Cannula transfer or cannulation is a set 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. [1]
Two methods of cannula transfer are popular: 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 (sg.: 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. Their bore is usually 16-22 gauge thick. [2] They are commonly made of stainless steel or PTFE for their 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 tips 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. [2]
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, although 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, although 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. [3]
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.
This technique has been described with illustrated detail. [4] [5] [6]
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.
Vacuum transfers risk drawing air into the system, spoiling the air-free environment. Loss of the fluid by evaporation is another problem, although less so where the fluid is a neat liquid, than a solution of known concentration.
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. [2]
Pressure transfers can be slow. 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.
While handling pyrophoric material (e.g. tert-butyllithium and trimethylaluminum), traces of the compound at the tip of the needle or cannula may ignite, and cause a clog. Some workers prefer to contain the tip of the needle or cannula in a short glass tube flushed with an inert gas, and sealed via two septa. [3]
Instead of exposing the needle tip to the air, it is withdrawn into the inerted tube. Where desired, it may be inserted into a flask via two septa (one on the tube, one on the flask). Used this way, needle tip fires are eliminated, reducing the obvious hazards. Also, there is a reduced tendency for the needle tip to clog due to the reaction of traces of the reagent with air to give salts.
Filtration is most easily accomplished using a syringe filter. PTFE filters tend to be most chemically resistant; nylon filters are less so.
Using a cannula, a filter stick Air-free technique#Gallery may be used. A filter stick is a short length of glass tubing sealed on one end with a septum, and sealed on the other with filter paper, or a sintered glass frit. [3]
For larger volumes, it may be preferable to connect the donor and receiving flasks via ground glass joints to a sintered glass filter tube.
Air-sensitive cannulas:
1: Pressure in (gas in) 2: Pressure out (oil bubbler orange) 3: Higher flask with transfer liquid (yellow) to transfer 4: Lower receiving flask/transferred liquid (yellow)
5: Liquid transfer cannula 6: Septum (orange) on transfer flask 7: Septum (orange) on receiving flask 8: Pressure-control regulator/stopcock
9: Tubing/ gas-line (not shown for clarity, arrows show connectivity) 10: Gas cannula 11: 2-way syringe stopcock 12: Gas-tight syringe
13: Gas/pressure removed from flask 4 14: Gas/pressure added to flask 3
O = Open stopcock; X = Closed stopcock; black-arrow = Gas flow direction, orange arrow = Liquid flow direction
Laboratory glassware refers to a variety of equipment used in scientific work, and traditionally made of glass. Glass can be blown, bent, cut, molded, and formed into many sizes and shapes, and is therefore common in chemistry, biology, and analytical laboratories. Many laboratories have training programs to demonstrate how glassware is used and to alert first–time users to the safety hazards involved with using glassware.
A pipette is a type of laboratory tool commonly used in chemistry and biology to transport a measured volume of liquid, often as a media dispenser. Pipettes come in several designs for various purposes with differing levels of accuracy and precision, from single piece glass pipettes to more complex adjustable or electronic pipettes. Many pipette types work by creating a partial vacuum above the liquid-holding chamber and selectively releasing this vacuum to draw up and dispense liquid. Measurement accuracy varies greatly depending on the instrument.
A syringe is a simple reciprocating pump consisting of a plunger that fits tightly within a cylindrical tube called a barrel. The plunger can be linearly pulled and pushed along the inside of the tube, allowing the syringe to take in and expel liquid or gas through a discharge orifice at the front (open) end of the tube. The open end of the syringe may be fitted with a hypodermic needle, a nozzle or tubing to direct the flow into and out of the barrel. Syringes are frequently used in clinical medicine to administer injections, infuse intravenous therapy into the bloodstream, apply compounds such as glue or lubricant, and draw/measure liquids. There are also prefilled syringes.
In biology, a septum is a wall, dividing a cavity or structure into smaller ones. A cavity or structure divided in this way may be referred to as septate.
A cannula is a tube that can be inserted into the body, often for the delivery or removal of fluid or for the gathering of samples. In simple terms, a cannula can surround the inner or outer surfaces of a trocar needle thus extending the effective needle length by at least half the length of the original needle. Its size mainly ranges from 14 to 26 gauge. Different-sized cannula have different colours as coded.
A funnel is a tube or pipe that is wide at the top and narrow at the bottom, used for guiding liquid or powder into a small opening.
A gas syringe is a piece of laboratory glassware used to insert or withdraw a volume of a gas from a closed system, or to measure the volume of gas evolved from a chemical reaction. A gas syringe can also be used to measure and dispense liquids, especially where these liquids need to be kept free from air.
