Derivatised silicas

Fig 1. Frontal loading adsorption isotherms of conventional "wide pore" derivatised silica (Brownlee RP-300) and macroporous divinylbenzene crosslinked polystyrene (Poros RII/H) supports. Protein 1 mg/ml solution in aqueous 0.1% TFA. Superficial linear flow velocities 347, 866, 1732 and 3465 cm/h. Temperature 25°C. (A) RP-300 2.1 mm ID cartridge. (B) Poros RII/H 2.1 mm ID column. 

In recent years, a small number of new phases have become available that are more suited to CEC. Phase Separations (Deeside. UK) have produced a SCX stationary phase. This is a strong cation-exchange material which contains aminopropyl-derivatised silica that has sulphonic acid groups covalently attached to the amino end of a short alkyl chain. The sulphonic acid groups are effectively ionised at all working pHs due to their low p/faS. Fig. 4.5 shows the dependence of EOF on pH of the mobile phase for a capillary packed with Phase Separations SCX stationary phase. The EOF is almost the same over the whole range. It increases beyond pH 7, presumably due to the added ionisation of the surface silanols. 

Most of the linkers described for the synthesis of 3 -aminoalkyl ohgonucleotides employ the carbamate (urethane) linkage to the polymer. A 2,2 -dithiodiethanol-derivatised silica gel support (Section 19.3.2.3) has been transformed by Asseline and Thuong [287] into the N-ro-hydroxyhexylcarbamate (Figure 19.12) via the consecutive treatments with l,T-carbonyldiimidazole and 6-amino-l-hexanol followed by the phosphoramidite synthesis of a DNA undecamer. Deprotection by 0.1 M DTT in concentrated aqueous ammonia unmasked the [3-mercaptoethyl carbamate, which fragments under basic conditions with the elimination of thiirane to furnish 3 -aminohexyl oligodeoxyribonucleotide. The structure of the latter has been confirmed by the reaction with acridine-9-isothiocyanate. 

The majority of work on organically modified solids as solid bases has been carried out on the simple 3-aminopropyl-derivatised silica. This material is established as an efficient catalyst for the Knoevenagel reaction.136-138 There are several interesting features about this application. One of these is the solvent, a... 

Advances in stationary phase technology have led to commercial availability of adsorbents such as high performance sihcas, aluminas, polyamides, celluloses and derivatised silicas [9,10], The development of automated method development (AMD) systems [127] now allow multi-step gradients of different elution strengths to be achieved in a relatively short time compared to earlier manual approaches. AMD systems are ideally suited for separation of complex mixtures with a wide range of polarities. Further improvements in sample resolution and reduced method development times in TLC include the use of two-dimensional development approaches [128] and forced-flow development by over-pressure liquid chromatography (OPLC) [129]. 

As has been discussed previously, a number of different interactions give rise to retention on HPLC columns that can be exploited to separate analytes. In a well designed column, normally one major type of interaction will be exploited, and other interactions will be suppressed in order to keep separations simple, predictable, and efficient. A good example of this is traditional RP HPLC, in which the main interactions are hydrophobic interactions between solutes and a nonpolar, C18 derivatised silica stationary phase. 

Uptake of metal on derivatised silica malri)< (umol/g)... 

Figure 14.7 Column plot of uptake of metals onto a derivatised silica matrix designed to be selective for uranium. Uptake measurements are after treatment of matrix with 0.9 mM uranyl nitrate solution containing other dissolved metal nitrates.

A GC analysis of amino acids requires a derivatisation step to increase the volatility of the amino acids. Generally, norleucine and/or norvaline are the internal standards added to the hydrolysate to check the derivatisation yield. According to the experimental method applied, the limits of detection (LOD) vary in the range 10 100 pg for each amino acid. Regarding the chromatographic columns, as most of the derivatives are esters barely polar compounds the most commonly used are fused-silica capillary columns with a low... 

A deactivated silica pre-column is also a fundamental tool when pyrolysis is performed, since it partially helps in avoiding the contamination of the analytical column with underivatised polar compounds. Blanks (that is the pyrolysis of the derivatising agent without sample) must be run between analyses to ensure the absence in the chromatograms of any signals that do not belong to the sample. 

Figure 3.1 Chemical structures of the current most successfully employed derivatised polysaccharide CSPs. (a) CHIRALPAK AD Amylose tris (3,5-dimethylphenylcarbamate) coated onto a silica support, (b) CHIRALPAK AS Amylose tris [(S)-a-methylbenzylcarbamate] coated onto a silica support, (c) CHIRALCEL OD Cellulose tris (3,5-dimethylphenylcarbamate) coated onto a silica support, (d) CHIRALCEL OJ Cellulose tris (4-methylbenzoate) coated onto a silica support.

The amount of fines (fine particles of silica gel) generated by different manufacturers cartridges varies. Ideally very little fines should elute from the column with the sample. Fine particles of silica gel can damage HPLC systems and interfere in derivatisation reactions used prior to analysis by GC. 

Mcdoe, J. E. G., Clark, J. H. and Macquarrie, D. J. Michael additions catalysed by N,N-dimethyl-3-aminopropyl - derivatised amorphous silica and hexagonal mesoporous silica (HMS). Synlett, 1998, 625-631. 

The material which is most commonly used for HPLC column packing is silica. It may be used either urunodified or after chemical derivatisation... 

One of the main reasons for the widespread use of silica as an HPLC packing material is that it may be used either unmodified or after chemical derivatisation of the silanol groups, allowing the surface chemistry to be altered to suit different modes of chromatography. 

Si MAS-NMR and CP/MAS-NMR spectra have been reported for me-soporous silica catalysts derivatised with bis(halogeno)dioxomolybdenum(VI)... 

The structure of hyperforin 10 was determined with extensive chemical degradation and derivatisation, as well as with spectroscopic means. Bystrov and co-workers described the isolation of 10 and systematically deduced its general chemical and physico-chemical characteristics [34, 35, 36, 37, 38] and suggested its stereochemistry [39, 40]. An acetone extract of St. John Wort was chromatographed over silica gel with gradient elution by petroleum ether-benzene mixtures. Further purification was performed by converting hyperforin 10 into the crystalline 3,5-dinitrobenzoate 11, from which the pure compound 10 was obtained by alcali hydrolysis. The molecular formula of hyperforin 10 was established by elemental analysis and mass spectrometry. From the IR-spectrum, the... 

Fig. 10.3. Steps in the chemical derivatisation of silica. X = Cl or OR and R is one of the commonly used bonded phase such as Cjg, Cg, C2, cyano, amino, phenyl, etc.

---