2.2. Sample extraction
Samples were extracted and concentrated before analysis using
an AutoTrace 280 automated SPE system from Dionex Corporation,
similar to previously published methods (Anumol et al., 2013).
Briefly, all samples were spiked with a solution of isotopically
labeled surrogate standards that contained all 12 stable isotopes
analyzed in this study at concentrations at 300 ng L1 in the sample.
Hydrophilic-lipophilic balance (HLB) cartridges (200 mg) from
Waters Corporation (Millford, MA) were first preconditioned with
5 mL of MTBE, followed by 5 mL of methanol and 5 mL of ultrapure
water. 200 mL samples were then loaded onto the cartridge at
15 ml min1, after which the cartridges were rinsed with ultrapure
water followed by nitrogen drying for 30 min. The analytes were
then eluted with 5 mL of methanol followed by 5 mL of 10/90 (v/v)
methanol/MTBE solution into 15 mL graduated conical tubes. The
eluent was concentrated to less than 500 lL using nitrogen
(TurboVap, Zymark Corporation, Hopkinton, MA)followed by reconstitution
to 1000 lL using methanol. This final extract was then
transferred to 2 mL autosampler vials for analysis. Samples were
filtered with 0.45 lm syringe filter prior to analysis