A rapid dispersive liquid-liquid microextraction of antimicrobial onion organosulfur compounds in animal feed coupled to gas chromatography-mass spectrometry.

A rapid analytical procedure is proposed for determining two antimicrobial onion organosulfur compounds, propyl disulfide (PDS) and propyl propane thiosulfonate (PTSO), in animal feed. The use of PTSO as a natural ingredient in animal feed is allowed due to its antimicrobial activity against pathogenic organisms. Two analytical methodologies using gas chromatography coupled to mass spectrometry (GC-MS) are compared. After the extraction of the compounds from animal feed with acetonitrile, dispersive solid phase extraction (DSPE) as a cleaning stage with C18, or dispersive liquid-liquid microextraction (DLLME), using 100 μL of CHCl3, was tried. Both the methods were validated using a pig feed sample and the best results were achieved by DLLME. This technique provided cleaner extracts, five-times greater linear ranges and lower detection limits than simple cleaning due to the enrichment factor achieved. The relative standard deviation decreased from 22% with DSPE to 13% with DLLME. The usefulness of the DLLME-GC-MS methodology was tested by analysing 10 different samples of chicken, calf, hen, cow and fish feed. The concentrations of PDS were in the 0.1-1.7 μg g-1 range and those of PTSO were between 0.09 and 2.1 μg g-1.