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change in volume can be harnessed to rapidly move the piston when a reservoir at
high pressure is allowed to relax to a lower pressure. This process is rapid, and the
pressurizing fluid does not need to go through a transition from liquid to gas to
deliver the needed force.
This entire action could be provided solely by the syringe pump. However, the
syringe pump plays a role immediately after the actual transfer step by rapidly
returning the sample to the required encapsulation pressure. This will be discussed
later in the example protocols. The end result is the syringe pump has the role of
maintaining the reverse micelle sample pressure over being used to drive the piston
directly. In addition, and perhaps more importantly, more of the precious deuterated
alkanes would be wasted using it as the fluid for this step.
Instead the secondary pressure source is provided by an internal fluid reservoir or
gas booster. It turns out that carbon dioxide has nearly identical compressibility as
liquid ethane. Thus the reservoir can be filled with CO
2
and this fluid pressurized to
the required level using liquid ethane delivered by the syringe pump. A separator
piston in the reservoir keeps the fluids apart. Once the transfer steps are performed,
the CO
2
fluid can be used to push any deuterated alkane out of the reservoir
keeping the losses to an absolute minimum.
The piston displacement literally rams the sample into the waiting NMR tube.
Because it might be possible to deliver more pressure to the piston than the tube
can withstand the dimensions of the piston and length of the connection tube to the
NMR cell were selected such that the piston displaces less than the volume of the
NMR cell plus tubing. For most situations this is not strictly necessary. However, for
the small fraction of cases where this might be important, it is suggested this
approach be maintained.
Since the volume displaced is less than the volume of the NMR cell, additional
solvent molecules must be added to the mixing chamber fill this extra space. This is
again done by taking advantage of the compressibility of liquid ethane. The sample
in the mixing chamber is over pressurized just enough that when it is released, and
the piston pushes the sample into the NMR cell, the expanded fluid volume will fully
fill the volume of the NMR cell at the encapsulation pressure. The method for taking
all these details into account is described later in this manual when outlining
potential sample preparation protocols.