Choices for SFC

Pure fluids. Carbon dioxide is often the mobile phase of choice for SFC, since it has relatively mild critical parameters, is nontoxic and inexpensive, chemically inert, and is compatible with a wide variety of detectors including the flame ionization detector (FID) used widely in GC and the UV absorbance detector employed frequently in HPLC (7). The usefulness of carbon dioxide as a mobile phase in many instances is somewhat limited, however, because of its nonpolarity (8), and many polar compounds appear to be insoluble in it. For a sample containing polar compounds, pure carbon dioxide may not be the proper mobile phase. The elution of polar compounds is often difficult and the peak shapes for these polar compounds are sometimes poor. This latter difficulty is commonly observed with nonpolar supercritical fluids and may be due to active sites on the stationary phase rather than any inherent deficiency in the fluid itself. 

Berger [340] has examined the use of pSFC in polymer/additive analysis. As many polymer additives are moderately polar and nonvolatile SFC is an appropriate separation technique at temperatures well below those at which additives decompose [300,341,342], SFC is also a method of choice for additives which hydrolyse easily. Consequently, Raynor et al. [343] and others [284,344] consider that SFC (especially in combination with SFE) is the method of choice for analysing polymer additives as a relatively fast and efficient sample preparation method. Characterisation of product mixtures of nonpolar to moderately polar components encompassing a wide range of molecular masses can be accomplished by cSFC-FID. Unknown polymer additives may be identified quite adequately by means of cSFC-FID by comparison with retention times of standards [343], However, identification by this method tends to be time-consuming and requires that all the candidate compounds are on hand. SFC-FID of some low-to-medium polarity additives on reversed-phase packed columns... 

There is a need for increased chromatography-FTIR sensitivity to extend IR analysis to trace mixture components. GC-FTIR-MS was prospected as the method of choice for volatile complex mixture analysis [167]. HPLC-FT1R, SFC-FTIR and TLC-FTIR are not as sensitive as GC-FTIR, but are more appropriate for analyses involving nonvolatile mixture components. Although GC-FTIR is one of the most developed and practised techniques which combine chromatography (GC, SFC, HPLC, SEC, TLC) and FUR, it does not find wide use for polymer/additive analysis, in contrast to HPLC-FTIR. 

Enantioselective separation by supercritical fluid chromatography (SFC) has been a field of great progress since the first demonstration of a chiral separation by SFC in the 1980s. The unique properties of supercritical fluids make packed column SFC the most favorable choice for fast enantiomeric separation among all of the separation techniques. In this chapter, the effect of chiral stationary phases, modifiers, and additives on enantioseparation are discussed in terms of speed and resolution in SFC. Fundamental considerations and thermodynamic aspects are also presented. 

Another technique is supercritical fluid chromatography (SFC), which is a chromatographic technique that in many ways is a hybrid of GC and HPLC. It is recognized as a valuable technique for the analysis of thermolabile compounds, which would not be amenable to analysis by GC or HPLC. Few applications have been reported for SFC in the field of OCP and OPP determination (16). The advantages reported for SFC are versatility in separation (by the addition of modifier or the choice of stationary phase) and detection (with LC or GC detectors). However, SFC is a little-used technique because it still presents a wide range of instrumental problems (14-16). 

Since the determination of polymeric anthocyanins is still an open problem, new analytical methods such as capillary electrophoresis and SFC will perhaps make the analysis of these polymeric pigments possible. Due to advantages such as speed and selectivity, it can be assumed that HPLC will stay the method of choice for the analysis of monomeric anthocyanins in the coming years. 

The first practical example of an on-line SFC-1 NMR separation was recorded by Dorn and co-workers [16] (Figure 7.2.14). Since up to 90% of a fuel is aliphatic, SFC-NMR on-line analysis is the matter of choice for separation and identification. Figure 7.2.14 shows a fuel mixture of isooctane, n-hexane, -nonane, dodecane, and n-hexadecane, separated by SFC and detected by on-line NMR spectroscopy. The SFC separation was accomplished with a flow rate of 2.0ml/min, a C18 250 x 4.6 mm column, operated at an isobaric pressure of 100 bar and a temperature of 323 K, using CO2 with 1% (w/w) CD3CN as solvent. Each NMR spectrum consists of 20 co-added transients at an acquisition time of 1 s per transient. The total separation occurred within 5 min. The first eluting isooctane can be easily identified by the methylene-to-... 

