Chromabond^® and Chromafix^®

Molecular interactions in SPE
SPE adsorbents are most commonly categorized by the nature of their primary interaction mechanism with the analyte of interest. The three most common extraction mechanisms used in SPE are reversed-phase (RP), normal phase (NP) and ion exchanger.

Typical Extraction Mechanisms:

• Reversed-Phase Extraction of hydrophobic or polar organic analytes from an aqueous matrix
• Normal Phase Extraction of polar analytes from nonpolar organic solvents
• Ion Exchanger Extraction of charged analytes from aqueous or non-polar organic samples

Types of Retention Mechanisms

	Nonpolar interactions Silica Based: C18 ec, C18, C18 Hydra, C8,… Polymer-based: HR-X, HR-P, Easy, PS-RP Interactions: Hydrophobic Sample: Mostly aqueous Elution: Solvents with lower polarity (compared to water) MeOH, CH2Cl2, CHCl3,… hexane
	Polar interactions Silica Based: SiOH, CN, NH2, OH (diol), C6H5,… Other: Alox, Florisil ® Interactions: Hyrdogen bonds, dipole-dipole, and π-π interactions Sample: Mostly organic Elution: Polar solvents (compared to sample solvent) (nonprotic) ethers, ketones (MTBE, THF, acetone,…) CH2Cl2, CHCl3
	Cation Exchangers Silica Based: SSA (SCX), PCA (WCX), PSA Polymer-based: HR-XC, HR-XCW, PS-H+ Interactions: Between charged analytes and functional groups of cation exchanger Sample: Aqueous (pH 3-5) Elution: Acidic- pH 2 (e.g. HCl, or 20% AcPH in MeOH/acetonitrile) Basic- pH 8-9 (e.g. 5% NH3 in MeOH/acetonitrile) Solvents or buffers with higher ionic strength and counter ions with high selectivity (e.g. Ca2+)
	Anion Exchangers Silica Based: SB (SAX), NH2, DMA Polymer-based: HR-XA, HR-XAW, PS-OH–,… Interactions: Between charged analytes and functional groups of anion exchanger Sample: Aqueous (pH 8-9) Elution: Basic- pH 10 (e.g. 20% NH3 in MeOH/acetonitrile) Acidic- pH 4-5 (e.g. HCl, or 5% AcOH in MeOH/acetonitrile) Solvents or buffers with higher ionic strength and counter ions with high selectivity (e.g. Citrate)

It should be noted, that in SPE the interactions described above are not found in pure form, but in combination. For example, modified silicas, unless they have been subjected to endcapping (silanization of residual silanol groups with short-chain silanes), still possess free silanol groups, which can enter into secondary interactions.

Sample Pretreatment
For direct extraction with adsorbents the sample matrix (sample environment) has to fulfill three conditions:

• The matrix has to be liquid, if possible with low viscosity
• Solids should be removed from the liquid matrix
• The matrix (sample environment) should be suitable for retention of the analyte

For solid samples there are different methods to convert the sample into a suitable matrix:

• Dissolution of the solid sample in a suitable solvent
• Lyophilisation of the sample and dissolution in a suitable solvent
• Extraction of the solid sample with a suitable solvent
• Homogenization of the sample in a suitable solvent

In order to find the suitable solvent, one has to consider all desired sample components. Also, a suitable solvent should enhance the retention of the analyte. For example, samples with large contents of solids are often homogenized in nonpolar solvents like hexane, while for samples with high water content dissolution in acids, bases, buffers or very polar solvents such as methanol are recommended.

Additionally, SPE allows one to alter the properties of the sample matrix. If, for example, natural products are extracted with methanol or acetone, the polarity of the extracts can be increased by dilution with water, in order to enhance nonpolar solid-phase extraction on the C₁₈ material.