Isomeric and isobaric

De novo sequencing seldom leads to a complete sequence becanse in most cases the y- and b-ion series are not completely present, some sequence ions might be lost in the noise, and/or ambiguities exist due to additional peaks (neutral losses of CO, HjO, NH3, etc.), and isobaric and isomeric amino acids (Table 17.2). 

Buhr, K., van Ruth, S., Delahunty, C. (2002) Analysis of volatile flavour compounds by proton transfer reaction-mass spectrometry fragmentation patterns and discrimination between isobaric and isomeric compounds. International Journal of Mass Spectrometry, 221,1-7. 

IM-MS is particularly useful for analysis of isobaric and isomeric lipid species, which possess different structures or configurations. For example, MALDI-IM MS has been used to analyze gangliosides, a class of complex glycosphingolipids... 

The aim of the LC step is the separation of the target compounds from each other and from matrix interferences. In bioanalytical LC-MS, this means that at least isobaric and/or isomeric target compounds must be separated from each other, and that the target compounds must be separated from metabolites and other related substances that may interfere. 

Ultimately, the definitive structure elucidation of unknown molecules is most often accomplished via NMR. NMR has traditionally been performed on the purified natural product isolated using bioassay-guided fractionation. With the advent of hyphenated techniques such as LC/NMR, these data can now be obtained prior to purification [130,131]. LC/NMR can prove useful even in the dereplication phase, particularly when LC/UV/MS data are insufficient for unambiguous peak identification. LC/NMR has played an important role in natural products structure elucidation, where several related compounds (factors) are often encountered in a single sample. For example, isobaric or isomeric mixtures that may prove difficult or impossible to differentiate by MS, can often be readily distinguished by NMR. Several thorough reviews of LC/NMR in natural products discovery and phytochemical analysis have recently appeared [132,133]. 

One of the main problems connected with DPMS is that in the case of polymers producing pyrolysis compoxmds, which may have isobaric or isomeric structures (such as the case of cyclic and open chain oligomers), it is not possible to correct the assignments of the corresponding mass peaks. The advent of high resolution and hyphenated methods such as tandem... 

A nuclide is an atomic species as determined by its atomic number (proton number) Z and mass number (nucleon number) A = Z+N, where N is the number of neutrons in its nucleus. Atomic species with the same nuclear composition but different nuclear energy states with measurable lifetime are considered independent nuclides in their own right. Nuclides can be classified in different ways. Nuclides having the same atomic number Z (but different mass number A) represent the same chemical element and are called the isotopes of that element. Nuclides with the same mass number A (but different atomic number Z) are called isobars. Nuclides of the same number of neutrons N (but different atomic number Z) are called isotones. Nuclides of the same nuclear composition but different nuclear states are referred to as (nuclear) isomers. The terms isotope, isotopic, isobar, isobaric, isotone, isotonic, isomer, and isomeric can also be applied to nuclei, but the terms nuclide and nuclidic can only be applied to atoms. 

As described in Chapter 15, correction for different stable isotopologue distribution due to differences in the number of carbon atoms between the species of interest and the selected internal standard(s) and correction for baseline should be performed prior to the comparison of the intensities. It should be recognized that in the majority of the studies in plant lipidomics after direct infusion, MDMS-type analysis is generally not conducted thus, the isobaric or isomeric species are not resolved as regrettable. A detailed protocol for profihng of polar hpids in plants after direct infusion can be found [9]. 

The identity of mass peaks or assignment of chemical structures to mass signals is complicated by the presence of isobaric fragment ions and isomeric structures of molecular ions. However, the presence of isobaric ions with a different composition can be investigated by HRMS, and pyrolysis product identification may further be supported by using MS-MS and/or GC-MS methods. Nevertheless, the combination of various ionization techniques, HRMS and, especially, MS-MS (Figure 8) can provide the extra... 

Giner, B. Artigas, H. Gascon, I. Cea, R Royo, F. M. Experimental data of isobaric vapour-liquid equilibrium for binary mixtures containing tetrahydrofuran and isomeric chlorobutanes. Rhys. Chcm. Liq. 

From the FIA—MS overview spectrum, speculation that there can be more than just one structurally defined molecule type behind an observable signal i.e. the presence of isobaric compounds, cannot be excluded whenever one signal defined by the m/z-ratio is examined in FIA-MS spectra. Consequently, the information obtained by FIA-MS is quite limited whenever we deal with complex mixtures of environmental pollutants rather than the analysis of pure products or formulations with a known range of ingredients. LC separation is inevitable when mixtures of isomeric compounds should be identified with MS-MS. Therefore, in FIA-MS-MS special attention has to be paid to avoid the generation of mixed product ion spectra from isomeric parent compounds. This would block identification by library search and may lead to misinterpretations of product ion spectra because of the fragmentation behaviour observed. 

However, tandem mass spectrometry, as a separation technique, does have limitations. It cannot easily differentiate between isomeric and isobaric species, and, in complex matrices, the presence of components with a high surface activity can suppress the ionization of components with a lower surface activity, leading to the nondetection of analytes (66). Therefore, the combination of MS-MS with a readily available chromatographic separation method such as TLC affords analysts real benefits. 

Different analytical chemistry methods are used for analysis of the metabolome (Figure 3). Direct-infusion mass spectrometry (DI-MS) on both, low and (ultra)high resolution MS, infuses raw metabolite extracts in the mass spectrometer without prior chromatography or electrophoretic separation and often uses high-resolution mass spectrometers. This method offers fast analysis with low duty cycles however, isomeric and isobaric substances cannot be... 

The advantage of GC/MS is the capacity to separate these isomeric (or isobaric) compounds. Its disadvantage is, among others, to introduce coeluting compounds, leading to uncertainties and to a reduced sensitivity (in the case where high resolution becomes necessary) this no longer occurs when MS/MS is utilized. 

The addition of mass information to mobility information opens new horizons for both ion mobility spectrometry (IMS) and mass spectrometry (MS). The combination of mass and mobility within one spectrum provides information on ion structure that is not possible with either method alone. By adding ion mobility information to mass information, the size of an ion as well as its mass may be measured. For example, MS is often blind to low levels of ions in complex mixtures due to chemical noise of the matrix and to isomeric/isobaric components of the mixture. Thus, the addition of size information to mass information expands the parametric space that can be used for ion detection. 

As Foe for each ion always depends on by Equation 4.51, the values of F e and Foe for any gap geometry will be ion-specific and the FAIMS spectmm will not transpose as a whole, even for isomeric or isobaric species. In this regime, ions will be filtered based on a combination of Fc and mF. While, resolution of a particular set of species may improve or worsen depending on those two quantities for each. 

The drop of correlation between m and a at higher n should allow HOD IMS to distinguish isomeric or isobaric ions better than DT IMS or FAIMS of equal resolving power (R). For the exemplary case of protonated Leu and He (3.1.7), the values of K (in N2 at room T) are 1.618 and 1.632 cm /(V s), and the 1% difference barely permits partial separation at the highest R 150 of present DT IMS systems. Under same conditions, the ai values (4.24 and 4.06 x 10 Td ) differ by a greater 4%, which still just suffices for incomplete separation by FAIMS of highest current resolution (3.1.7). In contrast, the difference of 560% between 02 values (0.12 and 0.79 x 10 ° Td ) should enable full separation even with a rudimentary HOD IMS capability.Same pattern for deprotonated Leu and He is clear from the data in Table 3.2. 

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