Fundamental studies in reversed-phase liquid-solid extraction of basic drugs; III: Sample matrix effects

Matrix effect and recovery terminology issues in regulated drug bioanalysis

Understanding the meaning of the terms used in the bioanalytical method validation guidance is essential for practitioners to implement best practice. However, terms that have several meanings or that have different interpretations exist within bioanalysis, and this may give rise to differing practices. In this perspective we discuss an important but often confusing term – ‘matrix effect (ME)’ – in regulated drug bioanalysis. The ME can be interpreted as either the ionization change or the measurement bias of the method caused by the nonanalyte matrix. The ME definition dilemma makes its evaluation challenging. The matrix factor is currently used as a standard method for evaluation of ionization changes caused by the matrix in MS-based methods. Standard additions to pre-extraction samples have been suggested to evaluate the overall effects of a matrix from different sources on the analytical system, because it covers ionization variation and extraction recovery variation. We also provide our personal views on the term ‘recovery’.

Theory and methodology of antibiotic extraction from biomatrices

A short review on the basic theory and practices of the extraction and clean-up of agricultural antibiotics from biomatrices is presented. For the analysis of residues of ionophores, beta-lactams, macrolides, chloramphenicol, aminoglycosides, tetracyclines and peptide antibiotics, the use of solid-phase extraction has become nearly ubiquitous as part of the basic extraction methodology. The majority of the methodologies for these compounds report recoveries greater than 70%, with relative standard deviations usually less than 15%. Each of the antibiotic classes, as well as antibiotics within each class, have unique chemistries that must be taken into account when developing a viable extraction method.

Application of reversed-phase liquid chromatography and prepacked C18 cartridges for the analysis of oxytetracycline and related compounds

The reversed-phase (RP) chromatographic separation of oxytetracycline (OTC) 4-epioxytetracycline (4-epiOTC), alpha-apooxytetracycline (alpha-apoOTC), and beta-apooxytetracycline (beta-apoOTC) has been accomplished on an Inertsil C8 column at ambient temperature. Using the simplex method of solvent optimization, a 0.1 M ammonium acetate buffer (pH 3.0)-acetonitrile-tetrahydrofuran (72.5:12.5:15, v/v/v) mobile phase was found to give excellent separation of the compounds. OTC, 4-epiOTC, alpha-apoOTC and beta-apoOTC were resolved in 35 min with calculated detection limits of 40, 20, 50 and 140 ng/ml, respectively. Solid-phase extraction (using RP C18 cartridges) of OTC and OTC degradation compounds from distilled water and porcine muscle was tested at four concentration levels ranging from 200 to 2000 ng/ml (g); overall mean recovery of OTC from distilled water and porcine tissue was greater than 90% and 70%, respectively.