Mass spectrometry isotope peaks

For example, little is known about the isotopic composition of formaldehyde in the atmosphere. Formaldehyde is a chemical intermediate in hydrocarbon oxidation. The carbon (8 C) and hydrogen (8D) isotopic composition of atmospheric formaldehyde is analyzed using continuous flow gas chromatography isotope ratio mass spectrometry." Isotope ratios were measured using GC-IRMS (Finnigan MAT 253 stable isotope ratio mass spectrometer, single-sector field with electron impact ion source and multiple ion collection) with a precision of 1.1 and 50%(lo ) for 8 C and 8D, respectively. The accuracy of the online continuous flow isotope technique was verified by calibrating three aliquots of the gas phase standard via the offline dual inlet IRMS technique. The concentration of formaldehyde in ambient air was determined on IRMS major ion peak areas (i.e., mass 44 for 8 C and mass 2 for 8D)." ... 

Carbon is naturally present in the forms of different isotopes. As indicated in Table 4.1, c is predominant and represents 98.9% of the total. "<1, commonly used for radioactive dating of ancient objects, is so scarce in nature that it cannot be detected with an ordinary mass spectrometer. C constitutes approximately 1.1% of the total carbon and is detectable by mass spectrometry. The peak corresponding to the isotopomer of C at m/z -e1 compared to that of C is as abundant as the number of carbon atoms of the ion. 

Isopropyl group (Section 2 13) The group (CH3)2CH— Isotactic polymer (Section 7 15) A stereoregular polymer in which the substituent at each successive chirality center is on the same side of the zigzag carbon chain Isotopic cluster (Section 13 22) In mass spectrometry a group of peaks that differ in m/z because they incorporate differ ent isotopes of their component elements lUPAC nomenclature (Section 2 11) The most widely used method of naming organic compounds It uses a set of rules proposed and periodically revised by the International Union of Pure and Applied Chemistry... 

This example can be used in reverse to show the usefulness of looking for such isotopes. Suppose there were an unknown sample that had two molecular ion peaks in the ratio of 3 1 that were two mass units apart then it could reasonably be deduced that it was highly likely the unknown contained chlorine. In this case, the isotope ratio has been used to identify a chlorine-containing compound. This use of mass spectrometry is widespread in general analysis of materials, and it... 

Isotopic cluster (Section 13.22) In mass spectrometry, a group of peaks that differ in mh because they incorporate different isotopes of their component elements. 

A further point about mass spectrometry, noticeable in the spectrum of propane (Figure 12.2), is that the peak for the molecular ion is not at the highest m/z value. There is also a small peak at M + l because of the presence of different isotopes in the molecules. Although 12C is the most abundant carbon isotope, a small amount (1.10% natural abundance) of 13C is also present. Thus, a certain... 

Fortunately, isotopic abundances as well as isotopic masses can be determined by mass spectrometry. The situation with chlorine, which has two stable isotopes, 0-35 and 0-37, is shown in Figure 3.2. The atomic masses of the two isotopes are determined in the usual way. The relative abundances of these isotopes are proportional to the heights of the recorder peaks or, more accurately, to the areas under these peaks. For chlorine, the data obtained from the mass spectrometer are... 

ESI mass spectra of mixtures are difficult to interpret, because each component produces ions with many different charge states. The most direct and reliable method to solve this problem is to use high-resolution MS and calculate the charge states by measuring the spacing of the isotope peaks. ESI mass spectrometry of (polymeric) mixtures with broad molecular weight distribution benefits from a prior separation that reduces the polydispersity of the analyte. 

Figure 3.2. Stable isotope labeling for quantifying differential protein expression. Cell populations are grown in either 14N or 15N containing medium. Protein lysates are fractionated and separated by 2D gel electrophoresis. Protein spots are excised, digested with trypsin and the mass of the resulting peptides is determined by mass spectrometry. The presence of 15N results in a shift and creates two peaks for each peptide. The ratio of intensities of the peaks is indicative of the relative expression levels of the proteins. Spot A contains a protein that is expressed at similar levels in both cell pools. Spot B contains a protein that is expressed at higher levels in cell pool 2. Figure adapted from Oda et al. (1999).

One can get an enormous amount of information from studying the region of the molecular ion in a mass spectrum. The mass of M+ is the molecular mass of the analyte. The ratio of the isotopic peaks (see below) allows one to roughly establish the elemental composition, while accurate mass measurements using high resolution mass spectrometry give exact elemental composition. The relative intensity of the M+ peak... 

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+. ... 

Isotopic mass spectrometry is used to establish 813C values. The sample is burned to C02 and the intensities of the ion peaks of m/z 44, 45, and 46 are measured. Then correction to eliminate the influence of 170 isotope is achieved. If using sector magnetic instrument and three detectors (for each mass) the standard deviation of the results will be better than 0.001%. The high accuracy of the measurements allows valuable results to be obtained. The ancient Europeans and Americans may be distinguished by the analysis of their remnants. The reason involves the fact that wheat constituted the basic food ration in Europe, while com played the same role in America. The difference in the isotopic composition of these plants forms several units of 813C scale [34],... 

Apart from the need for isotopic enrichment and synthesis there are other problems in applying whole molecule mass spectrometry to measure isotope ratios. Assume, for example, that we want to determine isotopic composition of chlorine from the spectrum of chlorobenzene presented in Fig. 7.7. The peaks at 114 and... 

Mass spectrometry is based upon the separation of charged ionic species by their mass-to-charge ratio, m/z. Within the general chemical context however, we are not used to taking into concern the isotopes of the elemental species involved in a reaction. The molecular mass of tribromomethane, CHBrs, would therefore be calculated to 252.73 g mol using the relative atomic masses of the elements as listed in most periodic tables. In mass spectrometry we have to leave this custom behind. Because the mass spectrometer does not separate by elements but by isotopic mass, there is no signal at m/z 252.73 in the mass spectmm of tribromomethane. Instead, major peaks are present at m/z 250, 252, 254 and 256 accompanied by some minor others. 

Mass Spectrometry. Mass spectrometric detection was used in early laboratory studies of H02 (103, 104) and has also been used in more recent investigations (105). The H02 peak at the mass-to-charge ratio mle = 33 is useful in laboratory identification and quantification, but in the atmosphere, most likely multiple species will interfere because of, for example, fragmentation of hydrocarbons, hydrogen peroxide, or oxygen isotopes. 

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