Cannula manipulations

Techniques based on solution transfer through stainless steel or Teflon cannulae permit rapid, convenient manipulation of air sensitive materials under an inert atmosphere. The basic equipment required for cannula manipulations is described, and their application to routine laboratory procedures such as recrystallization, sublimation, chromatography, and the preparation of spectroscopic and analytical samples is discussed. 

Septa. Two types of serum caps (septa) have been commonly used for cannula manipulations the older style have a smooth wall, but more recently Suba Seal septa with a ridged wall which provides better closure have become commercially available in this country. Suba Seals are available from Aldrich and from Strem, and in addition to providing a better seal they are made of harder rubber, have better solvent resistance, and can be penetrated more frequently before deteriorating. 

Subsequent manipulations of solvents are performed with nitrogen-purged syringes and/or a cannula, through degassed, flasks fitted with septa (see text). 

Cannula techniques have several advantages over other approaches to the manipulation of very sensitive materials ... 

Cannula techniques are complementary to other techniques for handling air sensitive materials. In particular, while they may be used to prepare NMR samples of air sensitive materials and to manipulate solids, it Is normally more convenient to carry out both these operations in an Inert atmosphere glove box. In our experience cannula techniques are at least as good as any other technique for the exclusion of oxygen from a reaction mixture (in fact we find them to be superior to inert atmosphere glove boxes), but glove boxes and high vacuum manipulation lines provide better protection from adventitious water. 

The objective of the present chapter is to describe the combination of cannula/Schlenk tube techniques which are currently used in our own laboratories for the manipulation of air sensitive materials. These techniques are not, of course, unique to our laboratories - similar techniques have been developed in many other groups, and many of the techniques to be described were initially developed elsewhere, or are based on conversations with colleagues. 

Recrystallization. The convenience of cannula techniques for the manipulation of solutions allows access to a wide variety of recrystallization techniques under inert atmosphere conditions. The general procedure in all recrystallizations is to extract a crude compound into a solvent with a polarity close to the minimum required to dissolve the material, filter the solution, and then reduce the polarity of the medium until the compound crystallizes out of solution. The product may then be collected by filtration, leaving small quantities of compounds with similar or higher solubilities in the solute mother liquor. 

Absolute qualitative identification can be assured only if samples are removed and analyzed. Two examples of such a procedure have been reported. The first was an attempt to determine if direct electrical stimulation of the caudate nucleus resulted in the release of dopamine as well as ascorbic acid from that tissue. Micro voltammetric and stimulating electrodes were micromanipulated into excised caudate tissue which was flushed with warmed, oxygenated buffer. Reference and auxiliary electrodes were nearby. Quantitative information was taken, stored, manipulated, and displayed by a minicomputer. Simultaneously a push-pull cannula device sampled the caudate and delivered the perfusate to an iced vial. Changes in the electrochemical signal that followed stimulation were correlated with changes in the dopamine and ascorbic acid content of the perfusate as determined via HPLC with electrochemical detection. It was found that little if any ascorbic acid was released as a result of electrical stimulation in these experiments. Although there is some question concerning the stability of ascorbate in an iced vial, the above example does illustrate this coincident analytical technique. 

Attachment of a rubber septum to a standard NMR tube (Figure 16) is the least expensive method for manipulating air-sensitive organometallic compounds. This method works well for sample preparations inside a glovebox. Like a screw-capped tube, one can inject or transfer liquids via cannula liquid into the tube. Caution must be used when attaching a septum to an NMR tube because the thin-walled tube has the potential of breaking. 

Since the butterfly needle uses a flexible tube, there is less chance causing damage if the patient moves during the manipulation. The wings allow grasping the needle very close to the end to ensure accurate insertion. Butterfly needles are only suitable for short term use because the steel cannula can dislodge and puncture the vein. For subcutaneous administration butterfly needles can be inserted into the skin. 

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