## 2 Sample Extraction
### Dynamic Headspace  
DHS was initially introduced in 1981 [64]. Because equilibrium is not required, it eliminates the sample matrix effect during the absolute VOC quantification. DHS is universally accepted as a non-polluting, easy-to-use, and flexible process. The central idea of this method is the extraction of VOCs by a continuous inert gas flow into the liquid matrix. An adsorbent or cryogenic trap is used to further enrich the extracted VOCs. The performance of different adsorbent materials on sampling wine VOCs was published in a recent study [65]. Since the concentration of the VOCs in the headspace decreases exponentially over extraction time, with proper mathematical modeling, it is possible to describe a total peak area that is proportional to the total amount of analyte existing in the original sample. In DHS sampling, ideally without a breakthrough, the analyte concentration in the sample can be expressed by the following equation:  
\[C_{i}=C_{0}\times\mathrm{e}^{-qt} \tag{1}\]  
The residual analyte concentration in the sample is denoted by \(C_{i}\). The initial concentration was \(C_{0}\) at the beginning of the sampling. \(C_{i}\) decreases exponentially with increasing sweep time (\(t\)). \(q\) is a constant that reflects the recovery efficiency. The peak area of the analyte (\(A_{i}\)) is proportional (\(k\)) to the concentration of the analyte in the adsorbent (\(C_{0}\times(1-\mathrm{e}^{-qt})\)):  
\[A_{i}=kC_{0}\times\left(1-\mathrm{e}^{-qt}\right) \tag{2}\]  
This area can then be linked to concentration by using an appropriate calibration procedure. Chemical traps of different sizes, sorbent particle sizes, and sorbent compositions are available on the market [66]. Tenax TA is the most commonly used adsorbent since it can adsorb a wide range of VOCs and has high thermal stability as well as good hydrophobicity [67].  
DHS sampling has been applied to VOC profiling for beer and wine [68; 69; 70]. DHS is suitable for the extraction of highly volatile compounds. Because of the long-term continuous extraction, the analytes in the sample can be recovered almost completely and injected into the GC. So, sampling with DHS provides good analytical sensitivity. In addition, the DHS sampling process has several adjustable parameters (e.g., temperature, time for incubation, trapping, and drying, and gas flow) that can be optimized according to the characteristics of the sample. Ideally, the sampling volume (gas flow multiplied by the trapping time) is smaller than the sampling breakthrough volume of the first eluting analyte from the sampler. And the real sampling gas flow can be obtained as a function of incubation temperature, sampling temperature, head pressure, outlet pressure, and outlet gas flow [71]. However, this flexibility in the parameters requires more instrumental investment, more maintenance, and complex optimization. Both sampling and GC injection can be automated, making it possible to guarantee reproducible analysis with high throughput.  
DHS covers a broader range of volatiles compared to the more commonly used SPME, such as acids, benzenoids, ketones, nitrogen, and sulfur-containing compounds [51]. As the VOCs in beer and wine vary from polar to apolar and from trace to high concentration, exhaustive sampling techniques--DHS coupling with GC\(\times\)GC would be perfect. However, the additional investment in the focusing equipment, such as the Cooled Injection System (Gerstel) and thermal desorption unit, limited the application of DHS in non-specialized VOC analysis laboratories [71].