A hermetic seal is any type of sealing that makes a given object airtight. The term originally applied to airtight glass containers, but as technology advanced it applied to a larger category of materials, including rubber and plastics. Hermetic seals are essential to the correct and safe functionality of many electronic and healthcare products. Used technically, it is stated in conjunction with a specific test method and conditions of use. Colloquially, the exact requirements of such a seal varies with the application.
Degassing, also known as degasification, is the removal of dissolved gases from liquids, especially water or aqueous solutions. There are numerous methods for removing gases from liquids.
A dropping funnel or addition funnel is a type of laboratory glassware used to transfer liquids. They are fitted with a stopcock which allows the flow to be controlled. Dropping funnels are useful for adding reagents slowly, i.e. drop-wise. This is desirable when the quick addition of the reagent results in side reactions, or if the reaction is too vigorous.
A stopcock is a form of valve used to control the flow of a liquid or gas. The term is not precise and is applied to many different types of valve. The only consistent attribute is that the valve is designed to completely stop the flow when closed fully.
A syringe filter is a single-use filter cartridge. It is attached to the end of a syringe for use. Syringe filters may have Luer lock fittings, though not universally so. The use of a needle is optional; where desired it may be fitted to the end of the syringe filter.
Round-bottom flasks are types of flasks having spherical bottoms used as laboratory glassware, mostly for chemical or biochemical work. They are typically made of glass for chemical inertness; and in modern days, they are usually made of heat-resistant borosilicate glass. There is at least one tubular section known as the neck with an opening at the tip. Two- or three-necked flasks are common as well. Round bottom flasks come in many sizes, from 5 mL to 20 L, with the sizes usually inscribed on the glass. In pilot plants even larger flasks are encountered.
The Schlenk line is a commonly used chemistry apparatus developed by Wilhelm Schlenk. It consists of a dual manifold with several ports. One manifold is connected to a source of purified inert gas, while the other is connected to a vacuum pump. The inert-gas line is vented through an oil bubbler, while solvent vapors and gaseous reaction products are prevented from contaminating the vacuum pump by a liquid-nitrogen or dry-ice/acetone cold trap. Special stopcocks or Teflon taps allow vacuum or inert gas to be selected without the need for placing the sample on a separate line.
A Schlenk flask, or Schlenk tube, is a reaction vessel typically used in air-sensitive chemistry, invented by Wilhelm Schlenk. It has a side arm fitted with a PTFE or ground glass stopcock, which allows the vessel to be evacuated or filled with gases. These flasks are often connected to Schlenk lines, which allow both operations to be done easily.
A gas bubbler is a piece of laboratory glassware which consists of a glass bulb filled with a small amount of fluid—usually mineral or silicone oil, less commonly mercury. The inlet to the bulb is connected to a ground glass joint, while the outlet is vented to the air.
Ground glass joints are used in laboratories to quickly and easily fit leak-tight apparatus together from interchangeable commonly available parts. For example, a round bottom flask, Liebig condenser, and oil bubbler with ground glass joints may be rapidly fitted together to reflux a reaction mixture. This is a large improvement compared with older methods of custom-made glassware, which was time-consuming and expensive, or the use of less chemical resistant and heat resistant corks or rubber bungs and glass tubes as joints, which took time to prepare as well.
Air-free techniques refer to a range of manipulations in the chemistry laboratory for the handling of compounds that are air-sensitive. These techniques prevent the compounds from reacting with components of air, usually water and oxygen; less commonly carbon dioxide and nitrogen. A common theme among these techniques is the use of a fine (100–10−3 Torr) or high (10−3–10−6 Torr) vacuum to remove air, and the use of an inert gas: preferably argon, but often nitrogen.
Laboratory funnels are funnels that have been made for use in the chemical laboratory. There are many different kinds of funnels that have been adapted for these specialized applications. Filter funnels, thistle funnels, and dropping funnels have stopcocks which allow the fluids to be added to a flask slowly. For solids, a powder funnel with a short and wide neck/stem is more appropriate as it prevents clogging.
A media dispenser or a culture media dispenser is a device for repeatedly delivering small fixed volumes of liquid such as a laboratory growth medium like molten agar or caustic or volatile solvents like toluene into a series of receptacles. It is often important that such dispensers operate without biological or chemical contamination, and so must be internally sealed from the environment and designed for easy cleaning and sterilization before use. At a minimum, a media dispenser consists of some kind of pump connected to a length of discharge tubing or a spout. Dispensers used in laboratories are also frequently connected to microcontrollers to regulate the speed and volume of the medium as it leaves the pump.