Fluorescent derivatization combined with aqueous solvent-based dispersive liquid-liquid microextraction for determination of butyrobetaine, l-carnitine and acetyl-l-carnitine in human plasma

• A rapid fluorescent derivatization is proposed using 4-bromomethylbiphenyl.
• The novel AS-DLLME method simplifies the clean-up of polar components in human plasma.
• Fluorescent derivatives were identified by LTQ Orbitrap in human plasma.
• The method increases the retention of extremely hydrophilic compounds during LC–MS/MS.

A novel aqueous solvent-based dispersive liquid-liquid microextraction (AS-DLLME) method was combined with narrow-bore liquid chromatography and fluorescence detection for the determination of hydrophilic compounds. A remover (non-polar solvent) and extractant (aqueous solution) were introduced into the derivatization system (acetonitrile) to obtain a water-in-oil emulsion state that increased the mass transfer of analytes. As a proof of concept, three quaternary ammonium substances, including butyrobetaine, l-carnitine and acetyl-l-carnitine, were also used as analytes and determined in pharmaceuticals, personal care products, food and human plasma. The analytes were derivatized with 4-bromomethylbiphenyl for fluorescence detection and improved retention in the column. The linear response was 10–2000 nM for l-carnitine and acetyl-l-carnitine with a good determination coefficient (r2 > 0.998) in the standard solution. The detection limit for l-carnitine and acetyl-l-carnitine was 4.5 fmol. The method was also successfully applied to a 1 μL sample of human plasma. In the linearity calculations for determining butyrobetaine, l-carnitine and acetyl-l-carnitine in human plasma, the determination coefficients ranged from 0.996 to 0.999. Linear regression exhibited good reproducibility and a relative standard deviation better than 7.50% for the slope and 9.06% for the intercept. To characterize highly hydrophilic compounds in various samples, the proposed method provides good sensitivity for a small sample volume with a low consumption of toxic solvents.