Phases octadecyl

Fig. 12. Tryptic map of it-PA (mol wt = 66,000) showing peptides formed from hydrolysis of reduced, alkylated rt-PA. Separation by reversed-phase octadecyl (C g) column using aqueous acetonitrile with an added acidic agent to the mobile phase. Arrows show the difference between A, normal, and B, mutant rt-PA where the glutamic acid residue, D, has replaced the normal arginine residue, C, at position 275.

The presence of these 12 stereoisomers was experimentally confirmed by the partial resolution of 12 peaks in HPLC chromatography using an achiral stationary phase (octadecyl polysiloxane, ODS) and different mobile phase compositions (CH3CN/THF 55 45-58 42) and temperatures (288-258 K). Figure 7 shows one of these chromatograms in which 10 partially resolved peaks are clearly distinguished. ... 

Normal-phase HPLC, using an amino-bonded phase, was used for determination of the PAHs of up to 7 rings. This type of separation results in elution by the number of n bonds. A special reversed-phase octadecyl column was used for PAHs of 7 through 12 rings. This HPLC packing, Vydac 201TP5, is well known for its orderly structure and separates the PAHs by their overall shapes. It has been compared to the liquid-crystal phases used in gas chromatography. It provides the best isomer specific separation of PAHs. 

Figure 33-2 shows plots of plate heights // as a function of average linear velocity u in cm/s for high-performance liquid chromatography and supercritical-fluid chromatography. In both cases, the solute was pyrene, and the stationary phase was a reversed-phase octadecyl silane maintained at 40°C. The mobile phase for HPLC was acetonitrile and water, while for SFC the mobile phase was carbon dioxide. These conditions yielded about the same retention factor (k) for both mobile phases. Note that the minimum in plate height occurred at a flow rate of 0.13 cm/s... 

Figure22.5 Reaction of (/ ,S)-metoprolol with (S)-fezr-butyl 3-(chloroformoxy)-butyrate and determination of the ratio of enantiomers in human plasma (reproduced with permission from A. Green, S. Chen, U. Skantze, I. Grundevik and M. Ahnoff, poster at the 13th International Symposium on Column Liquid Chromatography, Stockholm, 1989.) Conditions stationary phase, octadecyl silica mobile phase, phosphate buffer-acetonitrile (1 1) fluorescence detector 272/312 nm.

Recently, these parameters have been expanded to include a limited number of solvents for the reversed-phase octadecyl (C,g) support. It should be realized however, that ° is based on adsorption equilibrium and that adsorption (except in very specific instances) is not a predominant retention mechanism in reversed-phase separations. That having been stated, regardless of what system die ° parameter is used in conjunction with, it is at best an estimate of the solvent strength for an average solute on a generic support. 

Table 15. Recovery of trans isohumulone from some reversed phase (octadecylated silica gel) LC column.

Several N-substituted pyrroHdinones eg, ethyl, hydroxyethyl and cyclohexyl, are used primarily in specialized solvent appHcations where their particular physical properties are advantageous. For example, mixtures of l-cyclohexyl-2-pyrroHdinone and water exhibit two phases at temperatures above 50°C below that temperature they are miscible in aH proportions. This phenomenon can be used to facHitate some extractive separations. Mixtures of 1-alkyl-pyrroHdinones that are derived from coconut and taHow amines can be used at lower cost in certain appHcations where they may be used instead of the pure l-dodecyl-2-pyrroHdinone and l-octadecyl-2-pyrroHdinone. 

Dibenzyl-14-crown-4 (lithium ionophore VI 6,6-dibenzyl-l,4,8,ll-tetra-oxa-cyclo-tetradecane) [106868-21-7] M 384.5, m 102-103°. Dissolve in CHCI3, wash with saturated aqueous NaCl, dry with MgSOa, evaporate and purify by chromatography on silica gel and gradient elution with C6Hg-MeOH followed by preparative reverse phase HPLC on an octadecyl silanised silica (ODS) column and eluting with MeOH. It can be crystd from MeOH (v Br 120 cm , C-O-C). [7 Chem Soc Perkin Trans 1 1945 1986.]... 

It has been found that both bromination and chlorosulfonation reactions are very useful in coupling a wide range of bonded phases to the gels. It is possible to couple virtually any primary amine to brominated and/or chlorosulfonated PDVB gels. This has allowed the development of many other phases such as polyamino, octadecyl, polyethyleneimine, and quaternary amine. Figures 13.13-13.16 show an assortment of various bonded phases developed since the mid-1980s. 

J. Slobodnik, O. Ostezkizan, H. Lingeman and U. A. Th Brinkman, Solid-phase extraction of polar pesticides from environmental water samples on grapliitised carbon and Empore-activated carbon disks and on-line coupling to octadecyl-bonded silica analytical columns , J. Chromatogr. 750 227-238 (1996). 

The alternative technique for analyzing 1,4-dioxane is HPLC. Scalia proposed a method by solid-phase extraction using octadecyl-bonded silica cartridges and analyzed directly on a reverse phase column with UV detection at 200 nm and acetonitrile-water as eluent [328-330]. 

The basic polymer appears to be a hydroxylated polyether to which octadecyl chains have been bonded and so it behaves as a reverse phase exhibiting dispersive interactions with the solutes. An example of the separation of a series of peptides is shown in figure 15. The column was 3.5 cm long, 4.6 mm i.d. The solutes shown were (1) oc-endorphin, (2) bombesin, (3) y-endorphin, (4) angiotensin, (5) somatostatin and (6) calcitonon. The separation was carried out with a 10 min linear program from water containing 0.2% trifluoroacetic acid to 80% acetonitrile. 

ODS octadecyl silyl stationary phase used in high performance liquid chromatography... 

The k values of the barbiturates, secbutobarbitone and vinbarbitone, determined by using an octadecyl silyl (ODS) stationary phase and mobile phases of... 

Table 2.1 HPLC capacity factors for secbuto-barbitone and vinbarbitone with an octadecyl silyl stationary phase and mobile phases of methanoiyO.l M sodium dihydrogen phosphate (40 60) at (a) pH 3.5, and (b) pH 8.5. From Moffat, A.C. (Ed.), Clarke s Isolation and Identification of Drugs, 2nd Edn, The Pharmaceutical Press, London, 1986. Reproduced by permission of The Royal Pharmaceutical Society...

By far the most popular phase system at the present time is the one in which R n-octadecyl. Since the stationary phase is hydrophobic and nonpolar, while the mobile phase is relatively polar, this mode of operation is frequently called reverse phase... 

In comparison to GC, an advantage in using HPLC is that there is no decomposition of the acidic forms of cannabinoids. Commonly reversed-phased (RP) materials are used as the stationary phase. Mostly the octadecyl-type... 

Reversed-phase PLC precoated plates are based on silica gel matrices with chemical modifications in such a manner that the accessible polar, hydrophilic silanol groups at the silica gel surface are replaced by nonpolar, hydrophobic alkyl chains via silicon-carbon bonds. For preparative purposes, up to now only PLC precoated RP plates with C-18 modification are available. This abbreviation is often also designated as RP-18, meaning that an octadecyl alkyl chain is chemically bonded to the silica gel surface. 

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