Gas chromatography-mass spectrometry (GC-MS) is an analytical technique that couples the features of gas chromatography with that of mass spectrometry to identify different constituents within a sample mixture.
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GC-MS has a broad range of applications some of which include environmental analysis, drug detection, forensic investigations, airport security and identifying unknown samples.
GC-MS is generally accepted as the “gold standard” for forensic analysis and identification of samples due to its specificity in identifying a sample even of small sample volumes.
Gas chromatography (GC) contains a carrier gas, usually helium, nitrogen or hydrogen, known as the mobile phase. Helium gas remains the most commonly used carrier gas of GC instruments although hydrogen gas is used for improved separations.
The stationary phase is a microscopic layer of liquid or polymer on an inert solid support, inside a glass or metal capillary tubing called a column. The gaseous sample being analyzed interacts with the stationary phase within the column, which exposes each constituent in the mobile phase to the stationary phase.
Each constituent in the mobile phase will have a different retention time on the stationary phase within the column and will elute at different times. This is similar in principle to HPLC except for the mobile phase and stationary phase for HPLC techniques is a liquid and solid, respectively.
Mass spectrometry measures the mass to charge ratio of ions of a sample that can be solid, liquid or gaseous. The sample is ionized by subjecting the sample to a bombardment of electrons. The resulting molecules in the sample become charged and either fragment or retain their whole structure.
These charged fragments or non-fragmented molecules are separated according to their mass to charge ratio. This involves subjecting them to an electric or magnetic field and calculating the deflected path of the charged molecule to a detector.
The resulting spectrum is the signal intensity of detected ions as a function of the mass to charge ratio. It is used to determine the masses of fragments or non-fragmented molecules and to elucidate the chemical structure.
The hyphenated technique (GC-MS)
GC-MS is a hyphenated technique that combines the two methods of gas chromatography (GC) and mass spectrometry (MS). As discussed above, GC uses a capillary column to separate constituents in the sample by forcing a mobile phase containing the sample mixture through the length of the column containing the stationary phase.
Each constituent in the sample will have a different affinity for the stationary phase and will, therefore, elute from the column at different times known as the retention time. Each component of the molecule that gets eluted at different times from the column is captured by the mass spectrometer downstream, which is then ionized, subjected to an electromagnetic field and gets deflected to a detector.
The intensity of each charged fragment and non-fragmented component is calculated as a function of the mass to charge ratio. By coupling these two instruments together, it allows for more accurate identification than either instrument used independently.
The disadvantage of using mass spectrometry on its own is that it relies on the purity of the sample and that there is the possibility of two different molecular fragments sharing a similar ionization pattern.
In contrast, the disadvantage of using GC on its own is that typically it cannot differentiate between multiple molecules that have the same retention time and therefore elute at the same time.
Hence, combining the two instruments reduces the possibility of error and increases the accuracy of identifying the molecule of interest in the sample. Therefore, if a mass spectrum identifies a molecule that has a characteristic retention time in the GC, it increases confidence that the two techniques combined are identifying the constituent of interest in the sample.
Types of detectors for mass spectrometry
Typical detectors for GC-MS is the quadrupole mass spectrometer. This consists of four cylindrical parallel rods that use oscillating electrical fields to selectively stabilize or destabilize the path of charged molecules passing through the field created by the quadrupole.
Charged molecules are selectively passed through the system that has a mass to charge ratio of a certain range. The change in potential of the rods allows for a wide range of charged molecules to be rapidly swept through the system.
A quadrupole is effectively a mass selective filter and is similar to an ion trap, however, the quadrupole mass analyzer is designed to allow ions to pass through the system compared to an ion trap that collects the trapped ions.
Another common detector is time of flight (TOF), which uses an electric field to accelerate the ions through an electric field, and then calculates the time taken to reach the detector.
Thus, particles with the same charge will have identical energies and their velocities will only depend on their mass. This means that the first ions to reach the detector will be those with lower mass.
Applications of GC-MS
GC-MS has a broad range of applications covering many scientific disciplines. GC-MS is commonly used in environmental science subjects to quantify the levels of organic contaminants. Specifically, it has been to distinguish the varying types of hydrocarbons in a sample for monitoring and bioremediation purposes.
GC-MS has been used for forensic toxicology and criminal investigations as well as for law enforcement for drug detection.
The technique has also been used for explosives testing, anti-doping and detecting for performance-enhancing drugs and has also been used for astrochemistry for the analysis and investigation of samples taken from other moons in our solar system.
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