Isotope peaks, intensities

FIGURE 1. Graphical representation of the relative isotope peak intensities for (a) Zn+, (b) [Zn2], (c) [ZnCl]+ and (d) [ZnBr]+ ions as calculated by the AELITA software ... 

F. (This is a long exercise suitable for group work.) Relative intensities for the molecular ion region of several compounds are listed in parts (a)-(d) and shown in the figure. Suggest a composition for each molecule and calculate the expected isotopic peak intensities. 

One limitation on the use of isotope peak intensities to determine the molecular formula is that the molecular ion must be relatively intense, otherwise the isotope peaks will be too weak to be measured with the necessary accuracy. Difficulty may also arise from spurious contributions to the isotope peak intensities from the protonated molecular ion, from weak background peaks or from impurities in the sample. In any event the method is only reliable for molecules having molecular weights up to about 250-300. 

The minimum requirement for the organic chemist is the ability to record the molecular weight of the compound under examination to the nearest whole number. Thus, the spectrum should show a peak at, say, mass 400, which is distinguishable from a peak at mass 399 or at mass 401. In order to select possible molecular formulas by measuring isotope peak intensities (see Section 1.5.2.1), adjacent peaks must be cleanly separated. Arbitrarily, the valley between two such peaks should not be more than 10% of the height of the larger peak. This degree of resolution is termed unit resolution and can be obtained up to a mass of approximately 3000 Da on readily available unit resolution instruments. 

Development of the molecular formula starts with recognition of the molecular ion peak (Section 1.5). We assume the usual situation high-resolution MS instrumentation is not readily available. Let us also assume for now that the peak of highest m/z (except for its isotope peaks) is the molecular ion peak and is intense enough so that the isotope peak intensities can be determined accurately and the presence and number of S, Br, and Cl atoms can be ascertained. Look also at the fragmentation pattern of the mass spectrum for recognizable fragments. If the molecular ion peak is an odd number, an odd number of N atoms is present. 

In which case (low or high MW) is the monoisotopic peak intensity higher than isotopic peak intensity ... 

The relationship between the ratio of the isotope peak intensity to the molecular mass peak intensity (Ir) and the isotopic ratio of were then examined. C-labeled esters and limonene were added to non-labeled compounds, and the Ir values were measured. As shown in Fig. 2, a strong linear relationship was observed between the Ir value and the isotopic ratio. The value for the esters was greater than 0.96, except for decyl acetate. The accuracy with decyl acetate was rather low (R =0.85), because the intensity of the molecular ion peak followed the normal tendency of decreasing with increasing molecular weight. The difference in the intercept and gradient between octyl acetate and the other acetates may be explained by the difference between the isotope ratio of octanol and those of the other alcohols. 

Figure 10.8 Graphical representation of relative isotope peak intensities for any given ion...

By virtue of their different masses, the isotopes result in separate peaks in the mass spectrometer, with the intensities of these peaks directly proportional to the abundances of the isotopes. Isotope peak intensities therefore provide important information regarding the elemental compositions of ions. 

As a first approximation, the contributions to isotope peak intensities by various elements are additive. Each group of elements can be treated separately and the results summed. 

There is no contribution to isotope peak intensities by these elements. Therefore, weak isotope peak intensities for relatively high-mass ions are evidence for the presence of monoisotopic elements. 

In simple molecules containing only carbon, hydrogen, nitrogen, oxygen, and the monoisotopic elements, elemental compositions of ions may be calculated using isotope peak intensities. As a hrst approximation, one atom in an ion contributes an amount to the intensity of the isotope peaks that is equal to the relative abundances of the isotopes of that atom. When more than one atom is present, the intensity contribution is multiplied by the number of each atom present. 

The isotope peak intensity ratio will therefore be 9 6 1. 

When evaluating isotope peak intensities, the first step is to compare the intensity of the (P + 2) peak to the relative abundances of the heavy isotopes of bromine, chlorine, sulfur, and silicon. If there is a fairly close fit (with one or more... 

What isotope peak intensities are expected for the molecular ions of aniline (CgHyN), acetophenone (CjHjO), ethyl iodide (C2H5I), and ethylamine (C2H7N) ... 

Two serious complications arise in the determination of elemental compositions from isotope peak intensities in this manner. 

Intensity measurements made with modem rapid-scanning instruments cannot be relied on for better than plus or minus 1 to 2 percent accuracy. Unless care is taken to scan the ions of interest several times for averaging, the isotope peak intensities can be of only secondary importance. In conjunction with high-resolution exact mass measurements, however, the approximate isotope peak intensities can be of considerable value. This will become clear as you proceed through this book. 

Compare the isotope peak intensities with those in Table 2.4. 

Simple cleavage with the loss of 43 mass units may be indicative of an acetyl or a propyl unit in the molecule. Isotope peak intensities may help to identify the correct composition, but of greater value in this respect is high-resolution mass measurement. From the accurate masses of the molecular and fragment ions, the elanental composition of a neutral fragment can be determined. More about this important technique is discussed in Chapter 3. 

Since fragmentation is not present, isotope peak intensities take on added importance for identification purposes. 

Deisotoping. MALDI-TOF-MS instruments produce high-resolution data in reflec-tron mode. A peptide can be resolved into several isotopic peaks, determined by the number of isotopes of carbon, nitrogen, and others, that it contains. The isotope peak intensities are highly correlated across samples thus, for comparative analysis it is necessary to use a single monoisotopic peak for each peptide. Furthermore, for the identification and characterization of peptides using MS/... 

If more than one chlorine atom or bromine atom is present in a molecule, distinctive isotope cluster patterns are seen in the mass spectrum. Figure 10.8 gives a graphical representation of the isotope peak intensity patterns for Cl and Br. The numerical values for the peak ratios are given in Table 10.8. The patterns arise as follows ... 

Figure 10.8 Graphical representation of relative isotope peak intensities for any given ion containing the indicated number of chlorine and/or bromine atoms. The numeric values are given in Table 10.8. (From the NIST Mass Spectrometry Data Center accessed via http //webbook.nist.gov. 2011 U.S. Secretary of Commerce for the United States of America. All rights reserved. Used with permission.)...

In the present section we describe how to define a match value based on MS/MS data and present several examples that demonstrate how this additional information helps to determine the molecular formula. The main focus is on small and medium-sized molecules with a mass range up to approximately 1000 Da. Match values that reflect the consistency with MS isotope peaks and MS/MS fragment patterns are computed for candidate molecular formulas, and we demonstrate that these match values outperform methods based on isotope peak intensities alone. 

Peaks present far above the relative molecular mass Poor dynamics of peak intensities Wrong isotope peak intensities Tilted peak intensities... 

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