Natural Product and Metabolomics
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Published: Wed, 11 Apr 2018
Mass Spectrometry is often become very useful comes into the final confirmation stage of a chemical structure. This is because by noting the fragmentation pattern and the total molecular mass of compound, researchers will be able to confirm the compound’s chemical formula, followed by confirming the chemical structure proposed based on previous instrumental analyses. Nowadays, Gas Chromatography and Liquid Chromatography are coupled with mass spectrometry in order for researchers to obtain the molecular mass right after the compounds are separated chromatographically. However, GC-MS are more often being used to study natural product compared to LC-MS because LC-MS doesn’t have its database constructed like GC-MS due to the extensive range of secondary metabolites and it is hardly reproducible. (Lee & Yoon et al., 2013) Researchers can confirm the compound by matching the molecular mass obtained from GC-MS with the established database, while LC-MS users will have to refer to the data provided by previous researchers on that particular field of study. There are a lot of journals regarding the studies of primary and secondary metabolites being published and a lot of researches will insert their sample for GC-MS experiment and then compared their compound’s fragmentation pattern and also molecular weight to the National Institute of Standards and Technology libraries, which will tells researchers the structure of their compounds, even if there are no exact matching, researches will at least have a rough idea on their structure.
In the early 1990s, Solid-phase microextraction (SPME) was introduced to extract compounds from samples without the use of solvent; this method is later applied on samples before undergoing GC-MS process. This technique enables researchers to extract compounds from sample in a much simpler and cheaper way since it doesn’t include the use of solvent. SPME works by adsorbing compounds into its fibre coated absorbing phase from the sample, and then inserted into GC for desorption of compound into it and then separated. (D’agostino & Sanz et al., 2014) This technique is so convenient that researchers can even carry it along when going into a jungle to adsorb compounds from plant of interest and bringing it back to the lab to undergo GC-MS experiment, much more convenient than plucking a big bunch of plants sample to the lab for solvent soaking.
Samples can be injected into GC-MS through either split of splitless injection; split injection system will direct most of the samples out to the atmosphere and only allow approximately 1% of it to enter the column while splitless injection system allows most of the samples to enter the column. Split/splitless system are designed to give reproducible chromatographic retention times and also to keep the chromatographic requirements of column in place, to use split of splitless, it is depending on the sensitivity required. Though splitless system allows more sample to be injected in however, metabolomics researches are better be carried out with split system because metabolites are present in wide range concentrations and it allows researchers to analyze volatile compounds eluting near the solvent peak. (Cyril Jousse, 2009)
Besides plant’s metabolites, GC-MS is also very useful for bio-metabolics, some of the bio-metabolics researches are for example Chrysoula Christou et al. did a research on analysis of organic acids in human urine in clinical settings in order to develop a method for organic acid profiling in human urine, this can be useful to detect metabolics or other health disorders. Their sample was being separated by gas column and undergo quadrupole mass spectrometer in the electron ionization (EI) mode and as a result chromatogram and mass spectra shown 11 organic acids separated and identified by conducting similarity search on NIST library (3). A research was carried out by Ioannis Papoutsis and the team of research. where GC-MS method was used to determine amisulpride in blood. Amisulpride is an antipsychotic drugs which is prescribed to psychiatric patients, overdosing can cause arrhythmia, bradycardia and hypotension. (4) SPME was first used by them to extract compounds from the blood samples and then separated by using gas chromatography and finally detected by mass spectrometer, results show that GC-MS can be used to monitor amisulpride in the blood and this can be used in pharmacokinetic studies and also forensic investigation.
GC-MS is also used in assisting archaeology studies; it is especially useful when comes to determining the chemical constituents of the sample. In one of the recent archaeology research, M. Ménager, C. Azémard and C. Vieillescazes studied the mummification balms from the mummy skulls with FTIR and GC-MS. Twelve mummified heads were collected, the balms on the heads were extracted with organic solvent and water before introduced to GC-MS. Molecular components were eluted by using helium, and injected with split system. Samples were eluted by using helium as carrier gas with the flow of 1mL.min-1 and oven temperature was set to: 50 °C for 2 min, 50–250 °C at 8 °C·min−1, 250–350 °C at 3 °C·min−1, and 350 °C during 20 min. 1 μL of each sample was injected with a splitting system and the injection temperature was set at 250 °C. Mass spectra were generated by electron ionization mode with electron ionization energy of 70 eV, an ionization time of 25,000 μs and a mass range of 40–650 m/z. (5) As a result, those mummification balms were found to contain mixtures such as oils, fats, beeswax, resins, proteins, polysaccharides and minerals, these compounds are confirmed again by matching mass spectra with NIST library. With the use of only FTIR without confirmation by GC-MS, researchers will only be able to know the functional group in it without confirming the compounds.
In cosmetic industries, chemical compounds in every cosmetic must be carefully studied as it’s applied onto human’s skin and body, therefore any mistake made will cause allergy or irritation to end users. Synthetic husk has been widely applied in cosmetic industry due ti its long-lasting odor, however, it was found that two groups of synthetic musk can cause many health disorders. Many countries have also set up regulations on the amount of synthetic musk in order to protect end users, this has become another parameter in quality control of cosmetic products. In Beijing, China, Haifeng Dong and his team of researchers conducted a research on 7 synthetic musk in cream by using SPME and then GC-MS, cream is chosen as it is the most common form of cosmetic product. (6) Cosmetic creams contains moisturizing, nutritional agent and also many lipophilic compounds. Synthethic musks have low polarity and it is also one of lipophilic compounds, which cause synthethic musks can be hardly be separated out from other liphophilic compounds. Cream samples must undergo extra cleanup steps such as by matrix solid phase dispersion method (MSPD) in order minimizes the matrix effect. Since water and isopropanol was used to separate synthetic musks from lipophilic compounds, supported liquid extraction will need to be used in order absorb water and remove interferences to give better purification. Tandem MS was used in this research in order to improve the selectivity and decrease the detection limit. This research which deals with dream shows slightly different in the preparation method compared to other GC-MS research, however, overall concept is still similar.
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