# Comparison of Automated Extraction Techniques for Volatile Analysis of Whole Milk Powder
## 3 Results and Discussion
### The Selectivity of Each Extraction Technique  
Figure 3 is a series of Venn diagrams representing the selectivity of each technique in relation to (a) with salting out using the non-polar GC column, (b) without salting out using the non-polar GC column, (c) with salting out using the polar GC column, and (d) without salting out using the polar GC column. These figures highlight commonalities in relation to VOCs extracted by each technique and those recovered only by each individual extraction technique. It is immediately apparent that more VOCs were extracted using the non-polar GC columns (Figure 3a,b) than with the polar GC column (Figure 3c,d), as previously stated. However, these figures give a better insight into the discrepancies and commonalities with regard to the numbers of VOCs extracted across all four techniques. The greatest number of VOCs were associated with the two HiSorb techniques (DI-HiSorb and HS-HiSorb).  
More commonalities were evident in relation to specific VOCs between both Hi-Sorb techniques, which is not surprising as they both utilize the same sorbent phase (PDMS), despite the fact that, in general, PDMS is regarded as less useful for the recovery of polar VOCs [26]. Thus overall, Hi-Sorb was quite effective for the general recovery of VOCs in WMP. Considerably fewer synergies were evident between both Hi-Sorb techniques and HS-SPME and TD. As mentioned, PDMS is thought to be more effective for the recovery of less polar VOCs than HS-SPME with multiple fiber phases [23]. HS-SPME appeared to be very effective at recovering terpen and sulphur compounds independent of column polarity and salting out. While quite poor at recovering lactones, furan, and acids, this was also dependent upon GC column polarity and, to a much lesser extent, salting out. The DVB/CAR/PDMS multiphase SPME fibers tended to extract very volatile low boiling point VOCs more effectively [25]. In addition, as DVB is a polar porous coating, it is quite efficient in extracting polar compounds and, thus, was useful for sulphur VOCs [19; 25; 27], as evident in this study. The TEN components of the TD phase were less useful for very volatile VOCs, but compensated to some extent by the inclusion of CAR in the packing material [24].  
In summary, 12 VOCs were extracted by every technique (1-pentanol, 1-hexanol, hexanal, heptanal, octanal, nonanal, benzaldehyde, toluene, 2-heptanone, 2-nonanone, dimethyl sulfide, and D-limonene) (Table 2a) with salting out using the non-polar GC column (Figure 3a). Thirteen 13 VOCs (1-pentanol, hexanal, heptanal, octanal, nonanal, benzaldehyde, toluene, benzene, 2-pentanone, 2-heptanone, methyl isopropyl ketone, 2-nonanone, and D-limonene) (Table 2a) were extracted by every technique without salting out using the non-polar GC column (Figure 3b). Whereas, in relation to the polar GC column, only seven VOCs (pentanal, heptanal, nonanal, benzaldehyde, decanal, acetone,and 2-heptanone) (Table 2b) were extracted with salting out (Figure 3c), and only three VOCs (heptanal, benzaldehyde, and acetone) (Table 2b) without salting out (Figure 3d).  
The only VOCs extracted by all four extraction techniques independent of GC column polarity and salting out were heptanal and benzaldehyde (Table 2a,b). This likely reflects a combination of their relative abundance and chemical properties, which enabled them to be more easily recovered by each technique, despite the range of different phases and GC column polarities.