Aminopropyl-bonded silica

Many of the l.c. stationary-phases available may be packed into columns by the user at a fraction of the cost of a commercial column (see Section II,l,f). The most notable of these are the cation-exchange res-ins8.89 and the aminopropyl-bonded silica gels, which can be readily packed to provide high-resolution separations. The Cis-bonded silica gel and cation-exchange resin stationary-phases are especially useful, as large columns (2 x 25 cm) can be accommodated on analytical chromatographs, at flow rates of 1 to 5 mL/min, without any modifications of equipment. 

New advances in the l.c. of carbohydrates are likely to come from three general areas. The first is in the development of more-durable and stable, stationary phases. At present, a major limitation on the use of commercial columns, especially those of the aminopropyl-bonded silica-gel variety, is their short life-time and ease of fouling. More-durable, resin-based columns that operate with the same solvent system and selectivity as aminopropyl silica-gel columns are currently available, and will see further use and development. The development of improved phases for supercritical, fluid-type l.c. will allow this method to be of use for analysis of various carbohydrates. ... 

Sugar analysis by hplc has advanced gready as a result of the development of columns specifically designed for carbohydrate separation. These columns fall into several categories. (/) Aminopropyl-bonded silica used in reverse-phase mode with acetonitrile—water as the eluent. (2) Ion-moderated cation-exchange resins using water as the eluent. Efficiency of these columns is enhanced at elevated temperature, ca 80—90°C. Calcium is the usual counterion for carbohydrate analysis, but lead, silver, hydrogen, sodium, and potassium are used to confer specific selectivities for mono-, di-, and... 

Veuthey and Haerdi reported the separation of amphetamines using packed-column SFC [26]. The amphetamines were derivatized with 9-fluorenylmethyl chloroformate and chromatographed with a methanol or 2-propanol-modified carbon dioxide as the mobil phase. The separations were compared on bare silica and aminopropyl-bonded silica columns. Both columns gave comparable results and the separation of all five amphetamines (methylamphetamine, amphetamine, phenethylamine, ephed-rine, and norephedrine) was achieved in less than 5 min. Both methanol and 2-propanol-modified carbon dioxide gave comparable results. It was observed that the modifier concentration had more effect on the solvating power than the mobile-phase density. 

Later, Klemm and co-workers [86,87] achieved partial resolution of aromatic compounds by low-pressure chromatography on silica gel impregnated with TAPA. The separation was attributed to n-n complexation between TAPA and the enantiomers. Mikes et al. [88] used a column packed with an (i )-(-)-TAPA aminopropyl-bonded silica support to accomplish the full resolution of helicenes. The authors extended their study to other homologues of TAPA (Figure 22-19). These compounds were coated on silica gel or ion-paired to an aminopropyl-bonded phase, and they were used in the HPLC separation of helicenes. To describe the selective interactions that occur between the stationary phase and the helicenes, the authors assumed that the 2,4,5,7-tetranitro-9-fluorenylidene moieties of the selector are laying down on the silica surface, while the X groups point away from the surface and above the plane of the fluorenyl ring. 

In addition to the chromatographic supports already described, many other supports are available (both silica-based and organic polymer-based) that exhibit very different properties. The silica-based supports compnse those in which the surface silanol groups are bonded with a short-chain alkyl spacer group that terminates with one or more functional groups or a cyclohexyl or phenyl ring (see Table 2). Such supports can be very useful for preparative work, e.g., the use ofy-aminopropyl bonded silica columns for the isolation of polar materials such as carbohydrates. 

Early work in SFC was carried out using absorbants such as alumina or silica, or stationary phases insoluble in supercritical CO2, such as polyethylene glycol. Now, bonded non-extractable stationary phases such as octadecyl-silane and aminopropyl-bonded silicas, are usually used in packed columns. 

Ion-exchange and reversed-phase media are most commonly used as stationary phases in the LC analysis of water-soluble vitamins. Aminopropyl-bonded silica columns (e.g., LiChrosorb NH2, /tBondapak NH2) have been used as weak anion exchangers for... 

Suzuki et al. also separated phospholipids by using a combination of aminopropyl-bonded silica and an unbonded silica SPE to recover PC, PE, cardiolipin, phos-phatidylglycerol, and phosphatidylserine. Phospholipids... 

It is well known that even complex carbohydrate mixtures (found in food or urine samples) can be separated by TLC [10]. Although many different stationary phases are nowadays commercially available, there are in practice only three suitable layers for the separation of carbohydrates cellulose, silica, and aminopropyl bonded silica [3]. 

Pantothenic acid/calcium pantothenate in pharmaceutical products and vitamin premixes was also analyzed using low-wavelength ultraviolet (UV) detection (64,66). The vitamin was extracted from tablets or powdered premixes with 0.005 M NaH2P04 buffer (pH 4.5) and separated from other water-soluble vitamins on an aminopropyl-bonded silica column (LiChrosorb NH2) eluted with an acetonitrile-0.005 MNaH2P04 buffer (pH 4.5) (87 13, v/v) and detected at 210 nm. Quantitative recoveries (>95%) and relative standard deviations 0.79% to 2.2% were obtained for multivitamin tablets, vitamin premixes, fortified yeasts, and raw materials. The limit of sensitivity was approximately 1 mg/g sample. The results were compared with those obtained by the standard microbiological procedure. Low levels of calcium pantothenate (<3 mg per tablet) were more precisely analyzed by the HPLC procedure than by the microbiological method. 

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