Solid-Phase Extraction

The goal of solid-phase extraction (SPE) is to quantitatively remove an analyte from a sample matrix with complete recovery in a solvent suitable for subsequent analysis. Owing to the limitations of most commercially available SPE sorbents, this ideal is rarely achieved. The patented Diazem Dual-Zone SPE sorbents overcome many of the problems associated with conventional SPE products and provide near-ideal performance.

Ideally, SPE is a form of on/off chromatography that completely and selectively adsorbs an analyte under the load and wash steps and completely desorbs the analyte during the elution step. In the practice of analyzing drugs in biological matrices, however, it has been virtually impossible to selectively remove an organic analyte from biomolecule "contaminants" by a simple "on/off" mechanism.

Fouling Resistance Key to Consistency

Dual Zone SPE sorbents provide a near-ideal on/off, reverse-phase adsorption and desorption mechanism for nucleoside extraction from urine, as demonstrated by the steep slope of the elution profile in Figure 1. Nearly 100% of the nucleosides are recovered with a 20% MeCN elution solvent. The increased retentivity and shallow slopes of the elution profiles of nucleosides on competitive SPE sorbents are likely due to multiple retention mechanisms. Analytes are retained by both the bonded phase ligand and by matrix interferences which are bound to the phase.

The UT Proficiency Test uses GCMS to screen for commonly abused drugs in urine. A clean extract with good recovery is critical for reliable and reproducible detection of drugs at low concentrations. Liquid-liquid extraction has been preferred over conventional SPE sorbents because of the cleanliness of the extracts. Dual Zone SPE sorbents give results which show excellent correlation with liquid-liquid extraction. In some cases, the Dual Zone SPE method shows improved detection of certain analytes, such as morphine in UT-6, while using a simpler eluent system.

Sample preparation of Cyclosporin from plasma has been improved by Dual Zone sorbents. Cyclosporin is a 90% protein-bound drug for which conventional SPE sorbents rarely give recoveries above 40%. Owing to these difficulties, liquid-liquid extraction has often been used instead of SPE. The liquid-liquid extraction protocol, however, takes up to 45 minutes.

Fig. 3: The sample extraction shown here was accomplished in just five minutes using a Diazem SPE.