Forensic Chemistry - Methods

Methods

One particularly useful method for the simultaneous separation, identification, and quantitation of one or more individual components of an unknown substance or mixture is the use of a gas chromatograph-mass spectrometer (GC-MS). A GC-MS is actually two instruments that are attached together physically, and together comprising one of the so-called "tandem" or "hyphenated" techniques.

The gas chromatograph (GC) is essentially a hot (150-350°C), temperature-controlled oven holding a bent or coiled, specially packed or coated glass column between one and a few dozen meters long. A small volume (typically a few microliters) of a drug sample or other unknown substance that has been dissolved in an organic solvent (such as chloroform or methanol) is quickly injected into the hot column. Volatile components in the sample are vaporized by the heat of the oven and are forced toward the end of the column by the flow of an inert "carrier gas" (typically helium). The special chemical component(s) within the column bind to substances contained in the moving vaporized sample mixture with slightly different force. As a result, different substances eventually are "eluted" (i.e. emerge from the end of the column) in differing amounts of time, which is known as the "retention time". The retention time of various components so eluted can then be compared to those of known standard molecules eluted using the same method (column length/polarity, flow rate of carrier gas, temperature program). While this comparison provides (presumptive) identification of the presence of a particular compound of interest in the unknown sample, in general the GC portion of the technique is used as a separation and quantitation tool, not an identification tool.

To provide positive identification of the sample components, the column eluent is then fed into a mass spectrometer ("MS"). These highly complex instruments use one or more methods (bombardment with electrons, high heat, electrical force) to break apart molecules into ions. These ions are separated by their mass, commonly with the use of a quadrupole mass analyzer or quadrupole ion trap, and detected by an electron multiplier. This provides a distinctive fragmentation pattern, which functions as a sort of "fingerprint" for each compound. The resulting patterns are then compared to a reference sample for identification purposes.