## 3 GC\(\times\)GC Separation
### Modulator  
In order to achieve high-quality GC\(\times\)GC separations, it is critical to efficiently transfer the eluent from the first dimensional (\({}^{1}\)D) column to the second dimensional (\({}^{2}\)D) column while maintaining the separation obtained in the first dimension. The general criterion is that most of the peaks in the first dimension are cut into at least three slices, while no wrap-around occurs in the second dimension [36]. The \({}^{1}\)D and \({}^{2}\)D columns are connected by a modulator. The modulator periodically collects and focuses the condensable eluent for a short period of time and injects it into the \({}^{2}\)D column at once [115]. There are several modulator products on the market; they are either based on temperature regulation or gas flow control to achieve transfer [116]. There are two types of thermal modulator mechanisms: heating and cooling. The heating modulator collects the eluted \({}^{1}\)D analytes at temperatures close to those of the oven and performs the injection by raising the temperature. As such, a modulator needs to maintain a temperature difference of at least 100 \({}^{\circ}\)C from the GC\(\times\)GC oven [117], which limits the separation temperature of the GC\(\times\)GC oven and the boiling point range of the volatiles. It has been used less frequently today. The latter cooling modulator collects analytes with cryogenic cooling and performs the injection at the temperature of the oven. Using this type of modulator, a narrow injection band modulation period can be achieved in the second dimension for C\({}_{4}\) to C\({}_{40}+\) compounds to provide high detection sensitivity [118]. The flow modulator controls the transfer of analytes from \({}^{1}\)D to \({}^{2}\)D columns by using a branch line of high-pressure helium. When a \({}^{2}\)D column needs to be injected, helium flow will intercept the outflow from the \({}^{1}\)D column and inject the analytes stored in the modulator. This principle is known as Dean's switch and is well described in a paper [119]. When the injection is not required, the helium flow will cut off the path to the \({}^{2}\)D column, a portion of the eluates will remain in the modulator, and the excess will be expelled as waste [120]. The flow modulator is suitable for volatiles from C\({}_{1}\) to C\({}_{40}+\). However, the flow splitting will inevitably lead to a decrease in detection sensitivity.  
Both thermal and flow-based modulators have their advantages and disadvantages in areas such as detection limits, volatility range, flexibility of installation, hardware investment, and cost of operation [121]. There is no single modulator that would be the best choice for all purposes. Since non-target VOC profiling is the main task of GC\(\times\)GC in beverage analysis, a modulator should fit the demands of this task. Due to its technical characteristics, cryogenic thermal modulation is a superior platform for VOC profiling.  
Figure 1: The basic working scheme of GC\(\times\)GC.  
All eluted analytes from the \({}^{1}\)D column are collected, aggregated by the modulator, and injected into the \({}^{2}\)D column at extremely high speed. As a result, there is no loss of detection sensitivity and even an improvement in the peak shape if the proper \({}^{2}\)D column length is applied [119]. Besides, the method development of thermal modulation is less complicated than that of the flow modulator. It does not require optimization of the auxiliary helium pressure, equilibrium flow rate, or flow volume. The thermal modulator requires fewer connections, reducing potential leakage. The main disadvantage of the thermal modulators is the high level of cryogen consumption per analysis [122]. However, VOC profiling is required once (multiple technical replications) for each biological study, so this cost is affordable.