Gas chromatography-mass spectrometry (GC-MS)

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Contributors: Catherine H, Stephens, Alba Alvarez Martin
==Overview==
Technique: GC-MS
Formal name: Gas Chromatography-Mass Spectrometry
Summary description of this technique: GC-MS is a hyphenated technique which combines the separating power of gas chromatography (GC) with the detection power of mass spectrometry (MS). 
GC-MS analysis includes: 
(1)	Sample analytes separation: GC uses a carrier gas (mobile phase) to transport sample components through either packed columns or hollow capillary columns containing the stationary phase.
(2)	Ionization of sample analytes: ionization is the process whereby electrons are either removed or added to atoms or molecules to produce ions. The most common ionization methods are Electron ionization (EI) and Chemical ionization (CI) 
(3)	Separation and detection of gas phase ions: achieved within the mass spectrometer using electrical and/or magnetic fields to differentiate between ions based on their mass-to-charge ratio. 
Sample image of the data: 
(Total ion chromatogram of GC-MS analysis of essential oils. Image provided by Alba Alvarez Martin.) 
==Details==
What this techniques measures: GC-MS is a technique that can be used to separate organic analytes in a complex mixture. 
Limitations of this technique: 
(1) GC requires the analyte to have significant vapor pressure between 30 - 300°C, and also sufficiently thermally stable to survive the extended time the sample must spend in the gas phase. 
(2) Lack of definitive proof of the nature of detected compounds, identification being based on fragmentation pattern and retention time matching, analytes have to be in a database. 
Can/how can this technique be made quantitative? For quantification, various calibration methods such as external standard calibration, internal standard calibration, and method of standard addition can be performed. 
==Samples==
Phases it can be used to examine: gas, liquid, solid. 
Is this technique non-destructive? the degree of destructiveness will be limited by the type of sample preparation. 
How invasive is this technique? Depending on the type of GC-MS analysis (pyrolysis, thermal desorption, liquid injection, etc.), see description of the sample introduction techniques. 
Minimum size of sample necessary to use this technique? Sample amounts ranging from micrograms up to few grams. In some GC applications such as headspace or solid phase micro-extraction physical sampling is not required. 
Sample preparation methods: GC-MS samples often contain dirty, labile, and volatile compounds that sometimes need further processing before they are introduced into the gas chromatograph. Different sample extraction processes are often used prior to gas chromatography. The sample preparation will depend on the degree of selectivity required during sample preparation as well as the type of information required and the type of GC analysis. E.g. hydrolysis, derivatization, solid phase extraction, solid phase micro-extraction, solvent extraction, etc. 
Time to run one experiment? The oven program can last from a few minutes to more than an hour. The time required for sample preparation will vary from minutes to hours, please see sample introduction techniques (See: [introduction techniques for GC]). 
 
==Applications==
Examples of how this technique is used in the field? Characterization of lipids, resinous materials, protein paint binder, synthetic pigments, volatile organic compounds, pesticides, etc. 
Risks associated with using this technique? None aside from required sample size, noted above. 
==Budgetary Considerations==
Approximate cost to purchase equipment for this technique? GC-MS can range from ~$50k to > $1 million depending on the type of detector and the associated sample introduction methods. 
Annual cost to maintain this equipment? Annual service coverage costs are often in the 5-10% of the cost of the instrument. Consumable costs vary widely on the techniques that are used with the mass spectrometer. 
Sample analysis costs? Commercial labs would be expected to have a sample charge in the $100-$1000 range, highly dependent upon the specifics of the method and instrumentation required. 
==Case Studies== 
* Identification of non-cross-linked compounds in methanolic extracts of cured and aged linseed oil-based paint films using gas chromatography-mass spectrometry. van den Berg, J. D. J., van den Berg, K. J., & Boon, J. J. Journal of Chromatography A. 2002. 950(1-2), 195-211. 
* Historical mystery solved: a multi-analytical approach to the identification of a key marker for the historical use of brazilwood (Caesalpinia spp.) in paintings and textiles. Peggie DA, Kirby J, Poulin J, Genuit W, Romanuka J, Wills DF, et al. Analytical Methods. 2018;10(6):617-23. 
* Trimethylsulfonium hydroxide as derivatization reagent for the chemical investigation of drying oils in works of art by gas chromatography. Dron J, Linke R, Rosenberg E, Schreiner M. J Chromatogr A. 2004 Aug 20;1047(1):111-6. doi: 10.1016/j.chroma.2004.06.013. PMID: 15481466. 
==Additional Information==
* Complementary Techniques: Direct Analysis in Real Time (DART), liquid chromatography 
* Variations of this technique: See: [introduction techniques for GC]
==References==
* https://www.chromacademy.com/ 
* Organic Mass Spectrometry in Art and Archeology. DOI:10.1002/9780470741917. 
* Analytical Approaches Based on Gas Chromatography Mass Spectrometry (GC/MS) to Study Organic Materials in Artworks and Archaeological Objects. Bonaduce, I., Ribechini, E., Modugno, F. et al. Top Curr Chem (Z) 374, 6 (2016). 
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