Design and construction of Portable Columnless Gas Chromatography- Ion Mobility Spectrometery (PCGC-IMS) for identification of contraband materials

A mass spectrometer at normal pressure was the dream of the analytical people for a long time. It became realistic in the 1990s. Modern analytical IMS is an atmospheric pressure ionization method for detection of vapors. The primary ionization event is usually affected by a 63Ni (beta radiation) ion source. A series of ion-molecule reactions ensue involving primary, background, and analyte vapors, and an equilibrium mixture of ions is established. The ions are electrically injected into a drift region and move under the influence of an electric field through a reverse flow of buffer gas (air or nitrogen) until collision occurs with a Faraday cup detector. Charged ionic species move primarily according to their mass and shape (size) and are characterized by their drift time (typically several milliseconds) or ion mobility [1]. When detecting narcotics, it is necessary to vaporize compounds with a wide range of molecular weights. In order to do this, the desorber has to be set to a high temperature. A benefit of operating the desorber in this way is the evaporation of all chemicals in a very short time and, in turn, accommodation of the quick analysis requirement during field use. A drawback to this high temperature operation is the reduction of the detectability of each chemical and sometimes causes interference problems [2]. There are two ways to eliminate or to reduce this phenomenon of decreased detectability. One is to pre-separate mixed chemicals using a gas chromatography (GC) column placed before the IMS reaction region. GC-IMS instruments have been reported successful in the separation of chemical mixtures [3]. Although these approaches have demonstrated their potential in solving the separation problem, the total analysis time was more than a couple of minutes. This is unacceptable to users when performing daily routine analyses of samples obtained for illicit drug detection. Another way is to evaporate chemicals based on their vapor pressure by applying a temperature ramping process to the sample. In this way, overcrowding in the reaction region with chemicals is avoided. Columnless GC-IMS is a valuable alternative technique for the isolation of volatile compounds from non-volatile solid, semi- solid and, occasionally, liquid matrices. This paper addresses, the design and construction of a home-made novel portable columnless gas chromatography - ion mobility spectrometery (IMS) system for rapid screening of narcotics for the first time in institute of Materials and Energy of Isfahan. Desorber of this instrument works by controllably releasing analytes from a solid sample by thermally desorbing and releasing a selected subset of compounds through rapidly raising the temperature to a pre-determined value that has been chosen to desorb particular chemical compounds of interest, but not others. By providing a sufficiently long delay period in-between heating, and by selecting the optimum temperatures, then different chemical species can be controllably released in well-defined batches at different times to an IMS.