Sorbent selection

Is the sample matrix miscible primarily with water or organic solvents If the sample matrix is water soluble, is the analyte ionized or non-ionized  

If ionized, is the analyte permanently ionized (pH independent) or ionizable (pH dependent) is the analyte anionic or cationic  

If the analyte is nonionized or ionization can be controlled (by pH suppression or ion pairing), is it nonpolar (hydrophobic), moderately polar, or polar (hydrophilic)  

If the sample matrix is organic solvent miscible, is it miscible only in nonpolar organic solvents such as hexane, or is it also miscible in polar organic solvents such as methanol  

Is the analyte nonpolar (hydrophobic), moderately polar, or polar (hydrophilic)  

Distillation is usually the first choice for bulk separation of components in a liquid phase. 

Another option often used is air stripping. In both cases, the ease of separation is based 

1 The relative volatility between the key components to be separated is in the range of 

2 to 1.5 or less. In this case, distillation is not an obvious choice and the separation factor for adsorption could be very large based on criteria other than relative volatility. 

2 The bulk of the feed is a relatively low value, more volatile component, and the product of interest is in relatively low concentration. For this case, large reflux ratios (and large energy consumption) would be required if distillation is considered. This situation is often found in environmental applications where one has dilute concentrations of a pollutant in an air or water stream. 

---

The sorbents used for SPE are available in three basic formats, namely disks, prepacked cartridges, and syringe barrels, each with certain advantages. Solid media employed in SPE may be classified as nonpolar, polar and ion-exchange phases with mixed retention mechanisms (Table 3.41). The large sorbent selection ranges also... 

Method development required (choice of sorbent and optimisation of sorbent selectivity)... 

Analytical Method Development. From the point of view of sorbent selection, the important factors to consider in analytical method development are sorbent/solvent compatibility and the detection limit of the analyte. 

Silica (pure) Si02 Si-OH Adsorption Usually used with nonpolar mobile phase, since it is the most polar sorbent selectivity is based on... 

Appropriate SPE sorbent selection is critical to obtaining efficient SPE recovery of semivolatile organics from liquids. Henry [58] notes that an SPE sorbent must be able to sorb rapidly and reproducibly, defined quantities of sample components of interest. Fritz [73] states that successful SPE has two major requirements (1) a high, reproducible percentage of the analytical solutes must be taken up by the solid extractant and (2) the solutes must then be easily and completely eluted from the solid particles. The sorption process must be reversible. In addition to reversible sorption, SPE sorbents should be porous with large surface areas, be free of leachable impurities, exhibit stability toward the sample matrix and the elution solvents, and have good surface contact with the sample solution [68,73],... 

Thurman and Mills [75] point out that knowing the analyte structure is the clue to effective isolation by SPE. A sorbent selection chart (Figure 2.34) is a useful guide for matching the analyte with the appropriate sorbent. Most manufacturers of SPE sorbents provide such guidelines either in printed product literature or on the Internet. To use a sorbent selection scheme, the analyst must be prepared to answer the following questions ... 

Addition of surfactants, such as cetyltrimethylammonium bromide (CTMA), causes silica to mimic separations obtained with reversed-phase sorbents. The impurities of propranolol and pharmaceutical preparations of catecholamines have been chromatographically studied using this surfactant [310,316]. The one apparent advantage of using these surfactants is that the brand-to-brand variations in selectivity commonly seen for bonded phases is avoided [310]. For basic analytes, the addition of either methanol or acetonitrile changes the sorbent selectivity. Commonly, a retention minimum occurs at about 50% organic solvent content with increases in retention at either increased or decreased organic content [312]. 

To improve sorbent selectivity, and thereby to increase separation process efficiency, multifunctional sorbents have been developed. These kinds of sorbents have two functional groups that can offer mixed retention mechanisms (i.e., hydro-phobic, ionic and/or pi-cation interactions). Different interaction mechanisms give efficient retention of compounds with different polarities and acidities. Such sorbents are applied in analysis of some pharmaceuticals, food samples, biological fluids, animal tissues, and wastes. 

Figure 3.1. Solute structure (e.g., atrazine) is the key to sorbent selection.

Sorbent selectivity. This can be accomplished by three mechanisms (a) selective binding to the sorbent surface (equilibrium) (b) excluding certain components based... 

See www.markes.com for literature on sorbent selection for thermal desorption. 

The reactor can be substantially reduced in size and an appropriate stationary phase can be used for trace separations (including a stationary phase on which the retention of the main component is not selective) if the reactor (filled with a sorbent selective for the main component) is arranged in series with the chromatographic column only... 

---