Sample Preparation
The volatile compounds in serum
were first concentrated using solid phase extraction (SPE) to remove
the matrix effect. This concentration step also removes water from
the samples; water hinders the extraction of the headspace and can
lead to unintentional water (vapor) injection. SPE polymeric cartridges
(CHROMABOND HR-X (hydrophobic polystyrene-divinylbenzene resin), 45
μm particle size, 1 mL/30 mg, Macherey-Nagel AG, Oensingen,
Switzerland) were conditioned with hexane 3 × 1 mL, acetonitrile
3 × 1 mL, nanopure water 3 × 1 mL, and HIn order to associate candidate biomarkers of dairy intake to molecules
present in the food products, yogurt and milk samples were prepared
and analyzed in the same manner as serum samples: 5 mL of milk or
yogurt was mixed with 5 mL of nanopure water and vortexed in Falcon
tubes. The samples were then centrifuged (10 °C, 10 min, 2400
rpm); 5 mL of supernatants (without the cream for the milk samples)
were collected and frozen until preparation by SPE. The processing
of samples on SPE was the same as that for serum.Quality control
(QC) samples were prepared by combining 250 μL
of serum of each sample from batch 1 (162 serum samples). The volatile
compounds from the QC samples were extracted by SPE following the
sample preparation procedure for serum.