Extraction of low concentration levels of 6-MAM from urine with a mixed-mode SPE sorbent: Strata Screen-C

he screening for and confirmation of drugs of abuse and their metabolites present many challenges in a toxicological analysis. The analyst routinely handles complex biomatrices such as urine or blood searching for the presence of illegal drugs, which, if present, may be at very low concentration levels. Direct analysis of these samples is often hampered by the presence of endogenous compounds in the biomatrix. These interferences are often in higher concentration than that of the target analytes and thus may mask their presence. Solid phase extraction (SPE) is a sample preparation technique that is commonly used to purify and concentrate drug compounds and their metabolites prior to analysis (1). This application describes how Strata™ Screen-C, a new mixed-mode SPE sorbent from Phenomenex, successfully extracts and concentrates ppb concentration levels of 6-monoacetylmorphine (6-MAM) from urine. The Strata Screen-C sorbent is a mixed phase consisting of silica particles functionalized with C8 and benzenesulfonic acid, a strong cation exchanger (SCX). This stationary phase is excellent for the extraction of basic drug compounds and/or their metabolites. Since the pK a of the SCX is � 1, it is always negatively charged. In acidic solutions, the basic analyte will be positively charged and thus can be retained by ionic interactions with the SCX bonded phase (in addition to the Van der Waals interaction with the nonpolar C8 phase). This strong ionic retention mechanism allows the sorbent to be washed with relatively strong solvents such as methanol, which effectively remove anionic and neutral interferences without seriously affecting the recovery of the basic analyte. A mixture of organic solvent and ammonia disrupts the analyte‐sorbent interaction, resulting in the elution of the basic compound. T