Mass spectrometry high resolution

INTRODUCTION TO MASS SPECTRA INTERPRETATION ORGANIC CHEMISTRY 

Nevertheless, a resolution of 80,000 for a sector instrument does not mean that this mass spectrometer allows one to obtain separate peaks of the singly charged ions of masses 79,999 and 80,000. The registered mass is proportional to B2/V. An unlimited increase of magnetic field B is technically unreasonable, while decrease of accelerating voltage V decreases the resolving power. 

The exact mass of an ion (4 to 6 decimal points) reliably defines its elemental and isotopic composition, while the method is called high resolution mass spectrometry. The measurements are conducted manually or automatically (computerized). Manual measurements are based on the parallel acquisition of the peak of interest with the closest peak of an ion with the known composition. Any compound with an intense ion peak with m/z value in the region +10% may serve as a marker. The most widespread markers are perfluorokerosene, perfluorotributylamine, and other polyfluorinated compounds. The use of these compounds is based on their volatility, as well as on the fact that fluorine is a monoisotopic element. In the spectra of these compounds intense ion peaks randomly cover all the range between m/z 19 and M+.  

Actually, to get separate images of the ions with composition CO and C2H4 with masses 27.994915 and 28.03300 Da, respectively, one needs resolving power R = 28/(28.03300 27.994915) = 770, while to separate CO and N2 peaks (the 

High resolution mass spectrometry becomes indispensable for the analysis of biomolecules with ESI (see Chapter 2, Section 2.1.15) and MALDI (see Section 2.1.22) techniques. In these cases very high resolving power and accuracy of measurements are required to measure reliably the real masses of the sample molecules. 

High-resolution mass spectrometry involves the use of a detector that can measure tw/z values to four decimal places. This technique allows for the determination of the molecular formula of an unknown compound. In order to analyze the data obtained from high-resolution mass spectrometry, we must first review some background information. Specifically, we must discuss why atomic masses are not whole numbers (despite the fact that we have treated them as such until now). 

The mass of an atom is approximately equal to the sum of its protons and neutrons, because the mass of the electrons are negligible compared to the protons and neutrons. Originally, the mass of a proton was considered to be the same as the mass of a neutron, resulting in the relative atomic weights given in Table 15.4. With this simple model, the atomic 

Protons do not have exactly the same mass as neutrons  

As a result, a hefrum atom (which has two protons and two neutrons) is not exactly four times the weight of a hydrogen atom (which has one proton). 

Protons repel each other but will nevertheless bind together in the nuclei of atoms under the influence of the strong nuclear force. These protons possess an enormous amount of potential energy, which is achieved at the expense of some mass. According to Einstein s famous equation E=m )y matter and energy are interconvertible. When protons bind together to form the nucleus of an atom, some of their mass is converted into potential energy. As a result, two bound protons will have less mass than two individual protons. This explains why carbon, which has six protons and six neutrons, is less than six times the mass of a deuterium atom, which has one proton and one neutron. 

Element Atomic Weight Isotope (Nuclide) Exact Mass 

Double-focusing magnetic sector mass spectrometers and Fourier transform (FT) mass spectrometers (described in Chapter 9) are capable of mass measurements with high resolution. The most important use of high-resolution MS is the direct determination of molecular formulas by exact mass measurements. 

Element Atomic weight Isotope (nuclide) Exact mass 

Quantitative analysis can also be performed by external calibration or by the use of an internal standard that is not a labeled analyte molecule but a compound that is not present in the sample. 

In some cases, components in a mixture can be determined quantitatively without prior separation if the mass spectrum of each component is sufficiently different from 

2-butanol, and 2-methyl-2-propanol in the mixture. The three equations can be solved and the composition of the sample determined. Computer programs can be written to process the data from multicomponent systems, make aU necessary corrections, and calculate the results. 

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Fisher, I.P. and P. Fisher (1974), Analysis of high boiling petroleum streams by high resolution mass spectrometry . Talanta, Vol. 21, p. 867. 

