## 2 Results and Discussion
### Volatile Characterization
#### 2.1.2 HS-SPME-GC\(\times\)GC-TOF Analysis  
HS-SPME-GC\(\times\)GC-TOF analysis was applied to better elucidate the volatile profile, thus providing a tool for the direct comparison and visualization of volatile components and pointing out the presence of molecules not identified only with GC-MS. HS-SPME-GC\(\times\)GC-TOF analyses of the volatile fraction of onion products were submitted to advanced fingerprinting analysis of 2D chromatographic data.  
Sulfur-containing compounds were the most abundant in samples volatile fraction. Sulfur compounds are considered to be derived from the degradation of sulfur-containing amino-acids and are associated with alliaceous, sulfuric, sweaty, onion and cabbage aromas, contributing to the characteristic flavor of raw and processed edible _Alliums_. Table 3 reports the list of headspace volatile compounds of the onion products, 1 st D and 2 nd D retention times and the average peak volume  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **1-Propenyl** & **Allyl** \\  & **(\(\mu\)g g\({}^{-1}\))** & **(\(\mu\)g g\({}^{-1}\))** \\ \hline Flakes & 28.73 & 0.97 \\ Rings & 3.52 & 0.28 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Sum of the concentration (\(\mu\)g g\({}^{-1}\)) of the sulfur-containing volatile organic compounds (VOCs) with either the 1-propenyl moiety or the allyl moiety in the onion samples analyzed in the absence of ascorbic acid.  
from three independent determination. A split ratio of 1:5 during SPME injection was employed for a better separation in the 2D chromatographic space of the major co-eluting molecules not evidenced in the common 1D analysis. The most intense peaks corresponded to dipropyl disulfide and dipropyl trisulfide. Diethanol disulfide and allythiol were identified only by GC\(\times\)GC, probably owing to the co-elution with peaks deriving from SPME fiber bleeding and/or other molecules in mono-dimensional chromatography. Also peaks of _cis_-3,5-diethyl-1,2,4-trithiolane and _trans_-3,5-diethyl-1,2,4-trithiolane (which probably is a peak constituted by the two co-eluting enantiomers) were simultaneous separated by GC\(\times\)GC, in particular the _trans_ and _cis_ isomers were separated in the first dimension (29.900, 1.620 min and 30.150, 1.640 min, respectively). Another trisulfide, the propenyl propenyl trisulfide, was separated in the second dimension, eluting at 30.150, 1.380 min. Tentative identification was made on the basis of non-isothermal Kovats retention indices from temperature-programming from Chemistry WebBook (Table 1) and on the presence of the ion 151 which was present only in cyclic trisulfides (_cis_ and _trans_ 3,5-diethyl-1,2,4-trithiolane) and not in the linear ones (propenyl propyl trisulfide). Trithiolane compounds have been identified as important aroma-active compounds in cooked onions (_Allium cepa_ L.) showing cooked onion-like/fruity and blackcurrant-like/fruity odor impressions for the _trans_-enantiomers compared to the _cis_-isomer eliciting a meat broth-like, cooked onion-like aroma at a five- to ten-fold higher threshold [22].  
In Figure 2, \"contour plots\" from HS-SPME-GC\(\times\)GC-TOF analyses of the onion products are reported--each 2D-peak corresponds to a single volatile compound. In this case, SPME and comprehensive comparative analysis of 2D chromatographic data showed visual differences between samples. The main differences that emerged between the two samples could be summarized  
Figure 2: Comprehensive two-dimensional chromatography–mass spectrometry (GC\(\times\)GC-TOF) color diagram and comprehensive template matching fingerprinting with the main identified volatile compounds of onion flakes and rings.  
as follows--flakes showed higher intensity of sulfides peaks as reported by GC-MS; allythiol, 2-methyl-butenal and 4,6-diethyl-1,2,3,5-tetrathiolane were detected only in rings sample.