## 1 Introduction  
Significant analytical chemistry research efforts are currently aimed at developing suitable analytical methods to facilitate analysis at the location of the system under investigation. This is known as on-site analysis and it is a more efficient approach than laboratory analysis, as is common practice at present. On-site analysis offers many intriguing advantages such as error reduction, elimination of the possibility of sample changes and time delays associated with sample transport and storage, which results in more accurate, more precise and quickly available analytical data. Ion mobility spectrometry (IMS) is a gas-phase electrophoretic technique that allows analytes to be distinguished on the basis of their mass, charge and collision cross section and exhibits instrumental simplicity, low power consumption, real time monitoring, extremely fast response, low operation cost and so on [1; 2; 3; 4]. All of these characteristics make IMS feasible for on-site environmental analysis, airport security, safety monitoring and military applications [5; 6; 7; 8; 9; 10; 11]. However, a problem that remains is that the sensitivity and selectivity of conventional analytical methods face great challenges for direct measurement of trace analytes in complex samples. With regard to IMS, it makes difficult the characterisation of mixtures, because components present in high concentrations or with a high proton affinity will dominate the spectrum [4]. In addition, matrix components present together with the analyte in the sample may cause problems during analysis. Hence, a sample pretreatment step prior to instrumental analysis is usually necessary. The commonly applied sample preparation techniques, such as liquid-liquid extraction [12; 13] and solid-phase extraction [14; 15; 16], often require tedious operations and thus are not in favored in on-site sampling and IMS analysis. Solid-phase microextraction (SPME) is a simple sample preparation technique that integrates sampling, extraction, pre-concentration and sample introduction in a single step, which offers the advantages of simplicity, being solvent free, and easy coupling to the subsequent analytical instrument. In particular all these merits recommend the technique for numerous applications in on-site analysis [17]. Many researches have focused on the combination of SPME and IMS by means of simplified modifications of the original SPME sample holder for analysis of volatile organic compounds, explosives, and pharmaceuticals [18; 19; 20]. Moreover, the detailed interface exploration was reported to enhance the analytical performance. A thermal desorption SPME inlet constructed from a quartz glass tube and flexible electric heating tape with a hand-held IMS for rapid screening of chemical warfare agent precursors and degradation products in soil was demonstrated [21]. The transfer line/desorber unit which was made up of a low-thermal-mass silicosteel coated stainless steel tube for on-site applications [2]. On the other hand, the problems of the previous researches still remained unsolved. For instance, the IMS instrument was modified to introduce the SPME fiber; thetemperature of thermal desorption unit in SPME was non-tunable and this was not beneficial for desorption efficiency as it thus reduced the application scope; furthermore, the power consumption was still relatively high and unfavorable for on-site analysis. In our work, a novel thermal desorption-based interface was fabricated to integrate SPME with IMS for on-site water analysis. The self-made ultra-portable interface comprised an inner polished stainless steel tube and a low heat mass glass fiber. Chemical warfare agents (CWAs) were selected as the model analytes due to their high toxicity and the use in mass destruction and terrorist attacks, as well as the necessity to detect the presence of such weapons in both military and civil environments. Three CWAs and its two simulants were extracted in water samples and then imported into the IMS for the analysis through thermal desorption.