Carbon-dioxide mobile phase

Achiral-chiral chromatography has also been accomplished using subcritical fluid chromatography (Phinney et al., 1998). In this work, the structurally related [3-blockers, 1,4-benzodiazepines, and two cold medicines were separated using methanol or ethanol modified carbon dioxide mobile phases. The (3-blockers were separated using cyanopropyl and Chiracel OD columns connected in series. Likewise, an amino bonded phase and Chiracel OD column were used for the separation of the 1,4-benzodiazepines. Guaifenesin and phenylpropanolamine from cough syrup were separated on cyanopropyl and Chiralpak AD columns in series. 

The second approach was taken by practicing liquid chromatographers. They routinely dealt with thermally labile, highly polar molecules and frequently sacrificed resolution, and speed in their separations because of the aqueous mobile phases that were required. With the enhanced diffusion and decreased viscosity of supercritical fluids over liquids, chromatographic run-time and resolution could be improved when supercritical fluids were used. But solubility in pure carbon dioxide mobile phases, which has the solvating powers from hexane to methylene chloride under normal density ranges, was a problem for these polar molecules. To compensate for this, experimentalists started working with mixed mobile phases. These mixed phases were based on... 

Beyond the density changes that can be used to control method modifications in SFC, the mobile phase composition can also be adjusted. Typical LC solvents are the first choice, most likely because of their availability, but also because of their compatibility with analytical detectors. The most common mobile phase modifiers, which have been used, are methanol, acetonitrile and tetrahydrofuran (THF). Additives, defined as solutes added to the mobile phase in addition to the modifier to counteract any specific analyte-column interactions, are frequently included also to overcome the low polarity of the carbon dioxide mobile phase. Amines are among the most common additives. 

Actual operating conditions can be found in the figure caption. A given limitation of SFC, relative to HPLC, as described is the ability to dissolve samples in a solvent system compatible with the methanol/ carbon dioxide mobile phase. For this particular mobile phase, other compatible sample diluents that worked effectively are pure methanol,... 

FIGURE 4.2 The van Deemter curves for HPLC and SFC. Conditions (a) LC, 55 cm x 250 xm ID fused-silica capillary columns packed with 5 pm porous particles, nitromethane test solute, 25°C, UV detector (214 nm) (b) SFC, 75 cm x 250 pm ID fused-silica capillary columns packed with 5-pm porous particles, 45°C, 230 atm, carbon dioxide mobile phase, methane test solute, FID. (Adapted from Wu, N. et al. Anal. Chem. 71 5084-5092. With permission.)... 

The SFC analysis of barbiturates has been reported by Smith and Sanagi [16]. The method utilzed packed columns of ODS bonded silica (200 x 3 mm ID) or polystyrene-divinylbenzene polymer (150 x 4.6 mm ID) with a carbon dioxide mobile phase and FID detection. The barbiturates were strongly adsorbed on the ODS column and were not observed to elute. The drugs did elute with reasonable retention times on the polymer column. Unfortunately, there was peak tailing, and adsorption remained a problem with all the barbiturates studied. The authors were prevented from adding methanol as a polar modifier to the mobile phase because the detection was by FID. 

Edder et al. reported the capillary supercritical fluid chromatography of basic drugs of abuse, namely nicotine, caffeine, methadone, cocaine, imipramine, codeine, diazepam, morphine, benzoylecgonine, papverine, narcotine, and strychnine [25]. They compared the separation of these drugs on DBS and DB wax columns. The chromatographic conditions included a carbon dioxide mobile phase and a flame-ionization detector. It was noted that on the DBS column, all peaks other than methadone and cocaine were separated. With the exception of benzoylecgonine and papaverine, all other peaks were separated on a DB wax column. A reproducibility of less than 5% was obtained with an internal standard method. The detection limits obtained were within 10-50 ppm on both the columns. A linearity of >0.99 was obtained for methadone, codeine, and morphine in the concentration range 10-1000 ppm. 

Li and co-workers demonstrated the use of SFC in the analysis of panaxadiol and panaxatriol in ginseng, a famous traditional Chinese medicine [28 J. A capillary SB-cyanopropyl-50 column with a carbon dioxide mobile phase and flame-ionization detector was used for the analysis. Methyltestosterone was used as an internal standard for the quantitation. Figure 7.17 shows the SFC separation. The method was found to be linear (r > 0.999) in the range studied and the precision obtained was in the range 2.2-5.7%. 

