Gas chromatography–mass spectrometry

Gas chromatography -mass spectrometry is the coupling of a gas chromatograph (GC) with a mass spectrometer (MS ) (total procedure or device pairing shortening also: GC -MS or GC / MS or GC-MS ). In this case, the gas chromatograph for separating the mixture of substances to be examined and the mass spectrometer for identification and possibly also the quantification of the individual components is used. The separation column of a gas chromatograph consists of a thin (diameter 3-6 mm) stainless steel or glass tube or in most modern systems, of a 15 to > 100 m long fused silica or glass capillary. The former columns are operated as so-called packed columns and find even today often use the so-called process gas chromatography. The capillary columns are normally used in analytical testing of highly complex mixtures of substances (see below). For details on the types of columns and stationary phases ( separating liquids) used may be found in the contribution of gas chromatography. The column flow through the inert carrier gases such as nitrogen or helium as the mobile phase in the so-called temperature- oven chamber. In this gas stream, the vaporized fuel mixture is injected via the heated injector or injector head. Each component of the mixture has by their physico -chemical properties of a characteristic mobility in the separation column, which is determined, inter alia, the partition coefficient between stationary and mobile phases. As well as very complex mixtures can be separated into their components. If certain materials are not separated, it is called critical pairs.

Due to the physico-chemical characteristics of the gas chromatography only evaporable substances with correspondingly relatively low molecular mass ( m approx <1000 u) can be investigated.

After passing through the chromatography column, the separated substances are ionized. ( - Electron impact ionization electron impact), but also the CI (chemical ionization) or FI ( field ionization ), and there are several other ionization techniques used - to ionization of the substances in the ion source, the EI is usually the method are explained in the article mass spectrometry closer. Through ionization the molecules of a single substance are either destroyed (EI ) or protonated (CI ). From the mass numbers of Molpeaks (CI ), characteristic fragments ( EI) and the presence of isotope patterns can be concluded that the structural and molecular formula of the substance.

Since today are generally used capillary GC columns with low carrier gas flow, which do not disturb the required vacuum in the mass spectrometer, the devices are usually directly coupled via a heated "Transfer Line". Earlier typical further coupling methods such as " open split" or "moving belt" are no longer in use.

Typically ion trap or quadrupole analyzers come to recording the mass spectra with simple appliances. More complex devices have TOF (Time -of-flight ) or high-resolution sector field analyzers.

Since gas chromatograph can separate the substances with high temporal resolution ( low half- width of the peaks, lower -second range - for example, < 3 s - is state of the art ), it is sometimes a problem for the connected mass spectrometer to record the spectra in the required speed. To obtain the possible optimum desired information, compromising on the quality in terms of the spectra to be investigated mass range and / or the Nachweisempflindlichkeit must be made in still in use older devices. Modern devices of 2005, however, create a mass decade - that is, for example, 10 ... 100 u, or 50 ... 500 u - five and more complete mass spectra per second. Even faster can be scanned if one is interested in the purpose of quantitative analysis only for selected ions and only these measures (single or selected ion monitoring mode: SIM); Detection limits ( three times the background noise ) of 10-14 mol (equivalent to approximately 10 billion molecules or masses in the range of trillionth of a gram ) and more are possible per analytical run.

Mixtures of substances which can not be successfully analyzed by GC-MS, can be more closely examined frequently with LC -MS ( Liquid Chromatography). LC has the advantage of temperature-sensitive and / or high molecular weight substances do not need to be vaporized, but also the disadvantage that the above-mentioned half width of the peak is significantly larger, and therefore, the temporal resolution and thus the chromatographic separation of similar materials with similar retention time is less ( but even here recent developments have resulted from approximately 2003 to qualitative leaps ).

Applications of GC -MS

Exemplary responses to details of the qualitative and quantitative analysis and trace analysis see also the respective fields:

• Research tasks in the physiology
• Analysis in food chemistry, for the determination of food constituents and contaminants
• Flavor or fragrance analysis in sensor
• Environmental analysis for virtually all matrices
• Doping tests of urine and blood samples
• Clinical chemistry laboratory diagnosis, especially in hormone analysis and for low molecular tumor markers
• Analysis in toxicology, including in the fight substance analysis or into questions of bioterrorism
• Tasks in the analysis and research in pharmacy and pharmacology
• Analysis in the art (binders, coatings, etc. )
• Analysis of mixtures of substances in the chemical ecology ( pheromone compositions, plant fragrances, etc. )