Normal Phase-LC-SFC Applications

The low polarity of CO,-based eluents makes SFC a normal phase technique. Therefore, it is not surprising that most of the successful applications of chiral SFC have utilized CSPs designed for normal phase LC. Flowever, some exceptions have been noted. Specific applications of various CSPs are outlined in the next sections. 

SFE-GC and SFC-GC may replace normal-phase LC-GC. SFC-SFC may be adopted for a wide range of applications, particularly for solutes that are not amenable to GC and as a replacement for some coupled LC-LC separations. However, no applications to additives in polymers can be mentioned. 

Generally, better chromatographic performance is found with chiral separations in the normal phase for most column manufacturers. It is also likely that the easier solvent removal after collecting the isolated enantiomer, is what drove the industry to normal phase chromatography for chiral applications. It is advantageous to the chiral chromatographer that the majority of the commercially available normal phase LC CSPs and modifiers can be used on both LC and SFC instrumentation. This flexibility allows methods developed using one mode to be transferred to the other... 

An application of an LC-SFC system has been demonstrated by the separation of non-ionic surfactants consisting of mono- and di-laurates of poly (ethyleneglycol) (23). Without fractionation in the precolumn by normal phase HPLC (Figure 12.18 (a)) and transfer of the whole sample into the SFC system, the different homologues coeluted with each other. (Figure 12.18(b)). In contrast with prior fractionation by HPLC into two fractions and consequent analysis by SFC, the homologues in the two fractions were well resolved (Figures 12.18(c) and 12.18(d)). 

As discussed in Sect. 3.5, the interactions involved in the chiral recognition on Pirkle-type CSPs are mainly attractive forces, such as k-jt, hydrogen-bonding, and dipole-dipole interactions. Although bonded Pirkle-type CSPs have been used in reversed phase and polar nonaqueous mobile phase, most of the applications were found in normal-phase mode. With the introduction of SFC for the resolution of enantiomers [185], bonded Pirkle-type CSPs were among the most studied CSPs in the early application of chiral SFC [172, 175, 181, 186], Comparable enantioselectivity and the same elution order of enantiomers were usually observed for the enantioseparations of many compounds. Accordingly, similar chiral recognition mechanisms were believed to operate in both LC and SFC conditions [186]. However, when the enantioseparations of jt-acidic compounds on the n-acidic... 

Because of its intermediate position, SFC combines several advantages of both GC and HPLC. One significant advantage of SFC compared to HPLC is the fact that besides the normal LC detectors (e.g., UV detector), sensitive universal or selective GC detectors (e.g., FID, electron capture detector) are applicable as long as supercritical carbon dioxide is utilized as mobile phase. Furthermore in packed-column SFC, the same variety of stationary phases (selectivities) can be used as in LC (in contrast to GC), with the additional advantage that the analysis time is significantly shorter. 

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