In order to begin analysis of suspected substances, the elements must first be separated into organic and inorganic compounds. Organic compounds contain carbon in combination with hydrogen, nitrogen, sulfur, chlorine, and bromine. Most of the compounds analyzed in crime laboratories are organic.
SPECTROPHOTOMETRY
This analysis identifies substances by their absorption of different wavelengths of light. Spectrophotometry is a basic tool which requires the substance to be in an almost pure form. For this reason, it is not used as often anymore, since it's hard to get a substance in that state.
CHROMATOGRAPHY
This technique is used to separate the different mixtures of a substance. There is both a liquid phase and a gas phase, as well as a moving phase and a stationary phase. The molecules will separate into liquid (stationary) phase and gas (moving) phase. The materials in the moving (gas) phase will move forward at a faster rate than those in the stationary (liquid) phase, finishing the testing at different times. There are three types of chromatography: Gas Chromatography (GC), High-Performance Liquid Chromatography (HPLC), and Thin Layer Chromatography.
Gas Chromatography
" Gas Chromatography permits rapid separation of complex mixtures into individual compounds." A written record (chromatogram) is kept. The chromatogram will show peaks corresponding to the individual components. It can take a drug, which may be a mixture of several different components and break it down to each individual one. With this process, the sample being analyzed must first be vaporized then sent through a heated column. This is not a definitive test.
Mass Spectrometry
The biggest draw back to the Gas Chromatography, is that it is not a definitive (specific) test. However, when combined with Mass Spectrometry, it becomes definitive. The separation of the compounds are first done in the Gas Chromatography. After that's completed, it will flow into the Mass Spectrometry. This is a high vacuum chamber where the components will be hit with high energy electrons. The molecules then lose electrons, pick up positively charged ions, and break apart into smaller fragments. The smaller fragments then pass through a magnetic field where they will separate according to their individual masses. No two substances produces the same fragmentation pattern, so this test produces a specific "fingerprint" pattern.
High-Performance Liquid Chromatography (HPLC)
With HPLC, " As the liquid moving phase is pumped through the column, a sample is injected into the column." As the liquid is pumped through the column, it is separated into different components. The advantage to this technique is that it can be done at room temperature. This technique is good for analyzing heat sensitive explosives or drugs.
Thin Layer Chromatography
This techniques uses the solid stationary phase and the liquid moving phase to cause the separation of compounds. A thin layer plate is coated with a granular material and becomes the stationary phase. It's held into place with a bonding agent, such as plaster of paris, then placed upright in a closed chamber that holds liquid. Once separation occurs, the plates are either put under ultra violet lights in order to visualize them, or sprayed with a chemical reagent that causes the material to show up as spots. If the suspect sample and the control materials travel the distance at the same time, then suspected sample material will be identified as matching the control. This is not a definitive test though because there's always the chance that other substances can migrate up the plate in the same way.
ELECTROPHORESIS
Electrophoresis separates protein mixtures according to their migration rates. It does this by using electric current attached to a glass plate which forces the molecules to migrate across the plate. The proteins will migrate according to their electrical charge and size. The proteins in the blood, as well as DNA fragments, can be separated this way. DNA movement goes according to size. Smaller fragments will move across the plate at a faster rate than larger fragments. Once separated, they will show up in band patterns.
Republished from original Forensics Talk weblog
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