Supercritical fluid chromatography modifiers

The most common mobile phase for supercritical fluid chromatography is CO2. Its low critical temperature, 31 °C, and critical pressure, 72.9 atm, are relatively easy to achieve and maintain. Although supercritical CO2 is a good solvent for nonpolar organics, it is less useful for polar solutes. The addition of an organic modifier, such as methanol, improves the mobile phase s elution strength. Other common mobile phases and their critical temperatures and pressures are listed in Table 12.7. 

H. Daimon and Y. Hirata, Direct coupling of capillary supercritical fluid chromatography with superaitical fluid extraction using modified carbon dioxide , J. High Resolut. Chromatogr. 17 809-813 (1994). 

Supercritical fluid chromatography (SEC) was first reported in 1962, and applications of the technique rapidly increased following the introduction of commercially available instrumentation in the early 1980s due to the ability to determine thermally labile compounds using detection systems more commonly employed with GC. However, few applications of SEC have been published with regard to the determination of triazines. Recently, a chemiluminescence nitrogen detector was used with packed-column SEC and a methanol-modified CO2 mobile phase for the determination of atrazine, simazine, and propazine. Pressure and mobile phase gradients were used to demonstrate the efficacy of fhe fechnique. 

A gradient that runs with 30-80% methanol or acetonitrile is not uncommon. This amount of modifier is generally not needed in supercritical fluid chromatography to affect the same separation. Typical modifier composition in SFC is 1.0-10% and would achieve higher Hildebrand Solubility Parameter adjustment overall than the broader gradients found in LC. 

Snyder et al. [253] compared supercritical fluid chromatography with classical sonication procedures and Soxhlet extraction for the determination of selected insecticides in soils and sediments. In this procedure the sample was extracted with carbon dioxide modified with 3% methanol at 350atm and 50°C. An excess of 85% recovery of organochlorine and organophosphorus insecticides was achieved. These included Dichlorvos, Diazinon, (diethyl-2-isopropyl-6-methyl 4-pyrimidinyl phosphorothioate), Ronnel (i.e. Fenchlorphos-0,0 dimethyl-0-2,4,5-trichlorophenyl phosphorothioate), Parathion ethyl, Methiadathion, Tetrachlorovinphos (trans-2-chloro-l-(2,4,5 trichlorophenyl) vinylchlorophenyl-O-methyl phenyl phosphoroamidothioate), Endrin, Endrin aldehyde, pp DDT, Mirex and decachlorobiphenyl. 

Non-ionic surfactants of a commercial washing powder were separated by supercritical fluid chromatography (SFC) and determined by APCI-MS. The constituents were first extracted by supercritical fluid extraction (SFE) using C02 with or without methanol as a modifier. Variations of the conditions resulted in a selective extraction of the analytes, which could be determined without further purification. Six groups of surfactants were observed, four of which are alkyl-polyethoxylates. The presence of APEO could be excluded by identification recording SFC-FTIR (Fourier transform infrared) spectra [31]. 

ViUeneuve, M.S., Anderegg, R.J. Analytical supercritical fluid chromatography using fully automated column and modifier selection valves for the rapid development of chiral separations. J. Chromatogr. A 1998, 826, 217-225. 

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. 

More recently, some studies have reported the use of supercritical fluid chromatography (SEC) [479,480], Coupling SEC with SEE, sample extraction, preconcentration, and quantification can be performed in a single step. The mobile phase, carbon dioxide, can be modified by adding different... 

Supercritical-fluid chromatography has been applied by Ramsey et al. (213) for the determination of trimethoprim, along with three steroid hormones, in swine kidney. Separation was performed on a Spherisorb 5 amino-bonded column, using carbon dioxide with methanol modifier as the mobile phase. Detection at levels greater than 10 ppm was accomplished by tandem mass spectrometry using thermospray interface. However, this method lacks the sensitivity required to detect the low ppb levels likely to occur in milk and tissues. 

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). 

The scope of this review is limited to the hop-borne a- and /3-acids and to products derived from them that have appreciable commercial relevance. Thus, the liquid chromatography of the iso-a-acids and their chemically modified counterparts as well as of the hulupones will be considered. By far the major focus here will be HPLC analyses, but other methodologies that have been applied (countercurrent distribution, gas chromatography, supercritical-fluid chromatography, thin-layer chromatography, and micellar electrokinetic chromatography) will also be briefly considered. 

