Detection systems for explosives have to be tested and validated with realistic samples. However, at present, well defined and characterized and homogeneous test materials for the development and validation of trace detection systems are still missing. Drop-on-demand and inkjet printing technology has become a promising method of producing standardized chemically contaminated test materials with high precision, accuracy, scalability, and flexibility to allow for the inexpensive, high throughput production. Especially for the preparation of samples for optical stand-off technologies the drop-on-demand technique leads to samples of higher quality. Test samples with contaminations ranging from nanograms to micrograms have been prepared and analysed over several weeks of storage. The influence of plot parameters on the morphology and durability of printed samples of various common explosives have been investigated on different substrates. Our results show that this technique leads to samples of higher quality compared to already existing preparation methods of reference samples. For the evaluation of deposition uniformity, quantity and reproducibility samples on several different surfaces, with different solvents and areal density distributions were prepared and analysed. HPLC measurements were used to quantitatively evaluate the durability of printed samples. The morphology of the test samples has been characterized by optical microscopy and confocal Raman-microscopy. This poster focuses on the development of a proposed standardized methodology for the characterization of particulate trace explosive samples of all types. Micrographs were processed with ImageJ (National Institutes of Health), an open source image processing program designed for scientific multidimensional images, which has already been applied to the shape analysis of particles in various research areas and for different materials. The main problem is to find the right threshold to apply to segment the image into features of interest (particles) and background. It is certainly a goal of digital image processing to establish protocols that eliminate variability, but currently it is not possible to use only one threshold with a fixed value for a set of images of samples of various explosives. Manual methods have several limitations like reduced reproducibility, high user bias, tedious and time-consuming fiddling-around finding an "appropriate" cut-off value, etc. The open source software that is in use to process the images provides several methods to automatically apply a threshold. Based on the morphology of the particles and the applied threshold, the obtained results can be really different and, an incorrect selection of the automatic threshold could provide either an overestimation or an underestimation of the parameters to be evaluated, particle size distribution and area coverage.
Tougher security requirements and an increasing number of terror attacks have led to rapid advances in the field of explosive detection in the past few years. Detection systems have to be tested and validated with realistic samples and amounts as early as possible during the development. However, at present, well defined and homogeneous test materials which are a crucial requirement for development and validation of trace detection systems are not available.
Existing preparation methods of reference samples, such as drop-and-dry (drop casting) methods, present a range of variability and reproducibility issues, including inhomogeneous sample coverage and material waste. Especially for the preparation of samples for optical stand-off technologies the drop-on-demand and inkjet printing technology could be a promising method of producing standardized chemically contaminated test materials with high accuracy, precision, scalability, and flexibility to allow for the inexpensive, high-throughput production.
Test samples with contaminations ranging from nanogrammes to microgrammes have been prepared and analysed over several weeks of storage. The influence of plot parameters on the morphology and durability of printed samples of various common explosives have been investigated on different substrates.
HPLC measurements were made to quantitatively evaluate the durability of printed samples. The morphology of the test samples was additionally characterized by optical microscopy and confocal Raman-microscopy. Besides the occurrence of polymorphic phase changes, described in literature for low concentrated drop-on-demand samples of RDX, we observed changes in sample distribution by recrystallization of some explosives in the printed samples.
This paper focuses on the optimization of methods for the preparation of test samples and on the analysis especially concerning the effect of polymorphic phase changes caused by inkjet printing of samples.