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.)...

The basic SFC system comprises a mobile phase delivery system, an injector (as in HPLC), oven, restrictor, detector and a control/data system. In SFC the mobile phase is supplied to the LC pump where the pressure of the fluid is raised above the critical pressure. Pressure control is the primary variable in SFC. In SFC temperature is also important, but more as a supplementary parameter to pressure programming. Samples are introduced into the fluid stream via an LC injection valve and separated on a column placed in a GC oven thermostatted above the critical temperature of the mobile phase. A postcolumn restrictor ensures that the fluid is maintained above its critical pressure throughout the separation process. Detectors positioned either before or after the postcolumn restrictor monitor analytes eluting from the column. The key feature differentiating SFC from conventional techniques is the use of the significantly elevated pressure at the column outlet. This allows not only to use mobile phases that are either impossible or impractical under conventional LC and GC conditions but also to use more ordinary... 

Compatibility with different fluids under pressure programming conditions and with modifiers... 

An orthogonal array design and electronic pressure programming have been described for the optimisation of the GC determination of NP [76]. The characterisation of isomers of technical NP previously reported by Giger et al. [77] was further improved by GC-MS and GC-FTIR [78] identifying 14 isomers. On the other hand, Wheeler et al. [79] indicated the presence of five different groups of isomers of the 22 separated isomers. The isomeric composition of APEO mixtures and their structure elucidation are discussed in more detail in the next section. 

MAIN PROGRAM SETTS DIFFERENT VALUES FOR INITIAL GUESS OF TEMPERATURE AND DIFFERENT INITIAL STEP SIZES SPECIFIC CHEMICAL SYSTEM IS BENZENE/TOLUENE AT 760 MM HG PRESSURE PROGRAM MAIN... 

In the first section, the mechanisms involved in size exclusion chromatography are discussed this is an area where additional understanding and clarification still are needed. Data treatment with respect to statistical reliability of the data along with corrections for instrumental broadening is still a valid concern. Instrumental advances in the automation of multiple detectors and the developm.ent of a pressure-programmed, controlled-flow supercritical fluid chromatograph are presented. 

Pressure-Programmed Controlled-Flow Supercritical Fluid Chromatograph... 

In this paper, an instrument is described in which the inlet liquid flow rate is held constant and the pressure regulated by a pneumatically actuated flow control valve at the exit of the column. This approach permits the use of a wide-range pressure program with a controlled flow. Also, by selecting mobile phases that are liquids at ambient laboratory conditions, several types of conventional liquid chromatographic detectors may be utilized. 

Figure U, Separation of low molecular polystyrene "by pressure programming.

Figure 5 Pressure separation of 1,800,000 molecular vreight polystyrene by pressure programming.

Fio. 10. Effect of pressure programming on chromatographic separation. Stationary phase, nitrobenzyl-silica, SI 200 d, 35 /tm column, 50 x 2 mm eluent, n-hepune, temp., 23 C. Sample components 1. unretained 2, bromobenzene 3, toluene 4, naphthalene 5, anthracene 6, brasan 7, o,A-[Pg.50]

To a good approximation, the retention time is inversely proportional to the pressure drop along a column under otherwise constant conditions. A linear increase in the inlet pressme and, hence, in the flow velocity results in a linear decrease in the retention time (/, 72). Since the mtention times increase exponentially within a homologous series, it is moit expedient to employ exponential pressure programming to maintain a constant distance between peaks. 

The advantages of pressure programming are demonstrated in Fig. 10 by comparing isobaric separations at 30 and 100 bar with a separation obtained by pressure-programming at column inlet pressures increasing from 30 to 225 bar (73). At low inlet pressure the early peaks are separated and eluted within 5 min, whereas the last one is eluted with tailing after an additional 5 min. The linear flow velocity is 1.1 cm/sec At 100 bar the... 

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