Preparative scale SFC

Verillon and Boutant [136] presented a method of separating the enantiomers of some /I-adrenergic blockers at the preparative scale. SFC modified with carbon dioxide and standard HPLC columns having bores of 4 to 20 mm could produce from 10 to 500 mg of purified enantiomers per hour. The authors described the practical rules of their method and its performance. In another study, Leveque and Duval [137] described the fast and easy transportation of... 

The sorbent/sorbate ratio of 4 1 was adhered to for these optimization experiments, and preparative-scale SFC was accomplished in three steps followed by a sorbent bed reconditioning as described in the experiment section. The first SFC step removed the majority of the TAG and the phytosterol fat aq l esters. The second step was signed for maximnni FPE eruichment, and the third Auction was run to elute ar r remaining com bran extract fix>m the sorbent bed, preventing extract carryover to subsequent runs. Column reconditioning purged the colirrrm of atqr residual ethanol and com... 

The cumulative mass collected in the fiactions fiom the SFC runs yielded an average of 4.96 g, which represented an 82.7 wt% recovery of the sUuting charge of com bran oil. This is in contrast to earlier research on the analytical-scale SFF of com bran oil, whidi exhibited nearly quantitative mass recovery (50). However, this result is not atypical in preparative scale SFC as evidenced by prior investigators (10,39,40). For example, in the first two studies (10,39) involving the SFC of tocopherols, only parti recovery of the tocoph ls (76 to 87%) was obtained from silica gel. 

The purification of combinatorial libraries on a Berger system is deseribed by Farrell et al. at Pfizer for their parallel solution-phase syntheses. The overall process employs as well analytical SFC in combination with mass spectrometry and nitrogen chemiluminescence deteetion off-line of the preparative-scale SFC systems. Pre-purification analytical SFC/ MS/CLND allows the triage of samples for purification, and an in-house software package analyzes data for predicted quality based on an evaluation of UV and MS data for the potential of co-eluting peaks during purification. This same software package selects a collection time window for purification, which is necessary to limit the number of fractions per sample. This system accommodates the purification of samples up to 50 mg... 

Packed column SFC has also been applied to preparative-scale separations [42], In comparison to preparative LC, SFC offers reduced solvent consumption and easier product recovery [43]. Whatley [44] described the preparative-scale resolution of potassium channel blockers. Increased resolution in SFC improved peak symmetry and allowed higher sample throughput when compared to LC. The enhanced resolution obtained in SFC also increases the enantiomeric purity of the fractions collected. Currently, the major obstacle to widespread use of preparative SFC has been the cost and complexity of the instrumentation. 

A new brush-type CSP, the Whelk-0 1, was used by Blum et al. for the analytical and preparative-scale separations of racemic pharmaceutical compounds, including verapamil and ketoprofen. A comparison of LC and SFC revealed the superiority of SFC in terms of efficiency and speed of method development [50]. The Whelk-0 1 selector and its homologues have also been incorporated into polysiloxanes. The resulting polymers were coated on silica and thermally immobilized. Higher efficiencies were observed when these CSPs were used with sub- and supercritical fluids as eluents, and a greater number of compounds were resolved in SFC compared to LC. Compounds such as flurbiprofen, warfarin, and benzoin were enantioresolved with a modified CO, eluent [37]. 

Chiral SFC can be performed in open tubular [41,42], and packed column [43,44] modes. Packed column SFC can be further categorized into analytical, semipreparative, and preparative SFC [7, 8], Packed column SFC is more suitable for fast separations than open tubular column SFC, since a packed column generally provides low mass transfer resistance and high selectivity [45, 46], Packed column SFC also provides high sample loading capacity [27,47], which can increase sensitivity. Only packed column SFC is suitable for preparative-scale enantioseparation. This chapter will focus on chiral separation using packed column SFC in the analytical scale. 

Packed-column SFC also is suitable for preparative-scale enatioseparations. Compared with preparative LC, sub- or supercritical fluid chromatography results in easier product and solvent recovery, reduced solvent waste and cost, and higher output per unit time. Because of its reduced sample capacity, SFC usually allows the separation of 10-100 mg samples per run. Chromatographers can compensate for these sample amounts by using shorter analysis times and repetitive injections (Wolf and Pirkle, 1997). 

King, J.W. 1994. Application of Preparative Scale SFE/SFC to Food and Natural Products. Proceedings of the 3rd International Symposium on Supercritical Fluids, 421-428. 

Most of the history of SFC has involved analytical-scale chromatography. Recent advances have made both analytical and semipreparative chromatography highly desirable. The term semipreparative (semiprep) refers to the separation of milligrams to grams of materials to prepare modest amounts of material for testing. It is distinct from preparative-scale chromatography, where the intent is the production of the final product or an intermediate of the final product. 

Cyclone separators can be effective as collection devices in SFC. However, they tend to be best suited for semipermanent applications, like preparative-scale chromatography. They are not user friendly for the separation of a large number of different samples in a short time. Each fraction requires a separate cyclone separator. For modest scale separations, the cyclone separators are bulky, with a very large internal surface area, which is difficult to clean out. Because they are subjected to relatively high pressures, they are usually made of stainless steel. Connections are usually made with large-diameter stainless-steel tubing and the whole apparatus is bolted to some sort of rack mount. One could think of this approaches a scaled-down pilot plant. 

In spite of this critical note, the potential of SFC in analytical and preparative-scale enan-tioseparations has been already illustrated. Technical development in this field may open even more challenges for this technique. The advantage of SFC for preparative separations is that the high-pressure liquid carbon dioxide used as mobile phase can easily be removed from the product. In addition, carbon dioxide is non-hazardous and relatively inexpensive. On the other hand, this mobile phase creates the following problems the solubility of polar compounds is limited, and alcohols or other polar modifiers have to be used. Although this makes the technical advantage of SFC questionable, the method may offer some advantages for chiral compounds that may dissolve in SFC mobile phases. Selected examples of preparative SFC enantioseparations are summarized in Table 10 [168-171]. 

Previous SFF studies (30,31) using the SFE/SFC tq roach were performed on an analytical scale, and were designed to emulate a preparative-scale fiacdonation process. In this study, solute fiactimiaticm was acconqtlished in two stq . The first stq>, utilizing neat CO2, removed the majority of the TAG and the tosterol fidty acyl esters. The seomd elution stq> was designed for FPE enrichment and was achieved with ethanol-modified CO2. 

Before preparative-scale SFE/SFC trials were undertaken, it was necessary to conduct experiments for both the SFE and SFC stages in order to optimize the processes. The SFE runs yielded an average amount of extract equal to 5.8S g. This equaled to an average yield of 3.49 wt% with a relative standard deviation (RSD) of 1.9%. The oil content of the com bran was also determined in triplicate 1 the AOCS Official Method Ac 3-44, which uses petroleum etter as the extraction solvent in a Butt-type extraction apparatus. The organic solvent extraction yielded an average of 3.50 wt% with an RSD of 2.0%, in excellent agreement with the SFE result... 

SFC offers some advantages over HPLC for enantioseparations both on the analytical scale (wider choice of available detectors, higher peak efficiency) and on the preparative scale (easy removal of the mobile phase). However, the technique has not yet become a serious competitor to HPLC for either analytical or preparative scale enantioseparations. 

Preparative chromatography can be practised on two different scales laboratory (recovery of grams of sample) and production (recovery of kilograms of sample). Preparative Scale Supercritical Fluid Chromatography (PS-SFC) has been practised on the laboratory scale from the very beginning of SFC in 1962 [1], More information on these historical experiments has been gathered by Berger et al. [2] and Bevan [3]. 

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