Gas chromatography (GC)

DEFINITION: Separation technique used in the qualitative and quantitative analysis of evidence samples.

SIGNIFICANCE: Gas chromatography is widely used in forensic science for the identification of the specific makeup of a variety of different types of physical evidence, including drugs, body fluids, and chemical residues found in fire debris.

A basic gas chromatography (GC) instrument (gas chromatograph) consists of an injector, a column that is housed in an oven, and a detector. The inside of the column is coated with a thin layer of liquid known as the stationary phase. One end of the column is connected to the injector, and the other is connected to the detector. The mobile phase is a flow of pressurized gas, known as the carrier gas, that feeds into the injector and continually flows through the column and into the detector. A syringe is used to inject the sample (typically liquid) into the injector, which is heated to vaporize the sample. The vaporized sample then travels through the column in the flow of carrier gas.

Chemical substances in the sample are separated based on differences in attraction between the mobile phase (carrier gas) and the stationary phase (inside the column). Of these analytes, those that have greater attraction for the mobile phase will travel quickly through the column and reach the detector. Analytes with greater attraction for the stationary phase will spend more time retained on the column, and so their travel through the column to reach the detector will be retarded. Analytes are thus separated and arrive at the detector in order of increasing attraction for the stationary phase. Temperature and flow rate also significantly influence the separation.

The time it takes for an analyte to travel from the injection port to the detector is called the retention time of the analyte. A variety of different detectors are available, but the flame ionization detector (FID) is among the more common. The FID uses a flame to ionize the analyte, resulting in an increase in electric current, which is recorded as the detector response. For the FID, the response is proportional to the concentration of the analyte present. Other detectors include the nitrogen-phosphorus detector, which is sensitive to molecules containing nitrogen or phosphorus, and the mass spectrometer (MS) detector, which allows conclusive identification of analytes with the support of extensive MS spectra libraries for effective interpretation of the spectra. Gas chromatographs are paired with mass spectrometers for a wide variety of applications, such as in the analysis of small, volatile molecules via the separation of a mixture’s different components. They are also used to detect compounds by analyzing the relative gas chromatographic retention times and elution patterns of a mixture along with the mass spectral fragmentation patterns of a compound’s chemical structures.

A computer records the retention time and detector response for each analyte. This information is plotted in the form of a graph (known as a chromatogram), with detector response on the y-axis and retention time on the x-axis. Forensic scientists use chromatograms to identify analytes based on the comparison of retention time with known standards analyzed under exactly the same conditions. Comparison of retention times alone is not sufficient for conclusive identification of analytes; however, given the nearly infinite number of possibilities, it is conceivable that two or more analytes will have the same retention time.

Gas chromatography is widely used in forensic science to identify the chemical substances that make up particular drugs, whether in samples seized by police or in body fluids submitted for analysis. Similarly, it can be used in postmortem analysis to detect the presence of alcohol, drugs, or toxic compounds in the body at the time of death. The technique is also used in explosives analysis and fire debris analysis to determine the presence of any ignitable liquid residues, which are indicative of deliberately set fires. Forensic scientists likewise employ it in testing other crime scene evidence, such as inks, body fluids, hair, or fibers, in order to identify possible suspects or to develop theories regarding a victim’s or perpetrator’s prior whereabouts. Research studies have also been conducted to examine existing forensic practices, such as the application of cyanoacrylate gas to the inside of plastic zip-top bags containing drug samples, as well as to develop new techniques. Fire debris analysis commonly follows standardized Gas Chromatography-Mass Spectrometry (GC-MS) approaches such as ASTM E1618 for ignitable liquid residue identification. Other GC methods used in operational forensic fire debris procedures include Headspace (HS) sampling for volatiles (e.g., ethanol/solvents/accelerants) and split/splitless capillary inlets; sometimes thermal desorption for traces.

Bibliography

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