During analytical processes, adsorption of peptides, which is believed to mostly be due to non-covalent interactions and depending upon the experimental conditions, cannot only lead to significant loss of the analyte, but also to increased analytical variability. This undesirable aspect, however, has been given scant attention [1-2].Some cyclic depsipeptides are considered as mycotoxins, i.e. beauvericin (BEA) and enniatins (ENNs). To quantitatively evaluate this transdermal behaviour, ex-vivo in-vitro Franz Diffusion Cell (FDC) experiments are performed. The adsorption of the cyclic depsipeptide analytes to the FDC glass wall, of which the quality differs from analytical volumetric glassware, was not yet investigated.Therefore, BEA and ENNs were solubilised in six different solvent mixtures (i.e. different concentrations of ACN and EtOH in H2O) and exposed to FDC glass (in duplicate). After 24h equilibration at 25°C, an aliquot of each solution was assayed with a newly developed and validated UHPLC-MS/MS method in multiple reaction monitoring (MRM) mode. Responses were analysed using loglinear models to obtain recoveries. These were fitted using generalised estimating equations with unstructured covariance to account for correlation within the duplicates . QQ-plots confirmed the normality of the raw residuals in these models. Bonferroni corrected p-values and 95% confidence intervals were determined to account for multiplicity in the analysis of each compound separately.Our results showed that at an EtOH or ACN concentration of ≥ 50% there is no statistical or biomedical significant adsorption effect. At lower levels of organic solvents, however, a significant adsorption effect cannot be excluded, i.e. up to approximately 50% adsorption at 10% ACN.References: Herath, H.M.D.R.; Kim, R.-R.; Cabot, P.J.; Shaw, P.N.; Hewavitharana, A.K. Inaccuracies in the quantification of peptides – A case study using β-endorphin assay. LCGC North America, 2013, 31(1): 58-61. Pezeshki, A.; Vergote, V.; Van Dorpe, S.; Baert, B.; Burvenich, C.; Popkov, A.; De Spiegeleer, B. Adsorption of peptides at the sample drying step: influence of solvent evaporation technique, vial material and solution additive. Journal of Pharmaceutical and Biomedical Analysis, 2009, 49: 607-612. Liang, K-Y.; Zeger, S.L. Longitudinal data analysis using generalized linear models. Biometrics, 1986, 78(1): 13-22.