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The Result - A Chromatogram

Chromatograph

Fume Hood Use #2 - Qualitative Analysis

• More modern chromatography, such as gas chromatography, can also determine how much of the substance is present using “peaks” that are produced on a chromatogram.

• A chromatogram is produced by marking the separation of the components. In modern gas chromatography, as each component comes off of the stationary column, a detector sends a signal to an attached computer. The computer displays each signal as a peak on a chart.

There are many types and uses of chromatography, but all forms contain two components or phases: the stationary phase and the mobile phase.

  • The stationary phase is placed in the column and stays still (for example, a piece of plastic) while the mobile phase is pumped in and carries the analyte (the mixture that needs to be separated) through the stationary phase.
  • Based on the chemical properties of the analyte, certain components travel quickly and others get held up and arrive more slowly, allowing them to be separated.

Fume Hood Use #1 - Liquid Extractions

Scientists use a specific order of chemical reactions to separate, eliminate, and then test for and identify many different drugs and poisons. This type of analysis is often used to detect poisons and contaminates in liquids. It can also be adapted to identify multiple poisons or contaminants at once.

• The goal of chromatography is baseline resolution – where each substance comes completely through the instrument before the next one comes out.

• Example Procedure – Chemical SEPARATION of an unknown by liquid phase extraction …

Liquid extraction involves the separation of two or more substances in an analyte (substance being analyzed). Using polarity and/or pH, scientists use different solvents to attract different types of chemicals. The solvents are mixed with the unknown to separate parts of the liquid mixture. Then one solvent can be “poured off” to separate the parts of the liquid for further analysis.

Centrifuge

• Used for physical separation, based on density.

• The centrifugal force created by spinning separates mixtures based on particle size and differences in density. Solids tend to be heavier and fall to the bottom, while liquids tend to go to the top of the tube.

• Many techniques in chemical analysis and forensic biology call for centrifugation. In a lab, specific centrifuges will be set for certain speeds and often dedicated to one type of procedure.

• Examples of use: DNA concentration, separating components of blood and urine prior to drug testing.

Chromatography - Stationary and Mobile Phase

Liquid Phase Extraction

Solid – Phase Extraction Manifold

Solid - Phase Extraction

• A preparation process that separates dissolved substances from other compounds in the mixture according to their physical and chemical properties

• Solid Phase Extraction uses the attraction/affinity of solutes dissolved or suspended in a liquid (known as the mobile phase) for a solid through which the sample is passed (known as the stationary phase) to separate a mixture into desired and undesired components.

The result is the sample is "cleaned" for further analysis.

• Solid phase extraction can be used to isolate (clean) analytes of interest from a wide variety of samples, including urine, blood, water, beverages, soil, and animal tissue.

This can also be used to identify organic explosives on plastic surfaces, because it does not require that heat be used to cause components to vaporize.

Conclusion

Chemistry in the Forensics Lab

Common Uses for Chromatography in Forensics

• Common uses – separating drugs such as cocaine from the cutting agents, for accelerants in fire residues, and for blood alcohol tests (to determine the amount of alcohol present).

  • Gas chromatography is NOT used for explosive residues. High temperatures are required to convert the analyte components to vapors and then the vapors are pushed through the stationary phase by another gas. Those high temperatures would cause explosive residues to... explode.

• The two main goals of a chemist in a forensic lab are ...

  • separation/isolation of chemicals in an unknown substance and
  • identification of each chemical in an unknown substance.

Many time chemical evidence at the scene in in a mixture. There are many cases where forensic chemists are called upon to identify one or more components of a mixture. Often it is necessary to separate the mixture into individual substances before identifying them.

Technology has greatly enhanced the

ability of forensic scientists to develop

data that helps solve crimes - a fume hood,

centrifuge, chromatography machines, solid-phase extraction manifolds, and spectrophotometers are all essential to a

functional forensics lab.

Fume Hoods

Spectrophotometry

• Fume hoods are one of most important pieces of lab equipment in forensic labs, especially as most of the chemicals brought into the lab are “unknowns”, to ensure safety.

• For working with gases, acid fumes, and chemical vapors

• Designed to capture and exhaust (expel) contaminates resulting from working with chemicals. The design creates a capture zone in front of the hood to draw contaminates away from the worker and into the hood where they are exhausted/expelled.

• A mass spectrophotometer is widely used in forensic labs because it can IDENTIFY each component as it emerges from a

gas chromatography. NOTE - These two machines are often combined as the GCMS.

• In a mass spectrophotometer, the atoms of a chemical are evaluated based on the atomic mass.

• You only need a very small sample for a mass spectrophotometer, which is a tremendous

advantage.

• Each chemical would have a unique mass spec diagram, so scientists can use this to compare two samples to see if they match (such as fibers from a crime scene and fibers from the home of a suspect), or to compare an unknown sample to tested samples of known drugs, accelerants, poisons, etc. to identify a chemical. Entire databases of "known" chemicals allow scientists to make many identifications of chemicals found at a crime scene.

Forensic Chemical Identification

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