Experimental design to optimise the analysis of organic volatile compounds in cow slurry by headspace solid-phase microextraction–gas chromatography–mass spectrometry

The headspace solid-phase microextraction (HS–SPME) of some odorous organic compounds [volatile fatty acids (VFAs), phenols and indoles] from animal wastes was optimised using an experimental design approach. The variables affecting the HS–SPME process studied were as follows: type of fibre, pH of the slurry (3–7), addition of sodium chloride (0–1 mol L−1), sample volume (10–30 mL), exposure time (10–30 min), exposure temperature (25–50 °C), desorption time (3–10 min) and desorption temperature (230–310 °C). As for the sorbent, three different types of fibres were studied: 50/30 μm divinylbenzene/Carboxen/polydimethylsiloxane (DVB/CAR/PDMS), 100 μm PDMS and 85 μm CAR/PDMS. Methylation VFAs was observed when fibres containing PDMS were used. The analytes studied in the optimisation procedure were acetic acid, propanoic acid, butanoic acid, 3-methylbutanoic acid, pentanoic acid, phenol, 4-methylphenol, 4-ethylphenol, indole and skatole (3-methylindole). In order to study the influence of the variables on the responses of the compounds, a Plackett-Burman design was built using the Unscrambler program. The optimisation was carried out using real samples. In order to take into account the matrix effect on the calibration curves, standard solutions were prepared using a purged cow slurry free from the analytes of interest. Repeatability within and among days, accuracy and detection limits were calculated from these calibration curves.