## 1 Introduction  
Detecting vapour traces of explosives and other materials in the atmosphere is a potentially powerful method to reveal the presence of explosive devices and other materials like toxins, _etc._ The principle of the detection method is based on the fact that almost any material, including explosives, emits a rather small, but detectable number of different molecules constituting the target material. Numerous detection techniques have been developed that are capable of detecting explosive devices [1], but their common limitations are rather large size and weight, high power consumption, unreliable detection with false alarms, insufficient sensitivity and/or chemical selectivity, and hyper-sensitivity to mechanical noise associated with a very high price. There is a need for miniature, sensitive and chemically selective sensor systems, typically capable of detecting one target molecule among 10\({}^{+12}\) molecules of atmosphere in real time. Chemically surface modified Micro Electro Mechanical System (MEMS) sensors [2; 3; 4] are currently the most promising and most popular candidates for the ultrasensitive detection of low concentrations of target molecules in the atmosphere. The detection method is based on optical measurement of the deflection of the sensing micro-cantilever, which is caused by the adsorption of the target molecules on one of the surfaces of the cantilever [5; 6]. An alternative to the optical detection method is to measure the change in the capacitance or the resistance of the cantilever [7] or a MEMS device. A very sensitive detection is possible by this method, and Trinitrotoluene (TNT) vapour concentrations in N\({}_{2}\) of the order of 1:10\({}^{+9}\) have been reported [8]. However, in all cases, there are several severe problems that limit the application of MEMS in a real environment. The measurement of the cantilever deflection requires a precise optical system, and the detection apparatus is bulky when precision optics with a long optical path has to be used for the precision required. Furthermore, the measurement of cantilever bending is very sensitive to environment influences like vibrations, mechanical shock, and acceleration. Because the cross-section of the sensing cantilever is usually asymmetrical, the cantilevers act as bi-metal devices and are highly sensitive to the temperature changes. This makes chemically functionalized cantilevers very impractical for sensing vapour traces of hazardous chemicals. An alternative to MEMS is to use Complementary Metal Oxide Semiconductor (CMOS) capacitive sensors with micro-meter-size or sub-micro-meter size electrodes, which were developed recently for various lab-on-chip and bio-sensing applications. Here the surfaces of the capacitor's electrodes are chemically modified in order to enhance the surface adsorption of target molecules, resulting in a slight change of the capacitance of that capacitor. The capacitive method has obvious advantages over the MEMS method, because it is not sensitive to temperature changes and mechanical vibrations, and is also fully compatible with the CMOS production process. While the sensitivity of this method has been reported for lab-on-chip [9] and bio-sensing applications [10], there is no available literature data on the comparison of the sensitivity of comb capacitive sensors with respect to the MEMS optical detection system. In this study, we report on the sensitivity comparison of two explosive vapour trace detection systems developed by us: **C**hemo **M**echanical sensor with **O**ptical detection (CMO) and **C**omb **C**apacitive sensors with **E**lectronic detection (CE). Both systems were tested for TNT and Hexogen (RDX) explosive sensitivity in the N\({}_{2}\) carrier gas and in the air. We found that the sensitivity of our CMO system is in the range of approximately 300 molecules of TNT in 10\({}^{\text{-}12}\) molecules of N\({}_{2}\) gas, while the sensitivity of our CE system is of the order of 3 molecules of TNT in 10\({}^{\text{+}12}\) molecules of carrier gas N\({}_{2}\).