The proposed equipment is a trace detector aimed at aeronautic cargo explosives screening, although the technology is readily applicable to narcotic detection, maritime cargo screening, or passenger and luggage screening.
The technology proposed is based on the following elements and operational concept (see Figure above):
Main subsystems within the equipment are briefly presented in sections below:
The filter (see Figure below) is built from a lead matrix impregnated with Tenax. It is a conventional item, optimized in order to maximize vapor retention efficiency at air flows required for operation.
An Electro-Spray (ES) Ionizer (see figure below) is a device originally proposed by Fenn, where a liquid is continuously aerosolized into a fine spray of charged drops. Mixing this spray with an air sample containing explosives vapors (or particles) leads to the ionization of the explosives molecules, either by contact with the drops or by charge exchange with ions produced by evaporation of ES drops. This leads to the formation of molecular ions that can be analyzed in the DMA and the MS. Electro-Spray ionization is more often used for large molecular weight biological species, but is ideally suited also to trace detection of low volatility explosives for the following reasons:
A Differential Mobility Analyzer (DMA) is an instrument that separates ions as a function of their electrical mobility, just as IMS. The main difference between both instruments is that the DMA separates ions in space, while IMS does so in time. As a result, a DMA can be coupled to an existing MS with only minor modifications taking place at its atmospheric pressure source. In addition, a DMA combined with a triple quadrupole requires the same time in order to analyze a set of explosives than the triple quadrupole alone, yet, it increases its resolution by approximately a hundredfold. Figure below is a sketch of a planar DMA with clean sheath gas coming from the left and an electric field driving upwards the ions entering through an inlet slit above and sampled through the sampling slit below. The DMA combines a laminar flow field with an electric field. More specifically, our proprietary DMA is defined as combining (i) a large laminar flow field of sheath gas produced by a suitable laminarization system with (ii) an electric field generated by several conducting or semiconducting electrodes or grids charged at various points to various electrical potentials, while (iii) a narrow stream of ions with various electrical mobilities is injected into the large laminar flow through a narrow inlet orifice or slit with the help of either an electric field or a small flow of gas, (iv) these ions are separated in space according to their electrical mobility, whereby (v) ions of selected electrical mobilities reach one or several sampling or collecting devices. The DMA can be operated as a band-pass filter and transmit to the MS only a small selected class among all the ions ingested.
The DMA is used as an input filter to the MS, fulfilling a function similar to a Chromatograph, but requiring times to do so in the order of milliseconds, not minutes. Results from the DMA-MS setting will be presented in the following section.
While mass spectrometers have become one of the analytical mainstays of today's chemistry and biotechnology laboratories, they have historically been large, complex systems that occupied the volume of several filing cabinets and were operated by highly trained mass spectrometrists. More recently, with demand from lab chemists and technicians for instruments that could be used for routine analysis, automated, self-calibrating, auto-tuning, bench top units of reasonable size have become available. These instruments are generally coupled with a gas chromatograph or a liquid chromatograph at the sample inlet to improve chemical selectivity. Mass spectrometers utilize four steps for analysis: (1) vaporize the sample, (2) place an electric charge on sample molecules to form ions, (3) separate the ions based on their mass-to-charge ratio using an electric or magnetic field, and (4) determine the number of separated ions having a particular charge-to-mass ratio. The uniqueness of mass spectrometry lies in its chemical specificity. It directly measures a fundamental property of the target molecule -its molecular weight- and thus provides a highly specific means of identifying the molecule. The mass spectrometers used in the equipment is the Sciex API5000, of the atmospheric pressure ionization type (API-MS), where the ions to be analyzed are formed at atmospheric pressure. The API 5000 is a highly sensitive, triple cuadrupole mass spectrometer.
The following papers describe the technology used and main results obtained:
The proposed technology is at a Technology Readiness Level 7. As shown in Figure below, an equipment has been developed using internal funding and formal homologation in a qualified European institution will take place in 2014.
The equipment has been internally funded, and is presently been co financed by the following Agencies supporting R&D activities:
1 See for example J. Fernandez de la Mora S. Ude and B. A. Thomson, The potential of Differential Mobility Analysis coupled to mass spectrometry for the study of very large singly and multiply charged proteins and protein complexes in the gas phase, Biotechnol. J. 2006, 1, 988-997