## 2 Results and Discussion
### Comparison of Headspace Extraction Methods
#### 2.1.4 Static Headspace Using Multiple Solid Phase Microextraction Car/PDMS Fibers (S-HS-MultiSPME)  
HS-SPME with a Car/PDMS fiber allows the extraction of a large range of compounds including carboxylic acids but lacks sensitivity for most of the volatile compounds in human milk. Thus, considering the weak quantity of sorbent used in the SPME procedure and the fact that the more the thickness of the SPME fiber is increased, the more the sensitivity is improved [34], a way to increase the quantity of sorbent was sought. However, Car/PDMS fibers with higher capacity are not commercially available due to mechanical issues and the thermal stability of the sorbent film [58]. Headspace sorptive extraction (HSSE) methods were not envisaged, despite the presence of a higher quantity of sorbent. Indeed, stable polar sorbents are not commercially available, restricting the concrutative effect to apolar compounds. Hence, in this study, we used a greater number of Car/PDMS fibers in their original structure to increase the sensitivity of the method toward polar compounds. Thus, an extraction procedure in a static mode using 10 Car/PDMS SPME fibers was evaluated and referred to as the S-HS-MultiSPME method. The volatile profile of compounds obtained using 10 Car/PDMS SPME fibers in a static extraction procedure is characterized by a 6-fold increase of the TPA compared to the use of a single fiber. The peak areas obtained for some compounds with this procedure are close to those obtained with the D-HS-Tenax method due to the increase in the quantity of sorbent material. This increase in the sensitivity of S-HS-MultiSPME is significant for all chemical families except for O-heterocycles and ketones. A larger number of aldehydes, ketones, alcohols, N-heterocycles and one ester are additionally identified in this S-HS-MultiSPME extract compared to the extract obtained with the conventional HS-SPME procedure. The highest sensitivity improvement is noted for carboxylic acids. Four additional carboxylic acids [propanoic (LRI: 1547), pentanoic (LRI: 1746), heptanoic (LRI: 1968) and nonanoic acids (LRI:2172)] are identified when 10 SPME fibers are used instead of one. The proportion of these compounds reaches up to 62% of the TPA. For most of these chemical families of compounds, the recovery is increased from 2 to 10 times when the number of fibers is increased from 1 to 10. The lowest sensitivity increase is noted for terpen, but no additional terpene was detected with increased SPME fiber quantity. In sum, using 10 SPME fibers (instead of one) in a static extraction mode enables a strong amplification of the signal recovered without deeply altering the relative distribution of chemical classes. 2.1.5. Dynamic Headspace Using Multiple Solid Phase Microextraction Car/PDMS Fibers (D-HS-MultiSPME)  
SPME extraction was shown to be potentially more sensitive when used under dynamic conditions through a non-equilibrium sampling process [59]. Thus, a second variation was implemented with 10 SPME fibers used under dynamic conditions. The volatile profile obtained for the present D-HS-MultiSPME extraction is characterized by a 4-fold increase of the TPA compared to the HS-SPME using a single Car/PDMS fiber. This gain is lower than the gain observed when the 10 SPME fibers were used under static conditions. However, if using 10 SPME fibers under static conditions leads to better signal amplification, the dynamic conditions significantly modify the proportion of the chemical classes of compounds extracted (Figure 1). For example, carboxylic acids are recovered with a ratio 8-fold higher in the S-HS-Multi-SPME than in the D-HS-MultiSPME method. However, except for carboxylic acids and ketones, the use of SPME fibers in a dynamic configuration leads to a better recovery of the main chemical families of human milk compared to their use in the static configuration. Terpenes are thus recovered to a significantly larger extent under dynamic (21% of the TPA) versus static (<2% of the TPA) conditions. A similar tendency is noted for most alcohols.