## 4 Vapour Generator and Calibration  
### Vapour Generator  
The development and testing of explosive vapour trace detector requires a reliable gas generator, capable of producing known, controllable, and very low concentrations of explosive vapours. We have developed and tested a computer-controlled vapour generator, which allows for long-term and reliable generation of TNT and other vapours with concentrations in the range from 0.05 to 1 vapour pressure of the selected element. The schematic diagram is presented in Figure 16a and a model of a constructed vapour generator in Figure 16b. A N\({}_{2}\) carrier gas from a storage tank passes through a regulated resistive heater, which heats the flowing gas to a pre-set temperature with an accuracy of \(\pm\) 50 mK. The flow of this thermally stabilized gas is divided into three parallel lines, where the gas flow through each line is regulated by an electronic flow controller (Aalborg GFC17A, Orangeburg, NY, USA). Each flow line is connected to one of the three equivalent glass vessels, which are thermally stabilized to any pre-set temperature in the range 25 \(\,\)C to 65 \(\,\)C with an accuracy of \(\pm\)50 mK. One of the cylinders contains a known mass of the explosive under consideration, which was deposited on glass wool (Figure 17), while the other two cylinders were empty. The switching and precise control of the concentration levels of the explosive's vapour in the output gas is achieved by mixing the explosive-saturated gas with the pure carrier gas. Figure 16: (**a**) Schematic diagram of the vapour generator. N\({}_{2}\) gas from a storage tank is thermally stabilized (T) and divided into three parallel flow lines (\(\Phi\)1, \(\Phi\)2, \(\Phi\)3), each with electronic flow regulator (F1, F2, F3). The switching between the pure carrier gas and gas with the known concentration of the explosive's vapour, while keeping the total mass flow constant, is done by mixing the gas from an empty glass cylinder (\(\Phi\)1) with either pure gas (\(\Phi\)2) or with saturated gas (\(\Phi\)3). The mixing is controlled by three valves V1, V2 and V3. (**b**) A model of the actual vapor generator used in the experiments. Red, blue, and green colors represent individual flow lines. At all times, the total mass flow \(\Phi\) is always kept constant by matching the flows \(\Phi\)1 and \(\Phi\)3. The electronic 3-way valves V1, V2, and V3 control the output of the vapour generator: the mixing of a pure \(\Phi\)2 and \(\Phi\)1 gas results in a reference gas flow without any traces of explosive vapours, while mixing of a pure \(\Phi\)2 gas with saturated \(\Phi\)3 explosive vapours delivers gas with known concentrations of explosive. Additionally, the concentration of the vapours can be controlled by varying the temperature of the gas and the glass cylinders, thus varying the vapour pressure of the explosive. Great care was taken to avoid any contamination of the system. All the components were made out of stainless-steel, Teflon, or glass and thoroughly cleaned and flushed with carrier gas for a longer time prior to any usage in the detection experiments. The temperature stability of the output gas with a known concentration of the explosive's vapour was found to be better than \(\pm\)1 mK, and the concentration was stable over weeks of constant operation. Several methods for calibration of the concentration of explosive vapours in the output gas were tested. It turns out that the most precise and reliable method of measuring the very low concentrations of explosive vapours from the generator's output is also the simplest one. The output of the vapour generator is connected to a glass tube, and immersed in the liquid nitrogen at 77 K. As the gas flows through the cold part, the solid and liquid fractions present in the carrier gas gradually condense at the tube's surface. At a flow rate of 100 mL/min, one can clearly observe solid condensate and water droplets on the glass surface after several days of gas flow.