Stationary phase column selectivity

Many racemic mixtures can be separated by ordinary reverse phase columns by adding a suitable chiral reagent to the mobile phase. If the material is adsorbed strongly on the stationary phase then selectivity will reside in the stationary phase, if the reagent is predominantly in the mobile phase then the chiral selectivity will remain in the mobile phase. Examples of some suitable additives are camphor sulphonic acid (10) and quinine (11). Chiral selectivity can also be achieved by bonding chirally selective compounds to silica in much the same way as a reverse phase. A example of this type of chiral stationary phase is afforded by the cyclodextrins. 

Trying to determine which column is ideal for a specific analysis can be difficult with over 1000 different columns on the market [74]. A proper choice implies a definition of parameters such as column material, stationary phase (polarity), i.d., film thickness and column length. Guides to column selection are available [74,75]. The most important consideration is the stationary phase. When selecting an i.d., sample concentration and instrumentation must be considered. If the concentration of the sample exceeds the column s capacity, then loss of resolution, poor reproducibility and peak distortion will result. Film thickness has a direct effect on retention and the elution temperature for each sample compound. Longer columns provide more resolving probe, increase analysis times and cost. 

Some selected applications (chemical class, mobile phase, stationary phase, column, detector) are as follows fatty acids, C02, PEG or CN-PS, OTC, FID organotin, C02, PhMe-PS, OTC, FPD [305], ThepSFC-ICP system is an effective tool for speciation of metal-containing compounds [362], even in the presence of modifiers to control the retention of components. 

FIGURE 9.19 Effect of mobile phase composition on shape selectivity with a monomeric octadecylpolysiloxane stationary phase, column using (a) SRM 869a (b) triphenylene/o—terphenyl (c) chrysene/benzo[a]anthracene with column outlet pressure 20.0 MPa and flow rate 1 mL/min at pump head. (Reprinted from J. W. Coym, J. G. Dorsey, J. Chromatogr. A, 971 61 (2002). With permission.)... 

Separations were performed on either silica- or amino-bonded stationary phase columns, using carbon dioxide with various modifiers as mobile phases. Each column exhibited distinctly different selectivities to the examined sulfonamides, the amino-bonded column being much more sensitive to modifier variations. In a continuation of the sulfonamide study, packed-column SFC was further evaluated for possible application to the analysis of furazolidone, chloramphenicol, and lincomycin residues (82). Separation was effected on an amino-bonded stationary phase using carbon dioxide with methanol modifier as the mobile phase, whereas detection was accomplished by MS. 

Stationary phase Column emperature. Selectivity factor, a... 

Interference in sample. —> Check column performance with standards. Change mobile phase or stationary phase. Check selectivity. 

Resolution between peaks is dependent on the retention characteristics of each component (k), the ability of the stationary phase to selectively retain the components (a) and the overall efficiency of the column (N). The equation derived below was first developed by Purnell in 1959 to accurately relate the degree of separation attainable in a GC column in terms of the separating capability and efficiency [13-15],... 

The next device is the column, the heart of the unit, where the separation takes place according to the various separation mechanisms. The column is located - hopefully -in a column oven to guarantee a constant temperature and reproducible results. Columns can be filled with various materials. The stationary phase is selected... 

Stationary phase optimized selectivity liquid chromatography basic possibilities of serially connected columns using the PRISMA principle. /. Chromatogr. A, 1157 (1-2), 122-130. 

There are several factors to consider in selecting a stationary phase. General considerations include temperature limits of the stationary phase, column efficiency, and lifetime and detector compatibility. Since nonpolar phases generally give more efficient columns that also exhibit superior lifetimes, it is wise to use the least polar phase that provides satisfactory separation. Phases containing the specific element corresponding with element-selective detectors (e.g., cyanopropyl phases with an NPD detector trifluoro-propyl phases with an BCD detector) should be avoided where possible. These selective detectors will be substantially more sensitive to normal column bleed with such phases. 

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