Ligands bonded silica sorbents

Each of the types of SPE sorbents discussed retains analytes through a primary mechanism, such as by van der Waals interactions, polar dipole-dipole forces, hydrogen bonding, or electrostatic forces. However, sorbents often exhibit retention by a secondary mechanism as well. Bonded silica ion-exchange sorbents primarily exhibit electrostatic interactions, but the analyte also experiences nonpolar interaction with the bonded ligand. Nonpolar bonded silicas primarily retain analytes by hydrophobic interactions but exhibit a dual-retention mechanism, due to the silica backbone and the presence of unreacted surface silanol groups [72], Recognition that a dual-... 

For reverse-phase chromatography purposes, silica sorbents are commonly modified by hydrophobic ligands by means of silanes. These systems are therefore particularly suitable for water intrusion experiments [7]. We studied the influence of the chain nature (perfluorated, alkyl), the chain length and the type of bonding chemistry (mono or trifunctional, chloro- or alkoxy- silanes) on the hydrophobic properties of silica gels. 

The base matrices are divided into two groups. The inorganic matrices are based on silica or zirconia, on which the transition metal ion is complexed with the ligand bonded on the matrix. The inorganic sorbents have high column efficiency but low capacity and low chemical stability. The hgand can also be coated prior to, or during, the experiment, on the octadecylated silica (ODS) reversed phase, which presents the most inexpensive but efficient and selective sorbent for CLEC. 

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. 

Type IB sorbents are chiral ligand exchangers. Several columns are commercially available with either proline, hydroxyproline, or valine and Cu(II) bonded to silica [256]. The binding is via a 3-glycidoxpropyl spacer Cu(II) needs to be added to the mobile phase to minimize the loss of copper from the sorbent. Silica modified by L-( + )-tartaric acid has also been synthesized. These columns generally have poor efficiency and analytes are limited to bidentate solutes [256]. 

The main downside of silica-based sorbents is pH compatibility they are stable only at pH values from 2 to 8 at lower pH, the H3O+ ions catalyze the hydrolysis of the alkyl ligand at higher pH, the nucleophilic attack of Si-O bonds by OH results in the erosion of the silica surface. The formation of Si(OH)4 is signaled by a back pressure increase. Recently the chemical resistance of the silica particles has been improved by hybrid particle technology. Organic moieties such as ethyl groups were inserted close to the particle surfaces and columns stable between pH 1 and 12 under ultra-high pressure (1000 bar) were produced. 

Another issue that deserves consideration is the measurement of the surface area of a sorbent after derivatization with alkyl chains. It is generally accepted that the total surface area of a stationary phase after ligand attachment is lower than the surface area of the bare silica because bonded ligands occupy volume inside the pore [7]. 

Ir(IV), Rh(III), Pt(IV), Os(VIII), Pd(II), and Au(III) were sorbed from aqueous solution by silica gel, which was chemically modified with nitrogen-containing organic ligands. This was performed as a function of hydrochloric acid concentration, time of contact, concentration of the element, and the ionic strength. Extraction, followed by determination on the sorbent surface by x-ray fluorescence of Ir(IV), and Rh(III), from 10 to 10 M solutions, were achieved. An atomic emission method was used to determine Au(III) on the surface of the sorbent containing bonded diethy-lenetriamine groups. 

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