1H-nuclear magnetic resonance spectroscopy of reducing-residue anomeric protons of pertrifluoroacetylated carbohydrates.

Eight monosaccharides (L-arabinoside, L-fucose, D-galactose, D-glucose, D-lyxose, D-mannose, L-rhamnose, and D-xylose), eight disaccharides (cellobiose, gentiobiose, isomaltose, lactose, maltose, nigerose, sophorose, and xylobiose), and three trisaccharides (isomaltotriose, maltotriose, and xylotriose) were derivatized with N-methylbis-(trifluoroacetamide) in pyridine solution to form trifluoroacetylated derivatives. These were analyzed by 1H-n.m.r. spectroscopy to determine the characteristics of the spectra and distributions of the reaction products. Peaks corresponding to reducing-residue anomeric protons were located significantly downfield of all others, and were in general 0.4 p.p.m. or more downfield of equivalent signals from the same carbohydrates when they were free or derivatized with other groups. Neither the location of anomeric proton peaks relative to each other nor the degree of spin-spin coupling between H-1 and H-2 varied greatly with type of derivatization. Spin-spin coupling, however, decreased for some beta-pyranose forms of xylobiose and the three trisaccharides. In all examples except some where H-2 was oriented equatorially to a pyranose ring, the proportion of the alpha-pyranose was either enhanced or not changed in concentration by trifluoroacetylation.

Determination of off-flavor compounds, 2-methylisoborneol and geosmin, in salmon fillets using stir bar sorptive extraction-thermal desorption coupled with gas chromatography-mass spectrometry.

A sensitive and solvent-less method for the determination of musty and earthy off-flavor compounds, 2-methylisoborneol (MIB) and geosmin (GSM), in salmon tissue was developed using stir bar sorptive extraction-thermal desorption coupled with gas chromatography-mass spectrometry (SBSE-TD-GCMS). MIB and GSM were solid phase extracted using polydimethylsiloxane (PDMS) coated stir bars, analyzed by gas chromatography, and detected in full scan mode of mass selective detector (MSD). Using this method, the calibration curves of MIB and GSM were linear in the range of 0.3-100ng/L, with a correlation coefficient above 0.999 and RSDs less than 4% (n=4). The limit of detection (LOD, S/N=3, n=6) and limit of quantification (LOQ, S/N=10, n=6) of MIB and GSM were both ∼0.3 and 1ng/L, respectively. The recoveries of MIB and GSM were 22% and 29% by spike in 30ng/L standard compounds, 23% and 30% by spike-in 100ng/L standard compounds in salmon tissue samples with good precision (<8% of RSDs, n=6), respectively. The recoveries of MIB and GSM were better than reported methodologies using SPME fibres (<10%) in fish tissue samples. This method was successfully applied to monitor and characterize depurated salmon fillet samples (0, 3, 6 and 10 days).