By high-resolution mass spectrometry, ions of known mass from a standard substance can be separated from ions of unknown mass derived from a sample substance. By measuring the unknown mass relative to the known ones through interpolation or peak matching, the unknown can be measured. An accurate mass can be used to obtain an elemental composition for an ion. If the latter is the molecular ion, the composition is the molecular formula. 

High resolution mass spectrometry (qv) has been used with extracts of a series of coals to indicate the association of different heteroatoms (27). Various types of chromatography (qv) have also been used to identify the smaller species that can be extracted from coal. 

The degradation of 2,6-xylenol (2,6-dimethylphenol) by bacteria produces a metabolite with elemental composition C8///0O2 as determined by high-resolution mass spectrometry Which carbon skeleton and which relative configuration are deducible from the NMR experiments 44, all obtained from one 1.5 mg sample ... 

The C NMR spectrum of the metabolite shows 16 signals instead of 8 as expected from the elemental composition determined by high-resolution mass spectrometry. Moreover, aromaticity of the 2,6-xylenol is obviously lost after metabolism because two ketonic carbonyl carbon atoms (5c = 203.1 and 214.4) and four instead of twelve carbon signals are observed in the shift range of trigonal carbon nuclei (5c = 133.1, 135.4, 135.6 and 139.4) in the C NMR spectra. To conclude, metabolism involves oxidation of the benzenoid ring. 

Unlike the stable molecule N2O, the sulfur analogue N2S decomposes above 160 K. In the vapour phase N2S has been detected by high-resolution mass spectrometry. The IR spectrum is dominated by a very strong band at 2040 cm [v(NN)]. The first ionization potential has been determined by photoelectron spectroscopy to be 10.6 eV. " These data indicate that N2S resembles diazomethane, CH2N2, rather than N2O. It decomposes to give N2 and diatomic sulfur, S2, and, hence, elemental sulfur, rather than monoatomic sulfur. Ab initio molecular orbital calculations of bond lengths and bond energies for linear N2S indicate that the resonance structure N =N -S is dominant. 

The male sex hormone testosterone contains C, H, and O and has a mass of 288.2089 amu as determined by high-resolution mass spectrometry. What is the likely molecular formula of testosterone ... 

Camphor, a saturated monoketone from the Asian camphor tree, is used among other things as a moth repellent and as a constituent of embalming fluid. If camphor has M+ = 152.1201 by high-resolution mass spectrometry, what is its molecular formula How many rings does camphor have ... 

B Nicotine is a diamino compound isolated from dried tobacco leaves. Nicotine has two rings and M + = 162.1157 by high-resolution mass spectrometry. Give a molecular formula for nicotine, and calculate the number of double bonds. 

The molecular formulas of PMs (Table 9.5) obtained by high-resolution mass spectrometry in collaboration with Prof. Y. Kishi, Harvard University, indicate that PMs are formed by the condensation of three molecules of PS and two molecules of methylamine, with the removal of four water molecules. No study has been made on their conformational isomers. 

Both compounds were non-fluorescent. Compound KM-1 was similar to PMs in the absorption maximum (A.max 488 nm) but not in the spectral shape, whereas KM-2 was similar to PMs in the spectral shape but not in the absorption maximum at 515 nm (Fig. 9.10). The chemical structures of KM-1 and KM-2 have been determined by high-resolution mass spectrometry and NMR spectrometry (Fig. 9.11 Stojanovic, 1995). 

Structure determination of luciferin. Once a luciferin is obtained in a sufficient purity, the determination of luciferin structure should be attempted most of the important properties of luciferin are usually already obtained during the course of purification as a necessity. The structural study is considerably more straightforward than the extraction and purification, due to the availability of advanced methods, such as high-resolution mass spectrometry and various NMR techniques. If help or collaboration is needed in structure determination, the attractiveness of a luciferin will make it easy to find a good collaborator. However, the purified luciferin is usually an extremely precious material considering the effort spent in preparing it. To avoid accidental loss of the purified material, the chosen collaborator must have solid knowledge and experience in structure determination a criterion to be considered is that the person has successfully done the structure determination of at least one new natural product. 

