A sensitive liquid chromatographic analytical method using fluorescence detection was developed for the simultaneous determination of amoxicillin and ampicillin residues in raw and processed bovine milk. Aliquots of raw or processed milk (5 mL) were diluted to 40 mL with 0.01 M KH2PO4 (pH 4.5) buffer, and the soluble proteins were precipitated with the addition of sodium tungstate and sulfuric acid followed by centrifugation. The drug residues were concentrated by passing the supernatant through a C18 solid phase extraction cartridge. Amoxicillin and ampicillin were eluted from the cartridge and reacted with salicylaldehyde to form fluorescent derivatives, which were then analyzed with liquid chromatography and fluorescence detection. Average recoveries of spiked amoxicillin and ampicillin at 5, 10, and 20 ng/mL were >80%, with coefficients of variation (CV) <5%. The limit of detection (LOD) and limit of quantitation (LOQ) for amoxicillin were 1.1 and 2.4 ng/mL, respectively. The LOD and LOQ for ampicillin were 1.0 and 1.7 ng/mL, respectively. Keywords: Amoxicillin; ampicillin; antibiotics; milk; HPLC

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A liquid chromatographic (LC) method with fluorescence detection was developed for analysis of amoxicillin in catfish and salmon tissues. The tissue was extracted with phosphate buffer (pH 4.5), followed by trichloroacetic acid (TCA) precipitation of proteins and solid-phase (C18) extraction. Trace amounts of nonpolar interfering substances present after solid-phase extraction were removed by ether liquid-liquid extraction. The extract was reacted with formaldehyde and TCA at 100°C for 30 min. A fluorescent derivative was extracted with ether, concentrated, and analyzed by reversed phase LC with fluorescence detection. Average recoveries of amoxicillin spiked at 2.5-20 ppb were &gt;80% for catfish and &gt;75% for salmon muscle tissue, with coefficients of variation of &lt;6%. Limits of detection (LOD) and quantitation (LOQ) for catfish tissue were 0.5 and 1.2 ppb, respectively. LOD and LOQ for salmon muscle tissue were 0.8 and 2.0 ppb, respectively.

An on-line nonaqueous capillary electrophoresis-electrospray mass spectrometry (ESI-MS) technique was developed using a commercial ion spray interface. The nonaqueous capillary electrophoresis ESI-MS system was used to profile tricyclic antidepressants of similar structures and mass-to-charge ratios. We found that pure methanol can be used as a sheath liquid to obtain stable ion spray from nonaqueous capillary electrophoresis. The flow rate of the coaxial nebulizing gas affected baseline signals, separation efficiency, and migration times. Other nonaqueous capillary electrophoresis operating conditions and electrospray parameters were optimized for enhanced baseline separation and high sensitivity detection. The effect of sample stacking on separation and detection was evaluated. The calculated detection limits were approximately 3 pg injected onto the capillary. ESI mass spectra of tricyclic antidepressants from a single quadrupole MS were obtained and elucidated. The information was used to propose fragmentation pathways of the tricyclic antidepressants. The method was also used to analyze the metabolites of amitriptyline produced by the fungus Cunninghamella elegans. Sixteen metabolites were detected and most of them were tentatively identified as demethylated and/or hydroxylated, and/or N-oxidized products.