Silicas Lichrospher

Chiral stationary phases are also used for the separation of positional isomers. Isomers are normally well separated on straight phase silica or medium polar silica phases, e.g. cyano-modified silica. However, if the two or more chiral centers are in a certain distance apart, they might behave on the columns as if they were pairs of enantiomers. Therefore it is also worth testing CSPs if difficult isomer separations have to be performed. Figure 4.37a-c shows optimized separations of a pair of diastereomers on a straight phase silica (LiChrospher Si60), a reversed phase silica (Purospher RP-18e) and a chiral stationary phase (Chiralpak AD), respectively. Clearly, the separation with the CSP is much better and simplifies the preparative separation so that the higher costs of the stationary phase are compensated by improved overall process economics. 

Fig. 4. Principle of restricted access material (RAM) based on the silica LiChrospher 60.

Fig. 2. Effect of silica pore structure on separation. Stationary phases are 10 LiChrospher SI 100, SI 00, SI 1000, and SI 4000 having 100, 500, 1000. and 4000 A mean pore diameter, respectively. Rowrate and inlet pressure are 5 ml/min and 125 bar, respectively. Sample components I, benzene 2. diphenyl 3, m-terphenyl 4, m-quaterphenyl 5, m ipiliiqiicphcnyl 6, m-sexiphenyl. (Cotiuesy of Merck AO.)...

Silica is the most frequently used adsorbent and for use in HPLC it is sold under a variety of brand names such as LiChrosorb. LiChrospher, Nucleosil, Partisil, Porasil, Spherisorb, Spherosil, and Zorbax. For adsorption chromatography, silicas with a relatively large specific surface area (S0-4S0 m /g), a large pore volume (0.7-1.2 ml/g), ahd a moderate mean pore diameter (50-250 A) are used. 

FIG U RE 1.13 Gradient separation of polypeptides on silica rod column and particle-packed columns. Mobile phase velocity 4mm/s, gradient 5%-60% ACN in the presence of TFA, gradient time 5min, columns (a) silica rod column, (b) Capcellpak SG (5 pm), (c) LiChrospher WP 300 RP-18e (5 pm), (d) nonporous NPS-ODS-1 HPLC column (1.5pm) (e) polymer-based TSKgel Octadecyl-NPR (2.5pm). (Reprinted from Minakuchi, H. et al., J. Chromatogr. A, 828, 83, 1998. Copyright 1998, with permission from Elsevier.)... 

The catalyst-loading was in the range of 42 mg (Nucleosil 300-7) to 63 mg catalyst/g (Lichrospher). RCM reactions carried out with these two silica-supported catalyst versions allowed TONs of up to 90 for a series of simple Q ,cu-dienes [56]. 

LiChrospher Alkyl-diol silica Internal Cis, Cs or C4 Precolumn 25 4.0 mm 200 injections... 

Benzoin acetate, methylben-zoin, Troger s base, tra/w-stilbene oxide and 1,1 -binaphthyl-2,2 -diol Aminopropyl silica gel (LiChrospher 1000, 5 pm) coated with helically chiral poly(diphenyl-2-pyridylmethyl methacrylate) (PDPM) Methanolic solution of ammonium acetate (2.5 mM, pH 4.5) 300 mm x 100 pm i.d. 200 mm effective length, chiral separation 163... 

P-adrenergic blocking agent and various enantiomers Vancomycin bonded on LiChrospher diol silica, 5 pm Methanol-acetonitrile-acetic acid-triethylamine (80-20 20-80 0.1-0.3 0.1-0.4) 355 mm x 75 pm i.d. 265 mm effective length, chiral separation... 

The stationary phase is a C18 bounded silica, granulometry 15 pm LiChrospher E.Merck. Two experiments will be compared. The first experiment has been carried out in almost isocratic conditions without the system allowing to perform the pressure gradient. The pressure drop in the column is low (15 bar on the 8 columns). In the third experiment, we add a pressure drop between the columns, using analogical valves, in order to obtain a global pressure drop of 90 bar. 

A recent development has resulted in HPTLC layers with spherical silica gels 511. These silica gels have a narrow particle size distribution with 6-8 pm of LiChrospher Si 60 F254S spread over glass plates at 0.2 mm thickness and on aluminium sheets at 0.1 mm thickness. At the same time spherical silica is possible with a. 3-5 pm spread on aluminium sheets to a thickness of 0.1 mm for the use of separated spots in Raman spectroscopy. Such plates are labelled HPTLC Aluminium sheets Si 60 FjsaS RAMAN. 

Fig. 2 Comparison between normalized, frequency functions of size (F %) of silica particles for HPLC packing (5 gm LiChrospher, Merck, Darmstad) nominal distribution percentiles dig = 3.7 gm, J50 = 5.0 gm, dgg = 6.8 gm) curve 1 (full line) Fm % curve 2 (dashed line) Fcum % curve 3 (dashed line) 100 - %. (a) GrFFF/PSD Sample load 100 gg channel ...

