Restricted access packings

There are various types of restricted-access packings that have been developed for sample clean-up. Reviews by Unger (1991) and Haginaka (1991) discuss internal-surface reversed phases, shielded hydrophobic phases, semi-permeable surfaces, dual-zone phases, and mixed functional phases. The majority of these packings have been developed to purify drugs from body fluids. 

Nimura, N. Itoh, H. Kinoshita, T. Diol-bonded siUca gel as a restricted access packing forming a binarylayered phase for direct injection of serum for the determination of drugs. J.Chromatogr.A, 1995, 689, 203-210... 

Direct HPLC enantioseparation techniques, which are free of many disadvantages of GC, indirect and chiral mobile phase HPLC methods, have gained unequivocal prevalence in bio-analytical studies. Several methods have been advanced so much that they allow enantiose-lective determination not only of the parent chiral drugs but also of their pharmacologically relevant metabolites [121]. As already mentioned above, a direct injection of biofluids offers several advantages in terms of analysis time and sample recovery. Precolumns packed with achiral or chiral packings, or with the recently developed so-called restricted-access packing materials, may be useful in this case. 

Restricted access packing materials (RAMs) have both hydrophobic and hydrophilic sites. Also called internal-surface reversed-phase packing materials, RAMs have two functions the surfaces of particles (the end groups of bonded... 

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. 

Many other types of solid phase adsorbents, including those based on conventional and specialty materials like restricted access media (RAM), can increase analysis speed and improve assay performance. These types of materials, also known as internal reversed-phase packings, are especially useful for assaying target compounds in biological samples such as serum and plasma. They are chemically modified porous silicas that have hydrophilic external surfaces and restricted-access hydrophobic internal surfaces. The ratio of interior to external surface areas is large. Macromolecules such as proteins cannot enter the pores of the RAM (they are excluded from the hydrophobic internal surface) and they elute quickly through the column. However, the smaller analyte molecules that can enter the pores are retained via interactions with the hydrophobic bonded phase within... 

Many variations on this theme that allow repetitive, direct injection and chromatographic analysis of untreated sample matrices including tissue homogenates, milk, plasma, and saliva (107-112) have appeared since the original publication. These packings have been described generically as restricted-access media because they are generally characterized by a limited accessibility of macromolecular compounds to the adsorption sites of the porous supports. 

On the basis of the separation mechanism, restricted-access media can be classified into physical or chemical diffusion barrier types. The limited accessibility of the former type is due to the pore structure of the support that represents physical diffusion barriers for macromolecular compounds. The restricted access of the latter type is due to covalently or adsorptively bonded synthetic or natural polymers that cover the support surface, preventing macromolecules from being adsorbed on or denatured by the column packing material. 

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. 

The simultaneous HPLC-UV determination of sulfamonomethoxine (SMM), miloxacin (MLX), and OXO in serum and muscle of cultured fish was developed (153). A sample of muscle was extracted with MeCN-THF (95 5) after centrifugation, the supernatant was injected into the HPLC system. A Hisep column, used in this study, is packed with restricted-access materials (RAMs) consisting of the polymeric hydrophilic/hydrophobic phase bound to silica gel. This column did not require time-consuming and complex extraction procedures. The RAM sorbent could also be applied in short precolumns, which are combined directly on-line with the HPLC equipment. This approach is much more convenient than that applied in the present paper. The guard column had to be changed very often in order to protect the analytical column in a sufficient way. The extraction recovery was 79.5%, RSD of 6.0%. 

K. S. Boos and C. H. Grimm, High performance liquid chromatography integated with solid-phase extraction in bioanalysis using restricted access precolumn packings , Trends Anal. Chem. 18 175-180 (1999). 

The data of Fig. 11 thus indicate that restricted-access delocalization does not exist for amino-phase packings. Furthermore, the importance of the variation of eo with 6n, shown in Fig. 11, is insignificant, so far as calculations of e" are concerned. We will therefore assume that cb is constant for a given solvent B and amino-phase packings, in the calculation of ° for multicomponent mobile phases (Appendix). 

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)... 

Restricted-access material (RAM) columns combine the size-exclusion of proteins by the hydrophilic outer surface of the packing and the simultaneous enrichment by SPE of analytes that interact with hydrophobic groups at the inner surface of the packing. These columns allow the direct injection of plasma samples without protein precipitation. Often, on-line RAM-LC-MS is described, following a procedure identical to on-line SPE-LC-MS (Ch. 1.5.4). The use of RAM columns has been reviewed by Souverain et al. [29]. 

M. Petrovic, S. Tavazzi, D. Barceld, Column-switching system with restricted access pre-column packing for an integrated sample cleanup and LCHdS analysis of alkylphenolic compounds and steroid sex hormones in sediment, J. Chromatogr. A, 971... 

Solid-phase extraction (SPE), which can be done off-hne or on-line by means of cartridges, cartridge columns, or extraction disks (Ch. 11.6.4), and eventually by using special packing materials, such as restricted access materials (RAM) or turbulent-flow chromatography (TFC) (Ch. 11.6.5). 

Types of particulate packings include bonded, polymeric, chiral, and restricted access materials. 

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). 

Kimata, K., Hosoya, K., Tanaka, N., and Araki, T. 1991. Effect of stationary phase structure on retention and selectivity of restricted-access reversed-phase packing materials, 7. Chromatog., 558 19-30. 

This section discusses dispersive SPE, matrix solid phase dispersion, solid phase micro-extraction, micro-extraction by packed sorbent, stir-bar sorbent extraction, and the restricted-access materials. ... 

Special sorbents possessing restricted-access properties have been developed to allow the direct injection of biological matrices into the online SPE-LC systems. These sorbents, the so-called restricted-access materials, combine the size exclusion of proteins and other high-molecular-mass matrix components that are prohibited from entering the pores of the packing, and they are not well retained by the column. Therapeutic drugs and other small molecules permeate the pores of the column packing material where they partition and are retained by... 

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