Alkyl-diol silica

Z. Yu and D. Westerlund, Direct injection of large volumes of plasma in a columnswitching system for the analysis of local anaesthetics , II. Determination of bupivacaine in human plasma with an alkyl-diol silica precolumn , ]. Chromatogr. A 725 149-155 (1996). 

A wide variety of SPE materials and cartridges are commercially available for example, alkyl-diol silica-based restrictive access materials (RAMs) and a variety of silica- and polymer-based SPE materials of different binding abilities and capacities. Reversed-phase, size-exclusion, ion-exchange SPE, and turbulence flow methods will be discussed in this chapter related to real-world applications. 

RAMs packed in small cartridges are often used for online SPE of samples in biological matrixes. First developed by Boos et al. (1991), these alkyl-diol-silica (ADS) materials consist of large size particles (20 to 50 /.an compared to 3.5 to 5 /an for analytical columns) with hydrophilic electron-neutral surfaces that do not retain proteins and hydrophobic internal pore surfaces that allow only small molecules to enter and bind. 

Viehauer S. et al., 1995. Evaluation and routine application of the novel restricted-access precolumn packing material Alkyl-Diol Silica Coupled-column high-performance liquid chromatographic analysis of the photoreactive drug 8-methoxypsoralen in plasma. J Chromatogr B 666 315. 

Besides the above differentiation, restricted-access media can be further subdivided on the basis of the topochemistry of the bonded phase. Packings with a uniform surface topochemistry show a homogenous ligand coverage, whereas packings with a dual topochemistry show a different chemical modification of the pore internal surface and the particle external surface (114). Restricted-access media of the former type are divided into mixed-mode and mixed-function phases, bonded-micellar phases, biomatrix, binary-layered phases, shielded hydrophobic phases, and polymer-coated mixed-function phases. Restricted-access media of the latter type include the Pinkerton s internal surface reversed-phase, Haginaka s internal surface reversed-phase diol, alkyl-diol silica, Kimata s restricted-access media, dual-zone phase, tris-modified Styrosorb, Svec s restricted-access media, diphil sorbents, Ultrabiosep phases. Bio Trap phases, and semipermeable surface phases. 

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

Bayenes, W.R.G., Van der Weken, G., Haustraete, J., Aboul-Enein, H.Y., Corveleyn, S., Remon, J.P., Carcia-Campana, A.M., Deprez, P. Application of the restricted-access precolumn packing material alkyl-diol silica in a column-switching system for the determination of ketoprofen enantiomers in horse plasma. J. Chromatogr. A 871, 153-161 (2000)... 

Two other ISRP phases have found extensive application in the literature. The hrst is the alkyl-diol silica (ADS) format introduced by Boos and coworkers [68]. This phase dp 25 /zm) is internally derivahzed with either C-4, C-8 or C-18. The akyl-silica esters on the surface are then removed by lipase enzymahc hydrolysis to yield a hydrophilic diol moiety. This phase has found popularity in part due to its comparahvely robust operation. According to Boos, it is possible for ADS media to accommodate more than 200 injechons of plasma (500 /zL each) on a single column [69]. 

Boos, K.-S. Rudolphi, A. Vielhauer, S. Walfort, A. Lubda, D. Eisenbeiss, F. Alkyl-Diol Silica (ADS) Restricted Access Precolumn Packings for Direct Injection and Coupled-Column Chromatography of Biofiuids, Fresenius J. Anal. Chem. 352, 684-690 (1995). 

Schafer, C. Lubda, D. Alkyl Diol Silica Restricted Access Pre-Column Packings for Fast Liquid Chromatography-Integrated Sample Preparation of Biological Fluids, J. Chromatogr. A 909,73-78 (2001). 

The comparison of LiChrolut EN (40—120 Xm particles) performance in SPE trace enrichment of herbicides listed in Table 14.2 with that of a restricted-access material (P AM, 25 im), alkyl-diol-silica (25 Xm), reversed phase C-18 (10 Xm), monofiinctional C-18 (40—70 Xm), and, finally, styr-ene-DVB Empore extraction disks revealed a decisive superiority of the hypercrosslinked sorbent in the retention of very polar DIA and DEA [247]. However, the strong retention power of LiChrolut EN causes significant band broadening when analysis is performed in on-line... 

Sorbent functional groups Alkyl—octadecyl, octyl, ethyl cyclic— cyclohexyl aromatic— phenyl Cyanopropyl, diol, silica, amine... 

Nowadays, almost all commercially available HPLC stationary phases are also applicable to planar chromatography. In addition to the polar hydroxyl groups present on the surface of native silica, other polar functional groups attached to the silica skeleton can also enter into adsorptive interactions with suitable sample molecules (34). Silica with hydrophilic polar ligands, such as amino, cyano, and diol functions, attached to the silica skeleton by alkyl chains, all of which have been well proven in HPLC, have also been developed for TLC (34). 

