Omar Bennett
Mass spectrometry (MS) is a powerful technique used to identify molecules and compounds. However, when it comes to distinguishing between constitutional isomers, mass spectrometry alone often falls short. Isomers have the same molecular mass, so differentiation based on mass is not possible. Nevertheless, isomers can have unique mass fragments, and these differences in fragmentation patterns can be used to tell them apart. For example, in the case of N-methyl benzylamine, the loss of a methyl amine fragment results in a tolyl cation (MW 91), while for N-ethyl aniline, only the benzene fragment (MW 77) is observed. While mass spectrometry can provide valuable insights, other techniques, such as VUV spectroscopy, offer advantages in distinguishing isomers due to their ability to detect differences in electronic structures.
| Characteristics | Values |
|---|---|
| Chemical formula | Same |
| Number of atoms | Same |
| Molecular mass | Same |
| Physical and chemical properties | Different |
| Fragmentation patterns | Different |
| Mass accuracy | Insufficient to distinguish isomers |
| Mass resolution | Important factor in compound identification |
| Ionization/dissociation energy | Important for isomer analysis |
| Ion molecule reactions | Important for isomer analysis |
| Ion mobility mass spectrometry (IMS) | Important for isomer analysis |
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What You'll Learn
- Fragmentation patterns can be used to distinguish isomers
- Tandem-MS provides more information about isomers than intact mass analysis
- VUV spectroscopy can differentiate isomers due to differences in electronic structures
- Mass spectrometry imaging can identify isomers on biological sample surfaces
- FAIMS can be used to separate lipid isomers
Fragmentation patterns can be used to distinguish isomers
Mass spectrometry is a powerful technique used to identify unknown compounds and study the structural properties of molecules. When an organic molecule is fed into a mass spectrometer, it undergoes ionization, forming a positively charged molecular ion or parent ion. This ion is often unstable and can fragment into smaller pieces, resulting in various charged particles. The fragmentation pattern produced by an unknown compound can be compared to known standards or databases to assist in identification. This is particularly useful for distinguishing between isomers, which have the same molecular formula but differ in the arrangement of atoms.
For example, let's consider the isomers pentan-2-one and pentan-3-one. Due to their structural differences, they will fragment differently in the mass spectrometer, producing distinct patterns. In the case of pentan-2-one, two types of ions with a positive charge on the CO group are expected, resulting in strong lines at m/z = 43 and 71. On the other hand, pentan-3-one will produce only one type of ion, yielding a strong line at m/z = 57. These differences in fragmentation patterns allow for easy distinction between the two isomers.
Another example involves N-methyl benzylamine and N-ethyl aniline. N-methyl benzylamine exhibits a loss of a methyl amine fragment, forming a tolyl cation with a molecular weight of 91, which is characteristic of Ph-CH2 compounds. On the other hand, N-ethyl aniline displays the benzene fragment with a molecular weight of 77 as its characteristic peak. By recognizing these characteristic fragments, we can distinguish between these two isomers.
The interpretation of fragmentation patterns can be facilitated by understanding bond strengths. Typically, the weakest carbon-carbon bonds are the most susceptible to breakage during ionization. Additionally, the formation of secondary and tertiary carbocations is generally favored over primary carbocations due to their increased stability. These factors influence the fragmentation pathways and the resulting patterns observed in the mass spectrum.
In summary, fragmentation patterns play a crucial role in mass spectrometry, enabling the differentiation of isomers. By analyzing the characteristic peaks and fragmentation pathways, we can identify unknown compounds and gain insights into their structural arrangements. This technique is widely applicable in organic chemistry and compound identification.
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Tandem-MS provides more information about isomers than intact mass analysis
Tandem mass spectrometry (MS/MS) involves two stages of mass spectrometry to examine the fragmentation of a particular molecular ion. This process is performed within the mass spectrometer and involves a specific precursor ion (m/z) being isolated and fragmented, normally via collisions with a neutral gas such as nitrogen or argon. Tandem MS in space uses the coupling of two instrument components that measure the same mass spectrum range but with controlled fractionation between them in space. Tandem MS in time involves the use of an ion trap.
Tandem-MS provides significantly more information about isomers than intact mass analysis. However, highly similar fragmentation patterns are common and include cases where no unique m/z peaks are generated between isomeric pairs. Even in such situations, differences in peak intensity can exist and potentially contain additional information. Tandem mass spectrometry can be used for protein sequencing. When intact proteins are introduced to a mass analyzer, this is called "top-down proteomics". When proteins are digested into smaller peptides and introduced into the mass spectrometer, this is called "bottom-up proteomics".
Tandem mass spectrometry can produce a peptide sequence tag that can be used to identify a peptide in a protein database. A notation has been developed for indicating peptide fragments that arise from a tandem mass spectrum. Peptide fragment ions are indicated by a, b, or c if the charge is retained on the N-terminus and by x, y, or z if the charge is maintained on the C-terminus. The subscript indicates the number of amino acid residues in the fragment. Tandem mass spectrometry is highly informative for the mass determination of fragmented ions originating from peptides.
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VUV spectroscopy can differentiate isomers due to differences in electronic structures
While mass spectrometry is a widely used technique, it has some limitations. One of the major drawbacks is its inability to distinguish between constitutional isomers. This is because isomers have the same mass, and mass spectrometry cannot differentiate based on mass alone.
