Silicas chemically bonded silica

Buszewski, B., Gadza-la-Kopciuch, R. M., Markuszewski, M. L, Kaliszan, R. Chemically bonded silica stationary phases synthesis, physicochemical characterization, and molecular mechanism of reversed-phase HPLC retention. Anal. Chem. 1997, 69, 3277-3284. 

Stationary phase Chemically bonded silica, alumina, polymeric resins Polysiloxanes... 

The TLC process is an off-line process. A number of samples are chromatographed simultaneously, side-by-side. HPTLC is fast (5 min), allows simultaneous separation and can be carried out with the same carrier materials as HPLC. Silica gel and chemically bonded silica gel sorbents are used predominantly in HPTLC other stationary phases are cellulose-based [393]. Separation mechanisms are either NPC (normal-phase chromatography), RPC (reversed-phase chromatography) or IEC (ion-exchange chromatography). RPC on hydrophobic layers is not as widely used in TLC as it is in column chromatography. The resolution capabilities of TLC using silica gel absorbent as compared to C S reversed-phase absorbent have been compared for 18 commercially available plasticisers, and 52 amine and 36 phenolic AOs [394]. 

Reversed-phase chromatography is the predominant technique in HPLC, and chemically bonded silica gel supports are made specifically for the nonpolar stationary phase. In the last decade, as many as 60% of the published LLPC techniques refer to RPC. The reasons for this involve the significantly lower cost of the mobile liquid phase and a favorable elution order that is easily predictable based on the hydrophobicity of the eluate. 

Most HPLC is based on the use of so-called normal-phase columns (useful for class separations), reverse-phase columns (useful for homolog separations), and polar columns (used in either the normal- or reverse-phase mode). Since reverse-phase HPLC columns are generally easier to work with, almost all authors use high-performance reverse-phase liquid chromatography with octade-cyl chemically bonded silica as the stationary phase and nonaqueous solvents as mobile phases (so-called NARP, or nonaqueous reverse-phase chromatography). 

C-18 Novapak (Waters) or equivalent, packed with silica, chemically bonded with octadecyldimethylsilyl groups. 

Rykowska, I., Wasiak, W. Novel stir-bar sorptive extraction coating based on chemical bonded silica for the analysis of polar organic compounds and heavy metal ions. Mendeleev Commun 23(2), 88-89 (2013)... 

Figure4.3 Formation of chemically bonded silica stationary phases.

Layer Channeled preadsorbent Cig chemically bonded silica gel paper-lined glass chamber, 10 cm development. 

Chemically bonded silica gels with cyanopropyl, aminopropyl, and diol functional groups are also available and each has been used for normal-phase as well as reversed-phase separations. Polymeric-based packings are available and can be used for ion-exchange or reversed-phase applications. Cross-linked polymeric based packings have been combined with ion-exchange materials to create a more pH-stable stationary phase. Graphitized carbon... 

Heavy and transition metals may also be separated via ion-pair chromatography on macroporous PS/DVB-resins or chemically bonded silica phases, respectively [154], The mobile phase contains complexing agents and a respective ion-pair reagent. If these columns are equilibrated with a surface-active acid such as octanesulfonic acid, metal ions such as Cu2+, Ni2+, Zn2+, and Co2+ elute in the same order as on surface-sulfon-... 

The retention factor imder linear conditions is related to the initial slope of the isotherm k = Fdqf dC). It is easy to check that this relation applies when the isotherm coefficients are calculated. Deviations from this relation are not unusual, and can often be ascribed to the heterogeneity of the adsorbent smface that can be explained, for example, by a small concentration of underivatized sUanol groups on the surface of chemically bonded silica. It may be that an additional Langmuir isotherm term would be necessary to account for the contribution of these groups, term that could become saturated at very low concentrations [172]. 

