The other information we should have as an aid to identification is the relative retention time of the fatty acid derivative or equivalent chain-length value on the GC column of the GC-MS system. This together with the molecular weight data will usually suggest that some structures are more plausible than others. More often than not we wish to know where double bonds are located in an aliphatic chain, and the mass spectrum of the methyl esters are not always of assistance. Although we cannot do this directly with monoenes and dienes, it is fortunate that with the conventional series of polyunsaturated fatty acids, i.
These ions tend to be much smaller than in spectra from more saturated esters. The latter ion was best defined in a paper by Brauner and coworkers . It should be stressed that these ions are not infallible guides, and are not reliable when the double bonds are not methylene interrupted. Unfortunately, when there are five or more double bonds, the molecular ion can be of low abundance and difficult to identify.
If we want to have definitive information on double bond positions, we have to get away from the sledgehammer approach. There are two ways this can be done. Alternatively, with monoenoic fatty acids especially, it is possible to "fix" the double bonds by reacting them with appropriate reagents to give chemical derivatives that give distinctive fragmentations in the mass spectrometer.
- A Beginner's Guide to Mass Spectral Interpretation by Terrence A. Lee
A host of such derivatives have been described, usually involving oxygenation followed by further derivatization, but most of these fell by the wayside when dimethyl disulfide adducts were described . This is a simple single-step if odoriferous reaction involving reaction of dimethyl disulfide with an unsaturated ester in the presence of iodine as catalyst.
The nature of the reaction is shown in Figure 4. For example with methyl oleate,the molecular weight increases substantially from to but this is still in a comfortable range for GC analysis. This technique has now been applied to identify many monoenoic fatty acids and other aliphatic compounds in natural samples in a number of laboratories, and new applications continue to appear in the scientific literature. This is the best test of any method - often a new procedure is described with one or two model compounds and then falls into oblivion.
The technique is less straight-forward if there is more than one double bond in the molecule, as bis -adducts are only formed quantitatively if there are more than three carbons between double bonds. However, the reaction can be used usefully even with methylene-interrupted dienes, such as linoleic acid, if the reaction is carried out under mild conditions since two mono-adducts one double bond derivatized and one not are then formed and these are easily separable by GC for analysis by MS . There is a more substantial article on this aspect of the topic on the website here..
Introduction to protein mass spectrometry
Such methods add an extra complication to analysis, and it would obviously be much easier if a single derivative could be used for all purposes, as discussed in Part 2. Most GC-MS systems are sold with software that enables comparison of spectra of unknowns with a library of spectra of standards, and this can indeed be very useful. However, beginners should be very careful of how this facility is used for a number of reasons. While the library may contain , spectra or more, this is still a small number in comparison to the number of organic compounds in general and fatty acid derivatives in particular that might be encountered.
A Beginner's Guide to Mass Spectrometry of Fatty Acids
Then, the identification is made from the intensities of a relatively small number of ions, and ion intensities can be rather variable, depending on instrumental factors. The computer does not recognize that isomeric methyl octadecenoates of this kind have spectra that are identical for all practical purposes. Analysts should have a sufficient understanding of the principles of mass spectrometry to know when to ignore the computer and make informed decisions.
I do not discuss quantification by GC-MS in these pages, mainly because I believe that GC with flame-ionization detection and quantification is robust methodology with an appreciable linear range and with a relatively simple requirement for standards . While GC-MS can be used for the purpose, it requires careful calibration with a wide range of standards .
A description of the various types of mass spectrometers is next and a discussion of the important topics of mass accuracy and resolution - or how well we can tell differences between closely related compounds. Chemistry, sample prep and data handling are considered as well as the definition of some terms commonly in use in the most prevalent forms of MS practice today.
Before considering mass spectrometry MS , you should consider the type of analyses you perform and the kind of results you expect from them:. The ability to determine an analyte's character increases with instrument capability.
Part 1. Methyl Esters
Researchers and practitioners from various disciplines and subdisciplines within chemistry, biochemistry, and physics regularly depend on mass spectrometric analysis. Pharmaceutical industry workers involved in drug discovery and development rely on the specificity, dynamic range, and sensitivity of MS to differentiate closely related metabolites in a complex matrix and thus identify and quantify metabolites.
Particularly in drug discovery, where compound identification and purity from synthesis and early pharmacokinetics are determined, MS has proved indispensable. Biochemists expand the use of MS to protein, peptide, and oligonucleotide analysis. Using mass spectrometers, they monitor enzyme reactions, confirm amino acid sequences, and identify large proteins from databases that include samples derived from proteolytic fragments. They also monitor protein folding, carried out by means of hydrogen-deuterium exchange studies, and important protein-ligand complex formation under physiological conditions.
Clinical chemists, too, are adopting MS, replacing the less-certain results of immunoassays for drug testing and neonatal screening.
So are food safety and environmental researchers. They and their allied industrial counterparts have turned to MS for some of the same reasons: PAH and PCB analysis, water quality studies, and to measure pesticide residues in foods. Determining oil composition, a complex and costly prospect, fueled the development of some of the earliest mass spectrometers and continues to drive significant advances in the technology.
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