Interactions of sorbent and Analyte in sPE and selective Extractions Based on sorbent Chemistry

The interactions of a sorbent and an analyte fall into three classes: hydrophobic (also called dispersive or van der Waals interactions with associated energy of 1 to 5 kJ/mol), polar, and ionic. Polar interactions are further divided into dipole-induced dipole (2 to 7 kJ/mol), dipole-dipole (5 to 10 kJ/mol), hydrogen bonding (10 to 25 kJ/mol), and ion-dipole (10 to 50 kJ/mol). Ionic interactions are electrostatic with the highest associated energy levels of 50 to 500 kJ/mol. These energy values reflect the fact that when analytes interact with neutral sorbents only through hydrophobic interactions, a thorough organic wash (with 100% solvent) cannot be carried out and hence extracts may contain some contaminants or interference. On the other hand, sorbents possessing ion exchange functionalities can retain ionizable analytes via ionic mechanisms and are amenable to 100% organic solvent washes, thereby furnishing much cleaner extracts.58

Ion exchange resins based on poly(styrene-divinylbenzene) backbones display mixed mode retention mechanisms. The ion exchange functionality (sulfonic acid or carboxylic acid for cation exchangers and quarternary or primary, secondary, or tertiary amines for anion exchangers) contributes to the ionic mechanism and the backbone polymer to hydrophobic retention. This is exemplified

Tetrahydrocannabinol (THC) THC-COOH (main metabolite) FIGURE 1.1 Structures of THC and THC-COOH, its main metabolite from urine.

Tetrahydrocannabinol (THC) THC-COOH (main metabolite) FIGURE 1.1 Structures of THC and THC-COOH, its main metabolite from urine.

by a recent report demonstrating the retention of a hydrophobic molecule like THC carboxylic acid (a metabolite of THC, the major constituent of marijuana; see Figure 1.1) on a strong cation exchanger like strata-X-C even when subjected to a 30 to 40% acetonitrile wash without breakthrough.97

The mechanisms of retention of apparently basic analytes on either strong or weak cation exchanger resins depend upon the structures of these analytes and the intra-molecular interactions of the functional groups on these analytes. Thus, tetracycline and its analogs are not eluted from the sulfonic acid-functionalized strata-X-C resin with methanol containing 5% ammonium hydroxide or with acetonitrile containing 0.1M oxalic acid. However, these antibiotics are eluted from strata-X-C with acetonitrile containing 1.0M oxalic acid. On the other hand, they could be easily eluted from the carboxy functionalized weak cation exchanger strata-X-CW with methanol containing formic acid.

The differences in the elution patterns for the two sorbents have been explained98 by invoking the zwitterionic structures for the antibiotics under the basic pH conditions employed for strata-X-C

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