To date, typical SPE materials are based on silica gel or highly cross-linked styrene-divinylbenzene (PS-DVB). The former is functionalised with distinct chemical groups to yield various sorbents with non-polar or polar characteristics. Non-polar materials are modified with alkyl groups of different chain length (C18, C8, C2), while polar sorbents have cyano-, amino-, or diol-bonded groups. Ion-exchange phases have either anionic or cationic functional groups. 

An early synthesis of A5-palmitoy]-.S -[2,3-bis(palmitoyloxy)propyl]cysteine employed cysteine methyl ester, however, this leads to difficulties in the saponification step of the tri-palmitoylated residue. 96 The optimized procedure, in which the cystine di-fert-butyl ester is used, 90 is outlined in Scheme 6 after N-acylation with palmitoyl chloride, the ester is reduced to the cysteine derivative for S-alkylation with l-bromopropane-2,3-diol to yield chirally defined isomers if optically pure bromo derivatives are used. Esterification of the hydroxy groups is best carried out with a 1.25-fold excess of palmitic acid, DCC, and DMAP. The use of a larger excess of palmitoyl chloride is not recommended due to purification problems. The diastereomeric mixture can be separated by silica gel chromatography using CH2Cl2/EtOAc (20 1) as eluent and the configuration was assigned by comparison with an optically pure sample obtained with 2R)- -bromopropane-2,3-diol. 

A material prepared by anchoring titanium(IV) on to the walls of a high-area, crystalline mesoporous silica (MCM41) has been used as an alkene epoxidation catalyst with alkyl hydroperoxides.204 The effect of replacing one of the three O—Si= groups to which the Ti(IV) is bound by an O—Ge= group is reported to lead to an increase in catalytic activity of up to 18% in die epoxidation of cyclohexene, although no explanation is provided and it is notable diat the selectivity towards the formation of cyclohexene oxide (versus cyclohexenol and cyclohexane-1,2-diol) was inferior to that with the non-modified system.205... 

The covalent C-B bonds of organoboronic acids and esters are very inert to ionic and radical reactions, thus allowing functionalization of remote sites other than the B-G bond (Equations (94)-(97)). Bulky diols such as pinacol have been used as the protecting group of B(OH)2 because of their high stability to nucleophiles and water and silica gel in amination of 316,474 hydroboration-amination of 317,475 Wittig reaction of 3 18,476 and oxidation-alkylation of 319.477... 

Before workup of the reaction of the dibutylstannylene acetal of a diol with an electrophile such as an acyl, alkyl, or sulfonyl halide, the product present in nonpolar solvents has a halodibutylstannyl group attached to the nonreacted oxygen atom. This organotin derivative can be cleaved with water or mild acid, but chromatography on silica gel is usually sufficient to remove it. Some research groups have made use of the strong Sn-F bond by washing with fluoride ions. 

NPC has also found some apphcations in the held of carbohydrate analysis. Typical stationary phases used for this application are alkyl amine-, diol-, or polyol-bonded silicas [50-53]. Alkyl amino-bonded silicas are commonly used for the separation of saccharides and oligosaccharides in various matrixs, such as food or biological huids. Although water is used as part of mobile phase, the retention behavior of carbohydrate follows the NPC retention behavior. 

Oximes.—Oximes of some saturated ketones are reduced by aqueous alkaline sodium borohydride under reflux to give the corresponding alcohols. Selective reduction of a 3,17-dioxime is possible, at C(3). a -Oximino-ketones afford diols. Diborane, in contrast, reduces oximes to give alkyl hydroxylamines a recent variant using sodium borohydride on silica gel in benzene gave the... 

In a later study, Pesek et al. reported the separation of other proteins using a diol stationary phase [61-64]. The use of a diol stationary phase should result in a surface that is more hydrophilic than a typical alkyl-bonded moiety, like Cis or Cg. The overall results showed significant variations in retention times due to differences in solute-bonded phase interactions. Other factors, such as pH, could also influence this interaction, due to its influence on charge and protein conformations. Combining all these factors in the separation of peptides and proteins provides an experimentalist with many decisions to be made in the optimized experimental conditions to be used. Other chemical modifications of etched fused silica need to be studied in order to provide a better understanding of their interactions with proteins and peptides, as well as other classes of biopolymers. 

These sorbents possess, as functional groups, cyano, amino, and diol residues, bonded by short-chain hydrophobic spacers to the silica matrix. With respect to polarity, hydrophilic-modilied silicas range between nonmodilied silica and the nonpolar alkyl- or aryl-bonded phases ... 

The silicas were modified by grafting alkyl chains, diols, or polyethylene glycols) (PEG). Since the hydroxyl groups of the silica are weakly acidic, the grafting reaction corresponds to an esterification ... 

Enthalpies of Interaction of Polar Probes with Silicas modified with Diols. Whereas the treatment of silicas with alcohols leads to fixation of alkyl grafts, their modification with diols results in the grafting of hydrocarbon chains still having a... 

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