Fig. 1 Use of polymer-coated silica stationary phase with a neat carbon dioxide mobile phase. Column Deltabond SFC Methyl (150 X 2.0 mm) mobile phase CO2 pressure gradient 75 bar initial for 2 min, then 50 bar/min ramp to 180 bar, then 15 bar/min ramp to 300 bar flow 0.5 mL/min temperature 100°C injection 5 jjlL detection flame ionization detector sample solvent methylene chloride. Peak key 1 BHT 2 dimethyl azelate 3 triethyl citrate 4 tributyl phosphate 5 methyl palmitate 6 methyl stearate 7 diethylhexyl phthalate 8 Tinuvin 327 9 Spectra-Sorb UV531 10 tri(2- ethylhexyl) trimellitate 11 dilauryl thiodipropionate ...

Saturated solutions in methanol can actually be injected into methanol/ carbon dioxide mobile phases (although not recommended) without the solute falling out of solution. This is probably due to the extensive adsorption of the modifier onto the stationary phase (38), creating a form of reservoir of solvent to receive the injected sample. 

The aromatic hydrocarbon content of diesel fuel affects the cetane number and exhaust emissions. One test method (ASTM D-5186) is applicable to diesel fuel and is unaffected by fuel coloration. Aromatics concentration in the range 1-75 mass% and polynuclear aromatic hydrocarbons in the range 0.5-50 mass% can be determined by this test method. In the method, a small aliquot of the fuel sample is injected onto a packed silica adsorption column and eluted with supercritical carbon dioxide mobile phase. Mono- and polynuclear aromatics in the sample are separated from nonaromatics and detected with a flame ionization detector. The detector response to hydrocarbons is recorded throughout the analysis time. The chromatographic areas corresponding to the mononuclear aromatic constituents, polynuclear aromatic constituents, and nonaromatic constituents are determined, and the mass-percent content of each of these groups is calculated by area normalization. 

Figure 1 SFC chromatogram of soybean oil sample. Carbon dioxide mobile phase at 200 C, 10 m x 50 pm i.d. SB-Methyl-100 column, linear density programmed at 0.005 g/mL/min, FID at 375 C. Fatty acid group identification P-Palmitic, O-Oleic, S-Stearic, L-Linoleic, Ln-Linolenic.

S. Rokushika, K. P. Naikwadi, A. L. Jadhav, and H. Hatano. Polyacrylate liquid crystalline stationary phases in supercritical fluid chromatography with carbon dioxide mobile phase. Chromatographia, 22 209-212,1986. 

Figure 7.2 Supercritical fluid chromatogram of polymer additives listed in Tables 7.1 and 7.2. Conditions 10 m x 50 pm id fused-silica capillary column, crosslinked methylpolysiloxane stationary phase (0.25 pm film thickness) carbon dioxide mobile phase at 140 "C 15 - 35 MPa at 0.3 MPa/min after an initial 12 minutes isobaric period ...

Willian and Lilly [27] also used SFC carbon dioxide mobile phase with FID to monitor the separation of MMA oligomers and polymer additives. 

Biicherl and co-workers [48] has described an integral restrictive interface with jet separation for coupling capillary column SFC with carbon dioxide mobile phase with high resolution mass spectrometry. 

A small aliquot of the fuel sample is injected onto a packed silica adsorption column and eluted using supercritical carbon dioxide mobile phase. Monoaromatics and polynuclear aromatics in the sample are separated from nonaromatics and detected using a fUme ionization detector. 

Although analytical SFC was demonstrated in the early 1960s, it has only been in recent years that the availability of adequate high resolution packed and capillary SFC columns and instrumentation has led to renewed interest in the technique. Plasma emission is a natural development because of its use in GC and HPLC. A surfatron MIP sustained in helium has been employed for SFC detection, giving sulfur-specific detection at 921.3 nm with a 25 pg s limit for thiophene [28]. An argon high efficiency MIP has been interfaced with packed column SFC and the separation and detection of ferrocene and derivatives achieved with iron specific detection. Methanol modifier concentrations to 5% were tolerated in the carbon dioxide mobile phase [29]. 

The SIPV replaces the Rheodyne 7125 loop injection valve in the chromatograph shown in Fig. 8-5 when it is desired to dissolve samples in carbon dioxide mobile phases. The SIPV overcomes most of the problems encountered when trying to chromatograph relatively large volumes of sample in supercritical carbon dioxide. This facility allows the chromatograph to be used in a very flexible manner at the preparative scale. 

Sensitivity for the eluted solute peak is often much larger than with analytical SFC. In fact, as often happens in preparative HPLC, the variable wavelength UV detector must be de-tuned away from the wavelength of maximum absorption of the eluting species to prevent overload of absorption. For the vast majority of cases when using modified carbon dioxide mobile phases there is no need to look further than the simple UV detector. 

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