Density programming

Pressure or density programming is the most popular of the gradient techniques in SFC. Density is the important parameter with respect to retention but pressure is the physical property which is directly monitored by SFC instruments. If enough experimental density-volume-temperature data are available for the mobile phase then a computer-based algorithm can be used to generate specific density programs. Such data are available for only a few mobile phases, such as carbon dioxide and the n-... 

Figure 6.9 Separation of a poly(styrene) oligomer mixture of 2000 average molecular weight by open tubular column SFC using a linear density program (A) and an asymptotic density progreua in (B) with carbon dioxide as the mobile phase. (Reproduced with permission from ref. 115. Copyright Preston Publication, Inc.)...

Merkle, R., Savin, A., Preuss, H., 1992, Singly Ionized First-Row Dimers and Hydrides Calculated With the Fully Numerical Density Program NUMOL , J. Chem. Phys., 97, 9216. 

Although cSFC shows relatively poor figures of merit (speed, sensitivity, detection dynamic range and sample capacity) as well as a limited application area, its applications tend to be unique. These include solutes that can be solvated with pure SCCO2 and quantified with FID. Linear density programs typical in cSFC are ideal for homologous series found in surfactants, many prepolymers, etc. Selectivity in cSFC, which can be achieved by mobile phase density and temperature programming, relies on selective interactions with the stationary phase. Quantitative analysis in cSFC may be rendered difficult by small injected volumes the use of internal standards is recommended. 

Column pressure usually has little effect on enantioselectivity in SFC. However, pressure affects the density of the mobile phase and thus retention factor [44]. Therefore, similar to a modifier gradient, pressure or density programming can be used in fast separation of complex samples [106]. Later et al. [51] used density/temperature programming in capillary SFC. Berger and Deye [107] demonstrated that, in packed column SFC, the effect of modifier on retention was more significant than that of pressure. They also showed that the enhanced solvent strength of polar solvent-modified fluid was nof due fo an increase in densify, caused by fhe addition of fhe liquid phase modifier, buf mainly due fo fhe change in composition. 

Figure 17. Enantiomer separation by supercritical fluid chromatography of 7-chloro-2,3.4,5-tetrahydro-l-methyl-5-phenyl-1,4-benzodiazepin-2(l//)-one (dihydrodiazepam) on a 2.5 m x0.05 mm (i.d.) fused silica capillary column, containing immobilized octamethylenc-Chirasil-Dex [carbon dioxide at 90CC. density programmed from 0.31 g/mL (130 atm) at 0.0029 g/mL min 1 after an initial 2.0-min period at 0.31 g/ mL]130.

The output is the integrated intensity value for that particular reflection. FHKL - This is the structure factor calculating program. The input is a list of hkl s and intensity values. The output consists of E values and phase angles to be used as input to the electron density program. ELECDEN - Calculates the electron density and contours the E-map on a Tektronix 4662 digital plotter. PATTERSON - Used to calculate three-dimensional Patterson maps. 

Figure 7.16 Capillary supercritical fluid chromatogram of tetrahydrocannabinol and six metabolites. Conditions supercritical C02 at 120°C 15-m x 50-/itn ID SE-33 column density programmed from 0.40 g/ml after a 7-min hold to 0.56 g/ml at 0.01 g/ml/min FID at 280°C. (Reprinted from Ref. 27, J. Chromatogr. Sci. by permission of Preston Publications, a division of Preston Industries.)...

Syringe pump This pump is widely employed for both open-tubular-column and packed-column SFC applications. Microprocessor control allows reproducible SFC fluid pressure or density programming. 

SFC Mobile phase density Density programming pressure programming... 

Supercritical Fluid (Dense Gas) Chromatography/Extraction with Linear Density Programming, L. M. Bowman, Jr., M. N. Myers, and J. C. Giddings, Sep. Sci. Technol., 17, 271 (1982). 

Figure 11.4. Example of asymptotic density programming in SFC. Polystyrene oligomer mixture with avg. MW of 2,000. Reproduced from the Journal of Chromatographic Science by permission of Preston Publications, Inc.

Figure 11.6. Comparison of a coal tar separation by (a) GC and (b) SFC. Both separations performed on SE-54 OT columns with 0.25 p-m film thicknesses. GC column 20 m x 300 g,m, temperature programmed from 40°C to 265°C. SFC column 34 m x 50 (j.m, density programmed. Reprinted with permission from J. C. Feldsted and M. L. Lee, Anal. Chem. 1984,56, 619A. Copyright 1984, American Chemical Society. 

CSFC, although, resembles gas chromatography (GC) at high pressures, with the pressure (density) programming taking the place of temperature programming used in GC. Typical operating temperatures are up to 100°C. 

Hadj-Mahammed et al. analyzed a mixture of flavone, 5-methoxyflavone, and tangeretin by supercritical CO2 SFC on capillary columns with two types of detectors flame ionization (FID) and FT-IR. Peafe identification was achieved with the help of the FT-IR fingerprint of each compound. However, the separation was satisfactory only by the use of supercritical C02g density programs, without the use of a phase modifier. The separations were accomplished using a Carlo Erbaa SFC system equipped with a Model SFC 300 pump and Model SFC 3000 oven. The fused silica capillary column were BPl (12 mxO.l mm I.D. 0.1-pm film of di- methylpolysiloxane) and DB5 (15 mxO.l mm TD. 0.4-pm film of 94% dimethyl-, 5% diphenyl-, and 1% vinylpolysiloxane). The two supercritical CO2 density programs used in this work were PI [from 0.127 g/mIi->... 

Table 2 Comparison of retention times and capacity factors of flavones analyzed using capillary columns DBS and BPl with supercritical CO2 density program P2...

The overall density of the mobile phase is one of the most important parameters used to optimize separations in SFC with density programming as common in SFC as temperature programming in GC and eluent composition in HPLC [5]. Capacity ratios, k decrease roughly linearly at higher densities with different slopes for different classes of compounds, thereby affording changes in selectivity [5]. A similar effect is seen for the supercritical fluid elution of analytes from octadecylsilica sorbents, as seen in Fig. 2 [6]. 

On the subject of sample preparation, analytical SFC can save the analyst considerable time, as illustrated by the SFC profile of the composition in a nutraceutical capsule containing sawtooth palmetto berry extract (Fig. 12). In this case, the extract was dissolved and diluted with a minimal amount of hexane and directly injected into the chromatograph. By density programming the CO2 mobile phase, a high-resolution chromatogram can be facilitated. The complexity of the sawtooth palmetto berry extract is apparent and... 

Figure 2. SFC chromatograms of a 2000 MW polystyrene oligomer mixture. Conditions n-pentane mobile phase at 210 C, 10 m X 100 Ji fused silica capillary column coated with a 0.25- Jm film of crosslinked 50% phenyl metbylphenylpoly-siloxane, UV-absorption at 205 nm, (A) linear pressure program from 34.6 atm (0.12 g/mL) to 60 atm at 0.15 atm/min after a 5-min isobaric period (B) asyoiptotic density program from 0.12 g/mL (34.6 atm) to 0.35 g/siL according to the...

Resembles high-pressure gas chromatography. Pressure (density) programming generally used to change elution strength (temperature programming less common). Typical temperatures > 80°C. Carbon dioxide alone or possibly saturated with water or formic acid as a mobile phase. Mixed mobile phases rarely used. Initial conditions... 

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