VUV spectroscopy, on the other hand, can differentiate isomers due to differences in their electronic structures. VUV spectroscopy uses short-wavelength light (120-240 nm) to probe a molecule's electronic structure. This technique is highly sensitive to the electronic transitions that occur within molecules. The absorbance spectrum of a molecule provides a unique fingerprint, reflecting its electronic structure and functional group arrangement.
For example, when comparing the C4 benzene isomers, VUV spectroscopy can easily distinguish between them due to their distinct VUV spectra, despite their similar chemical structures. This is because the electronic structure of each isomer is different, and these differences are reflected in their respective VUV absorbance spectra.
The same principle applies to other isomers, such as the cis and trans isomers of fatty acid methyl esters (FAMEs). VUV spectroscopy can differentiate between these isomers, which is crucial in the analysis of compounds like butter and vegetable oils. The unique VUV absorbance spectra enable unambiguous compound identification and can also help shorten the analysis time.
In conclusion, VUV spectroscopy is a powerful technique that can differentiate between isomers due to their distinct electronic structures. This capability addresses a significant limitation of mass spectrometry, making VUV spectroscopy a valuable tool in analytical chemistry.
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Mass spectrometry imaging can identify isomers on biological sample surfaces
Mass spectrometry imaging (MSI) is a powerful analytical technique that enables the detection, mapping, and identification of molecules in complex biological samples. It is particularly useful in biological research as it provides valuable information about the distribution of compounds within a sample. While conventional mass spectrometry alone cannot distinguish between isomeric compounds, recent advances in MSI have made it possible to achieve isomeric resolution for certain molecular classes.
MSI allows for the localization and identification of molecules on biological sample surfaces. This technology can be applied to various molecular classes, including lipids, steroid hormones, amino acids, and proteins. By obtaining high-mass resolution spectra at specific x and y coordinates across the sample, images can be generated using dedicated software by selecting peaks of interest. Tandem mass spectrometry (MS/MS) can further support compound identification.
However, when structural isomers have identical fragmentation pathways, even high-mass resolution may not be sufficient to differentiate them. To address this challenge, different ionization methods and surface sampling technologies have been developed, such as matrix-assisted laser desorption/ionization (MALDI), secondary ion mass spectrometry (SIMS), and desorption electrospray ionization (DESI). These techniques enhance the ability to distinguish between isomeric compounds and improve the accuracy of identification.
The specificity of lipid identification in MSI analysis is crucial but challenging due to the structural diversity of lipids and the presence of isomers. Advancements in MSI have enabled the differentiation of steroid structural isomers in human adrenal glands, highlighting its potential for biomedical research. Overall, MSI with isomeric resolution provides valuable insights into the local biological activity of isomers, making it a valuable tool for biological research and lipid analysis.
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FAIMS can be used to separate lipid isomers
Mass spectrometry (MS) is a widely used technique for lipid characterization. However, the full characterization of lipid isomerism has been challenging due to the lack of rapid and effective analytical tools. Field asymmetric waveform ion mobility spectrometry (FAIMS), also known as differential ion mobility spectrometry (DMS), is a powerful technique that can be used to separate lipid isomers. FAIMS is an atmospheric pressure separation technique that distinguishes ions based on their mobility differences at high and low electric fields.
FAIMS has been shown to successfully separate lipid isomers such as sn-1/sn-2 positional isomers, cis/trans double bond orientation isomers, and stereochemical isomers (e.g. R versus S). In the sn-1/sn-2 separations, it was observed that if the smaller fatty acyl chain was in the sn-1 position, the lipid had a smaller structure. Similarly, cis double bonds were found to have smaller structures compared to trans double bonds. These structural differences can be effectively resolved using FAIMS, enabling the separation of lipid isomers.
FAIMS is particularly useful for the analysis of ubiquitous glycerolipids and phospholipids, including diacylglycerols (DG), triacylglycerols (TG), and phosphocholines (PC). These lipid species often form isomer pairs, and FAIMS can facilitate their separation and identification. By applying a high electric field, FAIMS can resolve a larger number of conformers for a given ion, enhancing the separation of lipid isomers.
Additionally, FAIMS can be combined with other techniques such as collision-induced dissociation (CID) and travelling wave ion mobility spectrometry (TWIMS) to further improve the separation and characterization of lipid isomers. The combination of FAIMS with these complementary techniques provides a comprehensive approach to lipid isomer analysis, allowing for the identification of specific structural features and functional groups.
In summary, FAIMS is a valuable technique for separating lipid isomers due to its ability to distinguish ions based on their mobility differences at different electric fields. Its high resolution, rapid analysis, and compatibility with other techniques make FAIMS a versatile tool in lipidomics research, contributing to a deeper understanding of the isomeric identity of lipids and their biological roles.
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Frequently asked questions
It is difficult to distinguish between constitutional isomers using mass spec because isomers have the exact same mass. However, unique product ions that distinguish specific isomers can be generated by fragmentation.
In the case of N-methyl benzylamine, you would expect to see a loss of a methyl amine fragment to give a tolyl cation (MW 91). In the case of N-ethyl aniline, you would only observe the benzene fragment (MW 77).
VUV spectroscopy can distinguish between constitutional isomers because, although isomers have the same mass, their electronic structures are different. Tandem-MS provides significantly more information about isomers than intact mass analysis.
Radical-directed dissociation (RDD) is a structure-sensitive MS/MS method that often yields highly distinct spectra for isomeric pairs.
