Instrumental analysis method of the hottest paper

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Instrumental analysis method of paper micro samples

Abstract: paper physical evidence is one of the common physical evidence in forensic science. The purpose of testing paper in court science laboratory is to narrow the scope of investigation, provide clues and directions for the detection of cases, and provide scientific basis for the confirmation of crimes. There are many methods to test paper evidence in forensic science, but most of them are unconventional micro instrumental analysis

key words: forensic science; Paper evidence; Instrumental analysis

is closely related to paper in various criminal activities involving handwriting materials. In the investigation of such criminal activities, the examination of paper plays an important role

there are similarities and differences between the inspection or identification of paper in forensic science and the inspection of finished paper by the paper industry department. The reason is that the final focus is different. Because cases are diverse, the nature of different cases requires different levels of inspection or identification. The purpose of paper inspection in forensic science is that the laboratory should use appropriate methods and procedures to carry out all testing work and other relevant business activities within its scope of responsibility (including sample extraction, disposal, transmission, storage and preparation, which reduces the scope of investigation, provides clues for the detection of cases, points out the direction, and provides a scientific basis for the confirmation of crimes. Because the paper samples provided by the case are limited, unconventional microanalysis is required in the test method. At present, many instrumental analysis methods suitable for the scientific and technological requirements of the court have been developed, such as thin-layer chromatography and high-performance liquid chromatography Plasma emission spectrometry, X-ray diffraction, etc

1 thin layer chromatography

the main component of paper is pulp, as well as a small amount of rubber, color and filler. As for the ingredients of glue, pigment and filler in paper, due to the different sources of raw materials used by various manufacturers, there are also differences in formula and production technology, so the organic components of the same kind of paper produced by different manufacturers must be different. This is the theoretical basis for us to use thin-layer chromatography to examine paper evidence

when testing paper by thin-layer chromatography, generally use silica gel gf254 fluorescent thin-layer plate, select appropriate extractant for extraction (usually ethanol), and conduct thin-layer chromatographic analysis after the extraction solution is concentrated. The commonly used developing agents are:

V benzene: V methanol =95:51 or V benzene: V methanol =90:5

if the TLC of two paper samples is different, it can be determined that the two paper samples are different kinds of paper or the same kind of paper produced by different manufacturers under the condition that the samples are free of pollution. If the thin-layer chromatography of two paper samples is the same, it is generally the same paper. Whether it is the paper produced by the same manufacturer, we also need to use the thin-layer scanner. Determine the relative content of each component of the paper sample, or use other methods for further analysis

2 high performance liquid chromatography

some of the same kind of paper produced by different places and manufacturers are not only the same in appearance, physical properties and fiber composition, but also basically the same in other organic components, which is difficult to distinguish and identify by general thin-layer chromatography. However, there are certain differences in the relative content of organic components in these papers. Using high performance liquid chromatography is an effective method to analyze the organic components in paper, such as pigment and rubber. Measuring the relative intensity of main chromatographic peaks can further identify different manufacturers and batch numbers of the same kind of paper

using high-performance liquid chromatography to test paper not only does not need to crack the sample, but also has a wide range of mobile phases. C18 reversed-phase column is used for analysis, methanol/water or acetonitrile/water is used as mobile phase, and the detector is UV detector with detection wavelength of 254nm. The specific method is as follows: cut 5cm2 paper sample, then cut into small pieces, add 1ml absolute ethanol, soak for 10h, and take the supernatant for chromatographic analysis. The experimental results show that the relative content of organic components of the same kind of paper from different manufacturers and different batches are different to varying degrees

3 atomic emission spectrometry

atomic emission spectrometry is generally used to analyze the elemental composition of paper, especially the types and relative contents of metal elements in paper. Then we can distinguish the types of paper and manufacturers. If ordinary atomic emission spectrometry is used, the paper sample needs to be ashed and mixed with carbon powder, and then put into the electrode slot, and then excite the sample to measure the wavelength and intensity of the emitted spectral line

q24 medium-sized spectrograph and AC arc voltage commonly used in China

there are two specific operation methods: the first method is to cut the paper into thin strips and clamp one end of the strip with self-made spectral pure graphite tweezers. When an arc is formed between the electrodes, the other end is slowly sent to the gap of the graphite electrode for spectrography. The second method is to cut the paper sample into pieces, put it in a quartz crucible, ash it at 500 ℃ in a Mafu furnace, and then heat it with 2ml6mhcl to dissolve it. After concentration, the solution is added to the flat head electrode, which is pretreated with 2% polystyrene benzene solution to prevent the sample solution from penetrating into the electrode. After the sample solution evaporates dry on the electrode, the spectrum can be taken. The second method has high analytical reproducibility