Figure 10.16 Comparison of calculated (solid line) and experimental (symbols) band profiles. General conditions L = 25 cm dc = 4.6 mm Fi, = 1 mL/min (a,c,d) or 2 mL/min (b) N = 5000 plates, (a) Benzyl alcohol on silica. Mobile phase solution of THF in n-hexane (15 85). Sample sizes (mmol) 1, 0.0025 2, 0.00625 3, 0.0125 4,0.025 5, 0.060 6, 0.075. (b) Acetophenone on silica. Mobile phase mixture of ethyl acetate and -hexane (2.5 97.5). Sample sizes (mmol) 1, 0.025 2 0.05 3 0.075 4 0.1 5 0.125. (c) Benzyl alcohol on oc-tadecyl silica. Mobile phase, methanol/water (20 80) sample sizes (mmol) 1, 0.02 2,0.05 3, 0.10 4, 0.15. (d) Phenol on octadecyl chemically bonded silica. MobUe phase mixture of methanol and water (20 80). Sample sizes 1, 0.015 mmol 2 0.03 mmol 3 0.045 mmol 4 0.06 mmole 5 0.075 mmol. Reproduced with permission from S. Golshan-Shirazi and G. Guiochon, Anal. Chem., 60 (1988) 2634 (Figs. 7 to 10). 1988, American Chemical Society.

We compare in Figures 10.16a to 10.16d [70] the experimental band profiles in overloaded elution (symbols) and the profiles calculated for elution performed in normal and reversed phase chromatography (solid lines). Figure 10.16a corresponds to the elution of large bands of benzyl alcohol on silica with a THF/n-hexane solution. Figure 10.16b corresponds to the elution of acetophenone on silica with a (97.5 2.5) mixture of n-hexane and ethyl acetate. Figure 10.16c illustrates the profiles of bands of benzyl alcohol eluted on CIS silica by a methanol/water solution. Figure 10.16d corresponds to the elution of phenol on C18 chemically bonded silica with a (20 80) mixture of methanol and water. In all four cases. 

Figures 13.4a and 13.4b compare chromatograms published by Heme et al. [23] and chromatograms calculated from the data formd in their paper, assuming Langmuir competitive isotherms (Eq. 13.3) and numerical values of the other parameters that permit the best approximation of the experimental results. In this experiment, a sample containing acetonitrile, N,N-dimethylformamide, ethyl formate, fso-butanol, ethyl acetate and 1-pentanol is eluted on a C18 chemically bonded silica phase (Nucleosil C18), using a water-methanol solution containing 0.00025 M salicylamide, the UV-absorbing additive. The agreement between experimental and calculated chromatograms is excellent. It demonstrates the validity of the theoretical approach. The system peaks associated with the first five components, those that are eluted before the additive system peak (peak 6), are...

Silica, and to a lesser extent alumina, are the most common stationary phases used for the separation of low molecular mass organic compounds. Chemically bonded silica sorbents are used for the separation of polar organic compounds in the normal-phase and reversed-phase modes. Wide-pore, chemically bonded sorbents, are used for the separation of biopolymers [18,22]. Some separations require specially prepared stationary phases, such as silica gel impregnated with silver nitrate for the isolation of unsaturated compounds capable of forming charge transfer complexes with silver [23] (section 10.6.1), or silica and chemically bonded phases coated with cellulose tris(3,5-... 

Solid phase extraction is used primarily to prepare liquid samples and extracts of semi-volatile or non-volatile analytes, but may also be used for solids pre-extracted into solvents. The choice of sorbent is the key factor in SPE, because this can control parameters such as selectivity, affinity, and capacity. This choice depends primarily on the analytes and their physicochemical properties, which should define the interactions with the chosen sorbent. However, results also depend on the kind of sample matrix and interactions with both the sorbent and the analyte. SPE sorbents range from chemically bonded silicas, such as with the C8 and C18 organic groups, to graphitized carbon. 

In addition to LLE, the two other major types of analyte isolation and recovery are sohd-phase extraction (SPE) and supercritical fluid extraction (SEE) extraction. SPE refers to those techniques that isolate the analyte from a sample matrix and partition the analytes of interest onto a chemically bonded silica surface. SEE refers to those techniques that isolate the analyte from a sample matrix and partition it into a liquid that has been heated and pressurized beyond its critical temperature and pressure. It is indeed overly simpHstic to think that a striped bass can be stuffed into a GC as a means to conduct TEQA ... 

This completes our digression into alternative sample prep techniques for solid matrices. We now return to aqueous samples and entertain a somewhat detailed discussion of the prime alternative to liquid-liquid extraction that emerged over 20 years ago, namely liquid-solid extraction using chemically bonded silica gel, commonly called sohd-phase extraction. 

Table 3.11 Comparison of the Percent Recoveries of Various OCs from Spiked Water Between a Cg and a Cig Chemically Bonded Silica Sorbent...

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