Gas Chromatography Mass Spectrometry

GC-MS or Gas Chromatography Mass Spectrometry is a hybrid analytical technique that is used to perform the separation, purification, and identification of a substance simultaneously. The Gas Chromatography component is used to perform the separation and purification of the sample whereas Mass Spectrometry helps in the identification of the substance.

In GC-MS two different individual techniques are coupled together to analyze a single substance. Earlier the two techniques were used individually for analysis purposes. In 1950, Fred McLafferty and Roland Gohlke, two Dow Co. researchers, dramatically enhanced the analytical power of GC by coupling it with MS. 

Adding MS allowed each component exiting the gas chromatograph to be analyzed separately. When taken together, the mass spectra and the chromatographic peaks allowed unambiguous identification of each component.

Principle of Gas Chromatography Mass Spectrometry

GC-MS uses the principles of both instruments and consolidates them to give a single analytical result of the analyte. 

The sample is injected into the Gas chromatograph, where the analyte is first vaporized and made to run into the chromatographic column with the carrier gas such as helium. Inside the column, the separation of the analyte is carried out according to the polarity and affinity of the analyte with the stationary phase and mobile phase.

The column is attached to a mass spectrometer through a heated transfer line that passes the separated analyte into the ion source of MS. The ion source ionizes the sample particles and transfers them to the next situated mass analyzer.

The mass analyzer acts as a filter that separates the ions according to their mass-to-charge ratio, which is further detected by a detector. The signals received by the detector are amplified and recorded on a computer system in the form of spectral peaks.

Instrumentation of Gas Chromatography Mass Spectrometry

The instrument of GC-MS is composed of:

  • Gas Chromatograph
  • Sample Injection
  • Gas Reservoir
  • Thermostat Oven
  • Chromatographic Column (open-tubular or packed column)
  • GCMS Interface
  • Mass Spectrometer
  • Ion Source

The analyte from the chromatographic column is transferred to the ion source which ionizes the sample particles and converts them into ions. Electron ionization(EI) and chemical ionization(CI) are the main ionization methods used. 

In Electron Ionization, a beam of electrons ionizes the sample molecules resulting in the loss of one electron. A molecule with one electron missing is called a molecular ion and is represented by M+.

When the resulting peak from this ion is seen in a mass spectrum, it gives the molecular weight of the compound. Due to a large amount of energy imparted to the molecular ion, it usually fragments producing further smaller ions with characteristic relative abundances that provide a ‘fingerprint’ for that molecular structure.

This information may be then used to identify compounds of interest and help elucidate the structure of unknown components of mixtures. 

Chemical Ionization begins with the ionization of methane (or another suitable gas), creating a radical which in turn will ionize the sample molecule to produce [M+H]+ molecular ions. 

CI is less energetic and yields less information than EI.

  • Mass Analyzer

The mass analyzer separates the ions according to their mass-to-charge ratio. The various mass analyzers are ion trap magnetic sector, time-of-flight, radio frequency, cyclotron resonance, and focusing.

  • Detector 

The separated ions are traced by the detectors which are then amplified and recorded by the recorders. The most commonly used detectors are Photomultiplier tubes, Electron multiplier tube Micro-channel plate detectors, and Faraday cups.

Sample Preparation

For GC-MS the analyte needs to be prepared before injecting into GC. The sample is made free from any impurities, dirt, or any complex matrix so that they don’t interfere with the analyzing process. Depending on the sample matrix, the degree of selectivity required and the initial cleanliness of the samples, different sample preparation methods can be used.

The various methods applied include- headspace sampling, pyrolysis, solid-phase extraction, solid-phase micro-extraction, solvent extraction, stir bar sorptive extraction, accelerated solvent extraction, etc.

Types of Gas Chromatography Mass Spectrometry

Depending upon the level of selectivity and sensitivity required, GC-MS can be classified as follows:

1. Single Quadrupole GC-MS

When the GC is coupled with a single quadrupole, the instrument is called single quadrupole GC-MS. It is usually used for the analysis of samples that require targeted selected ion monitoring or untargeted full scan acquisition. 

2. Triple Quadrupole GC-MS/MS

When the GC is coupled with a triple quadrupole, it is called GC-MS/MS. It provides a higher level of selectivity and is best suited to analyze where the highest sensitivity is required.

3. HRAM GC-MS/MS

In this instrument, GC is coupled with High-Resolution Accurate Mass MS which is useful for the analysis of samples with the high-confidence compound discovery

Advantages of GC-MS

  • Highly sensitive
  • High resolution
  • A combination of the two instruments can easily detect the analyte selectively
  • Isomers having similar mass spectra can be distinguished by GC-MS

Disadvantages of GC-MS

  • The instrument is expensive and requires high maintenance
  • Leakage and contamination in the system are common
  • Sample activity and degradation affect the analysis adversely
  • Ion source easily gets contaminated

Applications of Gas Chromatography Mass Spectrometry

  • Medicine:- GC-MS is used in screening tests to detect any metabolic disorders in an individual.
  • Pharmaceutical Industry:- GC-MS is used to detect the impurities in ointments, lotions, and creams. It is also helpful in testing the quality of a drug.
  • Environmental Monitoring:- GC-MS is a useful technique to monitor the presence of pollutants in the air, water, and soil.
  • Food Industry– The analysis of various aromatic compounds such as fatty acids, alcohols, esters, and aldehydes present in the foodstuff is analyzed by GC-MS.
  • Fragrance Analysis– The presence of various types of oils such as lavender oil, essential oils, syrups, allergens, etc. in the sample can be analyzed by GC-MS.
  • Forensic Science:- GC-MS is a widely used technique for the toxicological analysis in the blood or any other such sample. Pesticide examination is also possible by the instrument. The presence of any steroid or doping agent in the blood can be detected by GC-MS. It is also useful in an explosive examination, gunshot residue examination, paint, fiber, metallic ions, etc.

Conclusion

GC-MS is a hyphenated technique that enables the separation, purification, and identification of an unknown substance. It is a widely used analytical method in various scientific fields. By coupling the two techniques, the instrument has reached a high resolution and sensitivity which is useful for the analysis of a substance.

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