The use of supercritical fluid chromatography for carotene separation has been examined and optimized, especially in regard to temperature, pressure, and organic modifiers in the supercritical fluid (71). With an RP column it was possible to resolve an a-carotene-cis isomer from an all-trans carotene as well as two cis isomers of /3-carotene from an all-trans /3-carotene. As with HPLC, only polymeric C,8 columns were able to resolve the cis isomers of a- and /3-carotene from the all-trans isomers. Supercritical fluid chromatography offers the advantage not only of an efficient separation but also of fast analysis. Indeed, the use of SFC with ODS-based columns for the analysis of carotenoid pigments affords a threefold reduction of analysis time compared to HPLC (72). The elution order of carotenoids and their cis isomers was found to be the same as in RP-HPLC. The selectivity of the system could further be increased by adding modifiers (e.g.,... 

Cosolvent-modifled supercritical fluids are also used routinely in supercritical fluid chromatography (SFC) to modify solute retention times (11-20). In these reports, cosolvents are used to alter the mobile and stationary phase chemistries (16t17t20). However, distinguishing between such effects in a chromatography... 

In addition to high-performance liquid chromatography (HPLC), the chiral resolution using CMPAs was also carried out by supercritical fluid chromatography (SFC) [91] and capillary electrochromatography (CEC) [92-98]. Salvador et al. [91] used dimethylated /1-cyclodextrin as the mobile phase additive on porous graphite carbon as the solid phase for the chiral resolution of tofizopam, warfarin, a benzoxazine derivative, lorazepam, flurbiprofen, temazepam, chlorthalidone, and methyl phehydantoin by SFC. The authors also studied the effect of the concentration of dimethylated /1-cyclodextrin, the concentration of the mobile phase, the nature of polar modifiers, outlet pressure, and the column temperature on the chiral resolution. 

McNally and Wheeler [364] used supercritical fluid extraction coupled to supercritical fluid chromatography to determine sulfonylurea herbicides in soil. Klatterback et al. [365,366] used supercritical fluid extraction with methanol-modified carbon dioxide followed by high-performance liquid chromatography with UV detection to determine sulfonylurea herbicides obtained on a Cis solid-phase extraction disc. Alternatively the determination was carried out by gas chromatography of the dimethyl derivatives of the sulfonylurea herbicides, employing an electron capture or a NP detector on the gas chromatograph. 

Janicot et al. presented the separation of opium alkaloids using sub-critical and supercritical fluid chromatography [20]. Carbon dioxide-meth-anol-triethylamine-water mixtures were used as the mobile phase with packed aminopropyl or bare silica columns. The influence of aminated polar modifiers such as methylamine, ethylamine, and triethylamine was studied. Figure 7.15 shows the separation of six opium alkaloids narcotine, papaverine, thebaine, codeine, cryptopine, and morphine on a Lichrosorb Si-60 column. The method gave comparable results with HPLC. 

Steuer et al. demonstrated the use of supercritical fluid chromatography in the separation of enantiomers of 1,2 amino alcohols, namely pindolol, metoprolol, oxprenolol, propranolol, and DPT 201-106 using ionpairing modifiers [21]. The mobile phase consisted of carbon dioxide mixed with acetonitrile containing triethylamine as a counterion and /V-benzo-xycarbonylglycyl-L-proline as a chiral counterion. They found that the ca-... 

Masuda et al. have reported the supercritical fluid chromatography of retinol palmitate and tocopherol acetate using an octadecylsilyl silica gel column with ethanol-modified carbon dioide as the mobile phase [43]. Both retinol and tocophenol eluted within 5 min. The method was applied to the determination of these compounds in an ointment formulation. The method was found to be linear for both retinol and tocophenol from 0.5 to... 

W. Steuer, M. Schindler, G. Schill, and F. Erni, Supercritical fluid chromatography with ion-pairing modifiers separation of enantiomeric 1,2-aminoalcohols as diastereomeric ion pairs, J. Chromatogr., 447 287 (1988). 

O. Gyllenhaal and J. Vessman, Packed-column supercritical fluid chromatography of omeprazole and related compounds selection of column support with triethylamine and methanol-modified carbon dioxide as the mobile phase, J. Chromatogr., 628 215 (1993). 

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