Sorption mixed mode

The mixed mode of sorption of the dye l,l -dioctadecyl-3,3,3, 3 -tetramethylindocar-bocyanine pechlorate (Dil) at the interface of an ODS stationary phase and ACN-water mobile phase was studied by single-molecule resolution and fluorescence imaging techniques. The measurements indicated that minimally four types of adsorption sites are present on the surface of the ODS stationary phase. The desorption times of the dye are different at the different adsorption sites resulting in a deformed peak shape [152],... 

M.D. Ludes and M.J. Wirth, Single-molecule resolution and fluorescence imaging of mixed-mode sorption of a dye at the interface of C18 and acetonitrile/water. Anal. Chem., 74 (2002) 386-393. 

Lerch, R.N., E.M. Thurman, and E.L. Kruger (1997). Mixed-mode sorption of hydroxylated atrazine degradation products to soil A mechanism for bound residue. Environ. Sci. Technol., 31 1539-1546. 

Some SPE packing materials do not receive endcapping thus, it is important to check for endcapping when choosing an SPE sorbent for a specific analyte. For example, in the case when the R group is C-18, the solute would experience reversed-phase sorption to the C-18 and in some cases sorption to the hydroxyl groups. When the interaction between the two groups is intentional, this is called a mixed-mode application of SPE. More examples of mixed-mode SPE will be shown later in this chapter. 

Occasionally, one inadvertently may use a mixed-mode mechanism in an SPE method. A good example is the sorption of triazine herbicides onto C-18 bonded phases that are monofunctional and are not endcapped. In this case, the basic compound has the potential for hydrogen bonding to the silanol sites of the silica gel, as well as reversed-phase sorption into the C-18 bonded phase. 

Figure 2.15. Example of silica mixed-mode sorption on partially endcapped C-18.

One sorbent that has been commonly used for drug isolation and purification is reversed phase. In the reversed-phase approach, many organic components of body fluids may also be isolated. These interferences are sorbed to the C-18 column and co-elute with the drug. Because of these interferences, a new method of SPE was invented that would combine several sorption mechanisms. Out of the need for a more effective clean-up procedure for SPE of drugs from biological fluids, the mixed-mode sorbent was invented. 

Methamphetamine and amphetamine are used as stimulants and their structure is shown in Figure 8.8. Both are basic compounds and may be isolated and purified by mixed-mode sorption. Both compounds are analyzed as parent compound in urine or blood. 

NIDA specifications call for the analysis of two opiates, codeine and morphine, whose structures are shown in Figure 8.9. Both compounds are amphoteric compounds because of the phenolic hydroxyl group and the amine group. Therefore, they are easily concentrated and purified by mixed-mode sorption. The compounds are present in urine as glycoside conjugates through the phenolic hydroxyl group. Thus, they must first be hydrolyzed in acid before isolation by SPE. 

Implications of the Dual-Mode Sorption and Transport Models for Mixed Gas Permeation... 

Chem RT, Koros WJ, Sanders ES, Chen SH, and Hopfenberg HB. Implications of dual mode sorption and transport for mixed gas permeation. In Whyte T, Yon CM, and Wagener EH, eds. ACS Symposium Series 223 on Industrial Gas Separations. Washington DC American Chemical Society, 1983, pp. 47-73. 

Advantages of SPME to traditional extraction methods should promote advances in the field of herbicide chemistry. However, SPME has some limitations, such as analyte carryover, fiber damage at extreme pH, salt-related problems, and low sensitivity in some complex soil samples. Advancements are being made in the refinement of the SPME technique. The HPLC/SPME interface has then been improved, and new mixed phases based on solid/liquid sorption (e.g., CW-DVB and PDMS-DVB) have been developed in recent years for the analysis of compounds by HPLC. A modified accessory to the HPLC system, called in-tube SPME, was developed. This device aspirates and dispenses samples from vials with the syringe in the inject position and then desorbs with aspirated solvent in the load position. Returning the valve to the inject mode will transfer analytes to the analytical column. ... 

When chromatographic separations (7) are operated in a batch mode, a portion of the mixture to be separated is introduced at the column inlet. A solute-free carrier fluid is then fed continually through the column, the solutes separating into bands or zones. Some industrial operations such as mixed-vapor solvent recovery and sorption of the less volatile hydrocarbons in natural gas or natural gasoline plants are being carried out on pilot plant and semiworks scales. Continuous countercurrent systems designed along the basic principles of distillation columns have been constructed. 

It has been proposed that the extent to which mixed-cation hydroxide compounds actually do form in aquatic and terrestrial environments is limited more by slow rates of soil mineral dissolution, a necessary preliminary step, than by lack of thermodynamic favorability (57). Because the dissolution rates of clays and oxide minerals are fairly slow, the possibility of mixed-cation hydroxide formation as a plausible "sorption mode" in 24 hour-based sorption experiments (and also most long-term studies) containing divalent metal ions such as Mg, Ni, Co, Zn, and Mn and Al(III)-, Fe(III)-, and Cr(III)-(hydr)oxide or silicate minerals has been ignored in the literature 16,17). This study and others recently published (77), however, suggests that metal sorption onto mineral surfaces can significantly destabilize surface metal ions (A1 and Si) relative to the bulk solution, and therefore lead to an enhanced dissolution of the clay and oxide minerals. Thus, predictions on the rate and the extent of mixed-cation hydroxide formation in aquatic and terrestrial environments based on the dissolution rate of the mineral surface alone are not valid and underestimate the true values. 

Comparison of the differences between pure and mixed compment cases for the penmealnlities of each component calculated in the standard fashion and in the thennodynamically normalized Esshkm (P ) is revealing. The difference between the pure and mixed gas cases for the P columns of each conqxment corresponds to the apparent total depression in flux resulting fnmi both true dual-mode competition effects and nonideal gas-phase effects. The differences between the pure and mixed gas cases for die P cohimns are free of complications arising from nonideal gas-phase efli. Therefore, the differences b ween these columns are manifestations of the rather small competition efliect due to dual-mode sorption under these conditions. 

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