Table 3.11 Comparison of the particle size distribution data of a LiChrospher Si 100 silica. (Kindly supplied by Dr. K.-F. Krebs, Merck KGaA, Darmstadt, Germany.)...

The results of examining the bed structure of a dc = 50 mm inner diameter silica column with Kromasil and LiChrospher silicas were (Marme, 1991) ... 

Figure 9.9 Separation of co-bromoundecanoic acid [reproduced by permission ofMarcel Dekker, Inc.,from R. Schwarzenbach, J. Liq. Chromatogr.,2,205(1979)]. Conditions sample, raw material from esterification column, 25cm x 3 mm i.d. stationary phase, LiChrospher SI 100, Bpm mobile phase, 1 ml min hexane-diethyl ether (2 3) UV detector. (A) Untreated silica bromoundecanoic acid is eluted with extreme tailing and cannot be quantitatively determined. (B) Silica treated with citrate buffer (pH 2.8) bromoundecanoic acid appears as a distinct peak and the other components remain unaffected.

The newly introduced HPTLC-LiChrospher Si 60 F254S precoated plates in many cases show a still better separation efficiency than the HPTLC silica gel 60 precoated plates. For example. Fig. 12a (HPTLC silica gel 60) and Fig. 12b (HPTLC-LiChrospher Si 60254s) how the scanned chromatograms of eight pesticides, all in the solvent system petroleum ether 40-60 °C + acetone (70 + 30 ml, without saturation of the chamber) [13a]. Also in the analysis of frankincense (olibanum), better separations were obtained on plates precoated with spherical silica gel than on those precoated with irregularly shaped particles [13b-dj. 

Figure 12. Comparison of separation efficiencies of HPTLC silica gel 60 F254 and HPTLC LiChrospher Si 60 F254S precoated plates of one manufacturer...

The protected (S - S) (1-10) salmon calcitonin is purified by preparative chromatography on silica as stationary phase with a solvent mixture (dichoromethane / methanol / acetic acid (93 / 7 / 2 v / v / v)) as eluent phase. After precipitation with water the pure product is analysed by HPLC on Lichrospher 100 RP 18 (125 x 4 mm) 5 micron as stationary phase and with a mobile phase (methanol / water / N,N-dimethylformamide / trifluoracetic acid 70 / 30 / 5 / 0.4 v / v / v / v). The DMF is introduced in the eluent phase to solubilise the protected ( S - S) (1-10) salmon calcitonin. The HPLC profile of the pure protected (S - S) (1-10) salmon calcitonin is shown in Figure 8. 

In the isolation of spongistatin 4 (Scheme 31), the CH2CI2 fraction from 2409 kg of Spirastrella spinispirulifera was initially separated by HPLC employing a pilot scale HPLC system on a silica-gel column, 3 x 0.15 m, operated at 150 psi. This was followed by a series of Sephadex LH-20 separations and multiple HPLC steps using the following columns Merck RP-2, Prepex- RP-8, and LiChrospher 100 RP-18 to yield 10.7 mg of spongistatin 4 (53). 

The silica-based supports are supplied by Bio-Rad Labs (Richmond, CA, U.S.A., under the trademark Bio-Glass) by Electro-Nucleonics, Inc. (Fairfield, NJ, U.S.A.) and Pierce Chem. Co. (Rockford, IL, U.S.A. under the name CPG (controlled-Pore Glass)) by Waters Associates (Milford, MA, U.S.A. under the name Porasil) by E. Merck (Darmstadt, G.F.R., under the names Fractosil or LiChrospher) by DuPont Instrument Products Div. (Wilmington, DE, U.S.A., under the name SE Series or Zorbax), or by Rhone Poulenc (France, under the name Spherosil). The modified silica-based supports are supplied by Electro-Nucleonics, Inc. (Fairfield, NJ, U.S.A.) or Pierce Chem. Comp. (Rockford, IL, U.S.A., under the trademark Glycophase-... 

Adsorption isotherms on ordered mesoporous silicas such as MCM-41, which exhibits hexagonally ordered cylindrical pores [148], were used to verify and correct fundamental relations required for pore size analysis. For example, a series of high-quality MCM-41 materials was used to elaborate an accurate method for the determination of the pore-size distribution based on the BJH algorithm [67, 149]. Based on the nitrogen adsorption isotherms for the aforementioned series of MCM-41 materials, it was possible to obtain the t-curve for the silica surface from the nitrogen adsorption isotherm at 77 K on macroporous (virtually non-porous) LiChrospher Si-1000 silica. This standard adsorption isotherm was reported in tabular form by Jaroniec et al. [26]. It should be noted that the aforementioned... 

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