Pressure restrictor

In SFC, the mobile phase is initially pumped as a liquid and is brought into the supercritical region by heating it above its supercritical temperature before it enters the analytical column. It passes through an injection valve where the sample is introduced into the supercritical stream and then into the analytical column. It is maintained supercritical as it passes through the column and into the detector by a pressure restrictor placed either after the detector or at the end of the column. The restrictor is a vital component it keeps the mobile phase supercritical throughout the separation and often must be heated to prevent clogging both variable- and fixed-restrictors are available. 

The pressure restrictor that controls the pressure in the column is installed before or after the detector, depending on its type (before for an FID, after for a UV detector). 

One solution to the problem mentioned above is not to rely solely on flow symmetry, but to achieve equidistribution by pressure-loss adjustment by means of flow resistors (see Figure 4.98) [140, 141, 148). In the flow sequence consisting of a main stream tube, fluid inlet, damping tank, distribution tubes and micro device such as a micro mixer, the main pressure drop is nearly always on the last side. The separation layer mixer acts here as a pressure restrictor similar to the sparger mentioned above. This requires accurate control over structural precision of micro fabrication. 

Spikes on baseline Spikes on baseline (Figure 10.7c) are caused by air bubbles out-gassing in the detector flow cell. They can be eliminated by mobile phase degassing and/or by placing a pressure restrictor in the detector outlet (e.g., with 50-psi back pressure). Spikes can also be caused by poor signal wire connections (loose or damaged wiring) or malfunction of the detector or the data system. 

Baseline noise There is air in the system Add a pressure restrictor to outlet... 

Degassing buffos is usualfy not necessaiy if they are freshly made. If you encounter problems vnth air bubbles in the UV detector, put a pressure restrictor downstream. 

Solvents with low boiling point, such as pentane or diethyl ether, are less convenient. Usually, it is no problem to find substitutes with higher boiling point, namely hexane or methyl tert-butyl ether in these cases. If a low-boiling eluent needs to be used, it is advisable to install a pressure restrictor at the detector outlet This can be a Teflon tube of length 10 m and inner diameter 0.25 mm. 

On expansion the gas cools rapidly (Joule-Thompson effect) and tends to block the back pressure restrictor valve (particularly when the simple manual valves are used) with solid carbon dioxide and precipitated solutes. 

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