The choice of possible mobile phases is more limited. The critical properties (critical pressure pc and temperature Tc) should be within practical reach. Moreover, stable compounds are required, which do not show disintegration at elevated temperatures and pressures. Also, the mobile phase must not be too agressive towards the materials used in the column (usually silica-based phases) and the instrumentation (mainly stainless steel). Therefore, mobile phases that are extremely interesting from a chemical point of view, such as supercritical ammonia and, especially, supercritical water, have found little use so far. Table 3.9 lists some possible mobile phases for SFC together with their chemical properties. 

FIGURE 7 Enantiomeric excess of propanolol by packed-column SFC. Conditions are shown in the inset. Reprinted with permission from SFC as the Method of Choice for Chiral Separations of Molecules Soluble in Organic Solvents. Berger Instruments, Newark, DE. 

SFC as the Method of Choice for Chiral Separations of Molecules Soluble in Organic Solvents. Berger Instruments, Newark, DE. 

More than half of small druglike molecules are chiral. The Food and Drug Administration (FDA) requires testing of pure enantiomers. Such testing is most useful early on in drug development. SFC is dramatically superior to HPLC for chiral separations. SFC offers dramatically faster method development and should be the technique of choice for any molecules soluble in organic solvents (i.e., most druglike molecules). Further, unlike capillary electrophoresis, SFC is fully scalable. A method developed at the analytical scale should work equally well at the semiprep level. 

As in analytical-scale SFC, semipreparative-scale SFC should be the technique of choice for chiral separations of solutes soluble in organic solvents. 

Mass Spectrometric Methods for Capillary SFC-MS. A significant advantage associated with capillary SFC-MS methods, and in contrast to all mechanical (e.g., moving ribbon) HPLC-MS interfaces, results from the flexibility in selection of ionization methods. Although initial studies were conducted using chemical ionization, and it remains the method of choice for most applications, the DFI process is also compatible with electron impact ionization (37). 

Ethoxylated alcohols, an important class of non-ionic surfactants, and polyethylene glycols (PEG) are sufficiently soluble in CO2 without derivatization for SFC on capillary columns to be the analytical separation method of choice. The range of polyethylene glycols which may be eluted has been shown [22] to be extended if the samples are trimethylsilylated before analysis. PEG samples containing at least 55 units and ethoxylated allyl alcohol with 41 units have been separated at pressures up to 560 atmospheres by this method (Figure 6). 

White and Houck [37] have analysed a wax containing approximately 80% saturated linear alcohols ranging in carbon number from C30 to C50. Quality control of this wax was necessary, as it was incorporated into various polymers such as polystyrene and polyvinyl chloride. As the properties of the wax were related to the distribution of the alcohols in the material, SFC was the method of choice for the analysis. 

The SFC is a critical parameter for the fats and oils industry. The official American Oil Chemists Society (AOCS) wet method is dilatometery. Alternative wet methods are differential thermal analysis and differential scanning calorimetry (DSC). LR NMR was proved to be an alternative method for SFC determination in late 1950s. The early continuous wave LR NMR spectrometers rapidly found their way into the fats and oils industry, the method being accepted by the Instrumental Techniques Committee of the AOCS as early as in 1972. Presently the technical choice is radio frequency (RF) pulsed LR NMR. Pulse NMR spectrometers are more compact, very efficient, and relatively cheap. They have the advantage of exciting the protons in the whole sample at once. 

Polymer additives - mould release agents, plasticisers, antioxidants, and UV absorbers, with molecular weights extending beyond 1000 - are generally unsuitable for GC or LC analysis because of their low volatility, lack of chromophore, or thermal instability. SFC is now the method of choice for the analysis of such compounds. Figure 7.5 shows the chromatogram of a polymer containing Tinuvin 1130. 

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