Application of VMD in forensic science
Vacuum metal coating technology
The method of vaporizing or sputtering metal, alloy or compound in vacuum and making it solidify and deposit on the coated object is called vacuum coating. Currently, vacuum evaporation and sputtering coating are widely used in industrial fields.
The real application of vacuum coating technology was after the introduction of oil diffusion pump and mechanical pump in 1930. In 1935, the anti-reflection film was developed by vacuum deposition technology, and in 1945, it was applied to the coating of eyeglasses. Along with the continuous development of science and technology, vacuum coating technology and equipment developed rapidly and widely used in industrial applications, including architectural hardware, locks, tabulation, large work piece goods such as uniform coating, etc., in order to achieve such as increase the rate of light transmittance of optical lens or reflective, make hardware, locks, plastic products such as beautiful, durable, corrosion resistance, etc.
In the field of forensic science, coating materials generally use gold, zinc, silver and other metals, so it is also known as vacuum metal coating technology (VMD).
Application of vacuum metal coating technology in forensic science
Vacuum coating was first applied to forensic science in 1970 by the police science development department (PSDB), part of the HOME OFFICE, which successfully demonstrated the potential fingerprints of impenetrable objects such as polymer-material packaging films by evaporating gold and zinc. The UK also became the first country in the world to apply vacuum metal coating technology to physical evidence inspection.
In 1998, Fingerprint Development technology, compiled by police science Development department (PSDB), was regarded as the bible of Fingerprint Development technology, and it was widely used by police all over the world as a manual of Fingerprint Development standardization. The technical means and process of developing handprint by vacuum metal plating film are described in detail.
In 2014, the newly established CAST, Centre for Applied Science and Technology of the UK interior ministry re-revised Fingerprint Development Techniques, and compiled the latest Fingermark Visualisation Manual (ISBN: 978-1-78246-234-7) combining a large amount of on-site exploration experience and scientific research results. The manual appeared not only to the 1998 version of the print process has carried on the elaboration, and metal of vacuum coating technology court science and conducted a more detailed analysis, including the applicable object types, coating process, the different coating process of setting up the vacuum degree of cavity, the evaporation boat materials, specifications, etc., make inspection personnel appeared in the conditions and procedures for as long as strictly according to the manual operation can be realized the best coating effect.
*CAST is a core department of the UK government, consisting of more than 200 scientists and engineers, providing expert technical services, technological innovations and cutting-edge technical support from a scientific and technological perspective.
In recent years, vacuum coating equipment used in hand print development has appeared in the domestic market, most of which are modified industrial vacuum coating equipment. However, the vacuum coating equipment for industrial use is not designed for material evidence inspection, so it cannot meet the requirements of finger print development in the control of coating process. The basic principle of vacuum metal plating fingermark developing equipment for forensic science is also the potential fingermark on one metal or multiple metal developing material evidence under closed and high vacuum environment (10-4 millibar level). However, there are many essential differences between its product design and industrial vacuum plating equipment.
Excellent vacuum metal-plated fingerprinting equipment for forensic science shall have the following characteristics:
*Control the vacuum of the chamber according to the use of metals (such as gold, zinc, etc.).
* when the development process from gold plating to galvanized, can automatically adjust the vacuum in the cavity, to ensure the best development effect. Different metal atoms fly at different speeds in a high vacuum environment and have different ability to attach to the object surface. It is necessary to accurately control the vacuum degree in the cavity according to the atomic number of the metal. Too high or too low vacuum degree will affect the development effect.
* can ensure a better manifestation effect on different objects of physical evidence, including impervious object, semi-permeable object and permeable object. The physical evidence extracted in the field is rich and varied. Various objects do not have the characteristics of smooth surface of substrate used in industrial applications, but the surface is rough and uneven. Therefore, it is impossible to use the film thickness gauge technology of industrial products to develop the fingerprint by measuring the film thickness.
* it is equipped with a large observation window and a lighting source with high brightness and high color temperature in the cavity. By placing control samples in the cavity, the development effect is observed at all times to prevent excessive or insufficient development.
* does not affect subsequent DNA testing and prevents cross-contamination. If the left conditions of fingerprints are not good, we should consider the development of fingerprints after DNA testing. At this point, the precise control of vacuum in the cavity is the key, too high vacuum will cause the loss of shed cells. At the same time, in order to prevent cross-contamination of physical evidence, anti-corrosion materials should be used on the inner surface of the cavity, so that chemical and physical methods can be used to clean and inactivate DNA, without causing cross-contamination of DNA for the next appearance of physical evidence.
* abundant fixed devices for material evidence, suitable for different forms of material evidence; The evaporator boat in the cavity must be designed in the center to ensure uniform coating and achieve the best development effect.
* easy to operate, without complicated mechanical control, through a simple menu control can complete the fingerprinting process.
* high reliability of equipment. High vacuum degree in the cavity is a necessary condition for coating, so high performance, stable and reliable vacuum pump is a key part of vacuum coating equipment. High-end vacuum coating fingerprinting equipment is equipped with multi-stage vacuum pump, including mechanical pump and oil diffusion pump, which can achieve a vacuum degree of 10-4 millibar in a few minutes.
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