page_title: Forensic chemistry
section_title: Chromatography
hierarchical_section_title: Forensic chemistry / Methods / Chromatography
caption_attribution_description: English: HPLC readout for acetaminophen, aspirin, and caffeine mixture
context_page_description: Forensic chemistry is the application of chemistry and its subfield, forensic toxicology, in a legal setting. A forensic chemist can assist in the identification of unknown materials found at a crime scene. Specialists in this field have a wide array of methods and instruments to help identify unknown substances. These include high-performance liquid chromatography, gas chromatography-mass spectrometry, atomic absorption spectroscopy, Fourier transform infrared spectroscopy, and thin layer chromatography. The range of different methods is important due to the destructive nature of some instruments and the number of possible unknown substances that can be found at a scene. Forensic chemists prefer using nondestructive methods first, to preserve evidence and to determine which destructive methods will produce the best results.
  Along with other forensic specialists, forensic chemists commonly testify in court as expert witnesses regarding their findings. Forensic chemists follow a set of standards that have been proposed by various agencies and governing bodies, including the Scientific Working Group on the Analysis of Seized Drugs.
context_section_description: Spectroscopy techniques are useful when the sample being tested is pure, or a very common mixture.  When an unknown mixture is being analyzed it must be broken down into its individual parts.  Chromatography techniques can be used to break apart mixtures into their components allowing for each part to be analyzed separately.  
  Thin layer chromatography (TLC) is a quick alternative to more complex chromatography methods. TLC can be used to analyze inks and dyes by extracting the individual components.  This can be used to investigate notes or fibers left at the scene since each company's product is slightly different and those differences can be seen with TLC.  The only limiting factor with TLC analysis is the necessity for the components to be soluble in whatever solution is used to carry the components up the analysis plate.  This solution is called the mobile phase.  The forensic chemist can compare unknowns with known standards  by looking at the distance each component travelled.  This distance, when compared to the starting point, is known as the retention factor (Rf) for each extracted component.  If each Rf value matches a known sample, that is an indication of the unknown's identity.
  High-performance liquid chromatography can be used to extract individual components from a mixture dissolved in a solution.  HPLC is used for nonvolatile mixtures that would not be suitable for gas chromatography.  This is useful in drug analysis where the pharmaceutical is a combination drug since the components would separate, or elute, at different times allowing for the verification of each component.  The eluates from the HPLC column are then fed into various detectors that produce a peak on a graph relative to its concentration as it elutes off the column.  The most common type of detector is an ultraviolet-visible spectrometer as the most common item of interest tested with HPLC, pharmaceuticals, have UV absorbance.
  Gas chromatography (GC) performs the same function as liquid chromatography, but it is used for volatile mixtures.  In forensic chemistry, the most common GC instruments use mass spectrometry as their detector.  GC-MS can be used in investigations of arson, poisoning, and explosions to determine exactly what was used.  In theory, GC-MS instruments can detect substances whose concentrations are in the femtogram (10⁻¹⁵) range.  However, in practice, due to signal-to-noise ratios and other limiting factors, such as the age of the individual parts of the instrument, the practical detection limit for GC-MS is in the picogram (10⁻¹²) range.  GC-MS is also capable of quantifying the substances it detects; chemists can use this information to determine the effect the substance would have on an individual.  GC-MS instruments need around 1,000 times more of the substance to quantify the amount than they need simply to detect it; the limit of quantification is typically in the nanogram (10⁻⁹) range.
caption: HPLC readout of an Excedrin tablet.  Peaks from left to right are acetaminophen, aspirin, and caffeine.
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