relative to the donating flask causes the fluid to flow through the cannula.

The main drawback of vacuum transfer is that if there are any leaks, air will be drawn into the system and spoil the air-free environment. Loss of the fluid by evaporation is another problem, though less so where the fluid is a neat liquid, than a solution of known concentration.

The two ends of the cannula are connected similarly. The receiving flask is connected to its own gas bubbler, while the donating flask is connected to a source of inert gas. By increasing the inert gas pressure, the pressure within the donating flask is raised higher than the receiving flask, and the fluid is forced through the cannula.[1]

The main drawback of pressure transfer, is that transfer can be slow due to the small pressure difference between donating and receiving flasks. Inert gas lines are usually vented out of a gas bubbler placed in-line to prevent overpressure. The vents need to be isolated by capping the bubbler outlet, or stopping the egress of inert gas with a stopcock or pinch clamp, to ensure sufficient pressure to complete the transfer. The use of a mercury bubbler instead of one filled with oil used to be popular, but is out of favor due to the difficulty in dealing with mercury spills.

By carefully filling the cannula fully with either above techniques, then allowing the pressures within the vessels to equalize, a syphon may be set up. This arrangement allows the slow addition of a fluid to a reaction vessel; the rate of addition may be controlled by adjusting the relative height of the donor vessel.