Underivatized Silica

A large number of published articles describe the use of conventional underivatized silica columns. A summary of the most common products and their structural and technical properties is found in Table 7.1. 

A major advantage of underivatized silica for LC-MS(/MS) is the absence of ligands that may detach from the surface and show up as spurious peaks in the mass spectra. Recently introduced bonded phase materials seem to have addressed this issue, so that functionalized silica gels are as common today as bare silica in HILIC-MS applications. 

The organic groups in the BEH substrate form a surface that is more alkaline in nature compared to traditional silica. The different surface of the substrate, in turn, leads to a different charge state of the residual silanol groups, which can impact retention. Grumbach et al. [18] investigated the retention behavior of methacrylic acid, cytosine, nortriptyline, and nicotinic acid on both BEH and silica substrates, which turned out to be very similar under the experimental conditions being used (pH 5, lOmmol/L ammonium acetate/0.02% 

Brand name Manufacturer Particle size Pore size Surface area 

Fully porous materials Alltima HP Silica Grace-Alltech 3,5 100 450 

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B. L. Lampert and J. T. Stewart, Determination of non-steroidal anti-inflammatory analgesics in solid dosage forms by HPLC on underivatized silica with aqueous mobile phase, J. Chromatogr., 504 381 (1990). 

W. Naidong, Bioanalytical LC-MS-MS methods on underivatized silica columns with aqueous/organic mobile phases, J. Chromatogr. B, 796 (2003) 209. 

As mentioned above, silica is the most common stationary phase, and as such, it is available in a host of shapes and sizes. Underivatized silica is most commonly seen in one of three sizes low-performance, flash, or HPLC. Low-performance silica stationary phases have large diameters (and often with a large particle-size distribution) and are packed into low-pressure glass or stainless steel columns. HPLC silica is typically 10 pm particles that are packed in finely polished stainless steel tubes. Flash silica particles are 30-70 pm and have low enough pressure drop that laboratory nitrogen... 

There are also stationary phases that effectively partition solutes in either reversed-phase or normal-phase mode. These stationary phases are typically silica particles derivatized with cyano, diol, or amino functional groups. Particles with a cyano-functionality separate based on polarity utilizing nitrile interactions between the stationary phase and the solute. The amino group of typical amino stationary phases interacts primarily with anionic and organic acid portions of the solute. Diols utilize hydroxyl interactions similar to underivatized silica but offer a slightly different selectivity. These and other bonded-silica phases offer alternatives to underivatized silica, but they are used much less frequently. The mobile phases employed with these stationary phases are the same as used in standard reversed-phase or normal-phase chromatography. 

X Chen, H Zou, L Yang, H Wang, O Zhang. Optical resolution of alpha-alkyl phenyl acetonitriles by HPLC on cellulose triacetate chiral stationary phases coated on underivatized silica gel. Chirality 12 599-605, 2000. 

Simmons, B.R. Stewart, J.T. HPLC separation of selected cardiovascular agents on underivatized silica using an aqueous organic mobile phase. J.Liq.Chromatogr., 1994, 17, 2675-2690... 

The results of detailed studies require a partial relaxation of the hopeful assumption that all protons of underivatized silica systems are at the surface [18]. Recent NMR studies indeed show that moderately intense D2O exchange conditions leave about 3% of the original proton content in a silica gel sample. This residual 3% is attributable to trapped OH groups. On the basis of extensive... 

Micropacked columns are available with most of the bonded-phase packings used in HPLC. Porous and non-porous silica particles are optionally functionalized with covalently ound silanes or other strongly adsorbed materials. ALkyl-bonded silicas produce separations, generally based on solute volatility, but with the potential for selectivity differences based on interaction with silanol groups. Underivatized silica is popular for petroleum separations of aliphatic and aromatic hydrocarbons. Silver-ion-containing silica columns are selective for olefin separations. Huoroalkyl-bonded... 

Naidong, W. (2003) Bioanalytical liquid chromatography tandem mass spectrometry methods on underivatized silica column with aqueous/organic mobile phase. Journal of Chromatography. B, Analytical Technologies in the Biomedical and Life Sciences, 796,209-224. 

A series of twenty-nine substituted phenols (e.g., alkyl, phenyl, lluoro, diloro, bromo, nitro, and multisubstituted) was studied using a diol column and a 50/50 chloroform/iso-octane mobile phase [611]. In general, retention increased as the polar nature of the solute increased (e.g., in order of increasing retention 2,4,6-tributylphenol < 4-amylphenol < 4-methylphenol < phenol < 4-chlorophenol < 4-nitrophenol). As might be expected, this system showed good isomer selectivity (A for 3-nitrophenol = 4.20, for 4-nitrophenol = 5.50). However this selectivity was not as pronounced as that found using an underivatized silica support. 

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