In addition to high sensitivity, a requirement for any acceptable analytical method is high specificity because at very low levels few confirmatory procedures can be used to establish the identity of a particular compound. A method which uniquely combines high sensitivity with high specificity is high resolution mass spectrometry. We have used this method as the basis for an approach which we believe will make possible a meaningful assessment of TCDD levels in the environment. 

The following is a procedure recommended for elucidating the structure of complex organic molecules. It uses a combination of different NMR and other spectroscopic techniques. It assumes that the molecular formula has been deduced from elemental analysis or high-resolution mass spectrometry. Computer-based automated or interactive versions of similar approaches have also been devised for structural elucidation of complex natural products, such as SESAMI (systematic elucidation of structures by using artificial machine intelligence), but there is no substitute for the hard work, experience, and intuition of the chemist. 

Advanced techniques like molecularly imprinted polymers (MIPs), infrared/near infrared spectroscopy (FT-IR/NIR), high resolution mass spectrometry, nuclear magnetic resonance (NMR), Raman spectroscopy, and biosensors will increasingly be applied for controlling food quality and safety. 

State-of-the-art ToF-MS employs reflection lenses and delayed extraction [176] to improve resolution by minimising small differences in ion energies, and in these cases up to 12000 mass resolution (FWHM, m/z 600) is available. This is sufficient for most modern applications. Solid probe ToF-MS (or direct inlet high-resolution mass spectrometry, DI-HRMS) is a breakthrough. DIP-ToFMS is a thermal separation technique. Advantages of DIP-ToFMS are ... 

Figure 7.39 Structural assignments of a benzophenone derivative based on low- and high-resolution mass spectrometry. After Squirrell [258], From D.C.M. Squirrell, Analyst, 106, 1042-1056 (1981). Reproduced by permission of The Royal Society of Chemistry...

HRMS High-resolution mass spectrometry IGC Inverse gas chromatography... 

Further evidence consistent with the polar radical pair mechanism was provided by a crossover experiment (Scheme 6.26). A 1 1 mixture of labeled 8Z /8 and unlabeled 8Z/8E was heated in xylene at 125 °C for 2h and at 135 °C for 4h to afford hydroxypyrimidinones 3 and 3. Analysis of the products by high resolution mass spectrometry showed no crossover between the labeled and unlabeled fragments. This result reinforces the computational results discussed previously wherein PRP recombines to give product within the solvent cage (Scheme 6.24). 

If you are lucky, the ion with the highest mass to charge value will be the molecular ion. However, this is often not the case, as textbooks on mass spectrometry make clear. If it is possible to carry out high resolution mass spectrometry on the molecules in question, and the molecular ion is indeed observed, the exact mass can be used in combination with tables to obtain the molecular formula directly. Alternatively, you can use the internet (http //www.sisweb. com/cgi-bin/masslO.pl) to calculate and plot mass distributions for any molecular fragment you think may be present. 

Seifert, W.K. Teeter, R.M. Preparative Thin-layer Chromatography and High Resolution Mass Spectrometry of Crude Oil Carboxylic Acids, Anal. Chem. 1969, 41, 786. 

Kendrick, E. A mass scale based on CH = 14.0000 for high resolution mass spectrometry of organic compounds. Anal. Chem. 1963,35,2146-2154. 

Ferrige, A.G., Seddon, M.J., Skilling, J., Ordsmith, N. (1992b). The application of MaxEnt to high resolution mass spectrometry. Rapid Commun. Mass Spectrom. 6, 765-770. 

Organic compounds polycyclic aromatic hydrocarbons, in particular phenan-threne (C14H10), pyrene (Ci6Hjo) and chrysene (CisH ), which were detected using high resolution mass spectrometry. 

Jacobsen, R.B., Sale, K.L., Ayson, M.J., Novak, P., Hong, J., Lane, P., Wood, N.L., Kruppa, G.H., Young, M.M., and Schoeniger, J.S. (2006) Structure and dynamics of dark-state bovine rhodopsin revealed by chemical cross-linking and high-resolution mass spectrometry. Protein Sci. 15, 1303-1317. 

Field et al. [747] used ICP high-resolution mass spectrometry to determine vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, cadmium, and lead in seawater. Each analysis required 50 p,l sample and a 6 minute analysis time. 

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