3.2. FMGC×LPGC–qMSanalysisofmenhadenfishoilFAMEs
After confirming the possibility to perform modulation using MS-compatible flow conditions and demonstrating the chromatog- raphy benefits of using a longer accumulation loop, an FM GC × LP GC–qMS method was developed for the analysis of a very complex FAME mixture, namely menhaden fish oil. While the GC1 and GC2 columns were the same, other operational conditions were mod- ified (see Method II, Section 2.2). In particular, the temperature program was set at a higher initial temperature (150◦C) because the menhaden fish oil FAMEs are characterized by higher boiling points compared to milk FAMEs; additionally, a slower tempera- ture gradient (1.5◦C) was applied to enable the separation of as many as possible compounds on the polyethylene glycol dimen- sion. A slower temperature program generates wider peaks and so a longer modulation period was used: 4.6 s accumulation and 0.4 s injection. An accumulation loop of 51-cm length (with a 0.53 mm ID) was used for the four-stage modulation process. The ALV in the first dimension was 11.3 cm/s, corresponding to an initial flow of 0.37mL/min; the ALV in the second column was 179.4cm/s, corresponding to an initial flow of ∼7.5 mL/min. During the first accumulation period the solute band reached a length within the loop of approx. 13.3cm (ALV=2.9cm/s); the first injection pulse pushed the band another 23.7 cm approx. (ALV = 59.3 cm/s), with the front end reaching a distance of about 37 cm; after the second accumulation step, the band reached a distance within the loop of about 50.3 cm, and then was entirely pushed out from the loop with