Restricted access sorbents

Figure 11.6 (a) Schematic illustration of a restricted-access sorbent particle, (b) On-line... 

Rbeida, O., Christiaens, B., Hubert, Ph., Lubda, D., Boos, K.-S., Crommen, J., Chiap, P. (2005). Integrated online sample cleanup using cation exchange restricted access sorbent for the LC determination of atropine in human plasma coupled to UV detection. J. Pharm. Biomed. Anal. 36/5, 947-954. 

Gustavson et al. (2000) developed a convenient and novel solid phase extraction (SPE) method for the removal of methyl oleate from SPMD dialysates containing PAHs. A small SPE column (1 g or 0.5 g) containing a dual-zone silica (normal phase)-based restricted-access sorbent (Diazem, Midland, MI, USA) is used for the separation. The capacity of this sorbent to remove methyl oleate is about 1.8% (lipid/sorbent wt wt ). The PAHs are eluted with 19 mL of hexane and methylene chloride (97 3 VV ) and recoveries of all PAHs are typically >72%. 

Gustavson, K.E. DeVita, W. Revis, A. Harkin, J.M. 2000, A novel use of a dual-zone restricted access sorbent Normal phase separations of methyl oleate and polynuclear aromatic hydrocarbons stemming from semipermeable membrane devices. J. Chromatogr. A 883 143-149. 

Another example of restricted-access sorbents is the dual-zone phase (DZP). This consists of an outer zone enriched in a hydrophilic moiety, a... 

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%. 

Conversely, restricted access materials or restricted access media (RAM) retain small molecules while excluding macromolecules such as biological proteins in their presence (Figure 2.29). Small molecules are retained by sorption processes in the pores of the sorbent while the large molecules are excluded and elute at the interstitial volume of the sorbent. This separation leads to size-selective disposal of interfering macromolecular matrix constituents. 

Figure 2.29. Schematic representation of a sorbent particle for restricted-access media chromatography. This medium allows proteins and macromolecules to be excluded and elute in the solvent front, while small analyte molecules enter the pores and are retained. (Reprinted with permission from Ref. 100. Copyright 2000 Elsevier Science.)...

Ajfinity sorbents such as the restricted-access matrix sorbents, immunosorbents, or molecularly imprinted polymers (MIPs) have been introduced as well. These are based upon molecular recognition using antibodies with a high degree of selectivity or with molecularly imprinted polymer. 

Solid-phase extraction (SPE) is an alternative to LLE. In SPE the analytes are partioned between a solid and a liquid [57, 58], Generally, interfering compounds are rinsed off the solid adsorbent and the analytes are then desorbed with an eluting solvent [58], A range (e.g., normal-phase, reversed-phase, ion exchange, restricted access) of sorbents and formats are available for SPE and the SPE systems are easy to automate [59, 60], In order to accomplish the isolation of the products from the fermentation matrix, both SPE and LLE were evaluated for use in papers I and II. 

There are a host of new solid phases being brought to the market. Some of the most interesting currently include graphitized carbon, functionalized styrene-divinylbenzene copolymers, restricted access solid-phase reversed phase, affinity chromatography with antibodies, and molecular imprinted polymers. These five types of sorbents have different applications from environmental to biological sample preparation. 

Other sorbents include mixed mode sorbents, normal-phase SPE sorbents or restricted access matrix sorbents. 

An evaluation of the scientific literature reveals that over 500 papers on veterinary drug residue analysis were published in the 5-year period of 2005-2009. Liquid extraction (LE) and liquid-solid extraction (LSE) were found to be very popular sample treatment techniques that were used in 30% and 60% of the reported studies, respectively. Here, LE includes all liquid-based approaches such as liquid-liquid extraction (LLE), extrelut liquid-liquid extraction, liquid-liquid micro-extraction, and pressurized liquid extraction (PLE). LSE includes solid phase extraction (SPE) and all other sorbent-based extraction procedures, such as solid phase micro-extraction (SPME), stir bar sorptive extraction (SBSE), restricted-access materials (RAM), turbulent-flow chromatography (TEC), dispersive SPE (dSPE), and matrix solid phase dispersion... 

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. ... 

Restricted-access materials (RAM) are biocompatible sample preparation supports that enable the direct injection of biological fluid into a chromatographic system. The technique was introduced in 1991 by Desilets et al., who also established the acronym RAM. Sorbents used in RAM represent a special class of materials that are able to fractionate a biological sample into a protein matrix and an analyte fraction, based on molecular weight cutoff. Macromolecules are excluded and interact only with the outer surface of the particle support, which is coated with hydrophilic groups. This minimizes the adsorption of matrix proteins. Applications of RAMs have been reviewed by several research groups. 

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... 

Today, there is strong interest in the development of online sample treatment techniques that allow the handling of untreated biological samples. Thus, in online SPE-LC, deproteination of plasma and serum is required before extraction, especially if the same cartridge is used for repeated analysis. For this purpose, restricted-access materials (RAMs) have been developed, which combine size-exclusion and reversed phase mechanisms, allowing extraction and cleanup of samples in the same step. RAMs have became quite popular for the direct injection of biological fluids, since they prevent the access of matrix components (e.g., proteins) while retaining the analytes in the interior of the sorbent. 

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... 

SPE is an exhaustive and almost solvent-free sample preparation technique. Typically, a tube is filled with a sorbent, which can be porous particles or a polymerized monolith. Various interactions are used to extract analytes from complex samples. Many of the commercially available SPE systems are for single use, but some, like RAM (restricted access materials) and MIP (molecular imprinted polymers), are typically obtained as reusable extraction devices. As will be discussed in detail, the extraction sorbents mainly function as normal phase, reversed phase, cation exchange, anion exchange, or a combination of these. 

Restricted access materials (Figure 9.8) are mostly used with online sample cleanup and enrichment of protein-rich biological samples. The crude samples, even as complex as serum or plasma, can be injected without any pretreatment. The commercially available RAM cartridges are reusable. The sorbents (alkyl-diol sUica ADS) are hydrophilic on the outside, while the surface in the pores is modified with hydrophobic groups (C4, C8, or C18). 

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