Gas chromatography with mass spectrometry coupling

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Gas chromatography with mass spectrometry coupling is the coupling of a gas chromatograph (GC) with a mass spectrometer (MS). The overall process or the device coupling is also referred to for short as GC-MS , GC / MS or GCMS , in the case of tandem mass spectrometry GC-MS / MS or similar.

history

In the 1950s, Roland Gohlke and Fred McLafferty used a mass spectrometer as a detector for a chromatography method for the first time. Both coupled a gas chromatograph with a time-of-flight mass spectrometer. This method made it possible for the first time to separate and identify substance mixtures in a plant. Since the development of capillary gas chromatography in the 1970s, the devices have mostly been coupled directly to the mass spectrometer via a heated “transfer line”. Other coupling methods that were common in the past, such as “open split” or “moving belt”, are no longer in use today. Another coupling technique that became important around 2010 is GC- APCI- MS (also known as APGC), in which gas chromatography can be coupled with the source of an HPLC-MS at atmospheric pressure .

Measuring principle

The gas chromatograph is used to separate the substance mixture to be examined and the mass spectrometer to identify and, if necessary, quantify the individual components. The separation column of a gas chromatograph consists of a thin (diameter approx. 3–6 mm) stainless steel or glass tube or, in most modern systems, a 15 to> 100 m long fused silica or glass capillary . The first-mentioned separation columns are operated as so-called packed separation columns and are still often used today in so-called process gas chromatography. The capillary separation columns, on the other hand, are used in the analytical investigation of highly complex substance mixtures (see below). Details on the column types and the stationary phases (separating liquids) used can be found in the article on gas chromatography . The columns are in the temperature-controlled so-called furnace chamber of inert carrier gases such. B. nitrogen or helium flows through as the mobile phase. The vaporized mixture of substances is injected into this gas flow via the heatable injector or injection block. Each component of the substance mixture has a characteristic mobility in the separation column due to its physico-chemical properties. a. is determined by the distribution coefficient between stationary and mobile phase. In this way, even very complex mixtures of substances can be separated into their components. If individual substances are not separated, one speaks of critical pairs .

Due to the physico-chemical properties of gas chromatography, only vaporizable substances with a correspondingly relatively low molecular mass (m approx. <1000 u ) can be examined.

Example of a GC / MS coupling with ion trap MS

After passing through the chromatography column, the separated substances are ionized. For ionization of the substances in the ion source is usually the EI ( electron impact - electron impact ), but also the CI ( chemical ionization ) or FI ( field ionization ) and quite a few other ionization techniques used - the procedures are in the article mass spectrometry further explained. Through the ionization, the molecules of the individual substance are either broken up (EI) or protonated (CI). The structural and molecular formula of the substance can be inferred from the mass numbers of the molar peak (CI), characteristic fragments (EI) and possibly existing isotope patterns .

Since capillary GC columns with a low carrier gas flow that do not disturb the required vacuum in the mass spectrometer are generally used today, the devices are usually coupled directly via a heated “transfer line”. Other coupling methods that used to be common, such as “open split” or “moving belt”, are no longer in use.

Ion trap or quadrupole analyzers are typically used in simple devices to record the mass spectra . More complex devices have TOF (time-of-flight) or high-resolution sector field analyzers.

Since gas chromatographs can separate the substances with high temporal resolution (small half-width of the peaks, lower range of seconds - e.g. <3 s - is state of the art), it is occasionally a problem for the connected mass spectrometer to record the spectra at the required speed . In order to obtain the possible optimum of the desired information, compromises have to be made with older devices that are still in use in terms of the spectra quality with regard to the mass range to be examined and / or the detection sensitivity. Devices of the year 2005, however, already managed over a mass decade - that is z. B. 10… 100 u, or 50… 500 u - five or more complete mass spectra per second. Scanning is even faster if you are only interested in selected ions for quantitative analysis and only measure these (single or selected ion monitoring mode: SIM); Detection limits (three times background noise) of 10 −14 mol (corresponds to around 10 billion molecules or masses in the range of a trillionth of a gram) and better are thus possible per analysis run.

Mixtures of substances which can not be successfully analyzed by GC-MS, can often with LC-MS ( L iquid C be examined more closely hromatography). LC has the advantage that temperature-sensitive and / or high-molecular substances do not have to be vaporized, but also the disadvantage that the above-mentioned half-width of the peaks is significantly larger, and consequently the temporal resolution and thus the chromatographic separation of similar substances with a comparable retention time is worse (But here, too, recent developments have led to qualitative leaps from around 2003).

Areas of application of GC-MS

A GC-MS device with closed doors (2005)
Same GC-MS as above with the doors open (2005)

Exemplary mentions, for details of the qualitative and quantitative analysis or trace analysis, see also the respective subject areas:

literature

  • Hans-Joachim Hübschmann: Handbook of GC / MS, Fundamentals and Applications. 3. Edition. Wiley-VCH, Weinheim 2015, ISBN 978-3-527-33474-2 .
  • Helmut Günzler, Alex Williams (Eds.): Handbook of Analytical Techniques. 2. Reprint. Volume 1, Wiley-VCH, Weinheim et al. 2002, ISBN 3-527-30165-8 , chapter 10, 11, 20.

Individual evidence

  1. ↑ GC / MS / MS - What does GC / MS / MS stand for? The Free Dictionary. In: acronyms.thefreedictionary.com. Retrieved February 7, 2017 .
  2. ↑ What does GC / MS / MS mean? - Definition of GC / MS / MS - GC / MS / MS stands for Gas Chromatography / Mass Spectrometry and Gas Chromatography / Tandem Mass Spectrometry. By AcronymsAndSlang.com. In: acronymsandslang.com. Retrieved February 7, 2017 .
  3. ^ FW McLafferty: Mass Spectrometric Analysis Broad Applicability to Chemical Research . In: Analytical Chemistry . tape 28 , 1956, pp. 306 , doi : 10.1021 / ac60111a005 .
  4. ^ FW McLafferty: Mass Spectrometric Analysis. Molecular Rearrangements . In: Analytical Chemistry . tape 31 , 1959, pp. 82 , doi : 10.1021 / ac60145a015 .
  5. ^ RS Gohlke: Time-of-Flight Mass Spectrometry and Gas-Liquid Partition Chromatography . In: Analytical Chemistry . tape 31 , 1959, pp. 535 , doi : 10.1021 / ac50164a024 .
  6. ^ Roland S. Gohlke, Fred W. McLafferty: Early gas chromatography / mass spectrometry . In: Journal of the American Society for Mass Spectrometry . tape 4 , no. 5 , May 1993, pp. 367-371 , doi : 10.1016 / 1044-0305 (93) 85001-E .
  7. ↑ Dandenau, Raymond D. and EH Zerenner: An investigation of glasses for capillary chromatography . In: Journal of High Resolution Chromatography . tape 2 , 1979, p. 351-356 , doi : 10.1002 / jhrc.1240020617 .