Packed columns pressure programming

Figure 12.18 LC-SFC analysis of mono- and di-laurates of poly (ethylene glycol) ( = 10) in a surfactant sample (a) normal phase HPLC trace (b) chromatogram obtained without prior fractionation (c) chromatogram of fraction 1 (FI) (d) chromatogram of fraction 2 (F2). LC conditions column (20 cm X 0.25 cm i.d.) packed with Shimpak diol mobile phase, w-hexane/methylene chloride/ethanol (75/25/1) flow rate, 4 p.L/min UV detection at 220 nm. SFC conditions fused-silica capillary column (15 m X 0.1 mm i.d.) with OV-17 (0.25 p.m film thickness) Pressure-programmed at a rate of 10 atm/min from 80 atm to 150 atm, and then at arate of 5 atm/min FID detection. Reprinted with permission from Ref. (23).

Early work relied on the use of packed columns, but all modern GC analyses are accomplished using capillary columns with their higher theoretical plate counts and resolution and improved sensitivity. Although a variety of analytical columns have been employed for the GC of triazine compounds, the columns most often used are fused-silica capillary columns coated with 5% phenyl-95% methylpolysiloxane. These nonpolar columns in conjunction with the appropriate temperature and pressure programming and pressure pulse spiking techniques provide excellent separation and sensitivity for the triazine compounds. Typically, columns of 30 m x 0.25-mm i.d. and 0.25-qm film thickness are used of which numerous versions are commercially available (e.g., DB-5, HP-5, SP-5, CP-Sil 8 CB, etc.). Of course, the column selected must be considered in conjunction with the overall design and goals of the particular study. 

Figure 6.3 Conparlson of the separation of the octylphenol poly(ethylene glycol) ether, Triton X-16S on a packed column, left, and an open tubular column, right, using UV detection. For the packed column separation al0cmx2mmI.D. column packed with Nucleosil C g, d. 3 micrometers, temperature > 170 C, and mobile phase carbon dioxide (2 ml/min] and methanol (0.15 nl/rnin). pressure programmed from 130 to 375 bar in 12 min were used. For the open tubular column separation a 10 m x 50 micrometers I.O., SB-Biphenyl-30, temperature = 175°C, mobile phase carbon dioxide (0.175 ml/min) and 2-propanol (0.0265 ml/min) pressure programmed, 125 bar for 5 min, then ramped from 125 to 380 bar over 19.5 min, and held at 380 bar for 15 min. were used. (Reproduced with permission from ref. 57. Copyright Preston Publications, Inc.) ...

Figure 6.8 Separation of Triton X-114 by SFC using prograMmed elution on a 10 cm x 2 mm I.D. Nucleosil column, 3 micrometer packing, at 170 C with UV detection at 278 nm. The separation on the left was performed under isobaric conditions at 210 bar with a mobile phase of carbon dioxide -t- methanol (2 + 0. 5) ml/min. The separation in the center was obtained using a ccmt. sition gradient from 0.025 to 0.4 ml/mln over 8 min with other conditions as above. The separation on the right was obtained using a pressure program from 130 to 375 bar over 8 min with the same mobile phase used for the isobaric sepeuration. (Reproduced with permission from ref. 57. Copyright Preston Publications, Inc.)...

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. 

This equation may be used to characterize capillary columns or when employing programmed pressure or temperature conditions for packed columns. 

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. 

Sample introduction is a major hardware problem for SFC. The sample solvent composition and the injection pressure and temperature can all affect sample introduction. The high solute diffusion and lower viscosity which favor supercritical fluids over liquid mobile phases can cause problems in injection. Back-diffusion can occur, causing broad solvent peaks and poor solute peak shape. There can also be a complex phase behavior as well as a solubility phenomenon taking place due to the fact that one may have combinations of supercritical fluid (neat or mixed with sample solvent), a subcritical liquified gas, sample solvents, and solute present simultaneously in the injector and column head [2]. All of these can contribute individually to reproducibility problems in SFC. Both dynamic and timed split modes are used for sample introduction in capillary SFC. Dynamic split injectors have a microvalve and splitter assembly. The amount of injection is based on the size of a fused silica restrictor. In the timed split mode, the SFC column is directly connected to the injection valve. Highspeed pneumatics and electronics are used along with a standard injection valve and actuator. Rapid actuation of the valve from the load to the inject position and back occurs in milliseconds. In this mode, one can program the time of injection on a computer and thus control the amount of injection. In packed-column SFC, an injector similar to HPLC is used and whole loop is injected on the column. The valve is switched either manually or automatically through a remote injector port. The injection is done under pressure. 

Figure 10.3. Examples of chromatograms. (a) The separation of 20 essential amino acids (in derivatized form) by RPLC using 1 mm inside diameter by 150 mm long microbore column packed with 4 ft m silica support (with C18 CBP) particles and a water/ acetonitrile mobile phase gradient. (6) The fractionation of gasoline using SFC with C02 mobile phase (temperature and pressure programmed) in 0.25 mm x 50 cm packed column with 5 fim polymeric support particles. (Courtesy of Frank J. Yang.)...

Solvent modifiers and additives can be used to adjust the retention and selectivity of separation in packed-column SFC. Similar effects have been reported with open-tubular capillary SFC. The advantage of capillary column over packed column arises from the differences in permeability. Pressure ramps are much easier to use in capillary columns to modify the solvent strength (via density modification) as compared to packed columns. Therefore it should be entirely feasible, with capillary SFC, to combine the benefit of solvent density (pressure) programming with simultaneous modification of the solvent strength. ... 

Now, we consider the conditions at the top of the absorber. Run the packed-tower design program of Appendix D using the data for that part of the column from Example 5.2. Try different values of the pressure drop per unit packed height until the program converges to the tower diameter of 0.641 m already selected. Convergence is achieved at a gas-pressure drop of 228 Pa/m. The results at the top of the tower are ... 

This program calculates the diameter of a packed column to satisfy a given pressure drop criterium,and estimates the volumetric mass-transfer coefficients. Data presented are from Example 4.4. 

Figure 7.3. Separation of organotin compounds on a 10 cm x 1 mm I.D. column packed with Deltabond Methyl with supercritical fluid carbon dioxide saturated with formic acid as mobile phase. The separation was obtained at 60°C using pressure programming 0.5 min hold at 90 atm. Then programmed at 4 atm / min to 150 atm where the program rate was increased to 10 atm / min to 300 atm. Peak identification 1 = dibutyltin dichloride 2 = tributyltin chloride 3 = tetrabutyltin 4 = diphenyltin dichloride 5 = dicyclohexyltin dichloride 6 = bis(tributyltin) oxide 7 = triphenyltin chloride 8 = tricyclohexyltin chloride 9 = tetraphenyltin 10 = tetracyclohexyltin 11 = bis(triphenyltin) oxide and 12 = hexakis(2-methyl-2-phenylpropyl) distannoxane. (From ref. [42] Springer-Verlag)...

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