Analysis of avermectin and moxidectin residues in milk by liquid chromatography-tandem mass spectrometry using an atmospheric pressure chemical ionization/atmospheric pressure photoionization source

The avermectins-ivermectin (IVR), doramectin (DOR), eprinomectin (EPR)-and also the milbemycin moxidectin (MOX) are anthelmintic compounds that may be administered to cattle. Different ionization techniques, including atmospheric pressure photoionization (APPI), were evaluated for the detection of these residues in milk. The ionization response of these compounds using APPI was compared with that obtained by atmospheric pressure chemical ionization (APCI), a combination of APPI and APCI, and electrospray. It was found that the relative response of these drugs with different ionization protocols varied depending on the compound and the mobile phase. When monitoring negative ions, the use of the UV lamp increased the MS response. However, the best response for these compounds was obtained by operating the APCI/APPI source in the positive ion mode without any discharge current applied to the corona needle, whether the UV lamp was on or not. Using this mode of ionization, an MS-MS method was established to monitor the product ion scans of the sodiated molecular ions with an ion trap instrument. Milk fortified with these compounds (0.5-20 ng g(-1)) and milk samples from dosed animals were analyzed after isolating the residues with a simple solid phase extraction method. EPR, DOR and IVR were confirmed in all of the extracts analyzed. MOX was confirmed in all samples fortified at 5 ng g(-1) or higher. Acceptable recoveries (greater than or equal to60%) and relative standard deviations (R.S.D. values less than or equal to20%) were observed for the residues at the following levels: EPR and IVR (1-20 ng g(-1)); DOR and MOX (5-20 ng g(-1)). (C) 2004 Elsevier B.V. All rights reserved.