Open tubular columns pressure programs

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.) ...

A programmed temperature-vaporization (PTV) injector (with a sorbent-packed liner) was used to preconcentrate and inject the sample. Thermal desorption was performed and the analytes were passed to a primary column (16 m X 0.32 mm i.d., film thickness 5 p.m, 100% methyl polysiloxane) and separated according to analyte vapour pressure. Selected heart-cuts were transferred to a second column (15 m X 0.53 mm i.d., Al203/Na2S04 layer, open tubular column with 10 (im stationary phase) where final separation was performed according to chemical functionality. 

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. 

Now, as the flow is increased, the inlet pressure will also increase and, thus, the inlet/outlet pressure ratio (y) wUl change progressively during the program the mean flow rate will be reduced according to the pressure correction function and the decrease in elution rate will not be that which would be expected. Consider an open-tubular column from Poiseuille s equation. 

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. ... 

During this time there was a controversy between open tubular and packed column SFC. There was a widespread belief that carbon dioxide was much more polar than it actually is. Carbon dioxide is roughly as polar as pentane but probably in a different solvent family. Initially, efforts to extend SFC to more polar solutes were limited by the (largely unrecognized) low polarity of carbon dioxide, since there was also a reluctance to abandon the inherent instrumental simplicity of pressure programming a pure fluid. 

The rapid decompression of a packed or a porous-layer open tubular (PLOT) column can ruin the column. The sudden pressure reduction can dislodge packing from the walls of a PLOT column and with a packed column cause the packing to shift within the column. You should allow a slow reduction of pressure at the end of a pressure-programmed run to avoid damage to the column. 

---