How to solve the problem of co-elution between two compounds in liquid chromatography through the first UV derivative spectrum. A trial on alternative plasticizers to di(2-ethylhexyl) phthalate

To meet new regulations, alternative plasticizers to di(2-ethylhexyl) phthalate (DEHP) are now commonly used in the manufacturing of medical devices. These are: acetyl tri-n-butyl citrate (ATBC), bis (2-ethylhexyl)adipate (DEHA), dioctyl terephtalate (DEHT), di-isononylphtalate (DINP), diisononylcyclohexane-1.2-dicarboxylate (DINCH) and trioctyltrimellilate (TOTM). An HPLC-UV analysis was previously developed to characterize four of them. However, two compounds were systematically co-eluated: DEHP with DEHA and DEHT with DINP. The first derivative of UV spectra and photodiode array detection allow the quantification of DEHA and DINP. Moreover, for each plasticizer, maximum wavelength absorbance was chosen to be as specific as possible. Quantification ranged from 0.3 to 750µg/mL according to the plasticizer. The assays were validated by analysis of variance. Our method was validated by determining the following parameters: specificity, linearity, limits of detection and quantification. The relative biases were inferior to 5% for ATBC, DEHP, DEHA and DINCH and inferior to 10% for DEHT, DINP and TOTM. Plasticizers were extracted with tetrahydrofuran and methanol. The developed method was then used to determine the composition of plasticizers in several medical devices used in clinical service. The major plasticizers were quantified from 19% to 40% w/w, traces of DEHT were found in six medical devices and DEHP in five.