Chromatographic retention modes

Mathre, D.E. (1971) Mode of action of oxathiin systemic fungicides. Structure-activity relations. J. Agric. Food Chem. 19(5), 872-874. McCall, P.J., Swann, R.L., Laskowski, D.A., Unger, S.M., Vrona, S.A., Dishburger, H.J. (1980) Estimation of chemical mobility in soil from liquid chromatographic retention times. Bull. Environ. Contam. Toxicol. 24, 190-195. 

Oddly, IPC was also performed on a classical ion exchange colnmn to separate 17 anionic, neutral, and cationic arsenic species in a single chromatographic run, thanks to a multiplicity of retention modes on this packing material [39], Ion pairing proved also valuable in size exclusion chromatography of sulfonated lignins [40],... 

Application of RP-HPLC in isocratic mode. In HPLC, the most frequently used chromatographic retention parameter for characterising hydrophobicity is the logarithmic value of the retention factor (logA = log((fR - Ud/tn). where /r is the retention time of the analyte and to is the retention time of the unretained compound). The retention factor can be related directly to the chromatographic partition coefficients (A chr) according to Eiq. (12.6) ... 

Figure 8. Optimization of eluent composition for the LACCC separation of polyester (a) molar mass dependence of retention in different chromatographic separation modes and (b) corresponding calibration curves.

Data-dependent acqnisition (DDA) is a mode of operation, where the MS experiment performed in a particular scan is based on the data acqnired in a previons scan. In a simple form, a DDA experiment switches the instrument from full-scan MS acquisition to full-scan product-ion MS-MS when the total-ion intensity or a selected-ion intensity exceeds a preset threshold. This avoids the need to perform two consecutive injections for the identification of unknowns in a mixture first to obtain the m/z values for the intact protonated molecules of the unknowns, and second to acquire the product-ion MS-MS spectra of these unknowns in a time-scheduled procedure, switching between various preselected precursor ions as a function of the chromatographic retention time. The DDA was promoted by Thermo Finnigan upon the introduction of the API-ion trap combinations [44-46]. Similar procedures are available for other commercial ion-trap systems, as well as for triple-quadrupoles, e.g.. Information Dependent Acquisition (IDA) from Applied Biosystems MDS Sciex, Data-directed Analysis (DDA) from Waters, and Smart Select from Bruker. 

In RPLC, the influence of pressure on the chromatographic behavior is related to the hydrophobic interactions involved in the retention mechanism and to the change upon adsorption in the numbers of acetonitrile and water molecules in the solvent shells of the protein molecule and of the bonded layer. The importance of the changes in the retention factor and the saturation capacity with a change in the average column pressure will thus depend on the retention mode used and will vary with the hydrophobicity of the molecule [128]. In RPLC, it is larger with polymeric than with monomeric bonded phases [126]. 

Figure 4 Examples of retention mode, stationary phase, and typical mobile phase used in MIP-based liquid chromatographic separations (MeCN acetonitrile AcOH acetic acid).

Identification of volatile compounds in strawberries and sliced bread headspace was performed in full scan mode (m/z 30-550). Carvacrol and thymol were identified by a combination of the NIST mass spectral library and gas chromatographic retention times of standard compounds. The rest of volatiles were tentatively identified by their GC/MS spectra. In this sense, the compounds having 90% similarity with spectra in the NIST library were not taken into consideration. Chromatographic responses of detected volatile compoimds (peak area cormts) were monitored for comparative measurements of each compotmd in the studied samples. 

The larger the average pore diameter, the lower the specific surface area and hence the chromatographic retention, A 50-nm pore size silica exhibits a specific surface area of about 50 m /g, about five times lower than that of a 15-nm pore size material. However, due to the multisite interactions of a biopolymer with a bonded silica in HPLC modes such as reversed phase or ion exchanger, the value of the specific surface area is not a critical parameter for the resolution of biopolymers. It has been demonstrated that even the low, entirely external specific surface area of about 1 to 2 m /g of 1- to 2-pm nonporous silica particles is sufficient to retain and to resolve biopolymeric analytes [12]. 

Figure 11.24 Chromatographic retention of proteins on (A) HRP-imprinted monolithic column and (B) non-imprinted monolithic column under gradient elution mode. Mobile phase 0.1 M phosphate buffer, pH 7.4, switched to 0.1 M acetic acid at 18 min for HRP-imprinted column and non-imprinted column. Blank sample, 0.1 M phosphate buffer, pH 7.4. Sample 1 mg mL protein dissolved in 0.1 M phosphate buffer, pH 7.4. (Reproduced from ref. 134 with permission. Copyright 2014, Royal Society of Chemistry.)...

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