Supercritical fluid chromatography (SFC) has advanced over the past decade. SFC uses highly compressible mobile phases, which typically employ carbon dioxide (CO2) as a principle component. In addition to CO2, the mobile phase frequently contains an organic solvent modifier, which adjusts the polarity of the mobile phase for optimum chromatographic performance. Since different components of a sample may require different levels of organic modifier to elute rapidly, a common technique is to continuously vary the mobile phase composition by linearly increasing the organic modifier content. This technique is called gradient elution.
SFC has been proven to have superior speed and resolving power compared to traditional HPLC for analytical applications. This results from the dramatically improved diffusion rates of solutes in SFC mobile phases compared to HPLC mobile phases. Separations have been accomplished as much as an order of magnitude faster using SFC instruments compared to HPLC instruments using the same chromatographic column.
SFC instruments used with gradient elution also reequillibrate much more rapidly than corresponding HPLC systems. As a result, they are ready for processing the next sample after a shorter period of time. A common gradient range for gradient SFC methods might occur in the range of 2% to 60% composition of the organic modifier.
It is worth noting that SFC instruments, while designed to operate in regions of temperature and pressure above the critical point of CO2, are typically not restricted from operation well below the critical point. In this lower region, especially when organic modifiers are used, chromatographic behavior remains superior to traditional HPLC and often cannot be distinguished from true supercritical operation.
In analytical SFC, once the separation has been performed and detected, the highly compressed mobile phase is directed through a decompression step to a waste stream. During decompression, the CO2 component of the mobile phase is allowed to expand dramatically and revert to the gas phase. The expansion and subsequent phase change of the CO2 tends to have a dramatic cooling effect on the waste stream components. If care is not taken, solid CO2, known as dry ice, may result and clog the waste stream. To prevent this occurrence, heat is typically added to the flow stream. At the low flow rates of typical analytical systems only a minor amount of heat is required.
While the CO2 component of the SFC mobile phase converts readily to a gaseous state, moderately heated liquid organic modifiers typically remain in a liquid phase. In general, dissolved samples carried through SFC system also remain dissolved in the liquid organic modifier phase.
The principle that simple decompression of the mobile phase in SFC separates the stream into two fractions has great importance with regard to use of the technique in a preparative manner. Removal of the gaseous CO2 phase, which constitutes 50% to 95% of the mobile phase during normal operation, greatly reduces the liquid collection volume for each component and thereby reduces the post-chromatographic processing necessary for recovery of separated components.
Expanding the technique of analytical SFC to allow preparative SFC requires several adaptations to the instrument. First the system requires increased flow capacity. Flows ranging from 20 ml/min to 200 ml/min are suitable for separation of multi-milligram up to gram quantities of materials. Also, a larger separation column is required. Finally, a collection system must be developed that will allow, at a minimum, collection of a single fraction of the flow stream which contains a substantially purified component of interest. In addition, there frequently exists a compelling economic incentive to allow multiple fraction collections from a single extracted sample. The modified system must also be able to be rapidly reinitialized either manually or automatically to allow subsequent sample injection followed by fraction collection.
Sample injection valves in SFC introduce a measured sample into the mobile phase flow stream prior to entering a chromatography column. Common injection valves are fixed-loop multi-port injection valves with either internal or external loops. Direct fill loop injections are a normal means of sample introduction in SFC so that a packed column in SFC has similar quantitative reproducibility to LC using fixed loop injectors.
Injection valves used in SFC sample introduction present special hazards caused by the high pressures found in SFC systems. Sample is manually injected into the sample loop with a syringe through some type of fill port. During an injection, the valve loop has discharges sample contents into a mobile phase flow stream and the valve is returned to a load position. However, mobile phase from the flow stream becomes trapped inside the sample loop. Switching the injection valve loops, loaded with high-pressure mobile phase from the previous injection, back to a load position at ambient laboratory pressure exposes the sample fill port to highly compressed mobile phase inside the loop.
The compressed mobile phase will rapidly expand when exposed to atmospheric pressure. A hazard occurs when mobile phase from a trapped in an injection loop expands back up through an injection system and out into a laboratory. Dissipating this pressure to a waste line also causes greater attention to the injection process and time delays which slows the entire SFC sample processing time. There is a need for a cost-effective solution in an SFC system to prevent high-pressure blowback from a sample injection valve and to minimize time delays associated with venting high-pressure build-up prior to sample injections.
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