development and fixing: use the red extra hard or UV II dry plate produced by Tianjin photographic film factory, use developer D-11, develop for 5min (20 ℃), fixer F5, fix for 20min (room temperature), and then wash with water

analysis: the spectral line on the dry plate qualitatively determines the existence of elements according to the comparison of ferrography, and the relative content of each element is determined semi quantitatively according to the blackness of the spectral line, and a special development plan is formulated. Emission spectrometry can detect more than a dozen elements in paper, but the constant element is for the detection of some materials. We still need to pay attention to the dynamic elements of our company: calcium, magnesium, aluminum, iron and silicon, which reflect the characteristics of common paper fillers. When zinc oxide (ZnO), barium sulfate (BaSO4) and titanium dioxide (TiO2) are used as fillers, the content of zinc, barium and titanium is higher

in recent years, inductively coupled plasma atomic emission spectrometry has been used in the analysis of paper. Experiments show that more than 20 kinds of common elements in paper can be qualitatively and quantitatively analyzed by plasma emission spectrometry, and the purpose of distinguishing similar paper can be achieved

the specific operation method is as follows: take a 50cm2 paper sample, weigh it and cut it into small pieces, place it in a grinded conical flask, soak it in 1ml of 10% HCl solution for 2h, then add 4ml of deionized water, place it for about 20h, filter it and dilute it to 10ml before the test. Each sample is tested three times and the average value is taken

element content calculation and data analysis: the instrument can obtain the quantitative curve after the determination of standard samples. After the determination of paper samples and comparison samples, the concentration value expressed in ppm can be obtained, and then the percentage content can be converted

calcium, magnesium, aluminum, iron, silicon, potassium and sodium are common elements in paper, with the percentage content of more than 0.5%, while other elements are small or trace elements in paper

4 X-ray diffraction analysis method

monochromatic X-ray incident on the crystal plane of the sample can produce a diffraction pattern. The position and intensity of diffractive rays are closely related to the nature, composition and phase of the sample, and the intensity is directly proportional to its content. The inorganic fillers contained in the paper, such as talc powder, clay, titanium dioxide and magnesium sulfate, can form their own diffraction rays. Due to the different manufacturing processes of different manufacturers, the sources of inorganic fillers used are different, and the varieties and quantities used are not exactly the same, and the crystal structure and relative content of their components are also different. Therefore, the X-ray diffraction analysis method can be used to distinguish the same kind of paper produced by different manufacturers according to the position of the diffraction rays and the relative intensity between them, and can further identify the paper samples that are difficult to distinguish by other methods. When conditions are ripe, production time and batch number can also be distinguished

x-ray diffraction analysis belongs to non-destructive testing, which can be carried out directly on paper. The condition of paper inspection by this method is to use an automatic diffractometer, select a copper target tube, and the wavelength is 15418 × 10-14m, target voltage 40kV, target current 40mA, continuous scanning interval 3 ° ~60 °, scanning speed 4 ° per minute. Cut the paper sample into 4cm2 and put it directly on the sample rack. The amount of filler can be as low as one thousandth. When analyzing unknown objects, standard spectrograms can be used for retrieval

if the paper inspection materials allow, we should use as many methods as possible to systematically analyze and test the paper physical evidence. In this way, various test results can complement and confirm each other, so as to improve the accuracy and reliability of the identification conclusion. Anodized aluminum: coloring the metal. Various inspection methods of paper have a certain scope of application, and each has its advantages and disadvantages. In practical work, we should formulate reasonable inspection procedures according to the specific conditions and inspection requirements of the inspection materials, as well as the existing instruments and equipment and experimental conditions. Usually, nondestructive testing is done first, followed by destructive testing. Due to the wide variety of paper, paper mills all over the world, and a large range of use, the existing paper inspection methods generally can not make the same conclusion, but can only identify the type of paper. In the case of comparison samples, the place of origin, brand and batch number can be determined; Within a specific range, the same determination can be made

(Jiang Hong)

source: Zhonghua paper

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