Gas Chromatograph-Mass Spectrometer (GC-MS)
Gas Chromatography-Mass Spectrometry, abbreviated as GC-MS, is an analytical technique based on the use of a gas chromatograph coupled with a mass spectrometer.
This technique is one of the most advanced analytical methods and allows for the identification and quantification of organic substances in a variety of even complex matrices.
The concept of coupling arises from the fact that these two techniques perform complementary analytical functions:
> The gas chromatograph enables the separation of complex mixtures present in the sample,
> The mass spectrometer acts as a detector, allowing for the characterization and quantification of compounds.
The use of a mass spectrometer as a detector for gas chromatography offers numerous advantages:
- It is extremely specific in selecting ions for quantification.
- It is one of the most sensitive detectors overall.
- It is highly versatile.
Implementing GC and MS techniques requires adapting the characteristics of chromatographic and spectrometric instrumentation to achieve a sufficient level of compatibility.
Firstly, the chromatographic column operates under pressure, while the mass spectrometer operates under high vacuum. Additionally, the flow of chromatographic columns, especially packed ones, is crucial and not negligible, particularly in the context of liquid chromatography.
Another extremely important aspect arises from the need to scan very quickly: analytes must be detected as soon as they exit the column, or there is a risk of nullifying the separation. Furthermore, any column purging can create many issues.
The block diagram of the instrument
A schematic representation of a modern GC/MS includes: a chromatograph, a mass spectrometer, and a data collection and analysis system (Data System) that constitutes the system for the analysis and separation (qualitative and quantitative) of complex mixtures.
The chromatograph consists of an injector (auto-sampler), a system for controlling the column temperature, and a transfer line that allows the column effluent to enter the mass spectrometer.
The mass spectrometer consists of an ionization chamber (ion source), a mass analyzer (quadrupole analyzer), and an ion detector; all maintained under high vacuum by diffusion pumps supported by molecular pumps.
The sample to be analyzed is subjected to the flow of a transport gas (usually inert) and introduced into a chromatographic column. Within the column, substances capable of separating the various components of the gaseous mixture are present. At the column's exit, one or more detectors signal the arrival of different components to a processing and analysis system. The final product is a gas chromatogram in which the analyzed mixture is broken down into its various components.
The principle underlying mass spectrometry is the ability to separate a mixture of ions based on their mass-to-charge ratio, generally through static or oscillating magnetic fields. This mixture is obtained by ionizing the molecules of the sample, mainly by having them pass through a beam of electrons with known energy. The ionized molecules are unstable and fragment into lighter ions according to typical patterns based on their chemical structure.
The diagram that shows the abundance of each ion as a function of the mass-to-charge ratio is the so-called mass spectrum, typical of each compound as it is directly correlated to its chemical structure and the ionization conditions to which it has been subjected.