What is the difference between HPLC and GCMS?
HPLC is high performance liquid chromatography whereas GCMS is gas chromatography coupled to a mass spectrometric detector. The HPLC instrument is typically connected to a UV-visible absorption detector for analysis of cannabinoids, but other detectors are sometimes used for this and other applications. When a mass spectrometer is connected to a high-performance liquid chromatograph, the resulting instrument is typically abbreviated LCMS.
The liquid chromatograph provides separation of the components of a mixture by forcing a liquid through a narrow stainless-steel tube containing an absorbent that usually has hydrocarbon chains attached to the surface of small particles that have been packed into the tube. Substances dissolved in the mobile phase interact with the immobilized hydrocarbon chains if they have an affinity for them. The rate of their passage through the column is slowed by these interactions because they are not moving while they are attached to the hydrocarbon chains. Substances that do not interact with the hydrocarbon chains pass through the column rapidly and enter the detector where the molecules are measured. These substances are characterized by short retention times (i.e., the total time that they are in the chromatographic column). Two different substances that interact identically with the hydrocarbon chains will be characterized by the same retention time and will not be differentiated unless their absorption spectra differ. In the case of Delta-8-THC, Delta-9-THC, Delta-4(8)-iso-THC, and Delta-8-iso-THC, the absorption spectra are identical so they cannot be differentiated from each other unless their retention times differ. As noted earlier, they are not adequately resolved by reversed-phase chromatography so a different stationary phase or a different chromatographic method is required to resolve them.
Gas chromatography, in contrast to liquid chromatography, is an orthogonal technique to HPLC because the mode of chromatographic separation is completely different. In gas chromatography, a gas (known as a carrier gas) flows through a narrow but very long (15-30 meters) column that has been coated on its inner surface with a thin film of a stationary phase that contains immobilized functional groups with which molecules may interact. Those that interact weakly are characterized by short retention times whereas those that interact extensively are characterized by longer retention times. Substances are carried by the gas from the end of the column into the mass spectrometer where they are bombarded with a stream of electrons that cause the molecules to ionize and break into characteristic fragments that are measured by the mass spectrometer and reported as a mass spectrum.
Analysis of the cannabinoids Delta-8-THC, Delta-9-THC, Delta-4(8)-iso-THC, and Delta-8-iso-THC by gas chromatography on an appropriate column affords resolution of these substances and detection by mass spectrometry assures identification because the mass spectra are different although the molecular masses are identical.